Volume - i - Final

Volume - I Volume - I

IMPACT OF INCREASE IN AXLE LOADS ON

TRACK & BRIDGES ON INDIAN RAILWAYS

SHIV KUMAR*

1.1 BACKGROUND

The Indian economy entered the tenth plan with an expectation of 6%

to 7% annual growth in the GDP and consequently 7.2% to 8.0% growth in

the transport sector. These expectations placed heavy demands on the

already saturated road and rail transport system which coupled with the

inadequacies in the power sector posed a major constraint in the realization

of the projected economic growth. With airways, coastal shipping and inland

waterways being in the fringes, freight transport in India is basically shared

between road and the rail sectors.

The road network in India has grown from 4 lakh km in 1951 to over 30

lakh km now, second largest in the world. Post independence, Indian

Railways (IR) made a flying start almost doubling the transport output in the

first 5-Year Plan. There was, however, a perceptible slowing down from 1968

to 1980 followed by a revival in the last two decades climaxing to introduction

of heavier axle load trains (22.82 tonnes) on certain routes on IR with effect

from May, 05.

1.2 WAKE UP CALL

• Freight traffic on IR has grown 90 times from 5.5 to 500 billion tonne

km from 1951 till now.

• Passenger traffic on IR has grown 80 times from 23 to 1800 billion

passenger km in the same period.

• National and state highways comprising only 8% of the road network

carry 80% of the traffic

• IR’s share of freight traffic has declined from 89% in 1951 to 38% now.

• Golden Quadrilateral road network and induction of multi axle road

vehicles will further make a serious dent on the share of freight traffic

carried by railways.

• Even heavy bulk freight may not remain the exclusive preserve of the IR.

* Director /IRICEN/Pune


1.3 COMPARATIVE EVALUATION OF INDIAN RAILWAYS WITH OTHER

RAILWAYS

The world’s heaviest and longest freight trains run in Australia. With a

payload of 82,000 tonnes and gross load of 99,734 tonnes, the train is formed

of 682 wagons, hauled by eight 6000 HP diesel locomotives.

The 7.2 km long train transfers minerals in bulk from one part of Australia

to other crossing thousands of miles of largely uninhabited and desert areas.

Theoretically, just 20 such trains are enough to carry the entire volume of

about 1.8 million tonnes of freight moved every day by IR, which deploy

5,000 trains of varying capacities to do the job.

A comparison of the railway systems in China and India makes

interesting study. In the decade 1992 to 2002 the route Km on the Chinese

Railways (CR) has grown from minus 6% to plus 14% in comparison to that

of IR. The two railways carried almost the same volume of passenger traffic

both in 1992 as well as 2002. However, in respect of freight traffic, the volume

carried by CR was four and a half times that of IR. They have achieved these

results through more efficient exploitation of track, locomotives and wagons,

and by assigning lower priority to passenger services. CR has a larger

proportion of double line and has adopted automatic signalling more

aggressively than India. As a result, CR operates roughly twice the number

of trains on electrified double line tracks than the Indian Railways.

In 1993 the Ministry of Railways, China decided to increase maximum

freight axle load from 20.5 to 25 tonnes over the next 10 to 15 years at the

same time when mechanized track maintenance was being introduced. Theirs

is a large railway system with high capacity utilization, which has track

materials and components that are not as strong and durable as they should

be. The key to successful mechanization on lines which are being used to

full capacity, where maintenance today is performed manually between trains,

is the ability of mechanized gangs to obtain a sufficiently high quality of

work so that less time is required for track occupancy and maintenance

over a period of years. Stronger, more durable materials and track components

would be essential. Thus, CR planned to lay all major heavy haul trunk lines

with 60 kg or heavier rails. They would lay 29% of main tracks with continuous

welded rail. Higher strength concrete sleepers and matching higher-capacity

elastic fasteners would be introduced. New stone quarries have been opened,

and existing quarries either have been reconstructed to provide quality ballast

or closed. A track maintenance planning system with new technical standards

for track condition management is being implemented. CR are planning an

investment of US$ 200 million in the mega plan period from 2004 to 2020

basically aimed at network expansion, doubling, creation of dedicated

passenger and freight corridors.

1.4 MODERNISATION AND EXPANSION OF INDIAN RAILWAYS

IR have embarked upon a path of modernisation and expansion in a

big way as per vision of the Honourable Prime Minister of India.

Accordingly, Integrated Railway Modernisation Plan (2005-10) has

been formulated on IR with an objective to enhance capacity, improve rail-

port connectivity, introduce higher axle load wagons to carry bulk material

and development of Dedicated Freight Corridors (DFC) and running of freight

train @ 100 kmph on the high density Golden Quadrilateral and its diagonals

connecting the four metropolitan cities.

At present, predominantly running axle load on IR is 20.32 tonnes. Heavier

axle loads will enable carrying more payloads in one train, which in turn will

improve throughput substantially. However, before a heavier axle load is

permitted to run, the safety of infrastructure has to be ensured as it carries

passenger traffic also. Running of such heavy axle load trains on the existing

routes would cause very high stresses on the track and bridge structure, on

the requirements of its components, their maintenance and service life.

As a precursor to the introduction of heavier axle loads, IR took a

bold decision in May,05 to introduce a pilot project of running freight trains

with BOXN wagons loaded up to CC+8+2 tonnes on iron ore routes ( where

CC is designed carrying capacity of wagon). This resulted in introduction of

heavier axle load of 22.82 tonnes and Track Loading Density (TLD) of 8.51

tonnes/m.

Introduction of freight trains with 25 tonnes axle loads has also been

planned on two routes on IR by end of the year 2006-07.

1.4.1 ENHANCING TRANSPORT CAPACITY ON INDIAN RAIWAYS

We are in transport business. Trailing loads and operating speeds are

our principal efficiency indicators. Within the limitations of a loop length

of 686 meters and the existing track loading density of 8.25 tonnes

per meter, the options for enhancing transport capacity of IR are as

under:-

• Introduction of higher axle loads

• Increasing number of axles per wagon

Even though about 12% higher throughput with 22.82 tonnes axle

load and about 23% with 25 tonnes axle load could be achieved, yet

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track loading density increases to 8.51 tonnes/m with axle loads of

22.82 tonnes and 9.33 tonnes/m with 25 tonnes axle load. Since rail

wear is already a matter of concern, it may be aggravated by higher

axle loads.

Higher operating speed may not be possible with higher axle loads.

Increase in number of axles in a wagon means redesigning and

revamping the fleet of wagons which is cost intensive and immediate

solution is not possible. Therefore, to carry more traffic cost effectively,

IR went for the option to introduce higher axle load trains being run

with certain type of wagons (BOXN) and carrying bulk consumables

like iron ore and coal.

Current loading standards for bridges (MBG 1987) permit axle loads

up to 25 tonnes in case of locos and track loading density of 8.25

tonnes/m. The trailing loads of 8.25 tonnes/m translate into trailing

axle loads of 20.32 tonnes at the wheel spacing as in BOX-N wagons.

With introduction of higher axle loads of 22.82 tonnes, IR has been

poised into a select group of Heavy Haul Railways. This would be

the stepping stone for moving ahead towards the regime involving 25

tonnes axle loads and track load density of 9.33 tonnes/m on the

existing network without any hiccups.

The axle loads running on the Heavy Haul routes of American,

Australian, China and other advanced Railways are ranging from 30 to

40 tonnes. However there is major difference in scenario prevailing on

Indian Railway as unlike the other Railways where heavy haul freight

trains run on a dedicated heavy haul lines, in Indian Railways same

infrastructure has to carry both goods and passengers traffic. Golden

Quadrilateral and its two diagonals constituting 16% of route km (25%

of running track km) carry 55% of passenger and 65% of freight traffic

of the IR and are saturated on most lengths.

1.4.2 DEDICATED FREIGHT CORRIDORS ON INDIAN RAILWAYS

Dedicated Freight Corridors (DFCs) have been planned on IR with the

capability of carrying 32.5 tonnes axle load wagons in train formations

of over 15,000 tonnes hauled by multiple units of modern freight

locomotives at speeds of 100 kmph. The loop lengths would be 1500

meters or longer to permit accommodation and crossing of train lengths

of 120 wagons.

RITES has done a feasibility study for providing dedicated freight corridor

on two routes. The 819 km double track Eastern Corridor will run from

Khurja to Sonnagar via Mughalsarai, Fatehpur, Etawah. The 1493 km

double track Western corridor will run from Jawahar Lal Nehru Port in

Mumbai to Dadri in U.P. via Vadodara, Ahmedabad, Palanpur, Phulera,

Rewari & Tughlakabad. Eastern corridor will have an extension which

will run from Khurja to Ludhiana via Meerut, Saharanpur & Ambala.

Western corridor will have an extension to Dandarikalan Container

Depot near Ludhiana via. Rewari, Hissar & Jakhal.

Detailed planning is already underway on the above mentioned two

routes, which were approved in principle by the government in March,

06. They are intended to form the first phase of a network totalling

around 10000 km, which is to be developed over the next decade at a

cost of more than Rs.700 billion. Built to a larger loading gauge and

capable of accepting wagons with axle loads of 32.5 tonnes (Table I),

the corridors will permit the gradual segregation of freight and passenger

services, allowing existing mixed-traffic routes to be optimised as high-

quality passenger corridors.

Table 1. Proposed operating parameters on DFC

Train length 120 to 130 wagons

Trailing load 14000 to 16000 tonnes

Maximum permissible speed 100km/h

Length of loops 1500 m

Average distance between stations 45 km

1.4.3 THE BENEFITS OF HIGHER AXLE LOADS ON INDIAN RAILWAYS

Fewer wagons will be needed to haul the same load, leading to lower

capital cost and possible reduction in wagon maintenance cost, fewer

locomotives, lower fuel consumption per net tonne, reduction in train

wagon kilometre operated, and fewer crew deployment entailing savings

in wages. The railways in North America, Australia, South America,

South Africa and Sweden have all increased axle loads to obtain

significant savings in operating cost. These savings have been

achieved despite increased cost of maintaining track and bridges,

greater component damages and shorter component lives.

The increase of the axle load from 22.5 tonnes to 30 tonnes yielded

40 per cent savings in transportation cost in the US. This in turn

helped the railways in that country achieve significant reduction in

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operational cost of transporting containers and introducing customised

wagons to win back traffic from the roadways.

Boosting wagon productivity that is, how to carry more load per wagon,

or how to achieve higher payload per wagon, has become important

for the IR in view of the increasing threat from the various other modes

of transport, particularly roadways. Overall, IR now accounts for 38

per cent of the country’s total freight movement compared to more

than 89 per cent half a century ago. An analysis of the commodity-

wise market share shows that between 1991-92 and 2000-01, there

was a sharp drop in the rail coefficient for cement, POL, food grains

and iron steel while it improved for coal, iron ore and fertilisers.

Also, while the drop has been significant, not so the extent of

improvement. But, then, the lower axle load presents only one issue.

There are several other issues that need to be tackled along with the

raising of the axle load. Thus, along with higher axle load, the track

load density, that is, the maximum load permissible per metre length

of track (TLD) too needs to be increased. Any increase in axle load

without corresponding increase in TLD will have a marginal effect on

the throughput.

The other issues that deserve careful consideration in this connection

are track friendly bogies, smaller wheel size, and enhancement of

Maximum Moving Dimensions (MMD).

One of the biggest constraints on rail productivity is the restrictive

Maximum Moving Dimensions in IR. This has essentially remained

unchanged since 1913, and today railway systems operating on both

1000 mm and 1435 mm gauges offer higher cubic capacity and

payloads than IR’s 1676 mm gauge vehicles. As a result, according to

a study conducted by consultant David Burns, the unit cost of bulk

freight movement on the IR is many times that of the most efficient US

railroads when considered on the basis of purchasing power parity.

The payload to tare ratio of IR wagon fleet varies from 2 to 2.6, with

only the BOXNLW(2.96) and BOY (3.42) designs approaching the

range of 3.5 to 5 achieved on other railways. Apart from the restrictive

vehicle profile, a large wheel diameter of 1000 mm, a coupling height

of 1105 mm and a relatively low axle load of 20.32 tonnes have not

helped.

The construction of DFCs presents an opportunity to effect a quantum

leap in productivity by modifying the design parameters of freight stock

and liberalising the loading gauge.

Maximum Moving Dimension having a width of 3.66 m and height of

6.81 m has been proposed for DFC.This would permit double-stacking

of 9'-6" high containers. However, it will be important to ensure a

reasonable degree of interoperability between the DFCs and the

existing network, in particular on feeder routes. Accordingly,MMD

having a width of 3.50m and height of 6.81m on routes where double

stack containers will be running and 4.385m on other routes has been

proposed to ensure interoperability of wagons on feeder routes .

In the long term, IR would be expected to upgrade as much as possible

of its existing network to the new standards. During the initial stage,

however, the priority will be to enhance the carrying capacity through

marginal relaxation of the loading gauge coupled with an increase of

axle loads to 25 tonnes on existing network.

Proposed wagon dimensions are as under:-

(i) Width of wagon

Height above rail level (in mm) Width(in mm)

1000 3500

945 3250

<945 3135

Note: (a) It has been decided to reduce the height of goods platform to 840

mm for a width of 1850 mm from track centre on routes where

these wagons will be running.

(b) The width 3500 mm as proposed above is for door less wagons.The

width for wagons with doors will be 3250 mm.

(ii) Height of wagon

For open wagons 4025mm

For covered wagons 4025 mm at side

4385mm at centre

(iii) Wheel Diameter

Maximum 1050 mm and minimum 840 mm when new.

Table II shows the proposed design parameters for wagons able to run

on both new and existing lines. When handling bulk commodities

such as coal, iron ore or cement, it may not be possible to make full

use of the 32.5 tonne axle load within the existing height limits of

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4265 mm in the centre and 3735 mm on the sides. Therefore, proposed

height for open wagons would be 4025 mm and for covered wagons

4025mm at side and 4385mm at centre. This will call for selectively

cutting back the platform canopies on the existing network which

would be a major task but offers immense benefits in terms of enhanced

productivity. In conjunction with a wagon width of 3500 mm and smaller

wheels this would conveniently permit the operation of freight wagons

with gross weight in the range of 110 to 130 tonnes.

Table II. Proposed design parameters of wagons

Proposed Existing

Wheel diameter* mm Maximum 1050 1000

Minimum 840

Bogie wheel base** mm 1830 2000

Floor height **mm 1091 1260

Coupler height** mm 936 1105

Axle load tonnes 32.5 20.32

Track loading density tonnes/m 12.0 7.67

Payload to tare ratio 4.2 2.5

* When new

** A final decision is yet to be taken.

2.1 TRACK STRUCTURE ON HIGHER AXLE LOAD ROUTES

Proposed track structure is as under:-

Feeder routes

• 60 kg (90 UTS rails)

• Existing design PSC sleepers, 1660 per km

• 300 mm ballast cushion

• Flash butt welding joints to be used

• E-clip type elastic fastening with improved rubber pad, being developed

by RDSO shall be used after rail seat design of PSC sleepers is

modified (in future relayings)

• Thick web switches with weldable CMS crossings in turnouts. To start

with, curved switches on PSC layouts can be permitted.

• For sections where 52 kg, 90 UTS rails have been recently laid on

PSC sleepers with 1540/km, 25 tonne axle load my be permitted at a

restricted speed of 60 kmh.

(Authority: Rly Bd’s Track Policy Circular No. 2 of 2006 dt 19th July

2006)

DFC

• 68 kg (90 UTS or greater) Rail

• New design PSC sleepers, 1660 per km

• 350 mm cushion of hard stone ballast

• New PSC sleeper for 30 tonnes axle load to suit 68 kg rail are being

developed by RDSO

• Flash butt welding joints to be used

• E-clip type elastic fastening with improved rubber pad, being developed

by RDSO shall be used

• Thick web switches and swing nose crossings in turnouts

• PSC sleepers have also to be designed for points & crossings, level

crossings, SEJs, bridge approaches

• Formation width in embankment/cutting and centre to centre spacing

will be as follows:

� Single line 6.85 m (as existing)

� Double line 12.35 m

� Centre to centre spacing of DFC tracks 5.50 m

� Centre to centre spacing of DFC track from existing track shall be

6.00 m.

• Suitable thickness of sub-ballast/blanket will have to be provided.

Judicious use of reinforced earth construction will have to be made at

those locations where land width is not adequate to accommodate

slope in formation.

• Curvature will be limited to 700 m (2.50 degree)

• Ruling gradient will be 1 in 200 compensated.

2.2 TRACK MAINTENANCE ON HIGHER AXLE LOAD ROUTES

• A new approach based on mechanised track maintenance will have to

be formulated and implemented consisting of

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� TRC based track geometry measurement to decide need for

tamping

� Need based mechanised track maintenance

� Spurt car based USFD testing

� Rail grinding

� On foot inspection by PWM

• All maintenance will need a daily track possession of 4 hours.

• A service road will be required, all along for facilitating maintenance.

3.1 BRIDGES ON HIGHER AXLE LOAD ROUTES

Proposed loading standards for bridges on DFC is as under:

• Locomotives:

� Axle load 30.0 tonnes

� Tractive effort 60.0 tonnes per loco for double headed

locomotives

45.0 tonnes per loco for triple headed

locomotives

� Braking force 25.0 tonnes

• Train load:

� Axle load 32.5 tonnes

� Track loading 12.0 tonnes/m

density(TLD)

3.2 EXAMINATION OF EXISTING BRIDGES ON FEEDER ROUTES

3.2.1 SUPERSTRUCTURE

Existing structures have been examined by RDSO on the following

premises:

• Actual axle load and axle spacing for moving loads of WAG9

locomotive and 25 tonne BOXN wagons.

• Coefficient of Dynamic Augment (CFA) for limited speed of 60

kmh.

The results of analysis for superstructures of standard spans of bridges

built as per various loading standards are given in Table III.

Table III Results of analysis of standard span bridges

SPEED POTENTIAL OF BRIDGES FOR DIFFERENT

LOADING STANDARDS OF EXISTING BRIDGESSTANDARD

SPAN(m)BGML LOADING

Axle load 22.9 tonnes

TLD 7.67 tonnes/m

T.Effort 47.6 tonnes

RBG LOADING

Axle load22.5 tonnes

TLD 7.67 tonnes/m


MBG LOADING

Axle load 25.0 tonnes

TLD 8.25 tonnes/m


1-20 60 kmph 60 kmph 60 kmph

20-63 60 kmph 55 kmph 60 kmph

78.7 50 kmph 55 kmph 55 kmph

3.2.2 SUBSTRUCTURE

Substructure designs are based on local geographical features.

Therefore, substructure analysis has to be done of all bridges by the

zonal railways.

3.2.3 BEARINGS

• Bearings of standard spans spans designed for MBG loading have

been found fit for 25 tonnes axle load.

• Bearings of standard spans designed for BGML/RBG loading having

spans 31.9 m, 47.25 m, 63.0 m and 78.8 m (all effective spans) needs

strengthening.

• One time inspection of bearings apart from schedule inspection may

have to be done by zonal railways before permitting 25 tonnes axle

load.

3.2.4 BRIDGE INSPECTION & MONITORING

Following locations/members will have to be monitored as being critical

from fatigue consideration:-

� Connection of cross girder with stringer.

� Outstanding leg of the top compression flange of the stringer at

the junction with web rivet.

� Rivets connecting bottom flange angle of the stringer with web

at mid span.

� Vertical members at connection with top chords.

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� Rivers connecting bottom flange of cross girders with web at

mid span.

� Rivets of splice joints of bottom flange in plate girders.

• Physical condition of bridges needs to be certified by zonal railways.

• More intensive bearing & substructures inspection will have to be done

by zonal railways.

In this connection, directives for checking superstructure of non

standard spans including arch bridges, bearing of non standard spans

and substructure of all bridges have been issued to zonal railways by

RDSO.

4. CONCLUSIONS

• Experience of increased axle load on other major railway systems in

world has shown that fatigue life of rail comes down drastically with

increase in axle loads and it may take 5 to 10 years for heavy axle

loads to tell its effect on rails.

• Rail grinding is required to be undertaken to prevent Rolling Contact

Fatigue (RCF) and increase rail life.

• Till rail grinding is implemented, there is a need to safeguard against

rail fractures due to RCF by extensive and effective USFD.

• The rail life obtainable under present axle load itself is in range of 350-

400 GMT which is way below the prescribed life of 800 GMT for 90

UTS rails.

• Fatigue life of rails also needs a review. Life of rails subjected to heavier

axle load needs to be realistically brought down till grinding is

introduced.

• Installation of way-side vehicle condition monitoring systems for

identifying poorly performing wagons will go a long way in reducing

stress state of track by identifying defective rolling stock and removing

them from service. Some of these are:-

� Wheel Impact Load Detector 9WILD)

� Truck Performance Detector (TPD)

� Hot Bearing Detector (HBD)

� Dragging Equipment Detector (DED)

� Hot Wheel Detector (HWD)

� Truck Hunting Detector (THD)

� Wheel Profile Monitoring System etc.

• Development of track friendly, self steering bogie will be required in

the long run.

• A review on the conclusion of the pilot project to run 22.82 tonnes axle

load trains needs to be done in June 07 to decide its universalisation

all over IR.

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RATIONALE BEHIND INCREASE IN AXLE LOAD

ON INDIAN RAILWAYS & ROAD AHEAD

V.K. JAIN*

1.0 INTRODUCTION

After economic reforms and liberalization, Indian economy has shown

a robust growth; the rate of growth 7.5% during 2004-05, 8% during 2005-06

and projected growth for 2006-07 is 7.5% – 8%. This rate of growth will

result in about 10 – 12% growth in transport sector. IR must gear up to carry

this extra traffic.

Capturing this extra traffic will not be an easy task for IR. There are

two basic problems. First, there is considerable improvement and

augmentation in road sector. NHAI is adding state of the art road network at

a fast pace. High capacity trucks are available, which provide door to door

services. In the present scenario, even bulk movements are opting for road.

Railway’s market share have fallen during last 10 years.

Secondly, golden quadrilateral along with its two diagonals and grand

chord, which is only 16% of the network, carries 65% of the freight traffic.

This network is saturated in many sections, part of the sections is still

single line or non-electrified. Other important constraints are inadequate

development of terminals and warehousing capacity, less axle load, poor

tare to payload ratio and priority to passenger traffic.

IR is to gear up to find out technical solution of these constraints and

to accept the challenge from other mode of transportation, especially road.

2.0 WORLD SCENARIO

2.1 SWEDISH RAILWAYS

Sweden is located between latitudes 550 to 690 and faces very harsh

weather conditions. Labour cost is very high. Main freight traffic is

iron ore, steel, timber and paper. Most of the companies are global

and will prefer to shift the business in case cost is high. Therefore, to

keep the transportation cost low, Swedish Railways introduced 25 T

axle load on ore lines about 40 years back. Railway market share in

* Executive Director Civil Engg. (P), Railway Board

Sweden is 30%, which is double when compared with 15% of Europe.

After the deregulation of road haulage, net permissible pay load of

Swedish trucks has increased from 31 to 40 tonnes, which has cut

the average trucking cost per tonne by approximately 22%.

This has posed further challenge to Railways. To accept this challenge

from road sector, Swedish Railway has planned:-

(i) Introduction of 30 T on ore lines.

(ii) Universalisation of 25 T.

Two studies have been conducted to work out the economics of running

30T axle load; one by ZETA-TECH Associate, Inc. of USA and second

by internal team of Banverket/Jerbaneverket. These studies have

shown:-

(i) Increase in track maintenance cost (including early renewal

cost) will be 13 – 15%.

(ii) Saving in overall operation cost will be 27-30%.

These studies have recommended:-

(i) Rail grinding and lubrication.

(ii) Use of movable crossing.

(iii) Use of head hardened rails.

(iv) Strengthening of bridges.

Railway is adopting line by line approach to upgrade the network.

2.2 US RAIL ROADS

Total track kms is about 2 lac 75 thousand kms. Track gauge is

mainly 1435 mm. Most US Railways are private owned and run only

freight trains. Operation of long distance passenger trains is monopoly

of AMTRAK. Rail road employment has fallen drastically from 4 lac

50 thousand employees in 1982 to one lac 55 thousand in 2003.

Safety regulation is the responsibility of the Federal Rail Road

Administration (FRA).

Traffic on US Rail Road can be divided into four general categories.

(i) Unit train traffic (coal, ore, grain)

(ii) Inter-modal traffic (ocean containers and truck trailers)

(iii) Mixed freight traffic (car load, plastic, automobile).

(iv) Passenger.

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Coal, most of which move in unit trains, account for 42.9% of US Rail

Road traffic originates and about 20% of revenues. Inter-modal traffic

have gone up rapidly in recent years. Operation of double stack

container was introduced in 1977 and most container trains are double

stack. Main axle load is 30 T, although a number of rail roads have

begun purchasing stock with 32.5 T and 35.5 T axle load.

Average length of train is 7000 feet which is going to be extended to

8000 feet. Trains load is 10,000 to 11,000 tonne and locomotives are

typically of 3000 to 5000 horse power. Now 6000 horse power

locomotives are being offered by both General Motors and General

Electric.

Structurally 60 kg rails are sufficient to support 30 T axle load, but

there is considerable maintenance problem, USA is mainly using 67

to 71 kg rails on timber and concrete sleepers. Ballast cushion is

300 mm with hard ballast. Rail hardness is 380 BHN and more.

Rail grinding is must for higher axle load operation. One machine

with 120 stone is able to grind about 10,000 km in a year. Operational

speed is 20 kmph. Grinding work has been mainly outsourced.

On-board rail lubrication is used on curves sharper than 40.

New turn outs are mainly Thick Web Switches with swing nose crossing.

On freight routes, track is inspected by a staff twice a week and by

TRC @ 3 to 6 months depending on traffic density. No regular

inspection by Supervisors or Officers.

2.3 AUSTRALIAN RAILWAYS

Australian Railways has been reorganised into number of small units,

three important Railways are Australian Rail Track Corporation (ARTC),

BHP Billiton and Queensland Railways. ARTC is operating 30 T axle

load in Hunter Valley, BHP Billiton is operating 36 T to 40 T axle load

on ore lines and Queensland Railway is operating 26 T axle load on

coal lines.

Salient features about ARTC are –

� ARTC was formed in 1997. It is a Government owned company.

Total length of track under ARTC is about 10,000 kms and total

number of employee is about 25,000. It operates 30 T axle load

in Hunter Valley, which carries mainly coal traffic. 30 T axle

load was introduced in 1994 -1996. Maximum train length is

1550 m and maximum train load is 7,500 T.

� Hunter Valley is a dedicated freight line. Entire track is track

circuited and with bi-directional signaling.

� Total length of the track in Hunter Valley is 600 kms. Rails are

60 kg, out of which, 395 kms are head hardened, 90% sleepers

are concrete sleepers. Track is inspected by Keyman twice a

week and by TRC once in 3 months.

� Rail grinding is a normal feature and done through Service

Contracts. For sharp curves of radius less than 400 m, grinding

is done after passage of 10 GMT and for straight track, grinding

is done after passage of 20 GMT. Average life of 60 kg rails is

1200 MT.

� To enhance safety, ARTC has provided –

(i) In-motion weigh bridges at loading points,

(ii) Hot box detector.

(iii) Wheel USFD tester.

(iv) Wheel Impact Load Detector (WILD).

� Only one derailment in last 18 months.

2.4 SOUTH AFRICAN RAILWAYS

South Africa has a well developed transportation system. The South

African Railways and Harbour Administration, established in 1910,

managed the operations of most of the nation’s transport network; in

1985, it became the South African Transport Services (SATS). In

1990 SATS reorganized as the Public Commercial Company, Transnet.

Transnet has six business divisions – Spoornet to operate the rail

roads; Portnet to manage the country’s extensive port system;

Autonet, a comprehensive road transport service; South African Airways

(SAA); Petronet to manage petroleum pipelines; and a parcel delivery

service, known as “PX”. The Government is the sole share holder in

Transnet.

Spoornet, the National Rail Authority, manages a network of 21,303

km of 1065 mm gauge track and 314 km of 610 mm gauge track.

Spoornet has restructured its operation to compete with road sector.

COAL link and OREX, manage Spoornet’s coal and iron ore traffic

over the Richards Bay and Sishen – Saldanha lines respectively. The

general freight services divisions are grouped into 15 industry based

segments.

Since 1991, the Spoornet work force has dropped from nearly 1,20,000

to 43,736 in March 1999.

16 17


Other salient features :

Rails 60kg (1950 km), 57kg (4,370 km)

and rest 48kg, 40 kg & 30 kg.

Sleepers Concrete, Steel & wood.

Sleeper density 1538/km & 1440/km.

Fastenings (Pandrol) Fist, E 3131 chairs.

Maximum axle load Locomotive - 29 tonnes.

Wagons - 26 tonnes.

Traffic:- About 210 million tonnes.

Fast freight trains operate on eighteen routes nation wide at a maximum

speed of 120 kmph. However, common speed is 60 kmph.

In 1989, Spoornet created a World record by running a 71,600 – ton

train at a speed upto 80 kmph on 861 – kilometer Sishen – Saldanha

ore lines.

3.0 HIGHER AXLE LOAD ON INDIAN RAILWAYS

3.1 BOXN was introduced in 1982 with an axle load of 20.32 T and with a

speed potential of 75 kmph in loaded condition and 80 kmph in empty

condition. Minimum track structure prescribed was 90 R rails with

M+4 sleeper density. Rails were mainly 3-rail panels with large number

of bolted joints. Sleepers were mainly metal sleepers, which used to

be maintained manually. Over the years, not only the track structure

has improved but maintenance standard has also improved

considerably. 90 R rails have been replaced with 52 kg and 60 kg

rails. Section modulus, an indicative parameter of structural strength,

of 90 R rails is 235.65 cu cm while that of 52 kg rail is 285.50 cu cm

and that of 60 kg rails is 377.4 cu cm, i.e. section - modulus of 60 kg

and 52 kg rails are higher than 90 R by 60.15% and 21.15%

respectively.

In addition to improvement in rail structure, sleeper technology has

also changed completely. Metal sleepers have been replaced with

PSC (Pre-stressed concrete) sleepers. These sleepers are provided

25 cm - 30 cm ballast cushion and are maintained with the state-of-art

track machines. With the introduction of 52 kg/60 kg rails on PSC

sleepers, track modulus, a parameter, which indicates ability of track

as a whole to carry the axle load, has improved considerably.

Improvement of other infrastructures such as rolling stock is also a

continuous process. Based on day to day experience, the rolling

stock is also improved continuously to ensure better availability,

reliability and safety. BOXN wagons have been provided with elastomeric

pads and composition brake blocks which has improved its suspension

system and reliability.

Considering these improvements, Railway has been increasing the

loading capacity of wagons from time to time. In past the 2 T loading

tolerance was provided for BOXN for loose commodities. The

Permissible Carrying Capacity (PCC) or Chargeable Carrying Capacity

of BOXN wagon was enhanced by 2T in July, 1997.

BOY, BOBS and BOBSN wagons have been introduced on Indian

Railways with an axle load of about 22.9 T. These wagons are running

for a considerable time and no adverse effect has been noticed on the

track and bridges.

Taking the advantage of improved track structure, rolling stock (BOXN)

and experience of running of BOY, BOBS & BOBSN, Indian Railways

took a historic decision in May 2005 to increase the axle load of

BOXN to 22.82 T (CC+8+2 T) on selected iron ore routes as a Pilot

Project. In November 2005, Board has permitted CC+6+2 T for ‘E’, ‘F’

& inferior grade of coal in BOXN on identified routes as Pilot Projects.

Further, CC+6+2 T has been permitted on identified routes for other

type of wagons i.e. BOXNHS, BOBR, BOBRN, BCN, BCNA, BCNAHS

& BOST for all type of commodities. Presently, CC+8+2 T is running

on 42 routes covering eight Railways (ER, ECOR, SR, SER, SCR,

SECR, SWR, WCR). CC+6+2 T is running on about 50 routes covering

14 Railways (except SWR & SR).

3.2 PRECAUTIONS

While introducing this Pilot project, following additional precautions

have been taken:

(i) 90 R rails are to be replaced on priority. Till renewal, maximum

permitted speed of higher axle load trains on 90R track has

been restricted to 30 kmph.

(ii) Maximum permitted speed of higher axle load freight trains has

been restricted to 60 kmph in loaded direction on other than 90

R track.

(iii) Rails and welds are being tested by Ultrasonic Flaw Detecting

Equipment (USFD) for gauge corner fatigue.

18 19


(iv) Thorough physical inspection has been done for bridges.

(v) Selected bridges are also being monitored by instrumentation

to assess the actual axle load and their effects in coordination

with the Institute of Learning like SERC, Chennai, and CRRI,

Delhi etc.

3.3 INSTALLATION OF WEIGH BRIDGES

To ensure control on adherence of Permissible Carrying capacity, an

action plan has been prepared by Railways to install 101 additional

electronic in-motion weigh bridges, out of which 58 nos have already

been commissioned and rest are likely to be commissioned by

December 2006. This will ensure that no overloading beyond what is

permitted actually takes place.

3.4 MONITORING MECHANISM

The project is being monitored by a Multi-Disciplinary Core group

comprising of Heads of Department from Civil, Mechanical & Operating

Department and progress is being reviewed at the level of General

Managers of Zonal Railways. Quarterly Review Reports are being

submitted to Railway Board.

4.0 ECONOMIC TURN AROUND OF INDIAN RAILWAYS

Indian Railway is passing through best phase in her history. Only in

2001, an expert Committee had put Railway on the verge of financial

collapse. It had forecasted an additional financial burden of over Rs. 610

billion to Government of India. This has been proved wrong and Railway

has generated enough resources internally and has fund balance of more

than Rs.136 billion. The turn around of Indian Railways is real, touchable

and backed by record breaking figures. Indian Institute of Management,

Ahmedabad & Harword University are so impressed with this turn around

that they want to take it as case study for their students. Today Railway

has got sufficient fund to finance safety & throughput enhancement works,

staff welfare activities and projects of national importance. All this turn

around has been possible due to bold decision of Engineering Department

under the leadership and guidance of Shri R.R. Jaruhar, Member Engineering

to permit CC+8+2 T and CC+6+2 T in BOXN and other wagons. This has

resulted in extra freight loading without corresponding increase in operational

cost resulting in huge surplus for Railways. This has been well appreciated

by one and all, including media.

5.0 ROAD AHEAD

5.1 Detailed commodity-wise analysis of freight traffic carried in 2004-05

is as follows:

Commodity % of originating % of NTKM

tonnage

Coal 45.07 39.74

Foodgrains 7.73 15.36

Iron & Steel 3.05 3.85

Iron ore & other ores 15.99 10.18

Cement 8.93 7.09

POL 5.31 5.16

Fertilisers 4.78 5.33

Others 9.14 13.39

90% of freight traffic is from 8 major commodities i.e. coal, foodgrain,

iron and steel, cement, POL, fertilizers, iron ores and other ores.

Important type of wagons are BOXN, BRN, BOBR, BOBS, BCN and

Tank. 60% of traffic is carried in BOXN wagons. Its carrying capacity

is 58-60 tonnes and its pay load to tare ratio is only 2.35, which is

very less when compared to international standard.

5.2 Indian Railways is planning to construct dedicated freight corridors.

Phase – I will consist of two corridors i.e. Western and Eastern

corridors i.e. Jawaharlal Nehru Port (JNPT) to Tughlakabad/Dadri near

Delhi and Ludhiana-Sonepur-Kolkata. Formation and bridges on DFC

will be constructed for an axle load of 30 T and initially track will be fit

for 25 T axle load which will be upgrade to 30 T as and when wagons

and feeder routes are available for 30 T axle load operation. About 24

routes covering 4200 route km have been identified as feeder routes to

these DFCs which will also be upgraded for running of 25 T axle load.

Subsequently, four more dedicated freight corridors will be planned as

follows:

North – South : (1800 km)

East – West : (2000 km)

East Coast : (1500 km)

Southern : ( 900 km)

20 21


5.3 In addition, about 25 routes covering about 7000 kms have been

identified on iron ore circuits which will be upgraded for 25 T axle load

routes. Bhilai – Dallirajhara in SECR and Gua-Barazamda-Padapahar-

Banspani-Daitari-Cuttack-Paradeep section in SER and ECOR are

being upgraded to introduce 25 T axle load within this financial year.

Efforts are being also made to universalize CC+6+2 T.

5.4 Field staff working on these enhanced axle load routes, especially on

proposed 25 T axle routes have to be careful and alert. Both track

and bridges should be kept under close monitoring. To assess the

effect of higher axle load on track and bridges, one of the important

requirements for 25 T axle load is grinding of rails. Indian Railways is

already planning to procure two rail grinding machines. Till these

machines are procured and commissioned, track should be monitored

intensively with the help of USFD machines for any gauge corner

fatigue crack.

6.0 CONCLUSION

6.1 With the economic reforms and liberalization, there is surge in transport

sector. Railways has to compete with other modes of transportation

to carry this extra traffic.

6.2 Swedish Railways has introduced 25 T axle load on ores line about 40

years back and is now planning to introduce 30 T axle load.

6.3 On US Rail Roads, operation of double stack container was introduced

in 1977 and axle load is predominantly 30 T.

6.4 On Australian Railways, ARTC is operating 30 T axle load in Hunter

Valley, BHP Billiton is operating 36 T to 40 T axle load on ore lines

and Qeensland Railway is operating 26 T axle load on coal lines.

6.5 Spoornet is operating 26 T axle load on coal and iron ore routes.

6.6 Indian Railways has to adopt higher axle load on economic and

technical considerations. This will improve wagon productivity and

will reduce operational cost.

6.7 Pilot project of running of CC+8+2 T on identified iron ore routes and

CC+6+2 T for coal and other commodities has resulted in complete

financial turn around of Indian Railways. It has generated a surplus of

more than Rs. 136 billion. Now sufficient fund is available with Indian

Railways to finance safety & throughput enhancement work, staff

welfare activities and projects of national importance.

6.8 Since India Railways has got mixed traffic lines, precautions specified

for running of enhanced axle load must be adhered to.

6.9 Dedicated freight corridors will completely change the scenario of freight

operation in coming years.

22 23

Volume - I Volume - I*Executive Director/ Track, RDSO, Lucknow

ENGINEERING JUDGMENT AND RISK

MANAGEMENT BASED APPROACH FOR

INTRODUCTION OF HEAVIER AXLE LOADS

ANIRUDH JAIN*

SYNOPSIS

About 50 years ago, many railways in the world began to increase axle

loads to provide more efficient and lower cost transportation of bulk commodities.

Serious problems with rail, track, wheels, and rolling stock emerged. Numerous

companies and administrations undertook lot of research initiatives to overcome

these serious problems. Indian Railways have now decided to increase the axle

loads. Similar problems are expected. It is, therefore, imperative that a solution

to the anticipated problems is sought by looking into the experience of others.

Nevertheless, it is a fact that problems as well as solutions will differ from system

to system and lot of engineering ingenuity and Judgment is required to sail

through. An attempt has been made in this paper to cover various aspects of

Engineering Judgment and Risk Management to analyse suitability of a railway

track for higher axle loads. A similar approach can be developed for bridges and

rolling stock.

1. INTRODUCTION

1.1. A 30 ton axle load wagon carries almost 60% more commodity than

does the conventional 20.32 ton wagon with only a marginal increase

in the empty weight of the wagon. This results in an increase in the

efficiency of rail freight movement due to an improvement in net/tare

ratio.

1.2. If wagons can be loaded more heavily without significantly increasing

their tare weight, railroads stand to realize savings in:

• Capital costs (fewer wagons needed to move a fixed volume of

traffic).

• Fuel costs (reduced tare weight means an improved ratio of net

load to gross weight).

• Crew costs (increased carrying capacity may permit a reduction

in the number of trains operated).

• Locomotive costs (if train net load can be increased within the

same gross train weight, there is more revenue for the same

locomotive mileage).

1.3. In addition, longer track possessions will be possible with reduction

in number of trains.

1.4. Heavier wagons impose heavier axle loads on the track, which means

more frequent maintenance and shorter track component life. The

same is true for bridges. As a result, costs of maintenance and

rehabilitation of track and bridges increase. Yet these increases in

axle loads are attractive as the savings in operating and ownership

costs are significantly higher than the increases in track costs. This

is because heavier wagons move long distances on high-quality main

line track, and each km traveled means additional savings.

1.5. Many Railways in the world began to increase axle loads to provide

more efficient and lower cost transportation of bulk commodities, about

50 years ago. Many systems, particularly North American Railroads,

have introduced axle loads of 30 ton or heavier on their network,

extensively. In some countries like South Africa, Australia and Brazil,

such heavier loads have been implemented on certain identified routes

either by upgrading an existing line or by constructing a new dedicated

line.

1.6. Indian railways, in order to meet the growing demand of traffic as also

to reap the benefits of lower transport cost with heavier axle loads,

have decided to construct two Dedicated Freight Corridors, in Mumbai

– Delhi and Kolkata – Delhi Sectors. Standards for construction and

maintenance for these are to be decided. Also, with the construction

of these corridors, many sections of the existing network will be

required to feed the corridor. These sections will, thus, be required to

carry equally heavy axle loads and up-gradation strategies for them

are to be chalked out.

2. THE DEMAND

2.1. Armed with the information that in Australia, USA and to some extent

in South Africa, Heavier axle loads have been introduced without much

inputs, Indian railway’s policy makers are questioning the engineers

“When USA, Australia and SA can introduce 30 ton axle loads on rails

lighter than 90 Lbs, why not we?“

25


2.2. Though lot of experience is available on heavy haul with the railway

systems which have implemented it, the same can not be straight

away copied on to the Indian system. On some heavy haul railroads,

heavy haul operations constitute only a small fraction of total traffic on

the line. Some other systems have switched over from mixed traffic to

a dedicated heavy haul operation.

2.3. Solutions applicable to the case of a dedicated mine-to-port line with

dedicated locomotives and rolling stock are different from those to a

railroad with mixed traffic. There is no perfect solution that applies in

all the circumstances.

2.4. There are a variety of approaches that should be examined before

arriving at an optimum solution for a particular problem. As the approach

changes, maintenance practices will have to be up dated. Risk factors

will have to be identified and guarded against.

3. THE PRESENT INDIAN APPROACH ON RAIL MANAGEMENT

3.1. Presently, the approach is based on the logic that the section of rail is

so chosen that the stresses in rails remain within allowable limits at

all times and under all conditions, for the operating axle loads. Rail

wears out under repeated action of wheel, looses its section and is

required to be replaced due to loss of section and hence its ability to

carry the loads.

3.2. As the wheel moves on rail, it applies not only the vertical load but

also lateral forces due to various reasons. Due to dynamic effect of

moving wheel the vertical wheel load is to be enhanced by a certain

factor called dynamic augment. Value of the dynamic augment varies

with the type of vehicle, particularly its suspension. Maximum lateral

force, to be resisted by rail alone is limited at a certain value because

it is presumed that at forces higher than this, entire track frame will

move resulting into loss of alignment.

3.3. In addition, the rail is also subjected to stresses due to effects of

temperature, residual stresses, flexing of rails at curves etc.

3.4. To calculate the stresses in rail under wheel load the theory in use

assumes the track to be a continuously supported beam on elastic

foundation. The basic theory has remained the same though better

tools are now being employed to solve the equations with the use of

Numerical methods. Computers have also enabled a better modeling

of the track with more realistic values for stiffness and dampening

factors being used.

3.5. Thermal stresses and to some extent the residual stresses are added

to the Bending stresses calculated due to wheel load. A rail section is

presumed to be safe if the stresses, so arrived, are with in the yield

stress of the rail.

3.6. Stresses at the point of contact between rail and wheel had not been

questioned, till recent past.

4. ACTUAL EXPERIENCE WITH TRACK IN FIELD

4.1. If the above approach is entirely correct, no rail shall fracture except

in very adverse circumstances wherein the rail may have some inherent

defect. Alternatively, the fracture may be caused if a defective wheel

imposes an impact load which is beyond all predictable limits. In

practice, it is observed that rails are developing flaws in head and in

extreme cases rail failures are taking place.

4.2. Studies have indicated that the contact stresses at the point of contact

of rail and wheel are high. As the load is transferred from wheel to rail

in a very small contact patch, very high contact stresses are developed,

exceeding yield point. Normally observed levels of stress at various

levels of track structure are as follows:

• Contact stress > 700 MPa

> yield stress

• Rail bending stress < 180 MPa

• Sleeper Bearing stress < 1.4 to 3.0 MPa

• Ballast bearing stress < 0.4 to 0.6 MPa

• Sub grade bearing < 0.1 to 0.15 MPa

4.3. In extreme cases, contact stresses as high as 1.5 GPa are caused

leading to hydrostatic compression resulting in to surface and sub

surface cracks due to shear failures. Accumulation of such cracks

ultimately leads to rail failure. This phenomenon has not been taken

care of in the approach discussed in Para-3 above.

4.4. Similarly, Concrete sleepers are designed against bending failure but

heavier axles are causing problems due to rail seat abrasion.

4.5. Thus there is a need to have a new look at the entire process, identify

the problem areas, effects, remedies and to set standards for

construction maintenance of infrastructure for heavier axle loads.

26 27


Engineers are required to use best of their Judgment to face the

challenge.

5. ENGINEERING JUDGMENT

5.1. Looking at the anomalies brought out in Para 4 & 5 above, it comes to

mind that there is something more than just the theory. There is a

marked difference between theory and practice which calls for a fair

judgment on part of the engineer to decide the course of action. Father

of Soil Mechanics, Karl Terzaghi had once said, “I produced my theories

and made my experiments for the purpose of establishing an aid in

forming a correct opinion and I realized with dismay that they are still

considered by the majority as a substitute for common sense and

experience.” In complex engineering problems, theory is not an end it

is just an aid to help one in arriving at a decision.

5.2. Civil engineers, often face great uncertainties in their work.

Uncertainties about the loads that structures will have to withstand,

about the properties of the construction materials used, about what

kind of computer modeling to do and how faithful the model is to the

physical system. This problem is compounded by the fact that the

engineer of even a simple structure is faced with a huge number of

parameters that can be combined in many permutations, so choices

have to be made intuitively. An experienced engineer applies his or

her judgment to try and ensure that solutions will really work in practice,

allowing for the assumptions, variations and uncertainties of practical

systems.

5.3. The term engineering judgment means how an “Engineering Issue” is

analysed, inferred and concluded by an engineer or a committee of

engineers. The engineering judgment of different engineers could be

different, what really matters is to be able to defend ones judgments

on any engineering issue, based on solid engineering reasoning.

5.4. Engineering Judgment becomes more important in engineering

applications where one or more phenomenon is not known with a fair

degree of certainty. Problems in the fields of geotechnical engineering

and engineering geology require us to work with very limited data

about a complex environment where conditions can change radically

over a short distance and an engineer has to use lot of judgment even

with an element of calculated risk.

5.5. Railway track engineering is an equally complex subject. Firstly, the

forces applied by a moving wheel to the rail are not constant and

depend on a number of factors. Then the assumption of track being a

continuously supported beam on elastic foundation is not entirely

correct. If we consider the contact stresses, contact patch is always

moving and its area is varying. Any decision about suitability of a

track structure for a given axle load, thus, calls for a lot of engineering

judgment.

5.6. Engineering Judgment requires that we use our skills and

professionalism to bring about good solutions that recognise the

realities of their business context. There is an old adage that ‘an

engineer is someone that can do for half a crown what any fool can do

for a pound’. It is this statement which is important in the context of

increased axle loads on railway track. It is easy to decide that the

track is to be replaced but the effort shall be to manage without

replacing and ensuring safety at the same time.

5.7. To understand the complexity, I will like to tell a small story:

“A Mathematics teacher took his 8th class students out for picnic. A

small river was to be crossed. The teacher took a rod, surveyed the

river and calculated its average depth. He then calculated the average

height of his class, including himself to find that the average height

was at least 1½ feet more than the average depth. It was now easy for

him to decide that the river can be safely walked through by the bunch

of students. Result! Only 20% of the students could cross by walking

rest sank.

A family of Father, Mother and two kids was waiting by the side.

Observing the fate of the school teacher and his class, the parents

decided the course of action. Both the adults carried one child each

on their shoulders and all 4 crossed the river safely.

There was yet another couple waiting to cross the river. Encouraged

by the success of the family, they decided to walk through. Result!

Husband crossed safely but the lady went afloat and had to be rescued

by the man. Remember Archimedes!”

An analysis of the above story tells:

i. Give reasonable thought to all the aspects, don’t jump to a

conclusion based on what comes to mind in the first instance.

ii. History and experience of others shall be given due weightage

while making important decisions.

28 29


iii. What happened once may not be true always as the

circumstances may be different.

5.8. Engineering judgment is a complex whole of theoretical knowledge,

collection of data, past experience, prediction of behavior. Some people

feel that they have lot of theoretical background in a field, some feel

they have more experience than any body else in the field hence they

are right. Bur the need is, they should learn to respect each other and

their technical contributions.

5.9. Engineering has traditionally used imagination, judgment, reasoning

and experience to apply science, technology, mathematics and

practical experience to convert concepts into reality. Engineers must

be able to work in teams, with colleagues from a variety of disciplines,

and must learn how to think across these disciplines in order to solve

problems that are affecting them, in our case it is rail wheel interaction.

5.10. The process can be better explained through a flow chart, shown

below. This may some times be an iterative process particularly for

projects like ours’ where in it is possible to take a corrective course in

case it is observed that some decisions have not gone well.

6. THE IMPLEMENTATION STRATEGY

In our scenario the implementation strategy shall be as follows:

6.1. INVESTIGATE PRESENT PROBLEMS

Indian Railways are already faced with many problems. Prominent

amongst these are Rail Fractures, Weld Failures and in-adequate Toe

load. With increase in axle loads, the problems are going to increase.

We have to safeguard against the risks due to these problems while

simultaneously working for their redressal.

6.2. OBSERVE WHAT OTHERS ARE DOING

Railways around the world have increased the axle loads. They have

implemented some very good practices about prediction and

maintenance. IR must study all these and consider their

implementation. Some of the noteworthy practices are:

• Guaranteed, daily track possession for fixed hours for reliable

inspection and maintenance activities.

• Rail Grinding.

• Detection and Removal from Service of Defective Rolling Stock.

• Track Friendly Bogies.

• Rail Lubrication.

• Effective Ultrasonic Testing with Data Management, enabling

Rail Fracture Prediction as well as Detection.

• Track Inspection Aided with High Speed Video Cameras.

6.3. IDENTIFY ALTERNATIVES

6.3.1. When the aim is to increase the through put, first question that

comes to mind is why not the train length be increased? We have

our own problems of loop lengths, coupler forces, synchronization

etc.

6.3.2. Loop length is perceived to be that big a problem that even with

increased axle loads of 25 & 30 tons, pressure is on to accommodate

the volume in a wagon only as long as the existing one with 20.32

ton axle load.

6.3.3. It is now certain that axle loads are to be increased may be to 25

Ton in first step and 30 tons in the second. We have three options:

I. Relay the track first then allow heavier axle loads.

II. Increase the axle loads first then think of any relaying.

Analyse

Predict

Investigate

Try

Observe

Evaluate

Implement

30 31


III. Yet another option is to increase the axle load without any inputs

but then, to ensure safety of passenger trains, slow them down

to 50 kmph.

6.3.4. A mixed approach is better, lay a criterion for relaying, say all rails

lighter than 52 kg 90 UTS to be renewed before any increase in axle

loads, all 52 kg 90 UTS rails to be renewed with 60 kg 90 UTS rails

in due course and 60 kg rails can be trusted to carry axle loads up

to 25 tons.

6.3.5. Sleepers and fastenings also have a role to play particularly when

heavier axle loads are to be carried. Preferred sleeper type is PSC.

Fortunately, most of our heavy density routes are having PSC

sleepers. Other sleeper types are also acceptable but then their

densities will have to be increased for various reasons.

6.3.6. Also consider if the special track works like Points and Crossings,

Switch Expansion Joints etc can be trusted to carry the increased

axle loads. A sound judgment shows that PSC sleepers underneath

these track works have given us a greater confidence and we can

begin with the existing layouts.

6.4. PREDICT RISKS

6.4.1. We have the benefit of others' experience due to our late start.

Some of the problems faced by others are:

I. Increase in rail fractures.

II. Increase in weld failures.

III. Excessive creep.

IV. Sleepers going out of square, even PSC.

V. Rail seat abrasion on PSC sleepers.

VI. Rounding of ballast and rapid deterioration of track geometry.

VII. Increased incidence of Track buckling.

6.5. USE COLLECTIVE WISDOM

Railway Engineering is a specialized job. A Railway Engineer

accumulates lot of knowledge with his observation and experience.

Two railway engineers think in a similar way, give them a problem and

they will come out with same solution though they might not have

consulted each other. Put these engineers together and they will come

out with a better solution. An individual may not be prepared to take

the risk but collectively, a group of individuals may decide on a course.

Pooling of their experience and knowledge will help. Possible problems

shall be discussed and solutions suggested. A benefit of discussing

in a group is that many solutions may be discussed best of them may

get selected.

6.6. DECIDE THE MOST SUITABLE COURSE

The most suitable course shall be decided after completing the above

analysis. The requirements, the resources and the speed at which

the proposed new systems can be inducted must be given due

weightage in deciding the course.

7. SAFEGUARD AGAINST RISKS

Having identified the risks, suitable systems shall be implemented to

guard against them. Let us take the perceived risks, one by one:

7.1. RISK OF DAMAGES DUE TO OVER LOADING AND DEFECTIVE

ROLLING STOCK

7.1.1. With increased axle loads prevention of over loading becomes an

essential requirement. In addition, wheel and track imperfections

cause additional dynamic forces on rail. Other vehicle defects like

hot axle, hot bearing, poorly performing bogie (Truck), hunting bogies

etc also cause damage to track and may sometimes result into

derailments. In order to detect the defects in rolling stock, timely,

various way side condition monitoring systems have been developed

globally such as:

• Wheel Impact Load Detector (WILD),

• Truck Performance Detector (TDP),

• Hot Bearing Detector (HBD),

• Dragging Equipment Detector (DED),

• Hot Wheel Detector (HWD),

• Truck Hunting Detector (THD),

• Wheel Profile Monitoring System etc.

7.1.2. To integrate damage control strategies from different disciplines in

railway engineering a vehicle track interaction measuring system to

evaluate the performance of the vehicles moving over the track is

essentially required for the success of a heavy haul operation. All

these systems shall be installed and integrated into a centralized

data base so that poorly performing wagons may be identified and

taken out of service.

32 33


7.1.3. To begin with, In-motion weigh bridges and Wheel Impact Load

detectors shall be provided at critical locations so that every wagon

comes across a weigh bridge almost at the beginning of its journey

and across a WILD at least once during a single run.

7.2. ROLLING CONTACT FATIGUE

Rolling contact fatigue due to high contact stresses caused by non-

conformal rail and wheel profiles results into rail fractures. Rail grinding

has been successfully used by railways to control rolling contact

fatigue. We should also implement effective Rail Grinding Regimes.

7.3. INCREASE IN RAIL FRACTURES

To safeguard against Rail fractures, the strategy shall be to catch the

flaws when they are still small and observe its growth so as to predict

when a rail is going to fracture.

7.3.1. As a predictive measure, an effective Ultrasonic testing system shall

be put in place. The number of in service failures of rail is closely

related to the effectiveness of Ultrasonic inspection. In a risk

management approach, high inspection reliability is required

particularly in lines with mixed traffic. To ensure an effective rail

testing program, the test equipment must be properly designed and

calibrated to reliably indicate defects. The equipment logic shall be

so built that only those locations are indicated to the operator that

could be a rail defect. At the same time the operator must be

experienced and diligent.

7.3.2. With the increased incidence of gauge corner cracks, the Ultrasonic

equipment shall be capable of covering almost entire rail head, entire

web and the area immediately under the head.

7.3.3. In addition to test capabilities, the frequencies must be matched to

the growth rate of critical defects so that at least one test, is done

in the interval between the development of a rail defect from a

minimum detectable size to a size that represents a significant

chance of rapid fracture.

7.3.4. Reliability of the Ultrasonic testing regime, effectiveness of the

machine and the test interval, can be evaluated by counting the rail

failures occurring at locations where no defect was detected and/ or

by comparing the ratio of rail fractures to detected rail defects.

7.3.5. North American heavy haul railways detect an average of 0.4 defects

per track km, each year while inspecting at intervals of 18 GMT and

experience 0.06 rail failures per km. One service defect in two hundred

leads to a broken rail derailment. Rails are typically replaced when

total defects are occurring at a sustained rate of 1-2 per rail km.

7.3.6. In controlling risk, the most basic control variable is the test interval.

In North American heavy haul railway practice, risk is typically judged

to be sufficiently high to merit tightening test intervals when:

• Service defect rates exceed 0.06 service failures/km/yr (0.1

service failures/mi/yr).

• Service plus detected rail defects exceed 0.04 failures/km/million

gross ton (0.06 failures/mi./mgt).

• The ratio of service to detected defects exceeds 0.20.

7.3.7. Canadian Pacific Rail System uses a risk management approach

whereby rail-testing intervals are adjusted according to different

categories of risk. The basic testing interval is selected based on

tonnage. As the following Table shows, there are six basic testing

intervals based upon the level of tonnage. The testing interval for

each track segment may then be upgraded to the test frequency

corresponding to the next higher risk class if there is an additional

element of risk associated with the track segment.

Traffic

Density

(mgt/yr.)

Test Class (Time

between

successive

tests)

Traffic Type Rail Type Defects

< 0.5 5 yrs.

0.5 – 2.7 3 yrs.

2.8 – 7.2 2 yrs.

7.3 – 13 Annual

14 – 27 Twice a year.

> 27 Thrice a year.

4 times a year.

5 times a year.

Transportation

of

Hazardous

Materials

OR

Passenger

Traffic at

speeds > 70

km/h

Non –

Cooled

(Hydrogen >

2.5ppm)

OR

Rail Section

< 50 kg/m

Detected defects

> 0.7/km/yr.

OR

Service failure/

detected ratio >

0.2

7.3.8. For example, a line carrying 10 million gross ton per year would be

tested once per year. But if it also carried passenger traffic at speeds

>70 kmph, it would be tested twice per year. If in addition to this the

line was laid with rails lighter than 50 kg/m (100 lb/yd), it would be

tested three times per year, or after every 3 million gross ton of

traffic. If it also had a defect generation rate of >0.70 per km per

year, the test interval will come down to four times a year.

34 35


7.3.9. On Indian Railways, following test frequencies have been specified: pad will have to be carefully chosen to overcome this problem.

Composite rail pad with softer layer in contact with concrete may

provide the answer. Indian Railways have made significant progress in

this direction. This, however, highlights the need to increase the rail

seat area in the PSC sleeper. A new sleeper, under development for

30 ton Axle load shall provide an increased rail seat area.

7.7. DETERIORATION OF TRACK GEOMETRY AND ROUNDING OF

BALLAST ETC

More frequent track recording may be adopted in sections where

disturbance of track geometry is noticed on account of weak formation

or any other reason. Based on the TRC results, need based tamping

may be carried out. Heavier loads are likely to cause faster pulverization

of ballast resulting into rounding of stones. Rounded ballast may not

be able to retain packing. Good quality hard stone ballast will have to

be provided.

7.8. INCREASED INCIDENCE OF TRACK BUCKLING

With introduction of heavier trains, increased tractive and braking forces

will have to be encountered by rails. This, along with higher thermal

forces on account of heavier rail section will compound the risk of

track buckling ahead of a train at the time of braking. Track will have

to be maintained properly to guard against this.

8. IMPLEMENT IN STAGES

IR has already started its march towards heavier axle loads in stages.

Stages are both about increased axle load as well as geographical reach.

The axle load is being increased in stages of 22.82 tons, 25 tons and 30

tons. Geographically also it is being implemented in certain identified

sections. This is a very good approach as we can gather our own experience

while minimizing the risk of irreparable damage.

9. CONCLUSIONS

9.1. To conclude, it can be said that there is a need not only to have a re-

look at the theory behind track strength assessment but to use a

more pragmatic and judgmental approach. Enough experience is

available, globally, on heavier axle loads which can be utilized in

chalking out the course for Indian Railways.

9.2. In IR's case the most suitable course appears to be:

Traffic Density Test Class (Time Testing

Annual GMT Frequency, Once in

< or = 5 2 yrs.

> 5 < or = 8 12 months

> 8 < or = 12 9 months

> 12 < or = 16 6 months

> 16 < or = 24 4 months

> 24 < or = 40 3 months

> 40 2 months

7.3.10. The test intervals are smaller in case of IR mainly due to the fact

that it is a mixed traffic scenario and passenger trains are running

at speeds over 100 kmph. Based on the rail type and the rates of

flaw generation the testing intervals may be tightened to mitigate

the risk.

7.4. INCREASE IN WELD FAILURES

It has been an accepted fact for years that rail joint is the weakest

link. Over the years, such joints have been replaced by welds. Today

the position is that thermit weld is the weakest link. We feel comfortable

after encasing a thermit weld between joggled fish plates. Joggling of

all thermit welds in a higher axle load section wherever weld failure

rate exceeds 1 per 100 is a good safeguard against risk of weld failures.

7.5. EXCESSIVE CREEP

Creep may occur due to increased tractive and braking forces coming

from heavier loads. In case of PSC sleeper track chances of creep

shall be nil. Another related problem may be sleepers going out of

square. On South African railways, even PSC sleepers have been

reported to have gone out of square. Analysis of the problem has

revealed that the longitudinal resistance available between rails and

sleepers was higher than the ballast resistance. In Broad gauge, with

high ballast resistance, this problem shall not occur, still, creep may

be measured periodically as also the toe load of the fastening system

to safeguard against these aspects.

7.6. RAIL SEAT ABRASION ON PSC SLEEPERS

Heavier axle loads are known to have caused problems of rail seat

abrasion on PSC sleepers on US and South African Railways. Rail

36 37


9.2.1. All rails lighter than 52 kg 90 UTS to be renewed before any increase

in axle loads, all 52 kg 90 UTS rails to be renewed with 60 kg 90

UTS rails in due course and 60 kg rails can be trusted to carry

axle loads up to 25 tons.

9.2.2. Existing PSC Sleeper can be considered suitable for axle loads

up to 25 tons, keep a design ready for a heavier sleeper which

may be required for axle loads of more than 25 ton.

9.2.3. Fastenings with increased toe load shall be required in areas having

problem of creep.

9.2.4. Rail Grinding is a must even in present scenario in some of the

sect ions. For 25 ton axle loads, i t shal l be essent ial to

implement it.

9.2.5. More effective methods of rail flaw detection are required to be

deployed. Equipment shall be capable of covering almost entire

rail head, entire web and the area immediately under the head.

9.2.6. Effective steps shall be taken to control over loading like installation

of In-motion Weigh Bridges.

9.2.7. Steps shall also be taken to identify and take defective wagons

out of service by installing equipments like Wheel impact load

detector etc.

9.2.8. Frequency of USFD shall be related to the weight of rail, GMT and

flaw generation rates of the section.

9.2.9. Thermit welds shall be eliminated to the extent possible.

PREPARATION FOR HEAVIER AXLE LOAD

ATUL KUMAR KANKANE*

SYNOPSIS

Indian Railway system is carrying mixed traffic both passenger and freight

traffic. With the increase in population and growth potential of country, demand

of transporting more traffic by railways have increased. The most economic and

feasible proposition to carry more load is to increase the carrying capacity of

our goods train which requires upgradation of existing infrastructure. In this

paper, an attempt has been made to list out various items for up grading our

track and bridges. Precautions for heavy axle load operation have also been

identified.

INTRODUCTION

Benefits of running of heavy axle load specially for transport of high

density commodities have been well experienced in North America and

Australia where 30T axle load is a common feature. To contain the operational

cost and to get more profit, increase in axle load is the only option available.

With the more input, better North-South and East-West connectivity and

better quality of roads, increasing competition is being faced by Railways.

Better trucks with more carrying capacity have further worsened the situation.

To maintain our market share of freight transport, increase in the carrying

capacity of wagons is the only option available with us to maintain viability.

For past 50 years, the growth in Railways could not match with the

growth of economy. Our market share in freight segment has reduced to

approx. 40% compared to 80% about 50 years ago. Due to constant

modernization and upgradation in technology, other modes of transports

have threatened us with such a heavy blow that our survival is at stake. If we

do not wake up right now and prepare ourselves for heavy haul, we may not

survive in this competitive atmosphere.

NEED

30 T and more axle load is very common in many of the world railways.

* Dy.G.M. & Secretary to GM, West Central Railway38


World’s heaviest and longest freight train runs in Australia with a pay load of

82000 tonnes and gross load of 99734 tonnes. The train is formed of 682

wagons, hauled by eight 6000 HP diesel locomotives. The length of train is

7.2 Km used for transport of mineral.

At present gross load of a freight train in India is about 4800 tonne

with pay load of about 3600 tonne. During 2006-2007, Indian Railways have

envisaged Mission 800 MT for freight loading. By 2012, Indian Railway

plans to carry 1200 MT of freight. With existing loading capacity and axle

loads, it is impossible to achieve the target. Construction of Dedicated

Freight corridors (DFC) is one of the right decisions taken in this direction.

We have been discussing about higher axle loads for the past several years

but nothing perceptible has been done at field level. Because of our slackness

and lack of vision in foreseeing the future requirement, we, as Civil Engineers,

failed to give desired input to our Permanent Way infrastructure at right

time. Nation can not wait for our idleness, therefore decision has already

been taken to enhance the carrying capacity of the wagons on existing

track as a pilot project on certain identified routes.

BENEFITS

Running of heavier axle load has many advantages –

1. Better net weight/gross weight ratio leading to better use of hauling

capacity of engine.

2 Increased line capacity, requiring less number of trains for hauling

the same tonnage. This will also reduce the operational cost per unit

tonne of haulage.

3. Fewer wagons will be needed to haul same load. This will increase

the wagon productivity. This will also reduce the capital cost and

wagon maintenance cost.

4. Less number of trains will increase the terminal efficiency.

5. Less requirement of locomotives for transporting the same amount of

tonnage, which will reduce the loco maintenance cost and capital wit.

This will also result in lower fuel consumption and less requirement of

crews, thus saving highly precious fuel and manpower.

6. Transportation by rail is considered as the most environment friendly

means of transport. If we are able to increase our market share, and

able to carry more freight by rail, we will be greatly contributing to

better environment.

DISADVANTAGES

There are certain inherent disadvantages of increase in allowable

axle loads–

1. Increased cost of maintenance of fixed assets like track and bridges

because of increased wear and tear of track components such as

rails, sleepers, ballast and sub grade.

2. Increased rate of deterioration of geometry of track.

3. Increased wear and tear of turnouts and crossing bodies.

4. Increased maintenance cost of moving assets like locomotives and

wagons.

5. Increase in level of noise and ground vibrations.

6. Due to high momentum, longer braking distance will be needed requiring

the change in signal overlaps.

7. Reduced life of infrastructure components (Fixed assets as well as

moving assets) due to more fatigue damage.

8. Requirement of more powerful engines for hauling extra load.

9. Increase in axle load will increase the risk of derailment due to Rail

fracture and increased track and wagon irregularities, which may affect

the safety of passenger train also.

PRECAUTIONS FOR RUNNING OF HEAVY AXLE LOADS

For economic viability of Railway system, heavy axle loads are to be

introduced on existing bridges with adequate strengthening, replacement or

rehabilitation to increase the structural strength as well as fatigue life of

bridge.

To safeguard against passage of overloaded, unevenly loaded wagons

and to prevent excessive dynamic load due to flat tyre or due to defects in

track geometry, certain mandatory safeguards are to be ensured.

1. Use of in-motion weigh bridges to detect overloading, preparation of

RR on the basis of weighment by weighbridge.

2. Use of Wheel Impact Load Detector (WILD) to prevent passage of flat

tyre and unevenly loaded wagons on the bridges,

3. Maintain the bridge approaches with better track geometry.

4. Elimination / reduction of rail joints on bridges to reduce dynamic

impact on bridges.

5. Preventing over speeding of goods train.

40 41


In addition to above, Hot Bearing Detector (HBD) and overload and

Imbalance Load Detector (OILD) will prove to be beneficial in preventing the

derailment of heavily loaded train and subsequent extensive damage to track.

EFFECT OF INCREASE IN AXLE LOAD

Running of heavy axle load trains on existing track affects both track

and bridges. All the track components like rail, sleeper, fastening, ballast

and sub grade are affected due to higher dynamic load. High axle load causes

increased wheel wear and higher bending stresses in rail and sleepers,

higher bearing stresses in ballast and subgrade. Fatigue life of track

component is reduced considerably. In bridges substructure and

superstructure both are affected whenever heavy axle load is introduced.

Reiff, 1990 suggested from AAR (Association of American Railroads) studies

that routine maintenance demands increase by 60% for 20 percent increase

in axle loads. Ebersohn et al, 1993 reported that track maintenance increased

by 183 percent for a reduction in subgrade stiffness by half. Therefore,

maintenance input for running of heavy axle load should be planned

meticulously.

INFRASTRUCTURE IMPROVEMENTS

Strengthening and up-gradation of existing infrastructure (track and

bridges) is the only option for increasing the axle loads. The strength of

assets and fatigue life both are to be assessed with the help of sophisticated

instrumentation and need based attention is to be given at specific location.

(a) ATTENTION TO TRACK

To bear higher stresses, following issues are to be taken care of –

• More resilient design to reduce the sudden impact to reduce

the permanent damage.

• Use of materials that is more resistant to wear and fatigue e.g.

use of better steel for rail to reduce rolling contact failures or

fatigue related fractures.

• Better sharing of loads/stresses between track components.

• Improved design of track components for increased elasticity.

• Strengthening or removal of weak components.

• Better resistance to gauge widening. Concrete sleepers are

better equipped for this.

• Reducing the requirement of surfacing and alignment of rails.

All the above issues can be tackled if we are able to improve our track

performance. For better track performance, we have to improve in following

areas

(i) IMPROVEMENT TO FIELD WELDING PROCESS

• For reducing the failure rate.

• For obtaining equivalent performance as of parent rail.

• For higher fatigue resistance under heavy axle loads.

If we are able to improve the quality of our field welding, introduction of

more welds during repair of failed weld can be avoided, Higher wear

resistance will reduce battering of heat affected zone of weld. Battering

of welds causes sudden impact and more dynamic loading which is

very detrimental to rails in the vicinity.

(ii) INTRODUCTION OF SUITABLE WIDE GAP WELD

TECHNOLOGY

• To fill a gap of 75 mm instead of 25 mm.

• To prevent introduction of one additional weld while

repairing failed weld.

(iii) IMPROVEMENT IN POINT AND CROSSING AREA

• Higher quality of rail steel for manufacture of points and

crossings to prevent early failure.

• Thick web switches and moveable nose crossings to reduce

dynamic load

• Better bolt tightening practices.

(iv) IMPROVEMENT TO SWITCH EXPANSION JOINTS

• To reduce discontinuity of rail head at expansion area.

• Manufacturing special thick web section for fabrication of

new lay outs where expansion is distributed at two places

instead of at one place provided in conventional layout.

(v) IMPROVEMENT TO SUBGRADE

• Drainage improvement of embankment.

• Increasing the clean cushion of ballast and better drainage

through ballast.

• More ballast cushion which may necessitate widening of

embankments.

42 43


• Improving the quality of ballast with good interlocking

properties.

(vi) IMPROVEMENT TO TRACK GEOMETRY

• Better track maintenance to reduce abrupt discontinuity in

movement of wheels on rail ( rail wheel interaction) because

of sudden change of stiffness of track at bridge approaches,

SEJs and Point and Crossings, Level Crossings etc.

(b) ATTENTION TO BRIDGES

The methodology for attention to bridge should be able to extend the

life of bridge and carrying out the replacement and maintenance works

while safely carrying the traffic. An objective decision has to be taken

whether it is more economical to replace the bridge or to increase the

live load capacity through different means. The decision should be

taken on the basis of condition of each bridge separately.

(i) INCREASING THE LIVE LOAD CAPACITY OF BRIDGES

• Replacing weak fatigue damaged components of bridge.

• Strengthening fatigue damaged components.

• Reinforcing weak components.

• Enlarge bearing area.

• Reducing the dead load of bridge by using stronger material

in critical components.

• Post tensioning of bridge.

• Adding new member to existing structure.

• Converting pin joints to rigid joints.

• Converting beam action to truss or arch action for better

load distribution.

• Strengthening of riveted connections by replacement with

high strength bolts.

• Removal of short and discontinuous welds, removal of

intersecting and overlapping welds.

• Replacement of welded cover plate with bolted cover plate.

• Removal of notches and weld defects.

• Reduction of residual stress in the welds.

• Removal of corroded members.

(ii) ATTENTION TO BRIDGE SUB-STRUCTURE

• Older bridges may not be able to withstand increase in

axle load, therefore more thorough inspection with better

technique is required.

• Under water inspection of important bridges foundations

with most modern techniques where lower portion of pier

remains under water throughout the year.

• Bridge specific unique repair methodology with use of

sophisticated under water maintenance technique.

• Strengthening of substructure by grouting or jacketing as

per requirement at site.

(iii) BETTER INSPECTION AND MAINTENANCE PRACTICES

• More frequent inspection of critical bridges specially during

initial period of introduction of heavy axle load.

• Use of better materials, new equipments and gadgets for

more safe and comfortable inspection and effective repair.

• Better repair techniques for quick and good quality repair/

rehabilitation.

• Checking the strength of guard rail for proper functioning

during derailment of heavier trains.

• Instrumentation to measure the strength and fatigue life of

bridges.

• Planning adequate time allowance for repair of bridge.

• Special inspection strategy of steel bridge to detect fatigue

crack initiation in -

� Eye bars and pin plates.

� Crack in rivet holes.

� Welds at end of cover plate.

� Welded attachments.

� Corroded locations.

� Weld defects.

• Measurement of acoustic emissions is one of the such

technique which can tell about potential crack formation.

Incidence of more acoustic emission near some member

is indicative of crack initiation. These techniques should

be extensively used in steel structures.

44 45


CONCLUSION

To keep ourselves economically viable, reduction in operating cost is

the only alternative in present scenario. Cost and benefit analysis of running

of heavier axle loads is to be done. Assessment of cost of rebuilding /

rehabilitation and increased maintenance cost due to increased track and

structure damage is to be done objectively. This should be compared with

the reduction in operating cost. New line construction should be able to

withstand even 40 T axle load to cater for future requirement on certain

critical routes as cost implication during new line construction may be less

than the future rehabilitations. On Indian Railway increase in life of existing

bridges is more critical. Replacement and maintenance work has to be

done without affecting the safety of traffic. Closer co-ordination is required

with Mechanical Directorate for better design of suspension system of rolling

stock to reduce the dynamic impact of heavily loaded wagons on track and

bridges. If we are able to maintain rail and wheel profile, dynamic impact due

to heavier axle load is likely to be reduced considerably. Extensive installation

of various rolling stock performance detectors like Hunting Detector (HD),

Hot Wheel Temperature Detector (HWTD), Acoustic Bearing Detector (ABD)

and finding and fixing vehicle defects by remote vehicle performance monitoring

at later stage will go a long way in improving the safety performance of heavy

axle load operation. Extensive use of new sophisticated track machines

will improve the life of track component as well as better and safe running.

Consideration should also be given for running of heavy axle load at higher

speeds may be 100 Kmph. This will require more extensive study for

rehabilitation planning of existing track and bridges.

REFERENCES

1. Ing Rainer Wenty : The Asian Journal Vol.13 Number 1 July 2006.

2. Duane Otter, Shakoor Uppal ; Railway Age Nov.2003

3. Kalay, Semih : Railway Track and structures, Jan 2002.

4. Reiff S.F. 1990 : Proceedings of Workshop on heavy axle load Pueblow,

U.S.A, Paper-21, PP.21.0-21.5.

5. J.F. Unsworth TRB TRB 2003 Annual Meeting CD-ROM.

6. Ebersohn W., Vizo M.C., Sclig E.T., 1993 5th International Heavy

Haul conference, Beijing.

IMPLICATIONS AND SOLUTIONS FOR

RUNNING OF HIGHER AXLE LOAD ON

SPECIFIED ROUTES OF INDIAN RAILWAYS

K.K.MIGLANI *, JAGTAR SINGH**

SYNOPSIS

With the increase in freight traffic, it has become necessary for the Railways

to increase loading per wagon to achieve fullest capacity of wagons. Slowly,

Railways have introduced CC+4+2t, CC+6+2t & CC+8+2t loading. These

loadings have enhanced the through put. This has benefited the Railways in a

big way.

Increase in loading per axle has forced Engineers to think on design of

rail section, sleepers, ballast bed, formation and bridges. Because of increased

loading, the stresses in rails, sleepers, ballast bed, formation and bridges have

definitely gone up, resulting in early fatigue failures.

On track front, there will be requirement for increased maintenance

inputs, especially rail grinding, ultrasonic testing of rails, and handling rail

weld failures. Special attention would be required on fish plated joints, Points

& Crossings area & availability of clean ballast cushion.

Paper also gives suggestions for various aspects for maintenance of track on

higher axle load routes.

1.0 OVER VIEW OF TRANSPORT SECTOR

The Indian economy enters the tenth plan with an expectation of 6%

to 7% annual growth in the GDP and consequently 7.2% to 8.0% growth in

the transport sector. These expectations place heavy demands on the

already saturated road and rail transport system which coupled with the

inadequacies in the power sector could be a major constraint in the

realization of the projected economic growth. With Airways, Coastal

Shipping and Inland Waterways being in the fringes, freight transport in

India is basically shared between Road and the Rail sectors. The road

network in India has grown from 4-lakh kilometres in 1951 to over 30-lakh

kilometres now – second largest road network of the world.

Railways made a flying start almost doubling the transport output in

*Dy.Chief Engineer/TO, Northern Railway

**Dy.Chief Engineer/Land, Northern Railway46


the first 5 year Plan. There was however a perceptible slowing down from

1968 to 1980 followed by a revival in the last two decades.

1.1 TRENDS IN FREIGHT & PASSENGER TRAFFIC

• Freight Traffic has grown 90 times from 5.5 BTKM in 1951 to

over 500 BTKM now.

• Passenger Traffic has grown 80 times from 23 to 1800 BPKM

in the same period.

1.2 SHARE OF TRAFFIC WITH HIGHWAYS AND RAILWAYS

• National and State Highways comprising only 8% of the network

carries 80% of the traffic

• Railway share of Freight Traffic has declined from 89% in 1951

to 38% now.

1.3 CHALLENGES FOR RAILWAYS

• Golden Quadrilateral Road Network and Induction of Multi Axle

Road Vehicles will make a serious dent

• Even heavy duty Bulk Transport may not remain the exclusive

preserve of the Railways.

1.4 COMPARISON OF FREIGHT TRAFFIC WITH CHINESE RAILWAY

A comparison of the Railway Systems in China and India makes

interesting study.

In the decade 1992 to 2002 the route Km on the Chinese Railways

(CR) has grown from minus 6% to plus 14% in comparison to that of

the Indian Railways (IR). The two Railways carried almost the same

volume of Passenger Traffic both

in 1992 as well as 2002. However, in respect of Freight Traffic, the

volume carried by Chinese Railway is four and a half times that of

India. Chinese Railway operates roughly twice the number of trains

on electrified double line tracks than the Indian Railways

1.5 MODERNIZATION AND EXPANSION ON INDIAN RAILWAY IN A

BIG WAY

Integrated Railway Modernisation Plan (2005-10) has been made

which has objective to enhance capacity, improve rail-port

connectivity, higher axle load wagons to carry bulk material and

development of dedicated freight corridors, two intercity, corridors

Delhi-Patna-Howarh and Delhi – Channai to be developed to run 150

Kmph trains using latest technology high speed coaches.And running

of freight train @100Kmph on the high density Golden Quadrilateral

and its diagonals connecting the four metropolitan cities.

At present predominantly running axle load on Indian railway system

is upto 22.82 tonnes are operating. Heavier axle Loads will enable

carrying more payload in one train, which in turn improve throughput

substantially.

Railway Board has already taken decision to run BOXN and other

wagons with CC+8+2 loading on specified routes. This would result

in axle loads of the order 22.82t and TLD of 8.51t/m.

In this scenario, introduction of trains with 30t axle loads probably is

not quite far away.

Running of such heavy axle load trains on the existing track would

cause very high stresses on the track structure which would have

far reaching implications on the requirements of track components

and their maintenance and life.

1.6 TRANSPORT CAPACITY ENHANCEMENT

Indian Railway is in Transport business. Trailing loads and operating

speeds are the principal efficiency indicators. Within the limitations

of a loop length of 686 meters and the existing and proposed Track

Loading Density of 7.67 and 8.25 tonnes per meter the options are

• Higher Axle Loads

Axle Load in tonnes 20.32 22.9 25.0 30.0

No. Of Vehicles 58 58 58 58

Trailing Load in tonnes 4831* 5410 5800 6960

Track Loading Density t/m 7.67 8.25 8.82 10.58

*With CC+2t

• Increasing number of Axles per Wagon

• 11 % higher throughput with 23t Axle Load and 20 % with 25t

Axle Load can be achieved. With introduction of 30t axle load

throughput will increase by 30% approx.

• With 25 t Axle Load Track Loading Density would need to be

relaxed to 8.8 t/m and 10.58 t/m for 30t axle load.

• To carry more, cost effectively Indian Railways has gone in for

increase in loading per axle and infrastructure development of

dedicated freight corridors.

48 49


1.7 BENEFITS OF HIGHER AXLE LOAD

Fewer wagons will be needed to haul the same load, leading to

lower capital cost and possible reduction in wagon maintenance

cost, fewer locomotives, lower fuel consumption per net tonne,

reduction in train wagon kilometre operated, and fewer crew

deployments entailing savings in wages.

The railways in North America, Australia, South America, South Africa

and Sweden have all increased axle loads to obtain significant savings

in operating cost. These savings have been achieved despite

increased cost of maintaining tracks, greater track component

damages and shorter component lives.

The raising of the axle load from 22.5 tonnes to 30 tonnes yielded 40

per cent savings in transportation cost in the US. This in turn

helped the railways in that country achieve significant reduction in

operational cost of transporting containers and introducing

customised wagons to win back traffic from the roadways.

Boosting wagon productivity that is, how to carry more per wagon,

or how to achieve higher payload per wagon, has become important

for the Indian Railways in view of the increasing threat from the various

other modes of transport, particularly roadways.

2.0 DESIGN ASPECT FOR TRACK

Computation of stresses has been done, that would be induced in

Rail, Sleeper, Ballast and Formation due to introduction of 30 t axle loads

and the suitability or other wise of these components. Minimum track

structure for running of these heavier loads and its impact on maintenance

of P.Way has also been dealt with.

On Indian Railway the strength of the for running various locomotives

and rolling stocks at different speeds is assessed by calculating rail stresses

induced locomotives/rolling stocks running at contemplated speed, using

Civil Engg. Report No.C-100 rail wheel contact stresses on straight and

curved tracks due to axle load combined stresses in rail head, foot,

assuming rail wear of 5% are calculated on 52Kg rail and 60 Kg rail.

2.1. RAIL DESIGN

2.1.1. STRESSES IN RAILS

Rail is a very important and costly component of the permanent

way. Its failure will affect safety. Therefore, the rail is treated as a

continuous beam on closely spaced elastic supports and the

bending stresses are determined from the theory of Beam on

Elastic foundation (BOEF). The fact that the rail is actually

supported on sleepers at some distance apart introduces a very

little error and is neglected. The maximum rail stresses calculated

for 60 Kg rails with 100 Kmph speed work out to be are as under:

Permissible Stresses in 72 UTS 90UTS

LWR Track for 19.25 Kg/mm2 25.25 Kg/mm2

It can be seen from the above that even in 60 kg Rail stresses are

higher than the permissible limit. In other words, for 30t axle loads,

rails of higher pondage will be needed.

2.1.2. RAIL WHEEL CONTACT STRESSES

The contact between rail and wheel flange should be theoretically a

point. Hertz theory explains that in practice the elastic deformation

under higher axle load results in deformation of steel of wheel and

the rail creating an elliptical contact area. The dimensions of contact

ellipse are determined by the normal force on contact area, while

the ratio of ellipse axes a & b depends on the main curvature of the

wheel and rail profile. Inside the contact area a pressure distribution

develops which in a cross

section, is shaped in the form

of a semi-ellipse with highest

contact pressure occurring at

centre.

The concentrated load

between wheel and rail

causes a shear stress

distribution in railhead as

shown in fig. (5.18 & 5.19)

The contact problem is most

serious in case of high wheel

50 51


loads or relatively small diameters. Eisenmann has devised a

simplified formula to calculate the maximum shear stress in rail

head, which is as follow

Tmax = 4.13(Q/ R)1/2

Where T max = maximum shear stress in rail head

Q = wheel load +load due on loading due to curves.

R = Wheel radius (mm)

Since problem is one of the fatigue strength, the permissible shear

stress is restricted to 30% of UTS, which works out to be 21.60Kg/

mm2 for 72UTS rail and 27.00 Kg/mm2 for 90 UTS rails.

It is seen that maximum shear stress increases with increase in

axle load. It also increases with increase in curvature of track as

increase super elevation results in increase on loading of inner rail

when goods train ply on mixed traffic routes. The shear stress also

increases with wearing of wheels as the wheel radius decreases

with the wear of wheel. Thus it may appear that the problem of

increase axle load can be solved with increase in wheel diameter

but this is not possible as increase in wheel diameter means less

carrying capacity because of restricted overhead clearances.

Therefore only way to keep the maximum shear stresses within

permissible limits is to use the rail with higher UTS.

The contact stresses for BOY, BOB and BOXNHA wagons would be

as under. The diameter of wheel of Casnub bogie is taken as average

of new wheel and worn out i.e. (1000+925)/2=962.5 mm.

For 72 UTS rail the maximum allowable shear stress will work

out to 21.60 Kg/mm2 and for 90 UTS rail, it will be 27 Kg/mm2. It

there implies that 90 UTS rail will be required for running 30 tonne

axle load.

2.1.3 SLEEPER DESIGN

CONTACT STRESSES BETWEEN RAIL & SLEEPER

As an extension to the Beam on Elastic on Elastic Foundation

model proposed by Zimmerman for arriving at the stresses in the

Rail, the contact pressure between rail and sleeper is computed

based on based on the principle of discretely supported rail on springs

at specified intervals. Using this principle and Dynamic Amplification

Factor because of the dynamic interaction between rail and wheel

due to the speed of train, the maximum bearing force on a single

discrete rail support due to the wheel load is obtained from the

formula

• Based on Beam on Elastic Foundation Model

Bearing Force on Sleeper –

Fmax

= DAF * Pa/2 * (U/4EI)1/4

• Where DAF = Dynamic Amplification Factor, which depends on

the track quality, the train speed and a multiplication factor of

slandered deviation, depending on confidence interval.

• P =Effective Wheel Load (T)

• a =Sleeper Spacing (Cm)

• U =Modulus of Elasticity of Rail Support or Track Modulus (Kg/

Cm/Cm)

• E =Modulus of Elasticity of Rail Steel (Kg/Cm2)

• I =Moment of Inertia of Rail Section (Cm4)

Based on the charts developed by RDSO in their report no C-

100 for different Rolling stock based on experimentation, the

speed factor for BOX wagons for a speed of 100 Kmph comes

to 1.68, However , since the wagon of 30 t Axle load would be

52 53


different with different dynamic characteristics , the dynamic

effect due to speed is also checked based on the formula

proposed by Elisenmann for Dynamic Amplification factor

• As per Eisenmann’s Formula,

• DAF = 1 + t ø {1+(V- 60)/140}

t = Multiplication Factor Depending on Confidence Interval and

Ø = Factor Depending on Track Quality.

• For confidence t = 3, Ø =0.2 for average track quality and 100

kmph speed,

DAF works out = 1 + 3 x 0.2 {1 + (100-60)/140} = 1.78

• For 75 KMPH, DAF works out to 1.66

• Speed factor of 1.68 is adopted for computation of stresses

• In the absence of relevant data regarding the type of rolling

stock and the speed that would be permitted for the purpose of

computation of stresses on sleepers ballast and formation a

speed factor of 1.68 is adopted based on the above computed

values and RDSO.

• For 30 t axle loads, the effective wheel load ‘P’ would be 15t.

The Mean Contact Pressure Between Rail and Sleeper on the

most heavily loaded sleeper would than be computed from the

formula: -

óm = (F

o + F

max)ó/A

Where

Fo = The total pre-tensioning force of fastenings on rail

support (T)

A = Effective rail support area (mm2)

For concrete sleepers with elastic rail clips, Fo works out to

= 2x1, 000 =2000Kg = 2T for PSC Sleeper

“A“ would be 0.125 X the width of the rail foot, since the width of

the grooved rubber pad used below the rail is 125mm.

For calculation purpose, it is normal to presuppose that the

contact force is distributed evenly over the contact surface area.

The Contact pressure between rail and sleeper for 52 Kg and 60

Kg rail sections or 52 kg or 60 kg PSC sleeper and also for

sleeper spacing of 60 cm and 65 cm computed basis on the

above formulae are:

Permissible contact pressure between the rail and sleeper for

concrete sleepers is 4N/mm2.

But as seen from computations in the above table, contact pressure

value at the rail seat for a track with 52 kg rail either on 52 kg or 60

kg sleepers would be far excess of permissible value when 30 t axle

load rolling stock is introduced. Contact pressures are higher than

the permissible value even for a track with 60 kg rail on 60 kg

sleepers at 60 cm spacing. As computed above a track with 60 kg

rail on 60 kg sleepers at 43 cm spacing only fit for running of 30t

axle load rolling stock from contact pressure criterion on PSC sleeper.

This poses a very severe restriction, which would have far reaching

implications and hence needs to be examined thoroughly before

introduction of 30 t axle loads.

Presently, Indian Railways is looking for only allowing upto 25t axle

load on the present track. But, if the future is to introduce 30t axle

load, then it is necessary to redesign the sleepers.

2.1.4 BALLAST BED DESIGN

STRESSES IN BALLAST BED

Ballast bed and formation are conceived as a two-layer system for

the purpose of computation of stresses. Vertical forces on the ballast

bed due to wheel loads will be considered as the determining

stresses for the load bearing capacity of the layer system. Over

loading of ballast bed due to increased axle loads causes rapid

deterioration of the quality of the track when heavy axle load trains

are introduced.

The compresses stresses that the sleepers exert on the ballast

bed are considered evenly distributed for the purpose of calculation.

It means that the material from which the sleeper is made plays no

role. The maximum stress between the sleeper and the ballast bed

54 55


under the wheel load ‘P’ is expressed based on Zimmermann’s

theory and by applying a Dynamic Amplification Factor due the

speed of the Rolling stock as per Eisenmann’s model.

ó sb = { DAF* Pa/2(U/4EI)1/4}/Asb

= Fmax/Asb

Where

Asb = Contact area between sleeper and ballast bed for half

sleeper (mm2)

52 Kg and 60 kg sleepers differ only in respect of the distances

between inserts so as to accommodate higher rail section and in all

other respects, they are identical. Hence, there would be no

difference in ballast stresses due to the use of either sleeper and

the half sleeper contact area works out to 336,875mm2. Stresses

on the ballast bed due to the force on sleeper, competed for 52Kg

and 60 Kg rail sections and different sleeper spacing are tabulated

as under:

The permissible contact pressure on the ballast bed is taken as

0.50 N/mm2. As seen from the values in the above table, for the

present track structure, stresses on the ballast bed would be whining

the permissible value when 30 tonne axle load rolling stock is

introduced.

It can be gathered from the above equation that sleeper spacing and

the extent of ballast support area have an important influence on the

mean stress on ballast bed. A high value for foundation leads to high

value of stress on ballast bed, whereas heavier rail profile has a

positive effect in this respect. A heavier rail profile has a greater

influence on rail stress reduction. The effect of ballast stress however,

is approximately half of the effect on the rail stress.

2.1.5 FORMATION DESIGN

STRESSES ON FORMATION

The loads from rolling stock are finally transferred to the formation

through the ballast cushion, where the ballast bed and the formation

are conceived as a two-layer system. Introduction of higher axle

loads results in imposition of increased compressive stresses on

the formation. This would lead to faster deterioration of track and

call for more frequent maintenance schedules. The compressive

strength on formation should be always kept within the bearing

capacity of the formation, which depend on the modulus of elasticity

of the formation apart from other geotechnical characteristics of the

soil.

The stresses transmitted to formation primarily depend upon the

depth of ballast cushion and the effective bearing length of the sleeper.

From the criteria of the force on individual sleeper due to axle load,

compressive stresses on the formation are calculated from the

following formula:

Pmax = DAF. Pa/ ð DL. (U/4EI)1/4

= Fmax. (2/ðDL)

Where, D= Depth of ballast cushion (mm)

And L= Effective bearing length of sleeper at rail seat (mm). For

PSC sleeper, ‘L’ is taken as 1040mm.

Formation stresses for 52 Kg & 60 Kg rail sections and ballast

cushion of 250mm & 300mm, on account of introduction of 30t axle

Load, computed based on the above formula are tabulated below:

The modulus of elasticity and permissible stresses on the formation

for 2 million cycles of loading as indicated by Coenraad Esveld are

reproduced on next page:

56 57


Obviously, with introduction of 30 tonne axle load rolling stock, in

most cases, formation stresses would exceed the bearing capacity

of the formation.

3.0 IMPACT OF INCREASED STRESSES DUE TO HIGHER AXLE

LOADS

3.1 IMPACT ON RAILS

If the permissible stresses are exceeded, then :

• There will be plastic flow of metal at contact and development of

cracks in railhead will take place. These cracks grow gradually

due to combined effect of contact stresses resulting is surface

breaking. If allowed to grow, they have potential go subsurface

and cause failure by combining with already present defect.

Another implication is that if the surface cracking is severe, the

substantial amount of ultrasonic waves transmitted will be

reflected from these surface defects making it impossible for

the rail section to be reliably inspected for full depth.

• The most prominent defects in rails on the heavy haul routes

have been observed as Rolling Contact Fatigue defects

predominantly the Gauge Corner Fatigue.

• The maximum shear stress is developed not on the contact

surface but at a depth of 5-7mm below the railhead. It therefore

implies that use of head hardened rails will be effective only if

such hardening increases the UTS up to the depth of 6mm or

more from the railhead.

• It is interesting to know the effect of surface hardening and

lubrication in context of maximum shear stresses. If wear is not

dictating the life of rail, as on head hardened rail/ lubricated

rails, the maximum repetitive shear stress will always occur at

same point, thereby increasing propensity of fatigue failure and

shelling.

• On the other hand if the rail is allowed to wear, the point of

occurrence of maximum shear stresses will gradually shift

downwards making it less prone to shear fatigue failures or

shelling.

Therefore, it is paradoxical to say whether the use of head

hardened rails/lubrication of rails will actually enhance or reduce

the life of rails with heavy haul. Tests at Facility for Accelerated

Service Testing (FAST) have also shown that higher wear rates

of rail not only reduce surface defects but also suppress the

internal defects i.e. detail fractures and shelling.

RAIL FATIGUE LIFE

From the analysis of bending stresses and contact stresses it may

though appear that 52 Kg 90 UTS rail may suffice the requirements of

increased axle load, but in practice, the above stresses coupled with thermal

stresses and residual stresses set up cyclic stresses. From the theory of

fatigue, it is evident that such cyclic stresses may result in failure of material

at a stress level lower than what would normally require for failure.

Allan M Zarembski compared the rail life based on wear limits to rail

fatigue life for different axle loading environment and found that in lighter axle

Rail Fatigue Life vs Wear 136 RE Rail

58 59


loading environments, rail wear is dominant mode of failure while in heavier

axle load environments, the fatigue emerges as dominant replacement

criterion.

AREA Bulletin No. 685, Vol 83 reports of study made by Dr.

Allan.M.Zarembski on the effects of increasing axle loads on tangent track

on a continuous welded track. Two independent studies were conducted to

determine the fatigue life of rails with different axle loads. The results have

shown that heavier axle loads have resulted in a more severe occurrence of

rail defects.

The study had two conclusions:

(i) An increase in axle loading will result in decrease in the fatigue

life of rail, measured in terms of cumulative MGT and reduction

occurs for both heavier as well as lighter sections.

(ii) When the axle loads are increased from 27.5 tonnes to 33 tonnes

(corresponding to 70 tonnes and 100 tonnes freight cars), the

resulting decrease in life of rail was found to be 40 %.

Thus under Indian Railways context, it can be said that with increase

in axle load upto 25 tonnes, 52 Kg/m, 90 UTS rail may even though

be permitted from the considerations of bending stresses and

contact stresses, however, in the interest of long term economy

and from fatigue considerations, it will be more appropriate to use a

heavier section of 60 Kg/m if further increases in Axle loads are

imminent.

3.1.1 IMPACT OF FORCES ON CURVATURE AND TRACK GEOMETRY

• ORE 161 studies reports that Dynamic effects of 22.5 tonnes

axle loads for different speeds, track quality and radius of

curvatures

• Dynamic wheel force (DSQ) increases with increase in speed

• The lateral rail- wheel force increases with increase in curvature

and deterioration of quality of maintenance.

• Curves with radius sharper than 400m require a greater care of

track geometry with increase in axle loads.

• Poorly maintained track will have most pronounced effect, where

increase in the wheel force can be up to 22% of axle loads for

speed ranging bet. 60 to 100 kmph observed.

• Track quality was expressed in terms of standard deviation of

vertical profile and alignment

• Standard Deviation?<1mm very good, 1-2 mm good > 2mm

moderate

3.1.2 IMPACT OF WHEEL FLAT

• Relationship between the flat size and force is almost linear

• On Indian Railways, the permitted sizes of wheel flats are 50mm

for locomotives and coaching stock and 60mm for goods stock.

• Size of flat will depend on diameter of wheel (C2/8R).

• No consideration for size of flat in specifications of wheel flat.

The largest loads applied to the track from vehicles are those, which

60 61


arise from irregularities on wheel such as wheel flat. ORE 161.1/RP 3

reports of the tests carried out on flat tyres measuring the effects of

speed, size, sleeper type and axle loads. The results reveal:

(i) The forces at frequencies above 500 Hz referred to as P1 forces

increases continuously with speed, while the forces at

frequencies below 100 Hz, referred to P2 forces are more of

less independent of speeds. The P1 forces have bearing on

wheel rail contact stresses. This force, which causes most of

damage to rails and concrete ties, increases with increase in

speeds.

(ii) Increase of axle load from 20 t to 22.5 t (12.5%) caused the

increased wheel flat force of the order of 0 to 6%. Hence if go

from 22.5t to 30 t the increase in wheel flat force will be of the

order of 24%.

• Studies have also revealed that movement of wheels with

flats can generate dynamic forces, as high as six times the

normal static load, in extreme situations.

• On Indian Railways, the effect of rail/ wheel defects and

vehicle suspension, on static wheel load, is represented by

a speed factor (Rail stress calculations), which can assume

a maximum value of 1.75 for locomotives and 1.65 for

wagons.

• The problem assumes alarming proportions incase of thermit

welds (which have the impact strength of 7-10% of parent

rail) in LWR territories, during winter season, when the full

tensile stresses are present in rail section.

• Spate of weld failures due to running of flat tyres under

these conditions, is not uncommon.

3.1.3 IMPACT ON RAIL/WELD FAILURES

With increase in axle load, there will be increase in rail/weld failures.

All the AT welds would be required to be supported on wooden blocks

and joggled fish plated. The patrolling in rail/weld failure area should

be effected so ensure safety.

62 63


The frequency of USFD testing should be increased with latest

technology and should be done in the periodicity of 8GMT.

With the increased loading, gauge face corner cracking will increase

and will have grater impact on curves. Therefore, there will be need for

carrying out ultrasonic testing of gauge face side at shorter intervals.

3.2 IMPACT ON BALLAST AND FORMATION

The permissible contact pressure on the ballast bed is taken as 0.50

N/mm2. As seen from the values in the above table, for the present

track structure, stresses on the ballast bed would be whining the

permissible value when 30 tonne axle load rolling stock is introduced.


most cases, formation stresses would exceed the bearing capacity of

the formation. Blanketing is to be done for whish extra cost of Rs.10

lac/km will be involved.

3.3 IMPACT ON SEJ’S AND TURNOUTS

SEJ’S will have higher impact loads resulting in pre mature failures.

Design developed by Rahi Industries should be used for higher axle

loads.

There will be higher wear and tear of switches and crossings of turnouts.

Loosening of fittings, crushing of rubber pads would be there.

3.4 IMPACT ON MAINTENANCE OF TRACK

• S.Hammarlund, B. Paulsson, conducted Swedish model for

prediction of maintenance costs when increasing axle loads

from 25 t to 30 t. The paper shows that by increasing the axle

load from 25t to 30t, the deterioration mechanisms on the rails

was surface fatigue (60%), engine burns etc (15%), internal

defects found by USFD (10%), actual rail and weld fractures

and isolating joints (10%). Only 5% of the rail maintenance was

found to be due to wear on this part of line.

• The expected increase of track maintenance cost was 3%,

which is substantially lower than the increase in axle load (20%).

The possibility to reduce the maintenance cost was suggested

by doing rail grinding and better system of lubrication

• Aasho test showed that, increase in the stress on the ballast

bed due to increased axle loads would definitely result in faster

deterioration of track geometry quality. Though the relation is

still ambiguous, on the basis of AASHO Road Test for Road

structures, it is assumed that;

• Decrease in track quality= (Increase in stresses on ballast bed)

m Where the value of ‘m’ is generally taken between 3 & 4.

Thus, a 10% higher stress on ballast bed leads to 1.2 to 1.5

times faster reduction in track geometry quality and consequent

proportionate increase in maintenance. Introduction of 30 tonne

axle load rolling stock against the present loading of 20.32t

would therefore result in the increase of track maintenance effort

3 times.

While running 30 t axle loads, impact stresses on the ballast will

increase and this further lead to crushing of ballast. Therefore, deep

screening of ballast is required at close interval of time.

3.5 IMPACT ON BRIDGES

It is required to be thoroughly checked the design of all the bridges in

dedicated routes where 30 tonne axle load is to be introduced and

make necessary design changes as per HMLS loading. Necessary

speed restrictions should be imposed on safety considerations.

As per technical instructions no 4 (issued by Member Engineering Sh

RR Jaruhar,) on Load carrying capacity of masonry Arch bridges, Test

load application with observation or deflection, spread and residual

deflection is more appropriate with limiting value deflection as 1.25

mm and spread as 0.38 mm as criterion. It is considered safe for all

practical purpose to allow axle loads up to 30t with 60 Kmph speed

with proper physical condition of arch being assured. Utmost, an

additional ring can help fixed proper skewback on strengthened

abutment / pier. Such bridges must be thoroughly examined.

3.6 EFFECT ON SCHEDULE OF INSPECTION

With the increase in axle load to 30t the schedule of inspections of

officials and supervisors needs to have re look. Definitely frequency

inspections will be increased.

4.0 WHAT WILL BE REQUIRED - UPGRADATION OF THE TRACK,

BRIDGES AND FORMATION, TO WITHSTAND THE INDUCED

HIGHER STRESSES ON ACCOUNT OF HEAVY AXLE LOADS

4.1 RAIL SECTION

The maximum bending stress in 6o Kg rail on introduction of 30 tonne

64 65


axle load works out to be 27.70 Kg/mm2 for 90 UTS rails. Permissible

Stresses are 72 UTS 90UTS 19.25 (LWR) 25.25 (LWR) It can be seen

from the above that even in 60 kg Rail stresses are higher than the

permissible limit, in other words 60 kg 90 UTS Rail also not fit for 30

t axle loads.

The contact stresses for BOY, BOB and BOXNHA wagons would be

as under. The diameter of wheel of Casnub bogie is taken as average

of new wheel and worn out i.e. (1000+925)/2=962.5 mm.

depth of ballast cushion for 30t axle load is 25cm with sub ballast of

15cm for speed up to 100 Kmph. However a clean ballast cushion of

300mm may prove best solution for running 30 t axle loads.

4.4 FORMATION


most cases, formation stresses would exceed the bearing capacity of

the formation. This would necessitate provision of a blanket layer of

adequate thickness to improve the bearing capacity just beneath the

ballast bed. Provision of blanketing, in accordance with the recent

guidelines issued by RDSO in June 2003 vide Guideline No. GE: G-1,

appears to be the only solution for stabilising weak formations. It is

obvious that a yielding formation will result in rapid deterioration of

track geometry, which will make it unsafe of higher axle load trains in

addition necessitating increased and frequent maintenance efforts.

4.5 BRIDGES

The bridges are required to of HMLS designs for carrying 30t axle

load.

5.0 ALSO THINKING HAS TO GO IN FOR

5.1 MODIFICATION IN WAGONS

Introduction of 3-axle bogie for increasing the pay load carrying

capacity, while keeping the stresses on the track structure within

permissible limits

Increase in number of axle (3-axle bogie) is another strategy, which

can be thought of for increasing the pay Higher tare loads of the wagons

running on the Indian Railways as compared to those running on

developed countries is one area where a possible solution to the

problem of running increased axle loads lie. To cite an example of

BOXN has the payload to tare ratio of 2.61. In most of heavy haul

routes, this ratio varies from 3.5 to 5. TRANSWERK, South Africa has

developed 104 t gondola tippler coal wagon with a 4.2 tare ratio and

120t tippler iron ore wagon with payload tare ratio.

These options will be apparently more beneficial compared to the

resource intensive up gradation of the track, bridges and formation,

on account of introduction of heavy axle loads wagons.

We have opted for Broad Gauge (5 feet 6 inches between rails against

the standard gauge world wide of 4 feet 8.5 inches) yet our moving

For 72 UTS rail the maximum allowable shear stress will work out to

21.60 Kg/mm2 and for 90 UTS rail, it will be 27Kg/mm2. It therefore

implies that 90 UTS rail will be required for running 30 tonne axle load.

The world’s longest rails are now manufactured in India for a length of

120m. With the availability of 120 m long rails, there will be drastic

reduction of weld population in Indian rail tracks (from 160 welds per

track km presently to 17) resulting enhance safety and cost reduction.

4.2 SLEEPERS

As computed above with 60 kg rail on 60 kg sleepers at 43 cm spacing

only fit for running of 30 Ton axle load rolling stock from contact pressure

criterion on PSC sleeper. The 60 Kg Rail fails in bending stress criteria

and 71 kg rail suits as next option. 71 kg rail is having foot width of

160 mm against availability of 162mm width of MS insert. This poses

a very severe restriction, which would have far reaching implications

and hence needs to be examined thoroughly before introduction of 30

t axle loads considering difficulty involved in maintaining track with

such high density of sleepers i.e. 2326 sleepers/Km.

4.3 BALLAST BED

The contact pressure on ballast for 60 Kg sleepers at spacing of 43

cm works out to be 0.1635 Kg/mm2. The permissible contact pressure

on the ballast bed is taken as 0.50 N/mm2. For the present track

structure, stresses on the ballast bed would be within the permissible

value when 30 tonne axle load rolling stock is introduced.

As per Railway Boards letter No.95/w1/Genl/0/39 dated: 9.10.96, the

66 67


dimensions are highly restrictive. Similarly, productivity of our wagons

in terms of tare to pay load ratio is probably one of the poorest in the

world. We carry 450 kg of dead weight for moving every tonne of traffic

as against 170 kg in developed countries. The wagons should be

redesigned to increase the cubic content and the load carrying capacity

to fall in line with the international norms.

No tippling for unloading should be resorted to.

Wagons should be equipped with end of train telemetry.

Coupling height should be 851 mm instead of 1105 mm at present.

Lowering of wheel diameter and coupling height substantially increases

the volume available for the payload Bogie Mounted Electronic Brake

System should be adopted.

6.0 CONCLUSIONS

6.1 To carry more freight, cost effectively, presently the Indian Railway

has only option to increase loading per axle.

6.2 From the consideration of bending stresses and contact shear

stresses, even 60 kg, 90 UTS rail is not able to sustain the increased

stresses due to 30 tonne axle loads. 68.5 Kg AAR or 71 Kg UIC rails

seems to be a realistic solution.

6.3 Contact pressure between rail and sleeper would be higher than the

permissible value even on a track with 60 Kg rail on 60 Kg sleepers

(PSC-6) at 60 cm spacing (1660 sleepers/Km).

6.4 It is found that track with 60 Kg rail on 60 Kg sleepers at 43 cm

spacing (2326 sleepers/Km would only be fit for running of 30 tonne

axle loading from the consideration of contact stresses. Else, PSC

sleepers will require redesigning considering the difficulties involved in

maintaining a track with such high sleeper density.

6.5 300 mm clean ballast cushion would be required for running 30 tonne

axle load and the deep screening and temping needs to be done at

closer intervals.

6.6 Therefore, the track structure for new track should be 68.5 kg. AAR or

71 kg. UIC rails laid on prestressed concrete sleepers, 2326 sleepers

per km. with minimum 300mm of clean ballast cushion.

6.7 With the introduction of 30 Tonne axle load, in most of the cases,

formation stresses would exceed the bearing capacity of the formation.

This would necessitate provision of a blanket layer of adequate

thickness as per RDSO’s specifications for which extra cost of Rs. 10

Lac/Km will be involved. For running 30t axle load in the existing track

this will be most challenging job due to field difficulty in carrying out

the work.

6.8 Each bridge in the dedicated corridor should be evaluated regarding

safety vis-à-vis its physical condition and check for HMLS loading.

Impose speed restriction, if necessary particularly in arch bridges.

6.9 The cost of the maintenance of the track with increase in axle loads

from 20.32 t to 30 t is expected to increase by 3 times depending on

the formation and track quality as per AASHO test. This is still

ambiguous since Swiss model studies shows 3% increase in

maintenance cost after doing rail grinding and lubrication.

6.10 Maintenance inputs are required to be increased. Rail grinding is to

be carried out at predetermined intervals. Also lubrication must be

carried out regularly.

6.11 For running the higher axle loads in existing track, increased frequency

of inspections, patrolling and USFD testing are required. Normal USFD

will be required to be done at periodicity of less than 8 GMT and for

gauge face testing to detect corner cracking.

6.12 As an alternative strategy, use of wagons with high payload to tare

ratio and increased number of axles may also be considered. Wagon

dimensions must be changed with reduced wheel diameter. Signalling

system requires to be upgraded.

7.0 SUGGESTIONS & RECOMMENDATIONS

(i) Head hardened rails should be used. And use of 120m long

panels rolled by steel plants.

(ii) Extra ballast profile should be given at curves.

(iii) Lubrication of gauge face should be done at closed intervals.

(iv) Provision of a blanket layer of adequate thickness to improve

the bearing capacity just beneath the ballast bed.

(v) Codal provisions of tolerances of flat wheel require to be changed.

Monitoring of flat wheels should be done closely and en route

detachment of wagons with flat wheels should be done.

The US studies revealed that the defect size more than 15%

have direct implication due to wheel flat and failure rate is more.

The USFD should be carried out within the periodicity of 8GMT

68 69


so that defects of sizes more than 15% shall be detected in

time.

(vi) Strict tolerances for the track parameters should be kept.

Maintenance inputs are required to be increased. Use of superior

materials to increase the life cycle should be used. Mechanised

maintenance should be adopted. Deep screening and temping

should be done at closer interval than existing provisions.

(vii) Rail Grinding

A better solution to increase the life of the rails on Heavy Haul

Routes is rail grinding. Such grinding will remove the plastic

deformation on railhead thereby removing the surface cracks

before they propagate further into rail section. It also helps in

progressively lowering the point of maximum shear stresses

thereby increasing the life of the rail and prevention of sub surface

cracks due to fatigue.

(viii) Thick web switches should be used in the turnout taking out of

curve. CMS crossing should be used instead of built up crossing.

Maintenance of fittings should be of highest order.

(ix) Frequency of Ultrasonic Testing of be increased especially gauge

face side testing. Ultrasonic testing of welds.

(x) Handling & care of welds.

(xi) Special attention to Points & Crossings & SEJs.

(xii) Keeping bridges under continuous watch.

(xiii) Modification in wagons – Decreasing tare load and improved

suspension system.

8.0 REFERENCES

1. Esveld Coenraad, Modern Railway Tracks.

2. RDSO Guideline No. G-1, Guidelines For Earthwork in Railway Projects.

3. RDSO Civil Engg. Report No. C.55, Investigations on Determination of

Intensity of Pressure on Railway Formations- BG Tracks.

4. Eisenmann Dr. Ing. J., Vehicle- Track Panel- Ballast Stresses.

5. Harvey A.F., Sleeper Spacing and its effects on the Maximum

Permissible Axle Load.

6. Mohan M.S., Track Structure on heavy Mineral Lines.

70 71


7. International Heavy Haul Association, may 2001- Guidelines to Best

Practices for Heavy Haul Operations: wheel and Rail Interface Issues.

8. Tech. Paper No. 245: Report of Bridge Sub-Committee on track

stresses, 1925.

9. Tech. Paper no.323- Stress in Rly track, by Venkatramaya, 1950

10. Tech. Monograph No. 12: Track loading fundamental by C. W.

Clarke,1959.

11. Civil Engg. Report No. C-100: Dynamic augments of track loads, 1971.

12. 53rd TSC Report, 1977, Item No. 716.

13. IHHA-1997, Strategy beyond 2000. Sixth International Heavy Haul

Railway Conference Cape Town, south Africa,Pg.1155-1158.

14 . STRATEGIES FOR MEETING TRANSPORT DEMAND Role of

Railways by Ashutosh Banerji for RAILWATCH

15. Technical Report No.4, on load carrying capacity of masonry arch

bridges, By Sh. R.R. Jaruhar, Member Engg. Rly Board.

CHECKING THE SUITABILITY OF EXISTING

ROLLER-ROCKER BEARINGS FOR HIGHER

AXLE LOAD

RAVINDRA KR. GOEL*, HARI OM NARAYANA**

SUJEET NATH GUPTA***

1.0 INTRODUCTION

Conventional roller and rocker bearings are provided in open web girder

bridges since long. These bearings have been designed as per the loading

standard (mostly BGML or RBG) prevailing at the time of construction of the

bridges. As the traffic volume is increasing and extra revenue is proposed to

be generated, a policy decision has already been taken to make some of

the routes fit for running 25t axle load. The existing bridges are required to

be checked for carrying this increased axle load, on restricted speed, if

required. The implications of higher axle load are also to be examined on

existing bearings with respect to original design parameters of BGML/RBG

loading. It has been seen that zonal railways are not having complete

awareness about the various design parameters involved in design of rocker-

roller bearings and replacement of bearings are proposed without exercising

adequate design check. The paper describes in detail the relevant provisions

for analysis of such bearings and comments on the adequacy of existing

bearings of standard spans.

2. LOADS AND LOAD COMBINATIONS

2.1 LOADS

The loads to be considered for design/analysis of bearings are to be

taken from Clause 2.1 of Bridge Rules. The following loads are to be

invariably considered. It is seen that the lateral horizontal forces due

to wind (not due to Earthquake) are governing in steel girders. Therefore,

the effect of wind load is to be considered unless some special

conditions are found prevailing at the site of bridges.

a) Dead Load

* Director, B&S, RDSO, Lucknow-226001

** Sr. Section Engineer/Design, RDSO, Lucknow-226001

*** Section Engineer/Design, RDSO, Lucknow-22600172


b) Live load including raking force

c) Impact load

d) Longitudinal load

e) Wind load

f) Forces due to curvature and eccentricity

a) DEAD LOAD

Dead load can be taken from the design sheets or drawings of

the existing bridge to be checked. In the absence of reliable

documents the dead load has to be estimated on the basis of

detailed survey of the different bridge components and

connections.

b) LIVE LOAD

At present it is customary to use the concept of EUDL for working

out the live load effect on bearing. EDUL for shear for the

particular span (effective) is to be considered for calculating live

load on each bearing. Annexure 2 of RDSO letter No.CBS/

Golden/Q/Strength dated 8-8-06 can be referred for the purpose.

This EUDL table has been prepared for 2WAG-9/2WDG4 with

BOXN wagons of 25t axle load.

c) IMPACT LOAD

For working out the impact load impact factor CDA can be derived

from Clause 2.4.1 of Bridge Rules. Impact factor can be reduced

for reduced speed. Proportionately if the bearings are to be

checked for operation of traffic at restricted speed as per the

following expression.

d) LONGITUDINAL FORCE

It is an important design parameter, considerably affecting the

design of bearings. The longitudinal force actually applied at

the bearing depends upon the maximum tractive effort of loco

running over the route, maximum braking force applied by the

train, dispersion of longitudinal force through the girder and track.

Clause 2.8.3.2 of Bridge Rules (as per A&C Slip No.22) has to

be referred in this regard which is reproduced below for ready

reference.

“In case of bridges having open deck provided with through

welded rails, rail-free fastenings and adequate anchorage of

welded rails on approaches (by providing adequate density of

sleepers, ballast cushion and its consolidation etc., but without

any switch expansion joints) the dispersion of longitudinal force

through track, away from the loaded length, may be allowed to

the extent of 25% of the magnitude of longitudinal force and

subject to a minimum of 16t for BG and 12t for MMG or MGML

and 10t for MGBL. This shall also apply to bridges having open

deck with jointed track with rail-free fastenings or ballasted

deck, however without any switch expansion or mitred joints in

either case. Where suitably designed elastomeric bearings are

provided the aforesaid dispersion may be increased to 35% of

the magnitude of longitudinal force.”

Note: Length of approach for the above purpose shall be taken

as minimum 30m

As per this clause the dispersion in longitudinal force (tractive

effort or braking force) may be permitted upto 25% by ensuring

adequate anchorages of welded rails on approaches.

Longitudinal force can be taken from Annexure 5 of RDSO letter

No.CBS/Golden/Q/Strength dated 08-08-06 as the maximum

value of tractive effort or braking force for effective span under

consideration. These values are again for 2WAG-9/2 WDG4

loco with BOXN wagons of 25t axle load.

e) WIND LOAD

Wind load is to be taken from the existing design sheets which

is generally for a wind load intensity of 150 kg/m2. As per A&C

Slip No.34 of Bridge Rule, the wind load shall not be considered

acting alongwith live load if the wind load intensity is more than

100 kg/m2. Therefore, the wind load for analysis of existing

bridges for permitting higher axle load shall have to be modified

accordingly. Wind load has following two effects:

i) To apply overturning effect on bearings causing increased

vertical load on leeward side bearings.

ii) To apply a lateral force on bridge simultaneously with

longitudinal force. Resultant of the two forces is to be

resisted in shear by saddle bolts and anchor bolts.

74 75


f) FORCES DUE TO CURVATURE OR ECCENTRICITY OF TRACK

Usually, the girder bridges are on straight. However, whenever

they are on curve the extra load on one girder should be

calculated as per Clause 2.5.3 (a) of Bridge Rules and the extra

horizontal load due to centrifugal force should be taken as per

Clause 2.5.3 (b) of Bridge Rules. Eccentricity effects are usually

not considered in girder bridges unless they are large enough.

2.2 LOAD COMBINATIONS

Following load combinations are to be considered:

a) For checking bed plates and bearing pressure on bed blocks

iii) DL + LL + Impact

iv) DL + LL + Impact + Overturning effect due to wind

(Permissible stresses are increased by 16.67%)

v) DL + LL + Impact + Overturning effect due to wind &

longitudinal force. (Permissible stresses are increased by

33.3%)

b) For checking of anchor bolts and saddle bolts in horizontal shear

i) Resultant horizontal force comprising longitudinal force and

wind load. (Permissiblestresses are not increased in this

combination)

3.0 ASSESSMENT OF EXISTING BEARING

3.1 CHECK FOR ADEQUACY OF MAXIMUM BED PLATE PRESSURE

This check is conducted to ensure that the concrete/bed stone is

safe against crushing under the vertical loads. The maximum pressure

calculated should be with in the allowable pressure which can be

taken from Clause 3.16 of Steel Bridge Code. This clause is reproduced

below for ready reference

“Allowable Working Pressure under Bearings or Bed Plates

The area of bearings or bed plates shall be so proportioned that when

the eccentricity of loads due to combination mentioned in Clause

3.2.1 the maximum pressure on material forming the bed shall not

exceed the following limits: -

Granite … 36 kg/cm2 (33 tons/ft2)

Sand Stone… 29.5 kg/cm2 (27 tons/ft2)

Cement Concrete:

As laid down for permissible bearing pressure in Plain concrete

in Table III and III(a) of the IRS Concrete Bridge Code-1962.

Reinforced Concrete:

As laid down for permissible stress in direct compression for

the specified crushing strength at 28 days for ordinary Portland

cement (or the equivalent period of time for other cement) given

in Table III and III(a) of IRS Concrete Bridge Code-1962

The above-mentioned limits may be exceeded by 331/3 per cent

for combinations mentioned in clauses 3.2.2 and 3.2.3

The centre of pressure under flat bearing plates attached to the

girders shall be assumed to be at one-third of the length from

the front edge.”

3.2 MAXIMUM PRESSURE ON CONCRETE

Maximum pressure on concrete can be calculated from the formula:

Where

V = Vertical load with overturning effect of wind

La

= Lever arm, in the height from the center of knuckle to the

bottom of base plate or top of pier/abutment

L = Length of base plate

B = Width of base plate

LF

= Maximum longitudinal force

WL

= Wind load

3.3 CHECK FOR ANCHOR BOLTS

Anchor bolts are provided at the base plate to restrain the bed plate

against any horizontal movement in X & Y direction. These bolts are

checked basically to resist resultant force comprising of longitudinal

force and lateral force due to wind effect. The shear stress on each

bolt can be calculated by the following formula:

76 77


Where

n = No. of anchor bolts provided

d = Diameter of anchor bolts

3.4 CHECK FOR SADDLE BOLTS

These bolts are provided to connect the bottom chord of open web

girders with the saddle plate. These bolts provide an interface for the

transfer of horizontal force in X, Y direction. These bolts are checked

for shear stress which is equal to

Where

n = No. of bolts

d = Dia of saddle bolt

3.5 RELAXATION OF PERMISSIBLE STRESSES

Permissible stresses for retaining existing bridges can be relaxed as

per clause 3.20.2 of Steel Bridge Code which is reproduced below:

“Mild Steel, Wrought Iron and Early Steel Girders

Bridge spans other than open web girder spans may, if they are kept

under regular observation by the Bridge Engineer and his staff, be

retained in use, provided that if the impact effect-specified in clause

3 of the Bridge Rules (Revised 1964) for the maximum permissible

speed over the bridges is allowed for the calculated stresses for various

combinations of loads as laid down in relevant clauses do not exceed

the working stresses specified for those combinations by more than

11 percent. Under the same conditions, permissible shear and bearing

stresses on rivets may be increased by 25 per cent. This increase in

rivet stresses shall not be allowed if the stresses are calculated by

the method given in APPENDIX- E.

Under the conditions specified above, open web girder spans may be

retained in use, provided that the calculated tensile and compressive

stresses do not exceed the specified working stresses by more than

5 per cent. The permissible shear and bearing stresses on rivets may

be increased by 10 per cent.”

4.0 ADEQUACY OF BEARINGS OF STANDARD SPANS

The adequacy of existing roller-rocker bearings of standard open web

girder span of BGML & RBG loadings have been checked by following above

provisions and the results are shown in Table-1 (a) & (b). The analysis has

been done by taking into consideration wind load of 100 kg/m2 alongwith live

load as per A&C Slip No.34 of Bridge Rules with and without 25% dispersion

of longitudinal forces as per clause 2.8.3.2 amended vide A&C Slip No.22 of

Bridge Rules. The adequate anchorage of welded rails on approaches has

to be ensured accordingly. The checking of these bearings has been done

keeping in view, the speed for which the standard span has been cleared by

RDSO vide letter No.CBS/Golden/Q/Strength dated 08-08-06.

78 79


5.0 CONCLUSION

5.1 It is seen that the design of existing roller-rocker bearings is governed

significantly by longitudinal forces. Forces on bearing are considerably

reduced by allowing dispersion of longitudinal forces. Railways may

thus ensure proper anchorage of track on approaches to take advantage

of clause 2.8.3.2 of Bridge rules in this respect.

5.2 The existing bearings can be safely retained by changing saddle bolts

with higher class (Property Class 6.6 or 8.8) and providing extra anchor

bolts as found necessary.

5.3 As the longitudinal forces are to be resisted at the fixed end only, no

action is required to be taken at roller end, provided the physical

condition is other satisfactory.

6.0 REFERENCES

6.1 IRS Steel Bridge Code (1962), Research Designs & Standards

Organisation, Ministry of Railways, Lucknow (U.P.).

6.2 IRS Bridge Rules (1964), Research Designs & Standards Organisation,

Ministry of Railways, Lucknow (U.P.).

6.3 IRS Drawings and Design Documents of relevant Standard Spans,

Research Designs & Standards Organisation, Ministry of Railways,

Lucknow (U.P.).

6.4 RDSO letter CBS/Golden/Q/Strength dated 08-08-2006 with Annexures.

80 81

Volume - I Volume - IGeneral Manager/Bridges, IRCON, New Delhi – 110066.

STRENGTH POTENTIAL OF ARCH BRIDGES

FOR HIGHER AXLE LOAD

RAMA KANT GUPTA*

Most of the Arch Bridges on Indian Railways are of initial construction

i.e. having life more than 100 years. These Arch Bridges were designed for

loading standard of at that time, which was much lighter than the present day

loading. While allowing the higher axle load, particularly keeping in view its

age as well, field engineers are in doubt about its strength particularly when

elastic theory of Arch Bridge analysis reveals that enormous overstressing is

coming in case of allowing the higher axle load. In this paper, firstly, it has been

tried to build the confidence by giving example of other World Railways followed

by Load Testing experience of Indian Railways. Subsequently, it has been tried to

brief the modalities for allowing the higher axle load on arch bridges.

* This paper has been written based on the past experience of the author

as Executive Director, Bridge & Structures at RDSO, Lucknow

1. INTRODUCTION

Arch Bridges are existing all over the world since ages. It is one of

the oldest bridge configurations known to the civilization. Durability-wise,

no other bridge configuration is at par with the arch bridge configuration.

This is so since arch is the configuration that remains under compression

and as far as compression members are concerned, they are having much

more life than the tension members/members subjected to compression

and tension.

Quite an appreciable number of bridges on Indian Railways are still

arch bridges. They are mostly 100 years and above of age. Those arch

bridges were also designed for loading standard of at that time, which was

much lighter than the present day loading standard. Whenever, question of

running of higher axle load comes, first doubt goes towards safety of such

arch bridges.

Actually, arch bridges are not so week as we mostly perceive. Main

issue is about having fair idea of the strength potential of arch bridges.

In this paper, it has been tried to build the confidence by sharing the

statistics of arch bridges of other railways, sharing the test results of

arch bridges done in the past and arriving at the viable conclusion.

2. POPULATION OF ARCH BRIDGES ON INDIAN RAILWAYS

Arch Bridges still constitute a good percentage of the total bridge

stock of Indian Railways. Bridge statistics of Indian Railways based on old

zones is given in Table No-1

From the above table, we see that Central & Eastern Railways (Based

on its old jurisdiction) are having more %age of arch bridge than the average

figure of 17.5% of Indian Railways. Other Railways are having arch bridge

population almost in the same ratio as that of Indian Railways.

3. ARCH BRIDGE STATISTICS OF SOME OF THE EUROPEAN RAILWAYS

Arch Bridges are not only predominantly available on India Railways,

but the same are available even on higher percentage on other world railways.

Statistics of some world railways mostly spreading over Europe is available.

The same is given herewith:

3.1 BRIDGE POPULATION

From Table No.-2, it is shown that populations of arch bridges on

European Railways are more than the average arch bridge population

of 17.5% of Indian Railways.

83


3.2 AGE PROFILE OF THE ARCH BRIDGES OF EUROPEAN RAILWAYS

(Based on details of SNCF, RFI, NR, REFER, DB, RENFE & CD

Railways)

Table No. 3

The above figures reveal that major chunk of arch bridge population on

European Railways are in between the age group of 50 to150 years.

Even about 12% of bridges are having age more than 150 years.

Position of Indian Railways is not so bad as far as percentage of

bridge population having life in between 100-150 years is concerned.

World over, only age is not the criteria for replacement of a bridge.

Age cum condition is the rational criteria for replacement of a bridge.

If a bridge even being old and completed the codal life, but is in

sound condition and structurally safe, there is no need of its

replacement. Codal life is just for guidance and cannot be taken as

replacement criteria.

3.3 CONDITION OF ARCH BRIDGES OF EUROPEAN RAILWAY

(Based on details of SNCF, RFI, NR, REFER, DB, RENFE & CD

Railways)

The above figures reveal that majority of the arch bridges are either in

good or medium condition. Only small percentages of bridges are

having poor to very poor condition. Almost similar situation is with the

arch bridges of Indian Railways as well.

From the above figures, we further conclude that as far as condition

and age profile of arch bridges of European Railways and Indian

Railways is concerned, the same are almost matching. However,

populations of arch bridges on European Railways are higher than

that of Indian Railways. Only one major difference is there that

European Railways are mostly catering for the passenger trains while

Indian Railways is catering for passenger as well as goods trains

both.

It is further worthwhile to point out that European Railways are not

thinking of replacing those old arch bridges. Rather, they had initiated

one project under the umbrella of UIC to device rational way of strength

assessment of arch bridges including effective way of its maintenance.

Fortunately, Indian Railways is also member of that Arch Bridge

Working Group Committee of UIC

4. INITIATIONS TAKEN FOR RETENTION OF ARCH BRIDGES EVEN

DESIGNED FOR LIGHTER AXLE LOAD

As explained above, in Europe as well, arch bridges represent even

good percentage of total bridge population. Regarding retention of such old

arch bridges even designed for lighter axle load, first comprehensive step

was taken in UK. On behalf of the Ministry of Transport and the Ministry of

Supply, Building Research Station (BRS) of UK had conducted series of

84 85


b. To get further information on:

(i) Dispersion of load through the fills over the arch ring.

(ii) Strength contribution of the fill, the parapet and the spandrel

walls.

(iii) Ratio of the load causing initial crack to the intrados of the

arch and the load causing ultimate failure.

(iv) The mechanism and behaviour of arch under test at ultimate

failure.

(v) Effect of abutment movement etc.

Large numbers of arch bridges were tested and reports published.

References of some reports have been given in the Bibliography.

6. SUMMARY OF TEST RESULT OF ARCH BRIDGES OF THE

RELEASED GANDAK VIADUCT OF EC RAILWAY

Before coming on the test result part, let us share the salient features

of the arch nos. 71 & 73 (both identical) of Gandak Viaduct, as given below:

(i) Span : 15’-0” (Semi circular)

(ii) Thickness of arch ring : 1’-10½” Uniform.

(iii) Depth of cushion : 3’-0” from the extrados at crown

up to the bottom of sleeper.

(iv) Barrel length : 14’-0”

(v) Fill : Earth impregnated with stone

ballast and brick bats.

(vi) Haunch fill : Lime concrete.

(vii) Masonry : Brick in lime mortar.

(viii) Pier widt : 3’-9”

(ix) Pier height : 8’-0” above ground level.

(x) Pier length : 14’-0”

(xi) Foundation soil : Alluvial sand.

6.1 FIELD OBSERVATIONS

Detailed observations were taken and reports, reference of some of

them is given in the bibliography were published. From convenience

point of view, only summarised conclusions and readings in Table

No.-5 is given herewith.

tests on various types of spans ranging from 14 ft to 54 ft and the conclusion

is summarised as below:

“For a single span of arch bridge up to 45 ft when tested with a single

axle load of 20 tons, the deflection at crown should not be more than

0.05 inch (1.27 mm) and change in span (spread) should not be more

than 0.015 inch (0.38 mm). It was further assumed that if these values

are not exceeded, a load of at least 40 tons could be carried, the load

being either the weight of a tracked vehicle or the load from the rear

bogie of a wheeled vehicle. If the deflection is more than 0.05 inch

and the change in span more than 0.015 inch, the arch is considered

to be suspected and requires further investigation.”

Perhaps, it was the first research work of its kind. Series of research

works were followed after that. In this process, Chettoe and Henderson

conducted further study on arch Bridges with bogie load arrangement for

varying loads from 20 tons to 90 tons. Based on the results, they came to a

conclusion that with a better load distribution for 2 axles, the deflection due

to 40 tons bogie is only 50% more than the single axle load of 20 tons.

Although, BRS criteria state that the change in span (spread) should

not be more than 0.015 inch, Chettoe and Henderson, after the series of

tests came to conclusion that the limit can be relaxed. The relevant Para of

their report is reproduced below:

“It is not considered that changes in span are likely to be cumulative

in sound arches having adequate backing and sound filling. The amount

of movement to be expected might be estimated by comparison with

these and BRS test results. In order to allow for the unpredictable

nature of this movement, a value of, say 0.02 inch to 0.04 inch might

be assumed for single span arches of moderate span, having good

backing, although there are cases where it might be more. For bridges

with weak backing, a considerable increase in abutment movement

and reduction in strength is to be expected”.

5. TESTING OF ARCH BRIDGES PERFORMED IN INDIA

In India as well, large number of arch bridges were tested as a part

of research work. Among such testing work, RDSO, Lucknow and Highway

Research Station, Chennai did the pioneering work. Before start of the

testing work, objectives were defined by RDSO and the same are given on

next page:

a. To study how far the results of British studies could be applied

to the Railway Bridges in India.

86 87


CONCLUSIONS

a. The load deflection curves are regular up to 100 tons for crown loading

with the fill, spandrel walls and parapets in position and

b. The load deflection curves are regular up to 70 tons for crown loading

when fill, spandrel walls and parapets removed.

Loads for limiting values of deflection and spread as laid down in BRS

criteria and the maximum load achieved while testing

6.2 DISCUSSIONS ABOUT FIELD-TEST RESULTS

Thorough review of the field test results were made and observations

are given as below:

6.2.1 COMMENT ABOUT BRS CRITERIA

These tests show that the limiting values of deflection and spread

are reached at loads much higher than the test load specified in the

BRS criterion. It was further seen that the loads causing limiting

deflection (0.05 inch) and spread (0.015 inch) are well within the

elastic limit of the arch.

6.2.2 CONTRIBUTION OF AXLE LENGTH, PARAPET WALL, FILL

AND SPANDREL WALL

The following conclusions were drawn:

(i) BG axle is giving more safe load on the same arch than the MG

axle

(ii) Fill, spandrel wall and parapet wall add strength to the arch

Conclusion No.-(i) gives more confidence about those MG standard

arch bridges whose fitness for BG is required

6.2.3 OBSERVATION OF DEFLECTION ON THE ADJOINING ARCH

WHEN THE MIDDLE ARCH WAS LOADED

Observations reveal that the load on the arch under test gets

transferred to the adjoining arches due to the possible continuity

effect offered by the presence of the lime concrete haunch filling.

This continuity effect also makes it difficult to work out theoretical

deflections. Due to the existence of the unpredictable phenomenon

of the increase in span of the bridge and the part played by the

spandrel walls, haunch filling and fill in strengthening the arch, there

are considerable errors in theoretical calculation of the deflections

of the vaults under different conditions of loading. Conclusions drawn

88 89


are as under:

(i) The deflection is fairly proportional to load.

(ii) The deflection in both the adjoining vaults are fairly equal.

Thus, it is observed that continuity effect is present in the arches

under the load test.

7. TESTING OF ARCH NO. 76 OF THE SAME VIADUCT FOR FURTHER

CONFIRMATION AND GETTING SOME ADDITIONAL

INFORMATION

Arch No. 76 is having the same details as that of arch nos. 71 & 73.

This was tested with the aim to get confirmation of the earlier test results as

well as to get additional information. Test conducted on this bridge span is

given as below:

(i) Loading on crown up to elastic limit of arch.

(ii) Loading beyond elastic limit and up to the failure of the arch.

Under the first series i.e., loading of the arch up to the elastic limit,

tests were conducted under four different conditions of the arch constituents

as given below:

(i) With parapets, earth fill over the arch ring, spandrel walls and

haunch fill in position.

(ii) Parapets removed, while earth all over the arch, spandrel walls

and haunch fill in position.

(iii) Parapets, earth fill over the arch and spandrel walls, all removed

i.e., only haunch fill (of lime concrete) in position and

(iv) Parapets, earth fill, spandrel walls and haunch fill all removed

i.e., loading on the bare arch.

Under the second series i.e., loading beyond the elastic limit and up

to the failure of the arch, loading was done on bare arch. The details and

sequence of tests are given in Table No. 6

Deflections of the arch at crown and spreads of piers at the springing

level were measured with the load at crown.

7.1 OBSERVATIONS RECORDED DURING THE TEST

Cracks were first observed on fresh dabs of plaster of Paris at the

intrados of the crown at 40 tons of load when the maximum deflection

was 0.056 inch. As the load increased, cracks became wider and

deeper. At a load of 60 tons, cracks at the crown widened and became

clearly visible extending over the entire length of the barrel. At this

load, crown deflection was 0.0920 inch. At a load of 90 tons, cracks

were observed to a depth of 4½ inch on the extrados, near both the

quarter points of the arch. Cracks on the intrados at crown widened

further. At this load, crown deflection was 0.1620 inch. At a load of

100 tons, another cracks at extrados were noticed. Crack at crown

intrados widened further. At this load, crown deflection was 0.1980

inch. At a load of 120 tons, cracks on the extrados near quarter points

developed fully extending to the full depth of the arch ring. At this load,

deflection at crown was 0.2320 inch. When load was increased to 130

tons, it was observed that -

(i) There was a splitting sound.

(ii) The cracks at the extrados at near quarter points widened

considerably.

(iii) The cracks at the intrados at crown tended to close.

(iv) The crown arch voussoir showed crushing.

(v) The load could not be maintained at this stage as it started

dropping.

All dial gauges were removed which took about five minutes and hence,

no proper deflection readings could be obtained. Efforts were made to

maintain the load at 130 tons by pumping oil into the jacks, but the

arch failed. Masonry between the cracks near quarter points fell down.

A clear mortar face was seen near the quarter point.

From the graphs of load deflection curves plotted for bare arch, it was

observed that the curve is linear up to 40 tons. At this load, first crack

90 91


was observed. From 40 to 80 tons, even though the deflection curve is

linear, the rate of deflection is higher. As already stated, bare arch

completely failed at 130 tons. Thus, the ratio between the load causing

final failure and the load causing the first crack is 3.25 to 1.

According to BRS criteria, if the deflection exceeds 0.05 inch under a

test load of 20 tons single axle, the bridge is suspect. This limit was

reached at a central load of 84 tons.

7.2 SPREAD OF ARCH

Spread of the arch had been recorded by three sets of three dial gauges

A1 – E1, A2 - E2 and A3 – E3. Generally, spread increases with the

load. Movements of both the piers were not equal. Furthermore, it was

also observed that increase in span was not uniform along the length

of the barrel.

According to BRS criteria, if the spread exceeds 0.015 inch under a

test load of 20 tons single axle, the bridge is considered to be

suspected. The loads at which this limit was reached for the three

sets of dial gauges are given below:

Sets of dial gauges Load in tons

A1 – E1 at one ft from the end of barrel 52.50

A2 - E2 at the centre of the barrel 50.00

A3 – E3 at one ft from the other end of the barrel 45.00

Thus, average axle load producing 0.015 inch of spread = 49.2 tons

7.3 EFFECT ON ADJOINING ARCHES

Loading on test span affects adjacent arches. Deflections of the

adjoining arches were measured. Conclusions drawn are given as

below:

7.4 CONCLUSIONS

The conclusions drawn are as under:

(i) The arch behaves elastically up to certain limits as discussed

above.

(ii) The limits of deflection (0.05 inch) and spread (0.015 inch) as

per BRS criteria are reached at loads in excess of BRS test

load viz, 20 tons single axle load.

(iii) The various arch constituents viz. parapets, earth fill, spandrel

walls and haunch fill contribute to the strength of the arch.

(iv) In a multi-span arch bridge, loading on one span affects adjacent

spans.

Although, the following observations are not part of the report but can

be inferred from Table No.6:

� Contribution of haunch in strength addition = 10 tons

� Contribution of fill and spandrel wall in strength addition = 30

tons

� Safe load as per deflection = 80 tons

� Safe load as per spread = 49 tons

� Overall safe load = 49 tons

� If we follow the MEXE Method or Survey & Tabulation Method

or go by Elastic Theory of Analysis, this much safe load will not

come.

8. SERIES TEST OF SINGLE SPAN OF 20 FT ARCH BRIDGES

For better confirmation as well as to ascertain about so many additional

parameters, many single span 20 ft arch bridges were tested by RDSO,

details of which is given in Table No-7

8.1 CONCLUSIONS DRAWN AFTER THE TESTS

The conclusions drawn after series of the aforesaid tests are given as

below:

(i) Load versus deflection graph is generally linear for loads up to

80 tons indicating that the arches behave elastically.

92 93


Conclusions drawn after conducting the dynamic tests are given as

below:

(i) Tests indicate that natural frequency of vibration of the arch ring

is very large (30 to 43 cps) as compared to frequency of excitation

forces provided by the rolling stock (1 to 6 cps). Due to this

reason, resonance cannot take place on arch bridges.

(ii) There is no residual strain or deflection after passage of normal

working live load indicating that the arch behave as an elastic

structure.

10. COMPARISON OF THE WORK DONE BY RDSO AS COMPARED

TO OTHER WORLD RAILWAYS

From the above test result, it is seen that BRS criteria of UK is giving

much conservative strength potential of arch bridges than the actual one.

Based on the European studies, UIC has published UIC code 778-3 R .It

dictates about the MEXE method developed in UK. This method omits many

factors that add to the strength of the arch bridges but mostly, not taken into

design consideration. After the series of tests conducted by RDSO glimpse

of which has been given above, RDSO had concluded the following factors

which are enhancing the strength of arch bridges:

(i) Spandrel wall

(ii) Parapet wall

(iii) Fills above the arch

(iv) Haunches provided on the arch

(v) Availability of track in Railway Bridges and Metal Surface on

road bridges, etc

Accordingly, contributions of aforesaid factors were finalized. Based

on that, “SURVEY AND TABULATION METHOD” was evolved for arch bridge

strength assessment by Dr. S. R. Agrawal under leadership of whom, lot of

testing works were performed. (Dr. S. R. Agrawal is an IRSE officer retired

as Additional Director General, RDSO, Lucknow) This method was circulated

to all the Zonal Railways for strength assessment of arch bridges.

Subsequently, RDSO desired to legalise this procedure after discussion

through BRIDGE & STRUCTURES STANDARD COMMITTEE meeting.

Unfortunately, on account of non-submission of results by some of the

nominated railways, item lingered on and then after dropped from the BSC

agenda. Author of this article is of the strong opinion that this method should

(ii) Spread and deflection readings in the arch bridges are very small.

Even under a load of 80 tons (four times the load specified

according to the BRS Criteria), deflections and spreads are

smaller than those specified in the BRS criteria.

(iii) Lateral dispersion through the fill may be assumed to be at an

angle of 45°

(iv) Deflection observations were found to be vitiated due to

temperature effects.

(v) Spread observations were found to be vitiated due to temperature

and wind effects.

(vi) Strain gauge readings as recorded could not be used for

assessing the strength of arch bridges.

(vii) Subsequent tests have shown that allowances should be made

for temperature effect and wind effect. The wind effect may be

eliminated by making the trestles rigid and closing the barrel of

arch by tarpaulin. Allowances for temperature effect will have to

be carefully assessed by experiment.

9. DYNAMIC LOAD TEST OF ARCH BRIDGES

Till now, testing of arch bridges for static load up to the elastic limit as

well as up to the collapse load under various conditions have been discussed

and relevant conclusions have been drawn after field observations. A necessity

was felt about dynamic test as well. In this category, RDSO had tested

bridge nos. 41C and 42B of Kota- Bina Section and published its reports as

Civil Engineering Report Nos. C-75 and C-76 respectively. Brief description

of the bridges is given in Table No-8

94 95


invariably be used for strength assessment of arch bridges, which gives

better result nearer to the load testing result. As a member of UIC Arch

Bridge Working Group, author of this paper presented the testing results

during the 6Th Working Group Meeting on Arch Bridges of UIC held at Vienna

in October 2004. Working Group Members appreciated a lot regarding the

work done by RDSO under the umbrella of Indian Railway.

11. IRS CODAL PROVISION FOR LOAD TESTING OF ARCH BRIDGES

Provision of load test exists in Arch Bridge Code of the Indian Railway,

relevant extracts of the same is reproduced below:

The criteria for arriving at the safe load shall be:

(i) Under the proposed load, the crown deflection and spread do

not exceed 1.25mm and 0.4mm respectively.

(ii) There is no residual deflection or spread after release of load;

and

(iii) There is no crack appearing on the intrados of bridge.

Note: (i) The above criteria will be applicable to segmental and non-

segmental arches of span 4.5m to 15.m provided and span/

rise ratio lies between 2 and 5.

(ii) The load test shall be conducted on distressed bridge only

after completepressure grouting of the masonry.

FILE PHOTO OF ARCH BRIDGE UNDER TESTING WITH

THE HELP OF SIX NOS. OF STEAM LOCOMOTIVES

12. SHARING OF SOME EXPERIENCE AFTER LOAD TESTING OF

ARCH BRIDGES

After experience gained thorough field investigations, large numbers

of bridges were made safe which were found to be having overstressing even

in between 400 to 500 per cent. Even with such a high extent of over stressing

arrived after doing the theoretical calculation based on elastic method, some

of such bridges were load tested and all of them were found safe. Details of

some of the bridges of Mughalsarai-Allahabad section along with its

theoretical stress and load test results is given in Table No-9: (Tests were

performed during 1979-80)

Thus, we see that theoretical analysis is highly inadequate in strength

assessment of arch bridges. Actually, load testing is the practical way of

strength assessment and it should be followed invariably in case of any

doubt as well as a confidence building measures.

13**. FINALIZATION OF LOAD TEST CRITERIA

For determination of the test load, influence line diagram for causing

the maximum bending moment at crown is to be prepared. The maximum

bending moment caused by the type of loading permitted or proposed to be

run on the arch bridges on the section is to be determined based on Clause

5.1.2 of Arch Bridge code. The bogie load required to cause this bending

moment should be worked out from the influence line diagram. The arch

96 97


should be tested with this bogie load and if the deflection of the arch is less

than 0.05 inch and the change in span is less than 0.015 inch, then arch is

to be considered safe to carry the type of loading, which is to be permitted

on the arch.

However, as railway arches are to be tested with bogie loads greater

than 40 tons the change in span that occurs in some cases would be more

than 0.015 inch and, therefore, this limit may be relaxed to 0.03 inch. Main

factors to be considered when the limit is required to be relaxed are-

(i) Condition of masonry and its behaviour under test load.

(ii) Type of foundations and nature of soil under which it is founded.

(iii) Recovery in side sway after test load, which should be complete.

** It is as per remarks given in RDSO report.

14. COMMENTS ABOUT CINTEC, HELIFIX AND OTHER SIMILAR

METHOD OF ARCH BRIDGE STRENGTHENING

A few foreign firms in the names of CINTEC, Helifix, etc are in the

market claiming to be having authority in innovative method of arch bridge

strengthening. Author of this paper is neither against any new technology

nor against any particular firm. But, before adopting any technology, some

clarifications should be taken from such firms. Some of the recommended

clarification may be as below:

(i) What is the strength of existing arch bridge?

(ii) What will be the strength after strengthening?

(iii) Design calculation in support of the above

(iv) Has their technology been approved by any internationally

recognized institution?

(v) Has their technology been adopted by any World Railways and

if so, its performance certificate?

(vi) What is the likely life after strengthening?

(vii) For how much axle load, the strengthened arch bridge is fit?

(viii) Cost comparison in rebuilding of a bridge as compared to

strengthening cost?

As Executive Director, Bridge & Structures, RDSO, Lucknow, author

of this paper tried to get some answers from representatives of such firms.

Rational answers have not been supplied. RDSO has got test result of some

arch bridges strengthened by CINTEC method and tested in TRL laboratory

of UK. As ED/B&S, RDSO and Member or UIC Arch Bridge Working Group,

author of this paper analysed the TRL report and shared with other Working

Group Members of UIC. Based on comments of RDSO as well as other

observations of Working Group Members, such method was not approved.

For kind information of the readers, comment of RDSO is given in the

annexure. (Selected paras is only given on account of space constraints)

CONCLUSION

Arch bridges are having much more strength potential than its

theoretical value. If theoretical analysis reveals that the arch bridge is unsafe

for proposed axle load, then Survey And Tabulation method should be

followed. Final confirmation can be obtained by load testing, if required

BIBLIOGRAPHY

(i) Civil Engineering Report No. C-72 –Investigations on Strength of

Masonry Arches (July 1969) – Report on the results of investigations

on Arches Nos. 71 & 73 (semi-circular 15 ft span brick masonry arches)

on abandoned viaduct approach over the River Gandak near Sonepore

on North Eastern Railway.

(ii) Civil Engineering Report No. C-73 – Investigations on Strength of

Masonry Arches (April 1969) – Report on the results of tests conducted

on Arch No. 76 (semi-circular 15ft span brick masonry) on viaduct

approach over the river Gandak Sonpur on North Eastern Railway.

(iii) Civil Engineering Report No. C-74 – Static Tests on 20ft span masonry

arch bridges on Kota Bina Section Western Railway (March 1973).

(iv) Civil Engineering Report No.C-75 – Investigation on Strength of Masonry

Arches (February 1969) – Interim report on dynamic tests on arch

bridge no. 41/C on Kota-Bina Section of Western Railway.

(v) Civil Engineering Report No. C-76 – Investigation on Strength of

Masonry Arches (February 1969) – Dynamic tests on arch bridge No.

42/B on Kota – Bina Section of Western Railway.

(vi) Civil Engineering Report No. C-77 – Report on Tests on Arch Bridge

No. 270 on Poona – Miraj Section of South Central Railway (May

1969).

(vii) Civil Engineering Report No. C-79 – Report on Tests on Arch Bridge

No. 347 on Poona Miraj Section of South Central Railway (April 1969).

98 99


(viii) Civil Engineering Report No. C-80 – Report on Arch Bridge No. 513

over River Nira on Poona – Miraj Section of South Central Railway

(May 1969).

(ix) Civil Engineering Report No. C-83 – Investigation on Strength of

Masonry Arches (August 1969) – Static-cum-Destruction Tests on

Arch Bridge No. 41-C on Kota-Bina Section of Western Railway.

(x) Civil Engineering Report No. C-84 – Report on Bridge No. 376 (Victoria

Bridge0 12 X 50’0” Arches on Rajkot – Jamnagar Section of Western

Railway (MG) (August 1969).

(xi) Civil Engineering Report No. C-109 – Second Report on tests on Arch

Bridge No. 270 on Pune-Miraj Section of South Central Railway after

strengthening of the Bridge for BG Traffic (April – May 1970) (October

1971).

(xii) Civil Engineering Report No. C-110 – Second Report on tests on Arch

Bridge No. 347 on Pune-Miraj Section of South Central Railway after

strengthening of the Bridge for BG Traffic (April – May 1970) December

1971).

(xiii) Survey and Tabulation Method of Assessment of Strength of Arch

Bridges finalized by Dr. S.R.Agrawal and issued by RDSO to all the

Zonal Railways. Article can also be seen in the Journal of Institution of

Engineers(India) ,January 1973 issue

(xiv) UIC Code 778 – 3 R Recommendations for the assessment of the

load carrying capacity of existing masonry and mass concrete arch

bridges.

(xv) UIC Report: Assessment, Reliability and Maintenance of Masonry

Arch Bridges: Full Report of January’2004 of UIC

(xvi) Arch Bridge Code of Indian Railways

(xvii) IRC Special Publication No.37: Guidelines for Evaluation of Load

Carrying Capacity of Bridges.

ANNEXURE

Extract of Relevant Paras Prepared by the Author in his Capacity

as ED/B&S, RDSO and Member of Working Group of UIC Arch Bridge

Committee

Para 3: This office got copy of the TRL report from Indian representatives

of M/s Gifford. Comments about the testing work assigned by

M/s Gifford to TRL are given as below:

(i) TEST ARCH CONSTRUCTED IN THE LAB NOT REPRESENTING

THE ACTUAL FIELD ARCH

(a) Test arch was constructed with 10mm mortar thickness.

Normally, arch bridges are having very thin mortar thickness.

Even in building construction, mortar thickness is normally

provided as 6mm. World over, mortar thickness of 10mm and

that too, in arch bridge construction may not be there. Providing

more mortar thickness will result unnecessarily more

displacement, since it is comparatively more compressible

material than the bricks and will give erroneous test results, as

the same will be clear from later part of the reply.

(b) Arch was constructed by providing sand layers in between the

rings for resembling the same as ring-separated arch. Ring

separation is one of the defects, but such a ring-separated arch,

wherein all the rings are totally separated, is difficult to get in

the field.

(c) Modeling of the rings of the arch as separated on account of

mortar strength loss and not allowing such thing to the mortar

available at other locations (like in between the bricks of the

same ring) does not seems to be justified. Environmental decay

does not follow the selective approach like the same was adopted

in test arch.

(d) World over, load testing on the arch bridges reveals that there

are so many factors contributing strength to the arches but

mostly, not included in the design. This is the reason arch

bridges are having much more load carrying capacity than its

design value. I had elaborated the fact while my presentation

during the 6Th meting held at Vienna. Some of the factors

100 101


of that order might had occurred due to providing unnecessarily

much more mortar thickness and providing sand layer in between

the rings, which is not representing the real arch bridges available

in the fields.

(b) As per Arch Bridge Code of Indian Railways, while doing the

load test, safe load is considered to that load which results:

� Maximum 1.25mm Crown deflection and 0.4mm spread

(change in span at the springing point)

� Residual deflection after the load test should be zero.

Hence, as per Arch Bridge Code of Indian Railways, the above

tabulated test results are not showing the safe load in terms of

vertical deflection as well as residual deflection, both. Codal

provisions of other World Railways is not known to me. Opinion

of other WG members may kindly be taken in this regard.

(c) Spread (change in span at the springing point) is one of the

important criteria to be measured while load testing. The above

test result is silent about this.

(d) Instead of providing masonry arch pier, RCC pier was provided

which does not represent the field condition. Providing stronger

and more elastic support than the masonry will certainly affect

the result.

(iv) Strength of arch based on elastic method/modified MEXE method

was not done and hence, it is not possible to compare the result.

This office has got one report from M/s Gifford bearing No.B

1660 A/V 10/R 02 Rev C August, 2003. Vide this document, TRL

test result was compared with output result of ELFEN software

and claimed that TRL test result and the result obtained from

ELFEN software are within the range of 2% and thus, almost

matching. Even after so many lapses to the test arch, if ELFEN

software is claimed to be giving very close result, then adequacy/

reliability of this software also needs to be examined.

Para 10: For better illustration M/s Gifford should submit some design

calculation so that design aspect can be well appreciated.

Note: M/s Gifford is the Technical Hand / Partner of M/s. CINTEC

contributing strength to the arch bridges are reiterated as below:

� Provision of haunches

� Fill on the arches

� Spandrel wall and parapet wall

� Track or road surface for which the bridge is there

� Type of loading whether the same is axle load or single

point load

If many factors, contributing strength to the arch brides was not studied,

then the test result is not representing the actual arch bridge strength

potential.

(ii) DESIGN CRITERIA NOT DISCUSSED

Arch bridge was strengthened by providing 12 numbers of 55mm dia

CINTEC anchors. How the necessity of providing such number of

anchors required was not discussed. System should be rational as

far as input requirement regarding extra strength demand is there. To

work out the demand, firstly, strength assessment of existing bridge

needs to be done. All such aspects are missing in the report.

(iii) TESTING OF THE ARCH WAS DONE UP TO INELASTIC LIMIT,

VIOLATING SERVICEABILITY CRITERIA

Load test result mentioned in the TRL report is summarized as below:

From the test result, it was concluded that after strengthening, strength

of the test arch bridge was enhanced from 2.05 to 2.25 times.

Regarding the test results, comments are as under:

(a) I had gone through the various test results performed in India, in

TRL itself and in USA, most of the details can even be seen

from the Proceedings of First International Conference on Arch

Bridges held at Bolton, UK on 3-6.9.1995. In most of the cases,

maximum displacement remains very less and not of the order

as shown in the table. It is likely that maximum displacement

102 103


IDENTIFICATION OF CIVIL ENGINEERING

STRUCTURES BY VIBRATION ANALYSIS

FOR HEALTH MONITORING AND DAMAGE

DETECTION

S.C. GUPTA*

ABSTRACT

The paper describes main methods used in recording vibration

signatures of the structures and its use in damage detection of bridges. This

also clarify that simply recording of vibrations of the structure does not give

any useful information until and unless the information received from such

testing is used in correcting the basic Finite Element model.

1. INTRODUCTION

Technical systems are damaged by overloading, fatigue, ageing and

environmental influences. If structures of civil engineering are planned for a

finite life time, monitoring with respect to damage is one chance to uarantee

safe functionality.

The life time of a structure can be split into three main phases, which

are the design phase, the construction phase and the utilization phase.

With regard to their functionality these three phases differ as follows:

• The design of a structure deals with the specification of the

type of structural system, of loads and other influences. Such

specification depends, of course, on the demands made on the

structure, especially under safety and economical aspects.

• The construction phase covers the quality examination of the

building materials, the safety of the planned construction and

the safety and examination of the various stages of a structural

building, in order to realize the goals defined during the design

phase.

• The utilization phase starts with the release of the structure

and then the structure is exposed to manifold influences, e.g.

ageing and fatigue processes, as well as further planned and

non-planned external events.[1]

An appropriate instrument to guarantee structural safety and economic

efficiency is the monitoring of a structure with comparatively little cost for

maintenance and monitoring in contrast to the high cost for structural repair

or maintenance work, which would then be avoided.

Some decades ago, the major concern of Structural Engineers was

the development and automatic application of new and powerful numerical

methods for the analysis (static and dynamic) and design of large Civil

Engineering structures. In this context, the fast development of the finite

element techniques accompanied by the tremendous technological progress

in the field of personal computers allowed the structural designer to use

currently excellent structural analysis software packages, which enable to

accurately simulate the structural behaviour.

However, the design and construction of more and more complex and

ambitious civil structures, like dams, large cable-stayed or suspension

bridges, or other special structures, made structural engineers feel the

necessity to develop the appropriate experimental tools that might enable

the accurate identification of the most relevant structural properties (static

and dynamic), providing reliable data to support the calibration, updating

and validation of the structural analysis numerical models used at the design

stage.

Beyond that, the continuous ageing and subsequent structural

deterioration of a large number of existing structures made structural engineers

gradually more interested in the development and application of effective

vibration based damage detection techniques supported by structural health

monitoring systems, in which the regular identification of modal properties

also plays an important role.

Therefore, the first and natural tendency of Civil Engineering

researchers was to take some profit from important previous developments

made in System Identification and Experimental Modal Analysis in Electrical

and Mechanical Engineering, trying to accurately identify the main dynamic

properties of civil structures by applying well established input-output modal

identification techniques.[2]

The difficulty to excite large civil structures in a controlled form, as

well as remarkable technological progress registered in the area of

transducers and analogue to digital converters, made however feasible to

open a new and very promising road for the modal identification of large

structures, exclusively based on the measurement of the structural response

to ambient excitations and application of suitable stochastic modal

identification methods.

*Director/B&S/Testing RDSO/Lucknow 105


2. METHODS USED IN VIBRATION RECORDINGS

To experimentally identify the dynamic characteristics of a structure,

also referred to as a system identification two methods are available:

i) Forced Vibration Testing (FVT) also known as input-output

modal

ii) Ambient Vibration Testing (AVT) also known as output only

modals

2.1 FORCED VIBRATION TESTING

2.1.1 BASICS

With Forced Vibration Testing (FVT) the structure to be identified is

artificially excited with a forcing function in point i and its response

yk(t) to this excitation is measured together with the forcing signal

xi(t) (Fig.1). Transformation of these signals into the frequency domain

and calculation of all Frequency Response Functions (FRF’s) Hik

between the response and the forcing function time signals yields

the Frequency Response Matrix, also referred to as Transfer matrix,

H(iω) (Figs.2 and 4).

For linear systems, the Frequency Response Matrix is diagonal.

This means that it suffices to either determine one row or one column

of this matrix (Fig.4). The choice is to either keep the excitation

point constant and rove the response points over the structure or

vice versa. Because it is not so easy to move the exciters used in

civil engineering investigations, the first method is preferred here. In

mechanical engineering, where the structures to be tested are

comparatively smaller and easy to excite, e.g. with a hammer, the

latter of the procedures mentioned is more common.

Figure 3 Frequency Response Function

2.1.2 EQUIPMENT & TEST PROCEDURE

The Conventional Model Testing is based on the estimation of a set

of Frequency Response Functions (FRFs) relating to the applied

force and corresponding response of several pairs of points along

the structure, with enough high spatial & frequency resolution. The

construction of FRF requires the use of an instrumentation chain for

structural excitation, vibration measurement, data acquisition &

signal processing.

2.1.3 EXCITATION

The excitation can be introduced by impulse hammer or shakers

see figure 5. Generally speaking, the means of excitation has to be

chosen such as to

• excite all natural frequencies of interest,

The frequency Response Matrix contains all the information

necessary to determine the dynamic natural properties of the

structure under investigation (natural frequencies and the associated

mode shape and damping coefficients). Dedicated software

packages are available on the market to extract these modal

parameters from the results of a Forced Vibration Test.

Simplification of the equation is given in fig 3

106 107


• be significantly larger in effect than any other

“unwanted” excitation (because: the processing

procedures are based on the assumption that

the measured, artificial excitation is the only

source of excitation during the tests).

Broad-band vibration generators excite all natural

vibrations of the structure in the frequency band at the

same time. Examples are impulse hammers and servo-

hydraulic or electro-dynamic shakers generating

random or swept-sine type forces. Narrow-band

vibration generators excite one specific frequency at a

time. Mechanical devices using counter-rotating

masses can be mentioned here. Of course, hydraulic

or electric shakers can also be used as narrow-band

exciters.

Broad-band exciters are very time effective, but they

have to have (relatively) more energy disposable than

narrow-band exciters. These devices distribute their

2.1.4 RESPONSE

The type of sensor chosen for the response measurement has to fit

the requirement concerning sensitivity & frequency range. This is

usually measured with accelerometers as they are much easier to

apply and rove over a structure. Measuring displacement in many

points is a very cumbersome task for Civil Engineering Structures.

Velocity transducers are well suited for structures exhibiting

fundamental natural frequency f> 4.5 Hz, but the most Civil

Engineering structure exhibit lower frequencies. For frequency >1

Hz piezoelectric type otherwise for f<1 Hz Force balance type

accelerometers are the most suited.

The data acquisition and storage of measurement data involves the

use of an analogue-to-digital (A/D) converter inserted in a digital

computer. The digital raw data must be preliminary analysed and

processed, considering operations of scale conversion, trend-removal

and decimation. Afterwards, the acceleration time series can be

multiplied by appropriate time windows (Hanning, Cosine-Taper, etc.),

in order to reduce leakage effects, and subdivided in different blocks

for evaluation of average spectral auto and cross spectra estimates,

using the FFT algorithm[4]. The automatic evaluation of FRFs

requires appropriate software for analysis and signal processing,

which is already available in commercial Fourier analyzers.

As a next point, the measurement directions and the measurement

point grid density have to be chosen. The basic rule here is:

information on mode shapes is available in measured points and

direction only. These information can be made available from a

preliminary finite element modeling of the structure. So the FE

analysis is the first and most important step of the procedure,

because major goal of the experimental system identification was

to update the preliminary FE model based on experimental results.

This updated FE model could subsequently be used as a basis to

identify problem solution performing parameter studies.

3. AMBIENT VIBRATION TESTING

3.1 BASICS

No artificial exciter is used with Ambient Vibration Testing (AVT), also

referred to as output-only modal analysis or natural-input modal

analysis. The response of the structure to ambient excitation is

energy on many frequencies at a time. Using a narrow-band exciter is

very time consuming, but such a device concentrates all the energy

available into a specific frequency.

To excite Civil engineering structures, hydraulic and electric shakers

are better suited than hammers:

Compared with mechanical structures, the fundamental natural

frequency of a civil engineering structure is low. The average value,

e.g. for some 200 highway bridges in Switzerland is f=3 Hz[4].

Fig 5(i)-Impact

Hammer

fig-5(ii):- servo-hydraulic shakers

108 109


measured instead. With civil engineering structures, ambient excitation

can be wind, traffic or seismic micro-tremors. The more broad-band

the ambient excitation, the better the results. Otherwise, there is some

risk that not all natural frequencies of the structure are excited.

Generally speaking : The information resulting from the force input

signal xi(t) with FVT investigations is replaced with the information

resulting from the response signal yR(t) measured in a reference point

R (Fig.7). Spectrum of such modal is shown in fig.6.

The first software package to extract modal parameters from AVT

investigations has been developed by a civil engineer in the early nineties

of the last century. Today there are several packages on the market

making use of the frequency domain procedures shown schematically

in Figure 10. One of them offers more sophisticated method like FDD

(Frequency Domain Decomposition) and EFDD (Enhanced FDD), the

latter also including estimation of damping values [1] .

more than one reference point unless the structure to be tested is

very simple. The risk of the reference point sitting in a node of one or

more modes can thus be reduced significantly. If response

measurements are three-dimensional, at least one 3D-point has to be

chosen as a referenc point.

As a rule of thumb, the length of the time windows acquired should be

1’000 to 2’000 times the period of the structure’s fundamental natural

vibration. This is a simple but very important rule of thumb. Experience

shows that many investigators do not care about this. But: You can

not harvest feathers from a frog! Therefore: we should make sure that

our time windows are long enough!

However, the most recent signal processing tools are not based on an

analysis in the frequency domain. Stochastic Subspace Identification

(SSI) is a method working completely in the time domain. This method

has especially been developed for AVT investigations.

Concerning response measurement requirements, the same basic

rules apply as for FVT investigations. In addition, it is wise to use

Fig.6 – Output only model

3.2 EQUIPMENT & TEST PROCEDURES

Modern force balance accelerometers (figure 9) specially conceived

for measurement in the range 0-50 Hz and virtually insensitive to high

frequency vibration, have contributed very significantly to the success

of ambient vibration test. Number of points needed is conditioned by

the spatial resolution needed to characterize appropriately the shape

of the most relevant modes of vibration (accordingly to the preliminary

FE modeling), while the reference point should be far from the nodal

point.

3.3 MODEL IDENTIFICATION METHODS

There are several model identification methods available now a days

and it is easy to use any of these techniques with the help of computer

programme readily available. Schematic representation of model

identification methods is shown in Fig.10. Details of these methods

has not been discussed here as it is beyond the scope of this paper.

Figure 7 Ambient Vibration

Testing relationship Scheme;

R is a reference point, K is a

roving point

Figure 8 Calculation of the

cross relationship between the

reference point R and roving

response point k signals

110 111


Figure-9 force balance accelerometers

Fig.10 – Schematic diagram of model identification techniques

Figure-9 force balance accelerometers

Numerical techniques used:

FFT fast Fourier transform

SVD singular value decomposition

LS last squares fitting

EVD eigenvector decomposition

QR orthogonal dcomposition

112 113


4.0 FORCED VERSUS AMBIENT TESTING

The main advantage of AVT is the fact that no artificial excitation is

necessary. This makes such tests comparatively cheap. In addition, AVT

investigations can be performed without embarrassing the normal user. This

fact is very important e.g. for highway bridges.

Ambient excitation is of the so-called multiple-input type. Wind, traffic

and micro-tremors are acting on many points of a structure at the same

time. In the contrary, a forced vibration is usually of the single-input type.

For small, structures, this difference is not important. For large and complex

structures, AVT has an advantage on the excitation side. AVT offers multiple-

input excitation “free of charge”.

Ambient excitation being non-controllable usually results in a lack of

stationarity. This may lead to problems due to the non-linearity of the structure

(no civil engineering structure behaves in a really linear way). In case of the

excitation amplitude being significantly different for each of the setups, a

certain scatter in the results may occur. This is not the case for FVT where

the structural vibrations induced can be kept stationary.

EXAMPLES

4.1 FVT INVESTIGATION

For a short RCC/PSC Arch Bridge on the Aare River at Aarburg.

The results of a single input FVT analysis is shown in Fig10 & 11 [20]

shows the comparison between an FE analysis & FVT investigation

results. A similar study done on long bridge in Berlin (eight span

continuous beam). The details as shown in fig12&13and results also

discussed [21].

Figure 11- Bridge on the Aare River at Aarburg Figure 13 Westend Bridge Berlin.

114 115


Figure 13 is showing :

Frequencies and shapes of the first five (out of nine[3]) modes of the Westend

Bridge as derived from the FVT in vistigation and an up dated FE-model.

(MAC for the modes shown in the MAC=0.74 ... 0.83 range.)

It can be seen that :

a) the freqency of F1

is too low to properly be excited,

b) the second torsional mode f2 is not properly excited at the right-hand side of

the bridge,

c) the bending modes are well excited in the area of the shaker but not at the

right-hand side of the bridge.

d) the natural vibration dictated by the 31 mtrs. middle span is properly excited(f5)

Figure 13(A)

4.2 EXAMPLES OF AMBIENT

VIBRATION TESTING

A long bridge : Ganter Bridge

[23]

A two lane high way bridge with

a total length of 678m has

eight spans with a length

between 35m & 174m, height

of the tallest piller is 174m.

Accelerometers were placed

inside the box girder. Three 3D

& three 1D sensors were roved

in pairs along the structure test

took ten days. A total of 25

modes could be identified in

the frequency band f=0.40,

…..3.55 Hz. AVT proved to be

a very good method to identify

dynamic parameters of such a

large structure exhibiting very

low natural frequencies.

5. FINITE ELEMENT CORRELATION & UPDATING

The modal identification of bridges and special structures plays a

relevant role in terms of experimental calibration and validation of finite element

models used to predict the static or dynamic structural behaviour, either at

the design stage or at rehabilitation. After appropriate experimental validation,

Figure 14 Ganter Bridge

116 117


finite element models can provide essential baseline information that can be

subsequently compared with information captured by long-term monitoring

systems, in order to detect structural damage.

The correlation of modal parameters can be analyzed both in terms of

identified and calculated natural frequencies and in terms of the corresponding

mode shapes, using correlation coefficients or MAC (Model Assurance

Criterion) values. Beyond that, modal damping estimates can also be

compared with the values assumed for numerical modeling.

The accurate identification of the most significant modal parameters

based on output-only identification tests can support the updating of finite

element models, which may be a very interesting task in order to overcome

several uncertainties associated to the numerical modeling.

Such updating can be developed on the basis of a sensitivity analysis,

using several types of models and changing the values of some structural

properties in order to achieve a good matching between identified and

calculated modal parameters. This type of procedure has been recently

followed to study the dynamic behaviour of a stress-ribbon footbridge at

FEUP Campus ( Fig.16) [2]. For that purpose, initial finite element models

were developed idealizing the bridge deck as a set of beam elements with

the geometry considered at the design stage or measured through a

topographic survey.

Later on this model was totally changed after AVT considering partial

rotations between beam elements to simulate the lack of sealing of the

joints and reducing the area and inertia of the beam elements to simulate

the effects of cracking and lack of adherence between precast and in situ

concrete. After all these iterations, very good level of correlation between

identified and calculated natural frequencies and mode shapes was achieved,

as extensively described in ref. (16)

Beyond this type of sensitivity analyses, more automatic finite element

updating techniques can also be used [17]. In this context, a drawback of

output-only modal identification seemed to be the impossibility to obtain

mass normalized mode shapes. However, this inconvenience can be

overcome [18] by introducing appropriate mass changes.

6. CONCLUSION

Vibration signature techniques may be used under normal operation

conditions & can provide a solid basis for: (i) the development of finite element

correlation analyses, (ii) the finite element updating and validation by recording

dynamic parameters through vibration i.e. mode shapes, natural frequency

and damping; (iii) the definition of a baseline set of dynamic properties of the

initially non-damaged structure, that may be subsequently used for the

application of vibration based damage detection techniques; (iv) the integration

of output-only modal identification techniques in health monitoring systems;

(v) the implementation of vibration control devices.

Ambient Vibration Testing can be effectively used in the condition

assessment of Civil Engineering structures like bridges. However, this

technique shall be used in a method as described in the paper, otherwise

simply recording vibrations of a bridge does not give any useful data.

REFERENCES

1. Identification of Mechanical Systems by Vibration Analysis for Health

Monitoring and Damage Detection – by Armin Lenzen, University of

Applied Sciences, Germany.

2. From Input-output to output-only modal identification of Civil Engineering

Structures – by 1) Alvaro Cunha, Faculty of Engineering University of

Porto(FEUP, Portugal and 2) Elsa Caetano, Faculty of Engineering,

University of Porto (FEUP) Portugal

3. Experimental methods used in system identification of civil engineering

structures – by Reto Cantieni, rci dynamics, Structural Dynamics

Consultants, Switzerland.

4. Maia, N.et at. “Theoretical and Experimental Modal Analysis”,

Research Studies Press, UK, 1997.

5. Cunha, A., Caetano, E. & Delgado, R. “Dynamic Tests on a Large

Cable-Stayed Bridge. An Efficient Approach”, Journal Bridge

Engineering, ASCE, Vol.6, No.1, p.54-62, 2001.

118 119


6. Mc Lamore, V.R., Hart, G. & Stubbs, I.R. “Ambient Vibration of Two

Suspension Bridges, Journal of the Structural Division, ASCE, Vol.197,

N.ST10, p. 2567-2582, 1971.

7. Abdel-Ghaffar, A.M. “Vibration Studies and Tests of a Suspension

Bridge”, Earthquake Engineering and Structural Dynamics, Vol.6,p.473-

496, 1978.

8. Felber, A. “Development of a Hybrid Bridge Evaluation System”,

Ph.D.Thesis, University of British Columbia (UBC), Vancouver, Canada,

1993.

9. Prevosto, M.”Algorithmes d’Identification des Caracteristiques

Vibratoires de Structures Mecaniques Complexes”, Ph.D.Thesis, Univ.

de Rennes I, France, 1982.

10. Correa, M.R. & Campos Costa, A. “Ensaios Dinamicos da Ponte sobre

o Rio Arade”, in “Pontes Atirantadas do guadiana e do Arade” (in

Portuguese), ed.by LNEC, 1992.

11. Brincker, R.,Zhang, L. & Andersen, P. “Modal Identificatiion from

Ambient Responses using Frequency Domain Decomposition”, Proc.

18thInt. Modal Analysis Conference, Kissimmee, USA, 2001.

12. Brincker, R.,Ventura, C. & Andersen, P. “Damping Estimation by

Frequency Domain Decomposition”, Proc. 19th Int. Modal Analysis

Conference, San Antonio, USA 2000.

13. Rodrigues, J., Brincker, R. & Andersen, P. “Improvement of Frequency

Domain Output-Only Modal Identification from the Application of the

Random Decrement Technique, Proc.23rd Int. Modal Analysis

Conference, Deaborn, USA, 2004.

14. Cunha, A. & Calcada, R. “Ambient Vibration Test of a Steel Trussed

Arch Bridge”, Proc. Of the 18th Int. Modal Analysis Conference, San

Antonio, Texas, 2000.

15. Caetano, E, & Cunha, A. “Ambient Vibration Test and finite Element

Correlation of the New Hintze Ribeiro Bridge”, Proc. Int. Modal Analysis

Conf., Kissimmee, USA, 2003.

16. Caetano, E. & Cunha, A. “Experimental and Numerical Assessment

of the Dynamic Behaviour of a Stress-Ribbon Bridge”, Structural

Concrete, Journal of FIB, 5, No 1, pp.29-38, 2004.

17. Teughels, A. “Inverse Modelling of Civil Enginering Structures based

on Operational Modal Data”, Ph.D. Thsis, K.U.Leuven, Belgium, 2003.

18. Brincker, R. & Andersen, P. “A way of getting Scaled Mode Shapes in

Output Only Modal Testing”, Proc.21st Int. Modal analysis Conference,

2003.

19. Cantieni, R. “Dynamic Load Tests on Highway Bridges in Switzerland

– 60 Years Experience of EMPA”. EMPA Report No.211, (1983).

20. Cantieni, R., Deger, y., Pietrzko, S., “Modal Analysis of an Arch Bridge:

Experiment, Finite Element Analysis and Link”. Proc. 12th International

Modal Analysis Conference (IMAC), (1994) 425-432.

21. Deger, Y., Cantieni, R., Pietrzko, S.J., Rucker, W., Rohrmann, R.,

“Modal analysis of a Highway Bridge : Experiment, Finite Element

Analysis and Link”. Proc. 13th International Modal Analysis Conference

(IMAC), (1995) 1141-1149.

22. Felber, A.J.Cantieni, R., Introduction of a new Ambient Vibration

System – Description of the System and Seven Bridge Tests, EMPA

Report No. 156’521, (1996).

23. Felber, A.J., Cantieni, R., Advances in Ambient Vibration Testing :

Ganter Bridge, Switzerland, Structural Engineering International (6),

Number 3, (1996) 187-190.

120 121


USE OF POLYMER MODIFIED MORTARS AND

CONCRETES (PMM/PMC) IN REPAIRS &

REHABILITATION OF STRUCTURES

M.M. GOYAL*

INTRODUCTION

Polymers are long chain of organic molecules (having high molecular

weight) formed by combination of single units called monomers. A polymer

consists of numerous monomers, which are linked, together in a chain like

structure, and the chemical process, which causes these linkages, is called

polymerization. Polymers are classified as either thermoplastics or

thermosets. They form the basis of all plastics and elastomers.

In general polymers are inert materials having higher tensile and

compressive strength than conventional concrete. However, polymers have

a lower modulus of elasticity and a higher creep, and may be degraded by

heat oxidizing agents, ultraviolet light, chemicals, and microorganisms; also

certain organic solvents may cause stress cracking. Many of these

disadvantages can be overcome by choosing a suitable polymer and by

adding substances to the polymer, e.g., antioxidants to suppress oxidation

and light stabilizers to reduce ultraviolet degradation.

Polymers are added to conventional Portland cement mortar/concrete

to enhance their properties, such as reducing permeability, increasing bond

with the substrate, improving resistance to chemicals and damage from

freeze-thaw cycles.

The material used for repairs is described as ‘concrete’ where it is

practical to compact it properly. In situations where it is not possible to

adequately compact the material, it is described as mortar. This is one of

the reasons why many repairs are carried out with a mortar or shotcrete.

TYPES OF POLYMER CONCRETE

There are three principal classes of concrete containing polymers:

(a) Polymer impregnated concrete,

(b) Polymer concrete, and

(c) Polymer modified cement mortar/concrete (PMM/PMC).

The distinction between these three classes is important to the design

engineer in the selection of the appropriate material for a given application.

The salient characteristics of the three types polymer concrete are described

below.

Polymer Impregnated Concrete: It is produced by impregnation

under pressure of hardened cement concrete with a low viscosity monomer,

methyl methacrylate (acrylic plastic) and styrene (elastomer - synthetic

rubber) that polymerizes in situ to form a network within the pores.

Polymerization is achieved by thermal-catalytic means. Impregnation results

in markedly improved strength and durability in comparison with conventional

concrete. Principal applications include storage tanks for seawater,

desalination plants and distilled water plants, sewer pipes, tunnel lining and

swimming pools. The disadvantage is the relatively high cost of the polymer

and complicated production process. However, partial impregnation of

concrete members may be economically viable. For example, the shear

capacity of RCC beams, without shear reinforcement, may be increased by

about sixty percent and resistance to anchorage stresses is increased.

Partial impregnation of bridge decks will increase their flexural strength,

reduce deflection and improve water tightness and surface durability.

Polymer Concrete (PC): It consists of a polymer binder and mineral

fillers such as aggregate, sand and crushed stone dust. In PC, Portland

cement as a binder is replaced entirely by a synthetic organic polymer.

Early PCs were made with epoxy and polyester resin systems.

Epoxy is a high strength adhesive compound formed as a result of

polymerization of resin at ambient temperature in presence of a specified

proportion of hardener. Ambient-cured epoxy systems (thermosetting resins)

are a mixture of two components: the epoxy resin (component A), and the

curing agent (component B). Neither component is stable or commercially

useful separately. The components are manufactured separately and not

combined until ready for use by the person applying the material. Epoxy

resins react with the curing agents to yield the desirable flexible, semi-rigid,

or rigid thermosetting plastics. The curing agent, also known as the hardener,

chemically brings about the change from liquid, paste, or mortar consistency

to a solid plastic. It is in this state that the system is usually used, there

being limited usage in the uncured, non-cross-linked state.

PC is substantially more costly than conventional concrete and should

be used only in applications in which the higher cost can be justified by

superior properties in a particular job. It is also known as resin concrete. PC

123* Add. Member (Project), Retd. Railway Board


has higher strength, greater resistance to chemicals and corrosive agents,

lower water absorption and higher freeze-thaw stability than conventional

Portland cement concrete. The monomers and prepolymers most widely

used to produce PC are methyl methacrylate (acrylic), unsaturated polyester,

epoxy and furan-based polymer.

Polymer Modified Cement Mortar/Concrete (PMM/PMC): In these

materials, a part of the cement binder is replaced by polymer latex. The

process of making PMM/PMC is similar to that of the conventional cement

concrete. Most polymers, such as latexes are a colloidal dispersion of a

rubber resin in water, which coagulates on exposure to air. These are initially

mixed in water in required proportion and then added to the cement mortar

or concrete. The latex-modified mortar or concrete is placed in a manner

similar to normal concreting.

Polymer modification results in formation of continuous polymer film

when dried. This improves substantially the characteristics of PMM/PMMC

in both (a) the fresh state (e.g. workability, water retention property), and (b)

the hardened state (e.g. strength, creep, shrinkage, impermeability and

chemical stability) at a reasonable cost.

For repairs to spalled and disintegrated concrete and corroded

reinforcement, PMM/PMC are most often used these days and of the three

types of composite concretes, PMM/PMC is generally the preferred choice

of the material in a repair and rehabilitation job. This will be the main focus of

our attention in this paper.

MATERIALS EMPLOYED IN PMM/PMC

Cement and Aggregates: The materials used in PMM/PMC are the

same as those employed in normal mortar and concrete but for the polymer,

which is used as a modifier. Ordinary Portland Cement (OPC) is widely

used as the basic binder. Air-entraining admixture is not used in PMM/PMC

because air entrainment occurs due to latex addition.

The aggregates used for normal concreting operations are

recommended for latex mixes. The aggregates should be clean, sound, and

of proper grading. The basic principles in regard to design of concrete mixes

apply to the design of PMM/PMC as well.

Polymers: Polymer modifiers most widely used to enhance the

properties of the repair material are latexes based on poly methyl methacrylate

also called acrylic latex, and SBR (Styrene Butadiene rubber) latex. Latexes

are white, milky white liquids consisting of very small diameter particles

(0.05 – 5 µm) emulsified (colloidal dispersion) in water. See Figure 1 for

molecular structure of latex. They form continuous polymer films when dried.

Latex reduces permeability, and increases bond strength with the substrate.

Polymer latexes are copolymer systems of two or more different

monomers and their total solid content including polymers, emulsifiers,

stabilizers, etc. is limited to 40 ± 3 %. Higher percentage would adversely

affect concrete compressive strength.

All latex systems should ensure controlled foaming (air entrainment).

Commercially available polymers must contain proper amounts of anti-

foaming agents. One should be able to use them directly without addition of

anti-foaming agent during cement modification at site.

CHEMISTRY OF POLYMER MODIFICATION

Formation of Polymer Film: Most of the polymer latexes for cement

modification form continuous polymer film when dried. Latex modification of

cement mortar and concrete is governed by both cement hydration and

polymer film formation process in their binder phase. The cement hydration

process generally precedes the polymer film formation process. In due course,

a co-matrix phase is formed by both cement hydration and polymer film

formation processes. This yields a monolithic interwoven matrix of solidified

polymer and its continuous film with hydrated cement; this binds the

aggregates strongly.

It is important to understand the reactions that take place during cement

hydration and polymer film formation. Immediately after mixing, the concrete

when placed is a mixture of unhydrated cement particles, polymer particles

and aggregates with interstitial spaces filled with water (water phase) as

shown in Figure 2 (A). Polymer particles partially deposit on the mixtures of

unhydrated cement particles and cement gel in the first stage process of

hydration as shown in Figure 2 (B), completely enveloping them and forming

a membrane in the binder phase (Figure 2C). As hydration proceeds and

water is further removed from the pore solution, continuous polymer film

forms encapsulating the cement hydrates (Figure 2D) providing a strong

binder matrix having substantially reduced permeability.

The continuity of the polymer phase through the binder matrix is more

pronounced with polymer cement (p/c) ratio in the range of 0.1-0.2 by weight

of cement.

124 125


Reduction in Pore Size Distribution: Concretes modified with

polymer latexes generally show a different distribution of shape of pores. In

conventional Portland cement concrete the structure consists of large pores

surrounded by a number of small pores, whereas concrete with latex tends

to show only spherically shaped pores, usually at a smaller average pore

diameter. Capillary water uptake depends upon capillary pore diameter and

is effectively stopped at about 0.1 mili-micron. Polymeric modifiers are

designed to reduce the content of these over-sized pores. The net effect on

the impermeability characteristics of latex modified concrete is very favorable,

particularly regarding its resistance to chloride ion penetration.

IMPROVEMENT IN THE PROPERTIES OF PMM/PMC

Workability: Polymer latex modification of cementitious mixtures

improves workability and water retention property compared to conventional

mortar/ concrete. There is better resistance to bleeding and segregation

even though they have better flowability.

Resistance To Crack Propagation: Micro cracks occur easily in

the ordinary stressed/hardened cement paste. This results in poor tensile

strength and fracture toughness. In the latex modified mortar/ concrete, it

appears that the micro-cracks are bridged by the polymer film or membrane,

which prevents crack propagation and simultaneously, a strong cement

hydrate-aggregate bond is developed.

Increase in Tensile Strength: PMM/PMC with SBR latexes have a

noticeable increase in tensile and flexural strength but there is hardly any

improvement in its compressive strength compared to ordinary mortar/

concrete. An increase in the polymer content (defined as the weight ratio of

the amount of total solids in polymer latex to the amount of cement) leads to

increase in flexural tensile strength and fracture toughness. However,

excessive air entrainment and polymer inclusion cause discontinuities of

the formed monolithic network structure, whose strength is reduced.

Chemical And Abrasion Resistance: This depends on the type of

polymer, polymer cement ratio and type of chemicals used in the PMM/

PMC. Most PMM/PMC with SBR latex are attacked by strong organic and

inorganic acids and sulphate but they resist well alkalis and salts. Their

resistance to chlorides, fats and oils is also rated good, while they have

poor resistance to organic solvents.

PMM/PMC have better abrasion resistance than conventional mortar/

concrete.

Temperature And Shrinkage: PMM/PMC show rapid reduction in

strength with increase in temperature. Most thermoplastic polymers have

glass transition temperature (tg) of 80 – 100°C, the temperature at which a

reversible change occurs in an amorphous polymer when it is heated to a

certain temperature and undergoes a rather sudden transition from a hard,

glassy, or brittle condition to a flexible or elastomeric condition. Below tg,

molecules have little mobility.

Drying shrinkage of PMM/PMC may be larger or smaller depending

on the type of polymer and polymer cement ratio used. More is the polymer

ratio less is the drying shrinkage.

Permeability And Durability: PMM/PMC have a structure in which

the larger pores are filled by polymer. The sealing effect due to the polymer

film formation in the structure also improves water tightness (impermeability)

as well as resistance to chloride ion penetration, carbonation and oxygen

diffusion, chemical resistance, and durability against freezing and thawing.

Such an effect is promoted with increasing polymer – cement ratio up to a

point.

Generally these materials are bio non-degradable after total

polymerization takes place. However, certain polymers such as styrene tend

to disintegrate under any form of energy like ultra violet rays, heat, etc.

Acrylate based materials (acrylic polymers) are reported to be robust and

bio non-degradable.

Adhesion or Bond Strength: A very useful property of PMM/PMC is

their improved adhesion or bond strength to various sub-strata compared to

conventional mortar/concrete.

MIX PROPORTIONS

Polymer Modified Mortars: Mix proportion for most PMMs is in the

range of 1:2 to 1:3 (cement – fine aggregate ratio). The polymer latex (solid

contents): cement ratio ranges from 5 to 20% by weight. The water cement

ratio is of the order of 0.3 to 0.5 depending upon the requirement of workability.

The standard mix proportion for different usage is indicated in Table 1.

Polymer Modified Concrete (PMC): The mix proportion for latex

modified concretes cannot be easily determined in the same manner as

that of latex modified mortars. Because of many factors in design, normally,

the polymer latex: cement ratio ranges from 5% to 15% and water cement

ratio from 0.3 to 0.5.

126 127


GENERAL GUIDELINES FOR USE OF PMM/PMC

Polymer should be mixed with cement slurry or mortar in the proportion

recommended by the manufacturer for various uses. The following precautions

may be borne in mind while using PMM/PMC:

a) The speed and time of mixing should be properly selected to

avoid unnecessary entrapment of air;

b) The PMM/PMC have excellent adhesion even to metal and hence

all equipment should be washed immediately after use;

Table 1 Typical Applications and Standard Mix Designs of

Latex Modified Mortars

c) For resurfacing, flooring and patch repairing all loose and non-

durable materials including laitance must be removed by sand

blasting, wire brushing and blowing with compressed air. The

cleaned surface should be thoroughly wetted well before

placement of PMM/PMC. Before application, surface should be

in saturated dry (wet but no standing water) condition;

d) The choice of PMM/PMC depends on the thickness of coating

to be applied;

e) It is advisable to finish the surface by trowelling 2-3 times. Over

trowelling should be avoided;

f) PMM/PMC should never be placed below 5° and above 30° C.

The surface of the newly placed material should be protected

from rainfall or other source of water. The surface should be

immediately covered with burlap or plastic sheet;

g) In large area of application, it is advisable to provide joints 15

mm width at intervals of 3 – 4 m;

h) Polymers and latexes are non-toxic and safe for handling. They

should be stored in a cool dry room and should not be kept in

exposed areas.

Curing: Curing of PMM/PMC is different from that of conventional

concrete because the polymer forms a film on the surface of the product

retaining some of the internal moisture needed for continuous cement

hydration. Because of the film-forming feature, curing under water immersion

or under wet condition is detrimental to PMM/PMC. Moist curing of the latex

modified mortars/concretes is generally shorter than for conventional

products. Optimal conditions for strength development are moist curing for 3

days followed by dry curing at ambient temperature.

FIELDS OF APPLICATION OF PMM/PMC

Structural Repairs to RCC: PMM/PMC are used to make up the

damaged concrete or lost cover due to their better bond with substrate, and

the reinforcement.

Ultra Rapid Hardening Polymer Modified Shotcrete: It can be

classified into two categories depending on usage as under:

(a) Repairs to Leaky Liquid Retaining Structures: In this system,

polymerisable monomers are used that react with Ordinary

Portland Cement at ambient temperature to form protective cover

128 129


for concrete structures with leaking and flowing material. It uses

magnesium acrylate monomer and its setting time can be

controlled within few seconds or less.

(b) Urgent Construction And Repair Works: Ultra Rapid

Hardening Cement Concrete is used with SBR latex for urgent

construction and repair works subject to heavy traffic.

Polymer Ferrocement: For the purpose of improving the flexural

behavior and durability of conventional Ferrocement, polymer-Ferrocement

have been developed using latex modified mortars instead of ordinary cement

– sand mortars. Use of SBR and EVA modified mortars is found to be quite

effective in improving their flexural behavior, impact resistance, drying

shrinkage and durability. Incorporation of short fibers such as steel and

carbon fibers in the latex modified mortars is found to be further effective in

improving such characteristics.

Anti Washout Underwater Concrete: Major requirement for such

concrete is anti washout or segregation resistance, flowability, self-leveling

ability, and bleeding control. Anti-washout admixtures are water-soluble

polymers and are added in the polymer-cement ratio of 0.2% to 2.0% during

mixing of the ordinary cement concrete. They bond to a part of the mixing

water by hydrogen bonds in the concrete and disperse in a molecule form in

the mixing water. As a result, the mixing water is confined to the network

structure of the dispersed polymer and becomes very viscous. The very

viscous water envelops the cement and aggregate particles to impart anti-

washout character to concrete.

Protective Waterproofing Membranes and Repair Materials:

Polymer modified pastes or slurries with very high polymer: cement ratio of

50% or more have been widely used as liquid applied waterproofing membrane

and repair material. The constituents normally comprise Portland cement,

silica sand, water and polymer latexes such as SBR, EVA, PAE, SAE,

epoxy and asphalt latexes besides some other additives. Thickness of such

waterproofing membrane is 1.5 to 2 mm; they are generally available as pre

packaged products. The performance advantages of such membranes are

as under:

a) Safe application due to no organic solvent system,

b) Convenience of application as it does not require the surface to

be dry,

c) Good adhesion with the cementitious, metallic and most other

substrates,

d) Excellent elongation, flexibility and crack resistance,

e) Good waterproofness, and

f) Resistance to carbonation and chloride ion penetration.

An innovative water proofing material, which solidifies in water has

recently been developed in Japan for tunnels and dams. It has potential as a

shock absorbing, waterproofing backfill material.

Bond Coats (Structural Adhesive) And Grouts: Polymer modified

cement mortars (PMMs) as well as slurries are used as bond coats and

grouts due to their very good adhesive qualities on cementitious as well as

metallic surfaces.

130 131


LONGITUDINAL LOADS ON RAILWAY BRIDGES

SHYAM SUNDER GUPTA*

ABSTRACT

Increasing demand of traffic on existing infrastructure leads to higher

intensity of loads on bridges. To allow higher intensity of loading on existing

bridges, the alternatives are:

a) To exploit extra potential available on account of conservative

designs of early days of steam engines.

b) To allow heavy axle loads on existing bridges on physical condition

basis on theoretically overstressed structure.

c) To revise codal provisions based on instrumentation, field testing,

latest knowledge and actual requirements.

d) Strengthening/ rebuilding with minimum disruption to traffic at

least cost.

While alternative (a) and (b) are already being followed, the alternate

(c) and (d) are being discussed in this paper. For alternative (c), history of codal

provisions regarding longitudinal loads and retention of some deleted provisions

and amendments to existing provisions are discussed. For alternative (d), use of

STU’s for permitting higher longitudinal loads on existing bridges is discussed.

INTRODUCTION

Bridges on Indian Railways were constructed to lighter loading standard

in the earlier stage of developments of Railways, as the speeds as well as

the loads were low. Longitudinal forces were not even considered for design

of bridges prior to 1923. As the speed and loads were increased, the

longitudinal forces assumed importance. The first reference to provision of

longitudinal forces on Railway bridges in India is made in the Bridge Rules of

1923. By this time more than 60% of the bridges were built on Indian Railways.

The provision for longitudinal forces in earlier days (prior to 1975) was made

on empirical basis for design of bridges.

On bridges, longitudinal forces are caused by train running, i.e. tractive

efforts and braking force or due to temperature changes. In this paper

longitudinal force due to tractive effort and braking force only are discussed.

HISTORY OF PROVISION OF LONGITUDINAL LOADS IN BRIDGE RULES

Prior to 1892, bridges were built according to the British Board of

Trade Rules. The Bridge rules 1892 did not specify longitudinal loads.

Reference to the tractive and braking forces first appeared in 1923 Rules. A

historical note on longitudinal load provision as per Bridge rules is given in

annexure-I. The history of loading standards for bridges on Indian Railways

is also given in annexure-II.

As per 1923 rules, traction and braking forces were taken about 1/7th

of train loads. Detailed provisions were made in 1926 Bridge Rules, as given

in annexure-I. Since these provisions on braking force and tractive efforts

were copies of BESA rules, it was suggested in 1932 that there formulae be

amended to make them applicable to the standards of loading that were in

use at that time in India. Accordingly provisions were modified in 1933 rules.

It may be noted that provision for dispersion of longitudinal loads were

made for the first time in Bridge Rules of 1933.

In Bridge Rules of 1941 printed in 1960, a ‘note’ reproduced below is

appearing.

“Note – In the case of piers and abutments of RCC bridges with

ballasted floors, no longitudinal force need be taken into account for

spans upto and including 20 ft.”

This note is appearing as clause 2.8.2.3 of Bridge Rules 1964 but was

later deleted and does not exist in current Bridge Rules. This was an important

clause, which needs review.

In 1964 Bridge Rules, HM loading was deleted. It is also seen that in

1964 Bridge Rules Tables of longitudinal forces were modified by adding

dispersion allowance so that table gives longitudinal loads (without deduction

for dispersion). This is why the values of TE and BF for BGML bridges are

more than the MBG bridges for smaller spans.

The provision of TE and BF given in Bridges rules prior to 1975 are

based on empirical formulae.

BGML loading accounts for a maximum tractive effort of 47.6 t, which

was raised to 75 t in RBG loading introduced in 1975. Modified Broad Gauge

loading (MBG-1987) introduced in 1988 caters for 100 T of tractive effort for

coupled locos with TLD of 8.25 t/m.

*Chief Bridge Engineer, North Western Railway, Jaipur133


After the introduction of longitudinal force provision in Bridge Rules,

mainly three loading standards have been used on Indian Railways for bridges

(BG). There are BGML, RBG and MBG standard of loading. The comparative

statement of longitudinal loads for these loadings is given in annexure-III.

PROVISION OF LONGITUDINAL LOADS ON OTHER RAILWAY SYSTEMS

The provision regarding longitudinal forces i.e. tractive and braking

forces on foreign Railway systems are given in appendix-IV.

Maximum dispersion permitted as per BS 5400: 1978, is up to one-

third of the longitudinal loads for bridges supporting ballasted track, provided

that no expansion switches or similar rail discontinuities are located on or

within 18m of either end of the bridge.

For normal type RU loading which consists of four 250 kN concentrated

loads preceded and followed by a uniformly distributed load of 80 kN/m,

maximum tractive force is 750 kN for loaded length exceeding 25m.

As per AREA manual (1991) for Railway Engineering:

a) The longitudinal force (LF) from Train shall be taken as 15% of

the live load without impact and

b) Where the rail are continuous (either welded or bolted joints)

across the entire bridge from embankment to embankment, the

effective longitudinal force shall be taken as L/1200 (where L is

the length of the bridge in feet) times LF given in (a) above but

the value of L/1200 shall not exceed 0.80. So the dispersion of

longitudinal force will be

(1-L/1200) x100%, Subject to a minimum value of 20%.

It is seen that no distinction is made between tractive force and braking

force as per this manual.

UIC 776-1R

As per this code the term “accelerating force” has been used in place

of tractive force. The value of accelerating force is as under:

For L< 30m a) discontinuous track 33 kN/m

b) Continuous track 22 kN/m

For L > 30 m a) discontinuous track 1000 kN

(Uniformly distributed over a length of 30m)

b) Continuous track 660+1.25 (L-30) in kN

(May be distributed over a length of 30m)

For L>300m : 1000 kN constant

Where L is the loaded length

It is seen that for continuous track, part of the accelerating force is

considered to be dispersed away from loaded length.

Braking force (as per UIC 776-1R) acting on a structure is 0.25 times

the vertical force in the UIC loading calculated over the loaded length L. The

braking force shall be evenly distributed over the whole of the loaded length.

It is seen that no dispersion of braking force has been considered as

per UIC 776-1R Code. Also accelerating force for L> 30 m has been considered

to be distributed over a length of 30m whereas for braking force, it is

considered to be distributed over whole loaded length L.

DISPERSION OF LONGITUDINAL FORCES

Longitudinal forces generated due to traction/ braking of train are partly

dispersed to approaches and balance transferred to sub-structures. Amount

of dispersion of longitudinal forces to approaches depends on the stiffness

of approaches, type and condition of track structure, provision of guard rails

and their condition, type of braking, number and length of span of the bridge,

vertical load over the bridge etc.

Tests conducted in India and abroad under different conditions are

briefly summarized below:

TESTS CONDUCTED IN INDIA

Some tests were carried out by North Western Railway in the early

thirties to determine the dispersion of tractive force through the rails to the

approaches. These tests were conducted with steam locomotives. The actual

force applied to the piers in the case of bridges with one, two and three

spans of 40feet were measured.

It was found that about 70% of the TE is dispersed for 1x40 feet span

and for 3x40 feet span, 50% of the TE goes through the rails and balance

50% to piers. For a length of about 400-500 feet, it was found that nearly

whole of the TE is transferred to piers. Based on these observations it was

concluded that in bridges not exceeding 130 feet in total length, 50% only of

the specified tractive and braking forces need be considered as passing

through the girders to the piers and abutments.

134 135


These tests indicate that length and number of spans affects the

amount of dispersion to approaches.

Braking force tests on South Bassein Bridge (60x 60 feet) confirm

that in a long bridge, the whole of the LF is ultimately distributed amongst

the piers.

RDSO was entrusted with the work of investigation of dispersion of

longitudinal forces. The tests were conducted on Sone bridge of 14x76.3m

span with two shore spans of 30.5m near Chopan on Chunar- Chopan section,

Varuna Bridge No 32 of 6x 18.3m span near Varanasi on Varanasi-Zafrabad

section, Bridge No 3 of 2x 12.2m span near Shikohabad on Shikohabad –

Farrukhabad section and Kolab bridge No. 543 of 10x45.7m through spans

on SE Railway.

During the longitudinal force trials on Varuna Bridge of 6x18.30m span

(RDSO’s Civil Engg Report No C- 153), it was observed that dispersal through

track away from span under braking condition is much less as compared to

the dispersion under starting condition. Simultaneously, during trials on Kolab

Bridge (10x45.7m through span) on SE Railway (RDSO’s Civil Engg Report

No C- 235), it was observed that in case of multi-span long bridge with

spans supported on rocker and roller bearings when entire bridge is covered

under load and all the axles are simultaneously braked, the dispersion of

braking force away from some of the intermediate span is negligible.

TESTS CONDUCTED BY UIC

A number of tests have been carried out by office of Research and

Experiments (ORE) of the UIC. The results are reported in various reports

(RP1-15) on question D 101 “Braking and acceleration forces on bridges”

and some of the important conclusions are:

i) Guard rails transfer considerable longitudinal forces to

approaches.

ii) The sharing of longitudinal load between rails and other elements

of bridge is affected by horizontal stiffness of track on approaches

and on bridge. If track on approaches is stiffer than on the bridge,

a large share of horizontal load is taken by approaches. On the

other hand, if track on bridge is stiffer as compared to approaches,

a large share of load goes to bearings.

iii) All the vehicles/ axles don’t stop at the same instant. The rear

most vehicle/ axle stops first, followed by the other axles in a

sequence. The last one to come to a stop is the Loco or the

front vehicle. Due to the time gap between stopping of different

vehicles, the peak braking force is not the sum of individual

axle/ vehicle peak braking force. Due to the short duration of

the peak braking force at the moment of stopping, some vehicles

would have stopped while other would still be in motion, so the

peak force is not generated along the entire train at the same

instant but travels like a wave from rear to the front. The maximum

braking force will therefore, get distributed over adjacent spans.

iv) Track in the approaches up to 30m absorbs the longitudinal

forces and thereafter, there is hardly any affect on the approach

track.

v) Ballast in ballasted deck bridges does not transfer more than

3% of the horizontal load to approaches.

From the experimental results of tests concluded by UIC, it was found

that percentage of longitudinal force in case of tractive/starting forces, going

to substructure was less as compare to braking.

TEST CONDUCTED BY AAR

The Association of American Railroad (AAR) tested [10] a single span

(50feet) open deck steel girder to measure the longitudinal forces. The test

bridge is located on 1% grade and was built in 1908 for copper E-55 design

loading. The rail is continuously welded on the bridge and its approaches

also. The rail anchoring conditions were varied on the bridge and approaches

to study the effect of dispersion of longitudinal forces. The AC locomotives

used in the test have a maximum tractive effort per loco unit in excess of

1,50,000 pounds, while maximum dynamic brake effort is limited to about

80,000 pounds. 3- Unit set of six axle AC locomotive were used.

During tests, it was noted that:

(i) The force into the bridge increases as the TE applied to the rail

on the bridge increases

(ii) The maximum measured longitudinal force into the bridge is

roughly 25% of the locomotive weight applied to the bridge.

The highest forces measured during any of the tests were for the

condition when the rails on the approaches are minimally anchored. A force

into the bridge of nearly 100kips was measured for an applied TE of 135

kips. This force was noticeable reduced when the rail on the approaches are

136 137


tightly anchored. This allows more of applied TE to be dispersed to

approaches. “It was found that the measured longitudinal forces (i.e. 100

kips) into the bridge are considerably higher than the design values (<4

kips for 50 feet steel span) recommended by AREA for steel bridges.

IMPORTANT CLAUSES OF PRESENT AND PAST CODES NEEDING

ATTENTION

Arch Bridge Code: Clause 2.2.3 states “Horizontal loads on the

arch: The effect of the tractive effort and braking force may be neglected in

designing or analyzing arch covered by this code”.

Out of about 1,20,000 bridges on Indian Railways, about 18% are

arch bridges. As per Clause 2.2.3 of Arch Bridge Code, longitudinal force

due to TE and BF may be neglected. So as far as arch bridges are concerned,

there is no problem of longitudinal forces.

Bridge Rules 1933: In Clause No 22, it is mentioned that, “In the

case of single span bridges up to 80 feet long, the dispersion allowance

may be increased by 50%”.

The word “single span” was later deleted vide Correction Clip No 9/36

and the extra dispersion up to 50% for bridges up to 80 feet long was deleted

in Bridge Rules 1941.

It brings out an important issue to be considered that is whether for

single span bridges, dispersion of forces is more as compared to multi -

span bridges. This appears to be logical and needs further testing and

instrumentation to establish that dispersion in case of single span bridges

is more than multi-span bridges.

Bridge Rules 1941, Reprinted in 1960 (Incorporating CS- 1 to 13):

Note to clause 23 states, “Note: In the case of piers and abutments of RCC

bridges with ballasted floors, no longitudinal forces need be taken into

account for span up to and including 20 feet.”

The above note was reproduced in clause 2.8.2.3 of Bridge Rules

1964, but clause is missing in Bridge Rules of later years. This is an important

clause which is not finding place in current Bridge Rules.

It appears to be logical not to consider longitudinal forces for RCC

bridges of spans up to 6 m as most of the longitudinal forces will be transferred

to approaches.

Steel girders up to 6.1 m span are already being replaced by RCC/

PSC slabs. There is a need to establish by instrumentation the span up to

which longitudinal forces can be neglected.

NEW V/S EXISTING BRIDGES

It is pertinent to note that whereas new bridges are to be designed for

next 100 – 150 years for higher standard of loading expected in future, the

existing bridges need only to be checked for permitting present loads knowing

fully well, the existing condition at site. For new bridges, loading standards

catering for future expected loads for design life of bridges with higher factor

of safety needs to be considered, whereas for existing bridges, lower factor

of safety can be permitted taking into account the actual condition of bridge

structures by removing uncertainties with instrumentation and testing.

Higher percentage of dispersion of longitudinal forces to approaches

can be considered while checking existing bridges than what is provided in

current Bridge Rules whereas for design of new bridge, no dispersion of LF

be considered.

It is necessary that detailed guidelines are made available for checking

strength of existing bridges.

SINGLE V/S MULTI-SPAN BRIDGES

It needs to be kept in mind that percentage of dispersion of longitudinal

forces in case of single span bridges will be different than in multi-span

bridges of same length because part of the longitudinal forces is already

transferred to piers before dispersion to approaches in case of multi-span

bridges.

Experiments in nineteen thirties in North Western Railway and

investigations by RDSO between 1967-80 gives evidences that dispersion of

longitudinal forces in case of single span bridges is more as compared to

multi-span bridges.

This area needs further investigation to permit higher dispersion of

longitudinal forces in case of single span bridges than what is permitted as

per present codal provisions which does not distinguish between single

span and multi-span bridges.

TRACTIVE EFFORT V/S BRAKING FORCE

Longitudinal forces on bridges are generated on account of tractive

effort of locomotive during starting of train and braking force during braking of

trains. It is the maximum of tractive effort or braking force which is considered

for design of bridge structure for longitudinal loads. The maximum tractive

effort which can be generated is fixed by the design of locomotive.

138 139


Single WDG-4 locomotive can generate a maximum tractive effort of 52.9t

which is even more than MBG standard of loading which caters for a maximum

tractive effort of 50t for a single loco.

The maximum braking force which can be generated depends on train

load, speed, braking distance required, capacity of brake blocks and wheel

to absorb energy, loose or tight couplings etc.

Under normal operating conditions, the large longitudinal force occurs

when pulling a heavy train upgrade or when braking a heavy train downgrade.

Therefore, bridges in the vicinity of significant grades are more likely to

experience a large longitudinal force.

It is important to note that for bridges longer than a set of locomotives,

the higher longitudinal forces may be caused by train braking. For shorter

bridges, tractive effort will govern because of higher adhesion of the locos,

so braking force governs in case of longer spans, whereas tractive effort

governs in case of shorter spans.

USE OF SHOCK TRANSMISSION UNIT (STU)

A shock transmission unit (STU) or “Lock up device” (LUD) is a

fabricated component which is designed to link or connect separate elements

of bridge structure so as to form a rigid link under rapidly applied (short

duration) loads such as longitudinal forces due to traction or braking, seismic

forces etc. but move freely under slowly applied load, such as temperature.

This mechanism helps in load sharing of suddenly applied short duration

loads. After removal of these sudden dynamic loads, the device returns to

its original position and structure behave in normal manner.

Longitudinal force due to tractive effort and braking force are short

duration rapidly applied horizontal loads so STU/LUD’s can be advantageously

used to control the longitudinal forces coming on any pier/ abutment within

safe limits and train with higher TE/BF can be permitted to run.

PRINCIPLE

STU/LUD operates on the principle that rapid passage of viscous fluid

through a narrow gap/orifice generates considerable resistance, while slow

passage generates only minor resistance.

STU/LUD consists of a cylinder with a transmission rod that is

connected at one end to the structure and at the other end to loose fitting

piston inside the cylinder. The annular space is filled with silicone putty.

During slow movement caused by temperature change, creep and shrinkage,

the silicone is able to squeeze through the annular space. A suddenly applied

load causes the transmission rod to accelerate through the silicone putty

within cylinder. The acceleration quickly creates a velocity where the silicone

putty can not pass fast enough around the piston. At this point, the device

locks up usually within half a second.

LUD/STU’s can be used for strengthening of existing bridges or

economical design of new bridges.

STRENGTHENING EXISTING BRIDGES

Due to increase in longitudinal loads (tractive effort/braking force),

many existing bridges which are found unsafe for increase tractive effort/

braking force require either strengthening or re-building. STU/LUD’s provides

a convenient means of strengthening existing bridges by load sharing between

pier/abutment during suddenly applied loads i.e. tractive effort and braking

force. This load sharing will reduce net longitudinal force going to substructure,

with the result LF due to increased tractive effort/ braking force can be carried

by redistribution of load and without any need for strengthening of pier/

abutment/foundation. Also if any pier is found damaged (under water), the

longitudinal force on this pier can be reduced by providing suitably designed

STU’s on adjacent piers.

On Indian Railway, the use of suitably designed STU/LUD’s can go a

long way in not only solving the problem of strengthening of existing multi-

span bridges, for increased longitudinal forces due to increased TE/BF, but

also can be used for permitting LWR on existing bridges.

DESIGN OF NEW BRIDGES

The concept of distribution of longitudinal loads by means of STU/

LUD’s can be advantageously used in designing economical structures

resulting in saving in pier/ foundations. The new Bassein Creek Bridge located

on NH 8 crossing the sea at Mumbai, India is designed to take advantage of

STU’s for distribution of seismic loads. The use of STU’s reduced the size of

well foundation resulting in substantial cost reductions.

Use of STU/LUD’s and its effectiveness on Railway bridges in India

needs to be established considering safety and economical aspect and if

found appropriate, the use may be permitted after making suitable provisions/

amendments in clauses in relevant Codes like Bridge Rules.

140 141


RECOMMENDATIONS & CONCLUSION

Present clause pertaining to longitudinal forces in Bridge Rules must

be reviewed to:

a) Permit more dispersion for single span/ short length bridges as

compared to multi-span bridges.

b) Disallow dispersion for new bridges

c) Include a clause for neglecting longitudinal forces while checking

existing bridges in case of arches, pipes, slab bridges box

culverts and possibly all minor bridges with span up to 6.1m.

d) Clearly defining clauses for new bridges and existing bridges

separately for considering dispersion and longitudinal forces.

Since provision of guard rails and its condition, strength of approaches

helps in dispersion of LF for approach; these aspects must be kept in view

to increase dispersion of LF to approaches.

Since tractive effort as well as braking force is maximum at zero speed,

any speed restriction imposed on a bridge is detrimental from longitudinal

force consideration. So for any work in progress in substructure/foundation

or temporary arrangements, speed restriction of dead stop must be

prohibited.

Since TE required is more on gradients, requirement of TE for level

tracks be calculated and wherever, the bridge are on level track, the same

can be considered.

On smaller length bridges/single span bridges, more dispersion can

be permitted than what is provided in current Bridge Rules. So investigations

must be carried out to find out percentage of dispersion of LF to approaches.

The main reason for lesser dispersion of braking force in case of longer

span/ longer bridges is due to trailing loads. It appears reasonable to

separately consider/ investigate dispersion of braking force due to loco and

due to trailing loads.

Since TE govern short length bridges and BF governs longer bridges,

if a bridge is found to be unsafe for longitudinal loads on account of braking,

then STU/LUD’s may be provided on the bridge to distribute the longitudinal

load.

If a bridge is found unsafe due to tractive effort, then for actual condition

at bridge site like grades, detailed analysis be done for required TE.

Due to increase of axle load of wagons to CC+6+2 or CC+8+2, the

effect on longitudinal loads is mainly due to braking force of wagons,

considering the same locos. This will affect only large span bridges, where

use of STU/ LUD’s may be thought of.

REFERENCES

1. Tenth Report of the Bridge Standard Committee, 1931.

2. AREA Manual for Railway Engineering, 1991.

3. UIC Code:776-1 R “Loads to be considered in Railway Bridge Designs”,

1994

4. BS 5400: 1978 (Part-II)

5. Civil Engg. Reports C-64, C-153, C-235, RDSO.

6. Fourth Report of Bridge Standard Committee, 1927.

7. RDSO Report on BS-26, “Index of the Report of the Bridge and Structure

Committee”, 1999.

8. Agrawal, S.R.,“Dispersion of Tractive and Braking forces in Railway

Bridges”, PhD Thesis, University of Roorkee, 1973.

9. Patel, D.J., “Shock Transmission Units in Bridge Engineering”

Engineering World, October – November, 2000”

10. Otter, D.E. and LoPresti, Joseph, “Longitudinal Forces in an Open –

Deck steel- deck plate girder bridge”, Railway Track & Structures.

May 1997.

11. IRS Bridge Rules of year from 1923 to 2005.

12. IRS Bridge substructure and Foundation Code 2003.

13. IRS Arch Bridge Code 2000.

142 143


Annexure-I

HISTORICAL NOTE ON LONGITUDINAL LOAD PROVISION AS PER

BRIDGE RULES

Prior to the formulation of the Bridge Rules in 1892, bridges were built

to comply with the British Board of Trade Rules. These rules specified the

permissible stresses for wrought iron and steel but not the longitudinal loads.

After 1892, bridges were built according to the standards laid down in the

British Rules (which were revised first in 1903 and again in 1923, 1926,

1933, 1941 and 1964). Reference to the tractive and braking forces first

appeared in 1923 Rules. According to these rules “Traction and braking

loads are to be taken as horizontal forces, acting at the rails of each track in

the direction of moving train and are to be computed as directed in clause 9

of Part-3 of the British Standards Specification for girder bridges” .These

forces were approximately 1/7th vertical loads.

In 1926 Rules, the provision for longitudinal forces was made in clause

30 and 31 which are reproduced below:

CLAUSE NO 30 & 31 OF 1926 BRIDGE RULES FOR THE LONGITUDINAL

FORCES:

“30: Longitudinal forces: Where a structure carries a railway provision shall

be made for the stresses due to the tractive effort of the live load and

the braking effect resulting from the application of the brakes to such

load while passing there over, these forces being considered as acting

at rail level.

31 (a) For railway worked by steam or electric locomotives, the amount of

the tractive effort on one track shall be ascertained by multiplying

one and three quarter times the maximum end shear due to the live

load on that track by a factor equal to 20 ,L + 75

Where L= the span in feet. The factor shall not exceed 0.15 .

The braking effort shall be similarly determined using a factor

equal to 12 plus 0.75 L + 90

Also limited to a maximum of 0.15

(b) In case of lines worked solely on electrical multiple unit system,

the amount of the tractive effort on one track shall be ascertained by

multiplying the sum of the actual wheel loads on the span by a factor

equal to 3 plus 0.10

L- 10

Where L= the span in feet. The factor shall not exceed 0.20.

The braking effort shall be similarly determined by using a factor of

0.20 for all spans”

In the 1933 Rules the provisions for longitudinal forces were further

modified and a special clause was introduced to allow for the dispersion

of the horizontal forces through the track to the approaches of the

bridges on the basis of the experiments carried out with steam

locomotives on bridges. The relevant clause is reproduced below:

“22. Longitudinal forces: Where a structure carries a railway provision as

under shall be made for the stresses due to the tractive effort of the

live load and to the braking effect resulting from the application of the

brakes to such load while passing there over. These forces shall be

considered as acting horizontal through the girder seat where girders

have sliding bearings or through the knuckle pin.

For span supported on sliding bearings the horizontal forces shall be

considered as being divided equally between the two ends; for spans

which have roller bearings at one end the whole of the horizontal forces

shall be considered to act through the fixed end.

(a) Tractive Effort: For railways worked by steam or electric

locomotives the amount of tractive force on one track shall be

ascertained by multiplying the EUDL on one track taken from the

Table of loads for calculating bending moments by 32 .

L + 90

Where L= the length of the bridge in feet subject to a maximum of

120 feet for HM loading and 100 feet for ML and BL loadings.

For bridges or spans exceeding 120 feet in length for HM loading and

100 feet for ML and BL loadings the tractive effort shall be assumed

to be constant.

(b) Braking effect: For railways worked by steam or electric

locomotives the braking effect on one track shall be ascertained by

multiplying the EUDL on the track, taken from the Table of loads for

144 145


calculating bending moments, by

___32____ plus 0.03

145 + L

Where L= the length in feet of the bridge or the length of the train

whichever is less.

(c) In case of lines worked solely on the electric multiple unit

system, the following allowance shall be made:

Tractive effort: 25% of the sum of the driving axles on the bridge.

Braking effort: 20% of the sum of all axles on the bridges.

In all case the amounts calculated under heads (a), (b) & (c) shall be

reduced to allow for dispersion of the horizontal forces through the

track at the ends of the bridge by amount as given in the following

table:

Standard Allowance for dispersion of horizontal forces

through the track

BL 7.5 tons

ML 9.0 tons

HM 11.5 tons

In the case of single span bridges up to 80 feet long, the above

allowances may be increased by 50 percent. The word single span

was deleted in 1936.

In all cases the net horizontal forces after deducting dispersion shall

be assumed to be distributed equally amongst the spans in the length

L as defined.”

In 1934, an additional clause 137 was inserted as follows:

“For the purpose of calculating tractive and braking force on existing

Bridges Rules 22 shall be held generally to apply excepting that the

maximum tractive forces can be calculated as 25% of the axle loads

of the drivers of the actual engine under consideration and braking

force as 20% of actual braked engine axle loads and 10% of other

braked axles running or proposed.

In addition where the table of dispersion allowance specifies BL, ML

and HM, these shall be interpreted in terms of weight of rail in use on

the bridge, i.e. the dispersion allowance in tons per track can be

taken as one tenth of the weight of rails in pounds per yards. The 50%

increased dispersion on single span bridge up to 80 feet applies.”

In 1941, 50% extra dispersion of longitudinal force to approaches

incase of bridges up to 80 feet span provided in 1933 Bridge Rules

deleted.

In Bridge Rules of 1941, printed in 1947, dispersion clause was

changed as under:

HM loading – L> 100’ 11.5 ton

L< 100’ varying uniformly from 17.25 ton for L=0

to 11.5 ton for L= 100’

ML Loading – L > 100’ 9.0 ton

L < 100’ Varying uniformly from 13.5 ton for L=0 to

9.0 ton for L=100’

BL loading L > 100’ 7.5 ton

L< 100’ varying uniformity for 11.25 ton for L= 0 to

7.5 ton for L=100’

For TE, L shall not be taken to exceed 120 feet for HM and 100 feet

for ML and BL loading.

It is also noted that in Bridge Rules of 1941 printed in 1947, value of

net longitudinal forces (after deducting dispersion allowance) were

specified in Table given in annexure –F of Bridge Rules 1941 (Printed

in 1947).

In Bridge Rules 1941, Printed in 1960, A note to clause 23 as given

below is added.

“Note: In the case of piers and abutments of RCC bridges with ballasted

floors, no longitudinal forces need be taken into account for span up

to and including 20 feet.”

Clause 33 was also added for existing bridges as follows:

Tractive effort- 25% of axle load of coupled wheels of actual engines

under consideration.

Braking force- 20% of actual braked engine axle loads and 20% of

other braked axle loads.

146 147


Dispersion to approaches in tons if no rail expansion joint in track

would be (a) 10 % of weight of rail in lbs/ yd for loaded length L equal

to or exceeding 100’ and (b) increasing uniformly from 10% for L =

100’ to 15% for L= 0 for loaded length less than 100’

HM loading was deleted in 1964 Bridge Rules.

Dispersion of tractive and braking force problem was investigated by

field trials conducted on various bridges during 1967-1980. Based on these

investigations, dispersion provisions of longitudinal forces was modified as

under:

“In case of bridges, provided with through welded rails, rail-free

fastenings and adequate anchorage of welded rails on approaches

(by providing adequate density of sleepers, ballast cushion and its

consolidation etc. but without any switch expansion joints) dispersion

of longitudinal load P through the rails away from the loaded length

may be allowed to the extent of 25% of the value of longitudinal load

P as obtained through Appendices VII , VIII and VIII(a) of Bridge

Rules subject to minimum pf 16t for BG, 12 t for MGML and 10t for

MGBL. This will also apply to bridges with jointed track with rail free

fastenings but without any switch expansion of mitred joints. Where

suitably designed elastometic bearings are provided the aforesaid

relief may be further increased by 40% thereof”.

The total dispersion under above clause shall not exceed the capacity

of the rails for transferring the longitudinal load to the approaches nor

should it exceed the capacity of the anchored length of the track on

the approaches in resisting the longitudinal load.

After dispersion of longitudinal forces, distribution of longitudinal forces

in different supports is considered as given below:

For span supported on sliding bearings, the horizontal loads shall be

considered as being distributed between different supports as below:

(a) 2 supports directly under the loaded span; each 40% of the

horizontal load due to tractive/ braking effort after deducting

force dispersed as stipulated in clause above .

(b) Other two adjacent supports each 20%.

For spans which have roller bearings at one end, the whole of the

horizontal load shall be considered to act through the fixed end.

Modified Broad Gauge loading of 1987 introduced in 1988 envisaged a

maximum tractive effort of 100t with coupled operation with 25% braking

load for locomotives and 20% braking load for trailing load of 8.25t/m

which could be on both sides of locos. Tractive and braking force

values for various spans for the standard loading have been given in

the Bridge Rules. The provision of dispersion of longitudinal forces

remains unchanged.

The 20% braking force of Train Load was revised to 13.4 % in 1993.

Heavy Mineral loading -1995, introduced in 1998 for heavy minerals

routes, caters for maximum tractive effort of 135t and braking force of

13.4% of train load.

148 149

Volume - I Volume - I150 151


CONDITION MONITORING OF BRIDGES FOR

RUNNING HIGHER AXLE LOADS IN SOUTH

EAST CENTRAL RAILWAY

P. BARAPATRE*, AMIT GOEL**

SYNOPSIS

Railway Board has approved many sections for running of higher axle

load trains. Five such Pilot Projects have been approved in SEC Railway also.

Adequacy of existing infrastructure for carrying heavier loads has to be verified

for these sections. The bridges, being generally very old, require special

verification by checking of their designs, by verification of their physical condition

and by instrumentation & condition monitoring of sample bridges. This paper

deals with the various works done so far by SEC Railway for checking the

bridges in identified sections. The paper also discusses the instrumentation,

data recording and results of the first quarter test train run with 25 ton axle load

BOBS wagons, on one Plate Girder Steel Bridge in Durg-Dallirajahara section.

1.0 GENERAL

The growing demand of transport is one of the most important features

of the developing economy. In developing countries like India, Railways attract

major share of growing traffic, being the largest transporter. The volume of

traffic on Indian Railway has increased manifold and a quantum increase is

expected in the passenger and freight traffic in Indian Railways in the next

few years. The growing traffic demands not only necessiate expansion of

the existing infrastructure but also, the optimum utilisation of the existing

infrastructure.

One of the major steps taken for optimum utilisation of the existing

infrastructure was to permit the heavier axle loads on the existing system.

This also necessitated checking the capacity of the existing system and to

strengthen it, if required. Indian Railways has taken up several pilot projects

in recent past to upgrade many routes for running of higher axle loads.

2.0 ROUTES IDENTIFIED FOR HIGHER AXLE LOADS IN SEC RAILWAYS

Railway Board has approved many routes for taking up the pilot

projects for running of higher axle loads on identified routes on South East

* CBE/SEC Railway/BSP

** Dy CE/Bridge/ SEC Railway/BSP152


Central Railway as follows:

2.1 DURG- DALLIRAJHARA SECTION

This route was approved in May 2005 for running of BOBS wagons

with axle load of 25 tons. This line is about 88 kms. long and in a

length of about 78 kms. between Marauda and Dallirajhara BOBS

wagons are running presently with axle load of 22.9 tons. This line

carries iron ore from iron ore mines, which are situated on one end of

the section to Bhillai Steel Plant situated on the other end of this line.

Running of increased axle load of 25 tons on this section is yet to be

commenced.

2.2 JHARSUGUDA- KIRODIMAL NAGAR SECTION

This section was approved in February 2006 for running of CC+8+2

loaded BOXN wagons. This section of about 80 kms. length is situated

on Group ‘A’ route of Howrah- Mumbai. Running of increased axle

load was commenced in April 2006 with iron ore traffic originating from

Chakradharpur Division of South Eastern Railways.

2.3 KORBA-CHAMPA-BILASPUR-ANUPPUR-NKJ SECTION


loaded BOXN/BOBRN wagons. This section of about 404 kms. length

is situated partly on Group ‘A’ route of Howrah- Mumbai and partly on

other important routes. Running of increase axle load was commenced

in April 2006 mainly with coal traffic originating from sidings near

Korba in Bilaspur Division of S.E.C. Railway for further transportation

to W. C. and other Railways.

2.4 DUMRI KHURD- KANHAN- KAPERKHEDA/KORADI SECTION


loaded BOXN/BOBRN wagons. This section of about 50 kms. length

is situated partly on Group ‘A’ route of Howrah- Mumbai and partly

consist of Assisted sidings. Running of increase axle load was

commenced in April 2006 mainly with coal traffic originating from

sidings near Dumrighurd in Nagpur Division of S.E.C. Railway for further

transportation to two Power plants near Nagpur.

2.5 JHARUGUDA- BILASPUR-DURG-NAGPUR SECTION

This section was approved in June 2006 for running of 25 ton axle

load. This section of about 615 kms. length is situated on Group ‘A’

route of Howrah- Mumbai. Running of increase axle load is yet to be

commenced. Part of this route was sanctioned in the earlier sanctions

for running of CC+6+2 and CC+8+2 ton wagons.

3.0 WHY TO CHECK BRIDGES

Indian Railways has gone for modernization and upgradation of various

assets from time to time. In the field of Rolling stock the development of

ABB loco, development of Electrical multiple unit with Air break suspension,

high speed modern stainless steel coaches with very modern suspension

system etc. have brought the Indian Railways almost at par with International

standards. Similarly in the field of Signaling & Telecom, the Indian Railways

have been pursuing the most modern systems with high import content and

foreign collaborations almost on the sustained basis. Even in operations,

Indian Railways is going in for large-scale implementation of freight operation

information system. Upgradation of track has also been receiving considerable

attention for the last few years. However, nothing concrete has been done in

the field of Bridges mainly due to long life of the bridges and also due to the

fact that upgradation of the bridges was always considered to be a very

difficult task under the heavy traffic conditions.

Most of the bridges in the Indian Railways were constructed at the

time of construction of the line and have never undergone any replacement,

except for superstructure of some bridges. A large number of bridges in

Indian Railways are having life close to hundred years. The older age, the

need to permit higher density of traffic and also to permit heavier axle loads

on these bridges necessitated rigorous analysis and checking of these

bridges. In the approved routes of South East Central Railway also a very

large number of these bridges are very old, with a large population of bridges

constructed almost 100 years back. These bridges were constructed mainly

with BGML loading standard.

Checking the adequacy of existing bridges necessitated broadly

following items:

(1) Frequent physical inspection of the bridges with elaborate check

list

(2) Checking the design of substructure of the bridges for increased

axle loads

(3) Checking the design of superstructure of the bridges for

increased axle loads

(4) To identify the different works on short-term/ long-term basis

required to be carried out for running of higher axle loads

(5) Sanctioning and execution of various strengthening works

(6) Identification of the sample bridges for carrying out

instrumentation and condition monitoring

154 155


(7) Carrying out instrumentation and condition monitoring for seeing

the realistic response of some sample bridges for the increased

axle loads

(8) To assess the residual life of the bridges based on fatigue

consideration

3.1 PHYSICAL INSPECTION OF THE BRIDGES

For taking up the Pilot Project each and every bridge was considered

as an individual entity. Divisions were advised to carry out the initial

physical inspection of all the bridges and also to carry out physical

inspections at an increased frequency and also to report the bridges

warranting attention of the Headquarter and also to propose the works

required to be carried out on the basis of these physical inspections.

Divisions were also advised to keep a special watch on the weighment

records and the over speeding of the trains. Headquarter had identified

some sample bridges for instrumentation and condition monitoring.

Divisions were also advised to suggest bridges for the condition

monitoring basing upon their physical inspections.

3.2 CHECKING THE SUBSTRUCTURE OF THE BRIDGES FOR

INCREASED AXLE LOADS

The trains with increased axle loads have been approved at a sped of

60 kmph. The forces were calculated for Locos Double headed WDM2,

Double headed WAG5 and Single headed WAG7, which are sanctioned

for running in SEC Railway. Basic wind pressure was considered as

150 Kg/m2. It was found that EUDL was lesser than the EUDL

considered as per BGML standards. Hence, checking of substructure

was not considered necessary. However, sample checking was done

for a few bridges. Subsequently, while processing the application to

CRS for the proposed running of Double Headed WAG7 loco, it was

found that only for the bridges of 80 feet and above clear spans, the

longitudinal forces are exceeding the design longitudinal forces for

BGML loading standard. Hence, for the bridges of clear spans more

than 80’ replacement of bearings and jacketing of piers and abutments

was proposed for sanction to cater for future requirements.

3.3 CHECKING THE SUPERSTRUCTURE OF THE BRIDGES FOR

INCREASED AXLE LOADS

SEC Railway is mainly having following type of bridges in the identified

routes for higher axle loads:

(a) For the spans above 20 feet the bridges are mainly steel bridges.

Only some bridges are arch type or PSC girders with MBG

standard.

(b) For 20’ spans the bridges mainly are either steel girders or

RCC flat top bridges of BGML standard or PSC slabs of MBG

standards. Most of the steel girder bridges have been replaced

either with PSC slabs or are proposed/sanctioned for

replacement.

(c) For the spans below 20 feet bridges are mixed type i.e. RCC

flat top or small arches or PSC slabs or steel girders or some

very small span bridges (span upto 3feet) with pipeline culverts.

For the bridges of spans below 20 feet covered in item (c), the design

checking was not done and continuance of these bridges has been

permitted on the basis of their physical condition and with close

monitoring. For the bridges of spans of 20 feet with PSC slabs covered

in item (b), the design checking was not done since, 20 feet PSC

slabs are of MBG standards permitting the increased axle loads. For

the bridges of 20 feet span with steel girders, the design checking

was done and the bridges were found suitable for increased axle loads.

For the arch bridges of spans 20 feet and above covered in item (b)

and (a), the design checking was not done and continuance of these

bridges has been permitted on the basis of their physical condition

and with close monitoring.

For the steel girder bridges of different spans checking was done

either by manual calculations or by development of in-house software

or with the help of the guidelines provided by RDSO for different type

of bridges.

Development of Software for checking of steel girder bridges:

For checking the bridges for increased axle load SEC Railway has

developed software. This software is having following features:

(a) It includes the calculation of EUDL for various formation of trains,

locos and wagons

(b) It can check the bridges for rolling stocks as per RDSO’s general

criteria

(c) It can check Plate Girders for permissible stress criteria

(d) It can check the adequacy of Piers and Abutments

(e) For Open web and under slung type bridges, it can only check

stringers but it cannot check other members. Other members

have to checked through conventional manual calculations

156 157


The development of the software was mainly done adopting following

methodology/principles:

3.3.1 CHECKING OF BRIDGE FOR ROLLING STOCK FROM RDSO’S

GENERAL CRITERIA

(a) CHECK FOR MAXIMUM ALLOWABLE SPEED

This includes the calculation for maximum allowable speed on

the basis of EUDL for bending moment and EUDL for shear

force according to the formula

Vm = [LLD+(LLDxID)-LLT] Vt/(LLTxID)

Here, LLD is EUDL for BM/SF for which the bridge has been designed.

ID is CDA for 125 Kmph speed.

LLT is EUDL for BM/SF for which these bridges are required to

be checked.

With above calculations, the maximum allowable speed is to

be taken as 90 % of the minimum of Vm obtained for Bending

Moment and Shear Force. The maximum allowable speed should

be greater than the proposed speed.

(b) REQUIRED PERCENTAGE STRENGTH OF BRIDGE

This includes the calculation of %age strength required for the

span to move the train at a speed of 10% more than the proposed

speed on the basis of EUDL (BM) and EUDL (SF) according to

the following formula:

% Strength required = [LLT+(LLTxIT)] x 100 / [LLD+(LLDxID)]

Here, IT is CDA for speed 10 % more than the proposed speed.

Maximum % strength of the bridge is to be taken as maximum

of above two values.

(c) AVAILABLE IMPACT FACTOR

CDA available shall not be less than 0.1.This can be calculated

by the formula [LLD+(LLDxID)-LLT]/LLT.

Maximum CDA available is taken as minimum of the values

obtained on the basis of EUDL (BM) & EUDL (SF).

(d) LONGITUDINAL FORCE CRITERIA

It includes the comparison of induced tractive effort/braking force

(max. of the two) with the longitudinal force for which the bridges

have been designed. As per clause 2.8.3.2 of IRS Bridge Rules,

dispersion may be allowed to induced longitudinal force.

3.3.2 CHECKING OF GIRDER FROM PERMISSIBLE STRESS CRITERIA

3.3.2.1 CHECKING OF PLATE GIRDERS

(a) Calculation of dead loads including the self-weight of girder,

weight of bracings, track load, weight of the fastenings.

(b) Calculations of live load for given span, train formation and loco

combination including dynamic augment. For the calculation of

bending stresses, EUDL for BM and for the calculation of shear

stresses, EUDL for SF should be adopted.

(c) Calculation of maximum bending moment developed at mid span

and maximum shear force developed near support. If W1 is the

total of loads in (a) & (b) above for BM & W2 is the total of loads

in 1&2 above for SF, then:

BM=W1xL/(2x8)=M

SF=W2/4=V

(d) Calculation of section properties of girder such as its moment

of inertia (Ixx), gross area (Ag) and net area (An)

(e) Calculation of bending stresses on the basis of gross area and

net area

6g=(Mxy/Ixx).

6n= (6gxAg)/An

(f) Comparison of net area stress with permissible tensile stress

for fatigue criteria. Permissible stress from fatigue criterion can

be obtained from the appendix –G of Steel Bridge Code on the

basis of no. of cycles and the ratio of fmin

./fmax

..Here fmin

. Is the

stress developed in girder without live load and fmax

. Is the stress

developed in girder with live load.

(g) Calculation of shear stress by: ô= V/ (dxt). Here d & t are the

depth and thickness of web of the girder.

(h) Comparison of shear stress calculated above with the permissible

shear stress.

(i) Checking for wind forces.

3.3.2.2 CHECKING OF OPEN WEB GIRDERS AND UNDER SLUNG TYPE GIRDERS

(a) Checking of stringer and cross girder can be done as per the

criteria given above in Para 3.3.2.1

(b) Checking of the other members of Open web and Underslung

type girders can not be done by the software. These were

158 159


checked manually as per following methodology:

(j) Drawing of influence lines for all the members of open web

girder.

(ii) Calculation of dead loads for unit length, live loads for unit

length, longitudinal forces, wind forces including portal and

sway effect. Span for calculating EUDL should be decided

on the basis of influence lines.

(iii) Total dead load, live load, may be obtained by multiplying

the corresponding loads for unit length by the corresponding

area of influence lines.

(iv) Calculation of actual axial stresses developed in

compression members on the basis of gross area.

(v) Calculation of actual axial stresses developed in tension

members on the basis of net area.

(vi) Permissible compressive stresses for compression

members may be calculated on the basis of its slenderness

ratio or fatigue criterion, whichever is critical.

(vii) Permissible tensile stresses for tension members may be

calculated on the basis of fatigue criteria or normal

permissible tensile stresses, whichever is critical.

(viii) Comparison of actual compressive stresses with permissible

compressive stresses and tensile stresses with permissible

tensile stresses.

(ix) Calculation of bending stresses in the top chord members

in case of that type of under slung girders in which sleepers

are resting directly on the top chord members.

(x) Checking for combined direct and bending stresses according

to:

(6c’/6c)+(6b’/6b)< or = 1

3.3.3 CHECKING OF SUBSTRUCTURE BY SOFTWARE

The software can also be used for checking of Piers and

Abutments. For checking of Piers and Abutments following

principle/methodology was followed in the software:

3.3.3.1 CHECKING OF PIERS

(a) Calculation of sectional properties of pier.

(b) Calculation of direct load, bending moment about X-X axis and

bending moment about Y-Y axis on the basis of dead load, live

load, longitudinal forces due to water currents, forces due to

wind and seismic forces etc.

(c) Calculation of combined maximum and minimum stress at the

base of pier due to direct load and bending moments calculated

at (b) above.

(d) Comparison of stress calculated in step (c) above with the

permissible stresses.

3.3.3.2 CHECKING OF ABUTMENTS

(a) Calculation of vertical loads, earth pressure, surcharge loads

and moments of all these forces about toe of abutment base.

(b) Checking against overturning. Factor of safety against overturning

>2.

(c) Checking against sliding. Factor of safety against sliding>1.5.

(d) Checking for maximum and minimum base pressure.

3.4 RDSO GUIDELINES FOR RUNNING 25 TON AXLE LOAD

In the month of August 2006 RDSO has issued EUDLs for different

spans for 25-ton axle loads and has also given clearance to certain

standard RDSO type spans for running of 25-ton axle loads at a speed

of 60 kmph. Hence, for running of 25-ton axle loads between

Jharsuguda- Bilaspur-Durg and Nagpur, which was approved at a later

stage, the standard spans cleared by RDSO were not further checked.

Only, those spans, which were not included in the RDSO list, were

checked at Zonal level.

3.5 RESULTS OF CHECKING OF DIFFERENT SPANS

Basing upon the checking of different type of girders either through

software or manually following results were found for different type of

spans:

(i) 40’ plate girders confirming to RDSO Drg. No. BA 1056,BA11003

have been found safe.

(ii) 60’ plate girders confirming to RDSO Drg. No. BA 1057,BA11004

CE’s Drg. No. 10291, CE’s Drg. No. 11237 have been found

safe.

(iii) 80’ plate girders confirming to RDSO Drg. No. BA 1058 has

found to be safe.

160 161


(iv) 100’ girders confirming to RDSO Drg. No. BA 1059(Plate Girder),

BA 6081 (Under Slung Type) has been found safe. 100’ plate

girder confirming to RDSO Drg. No. BA 1518 has been found

safe for 50 Kmph speed in 2 million cycles for BOBS wagons

running in Durg- Dallirajhara section. This type was found unsafe

for 10 million cycles. These types of Plate Girders have been

found safe by RDSO for BOX-N wagons at a speed of 70 kmph.

(v) 150’ open web girders confirming to RDSO Drg. No. BA

5025,BA11101 have been found safe. Cross Girders and

Stringers for these girders have also been found safe.

4.0 BRIDGE WORKS PROPOSED

Basis upon the design checking and other items like type of bearings

provided on the existing bridges, following works have been mainly planned

in SEC Railway:

4.1 First Pilot project in this Railway was approved in Durg- Dallirajhara

section in May 2005 for running of BOBS wagons with axle load of 25

tons. Following bridge works were sanctioned in Pink Book 2005-

2006 for this section:

(a) Regirdering of Br. No. 105 with Spans 5 x 100’ and 2x40’. This

bridge is provided with Plate girders and as per the theoretical

calculations, 100’ girder failed from fatigue consideration. Hence,

all the spans were sanctioned for regirdering. However, as per

the instrumentation and data recording for the first quarter, actual

stresses are measured to be much lesser than the theoretical

and fatigue stresses. It appears at this stage that with further

analysis after getting second quarter report and after getting

the report for residual life analysis, this bridge, may not require

replacement and should be able to give service for many decades

with increased axle load.

(b) Strengthening of Plate Girder bridges of 40’ span by providing

thicker flange plates.

(c) Minor works of strengthening on small span bridges.

4.2 On Jharsuguda- Bilaspur- Durg- Nagpur section running of 25 ton axle

load wagons has been approved in June 2006. On this route, bearings

are provided as per BGML standards and no replacement or

strengthening was done for running of Double headed WAG7 wagons.

For the bridges of spans 80’ and above replacement of bearings and

jacketing of Piers and abutments has been proposed for sanction in

PWP 2007-2008.

5.0 NEED FOR INSTRUMENTATION OF BRIDGES

The theoretical checks done are based on many assumptions and

therefore the actual stresses are best checked by means of instrumentation.

There are many items of concern, which are not even amenable also to

theoretical analysis. Similarly, the inspection carried out in the conventional

way are more of subjective in nature and to have an objective assessment of

the deterioration in the condition of bridge, or otherwise, due to running of

enhanced axle loads, it is required that condition monitoring of sample bridges

is done by suitable instrumentation.

Instrumentation is a monitoring mechanism to assess the effects of

increased longitudinal loads & axle loads with respect to measurement of

settlement of foundations, tilting of piers/abutments, loads on bearing,

deflections & stresses at critical points. The monitoring mechanism consists

of using various instruments & equipments to record desired data at different

intervals and analysis of the same.

5.1 IDENTIFICATION OF THE BRIDGES FOR CARRYING OUT

INSTRUMENTATION AND CONDITION MONITORING

Due to the increased axle loads, the existing bridges will be subjected

to additional vertical & longitudinal loads. In order to assess the actual

effect of these increased loads on the bridges, Instrumentation and

measurement of actual stresses developed in the different members

is essential. This is also required for assessing the residual life of the

bridges & for planning the Rehabilitation/Strengthening of the same.

As per the different instructions issued by the Railway board sample

bridges containing different types/spans and also the vulnerable bridges

were to be selected for instrumentation and condition monitoring.

Bridges for instrumentation were identified mainly on following

considerations:

a. Type of girders

b. Type of spans

c. Age of the bridge

d. Condition of the bridge

e. Distribution of selected bridges in the different routes identified

for higher axle loads

162 163


5.2 BRIDGES SELECTED FOR INSTRUMENTATION AND CONDITION

MONITORING IN SEC RAILWAY

(1) Bridge No. 105 in Durg- Dallirajhara section (5x 100’ + 2x40’

Steel Plate Girders)

(2) Bridge No. 154 Up in Jharsuguda- Kirodimal Nagar section (4x

20’ Arch + 1x100’ Steel Plate Girder)

(3) Bridge No. 91K in Bilaspur- Anuppur section (7x60’ Steel Plate

Girders)

(4) Bridge No. 175 K Up in Anuppur- Katni section (2x100’ Under

slung type Steel Girders)

(5) Bridge No. 34 Up in Kamptee- Kalumna section (2x 60’ Steel

Plate Girders + 7x150’ Open Web type Steel Girders)

6.0 FIXING AGENCY FOR INSTRUMENTATION

Railway board had issued some guidelines in the month of June

2005.With these guidelines Board had pinpointed some specialized agencies

for different zones. Since no particular agency was suggested for SEC

Railway, a Special List containing all the nine specialized agencies suggested

by Railway Board in their different letters was prepared and Special Limited

Tenders were invited from these nine tenderers. Summary for the tendering

is as follows:

6.1 One tender was initially invited for Bridge no. 105 in Durg – Dallirajhara

section in March 2006. This was awarded to M/s Sharma Associates

in April 2006. So far, Data Recording for two quarters has been done

in this contract and report has been received for the Ist quarter testing.

6.2 Second tender was subsequently invited for four different bridges in

May 2006. This was also awarded to M/s Sharma Associates in

August 2006. So far, Data Recording for first quarters has been done

on one bridge in this contract and on the second bridge it is planned

to be done in second week of December 2006.

7.0 THE STAGES/ACTIVITIES INVOLVED IN INSTRUMENTATION & MONITORING

7.1 PREPARATION OF REALISTIC COMPUTATIONAL NUMERICAL MODEL

Realistic computational numerical model is prepared by using finite

element analysis system (structural analysis system, SAP) depending

upon the following aspects:

(a) Type and detailing of structure

(b) Type of loading

Numerical model is necessary for having an idea about the following:

(a) To have a knowledge about the theoretical values for stress/

strain, deflection etc. at various points of the structures.

(b) To have an idea about the theoretical values of various dynamic

characteristics of the structure.

(c) The model is used to identify the critical components/ locations

for doing the instrumentation.

7.2 CARRYING OUT STRUCTURAL ANALYSIS ON NUMERICAL MODEL

This is done for both static and dynamic load cases under IRS Bridge

Loading Standards and proposed heavier axle loads and increased

longitudinal load cases. Basing upon this, values for the various

parameter like:

(a) Theoretical maximum stresses in bending

(b) Theoretical maximum stresses in shear.

(c) Theoretical maximum deflection

7.3 CARRYING OUT INSTRUMENTATION & RUNNING OF TEST TRAIN ON BRIDGE

This is done to know about:

(a) Actual maximum bending strains/ axial strains at different critical

locations..

(b) Actual maximum deflection at centre of girder.

(c) Actual maximum shear stress at support in case of plate girders.

(d) Actual longitudinal strains in rails and on bearings

(e) Actual settlement and tilt of pier.

(f) Strains in pier

(g) Forces in members showing signs of distress/ failure etc.

7.4 DATA COLLECTION, RECORDING AND STORAGE

This process will be done quarterly. In this process the data is recorded

using data loggers of required channel capacity. In SEC Railway,

recording has been done using data logger for 32 channels. The raw

data is stored in a computer or transmitted to the server. The data is

then processed using suitable softwares. After analysis and

interpretation, reports are prepared which give the value of various

parameters and their exceedance, if any recorded.

164 165


7.5 ASSESSMENT OF RESIDUAL LIFE OF THE BRIDGE

Residual Life Analysis of the Bridge is done based on selection of

appropriate SN Curve, which depends on the physical and metallurgical

properties of the material. In the SN curve N is the number of cycles,

of the varying stress. Value of ‘N’ for any given material depends on

‘S’ (Stress range), which is difference of maximum and minimum stress

in fatigue cycle. Due to introduction of higher axle loads, the stress

range for any member will increase, which will reduce the fatigue life

of the member. By strain gauging of the various members of the steel

girder, the number of cycles due to any trainload passage and the

stress pattern get recorded. Using this information, the fatigue damage

done by one train passage is arrived at. The traffic history over the

bridge is required to be obtained to get the fatigue damage already

done for the steel structure and this is used to get the residual fatigue

life for various members.

7.6 SUBMISSION OF REPORTS

Perform numerical structural modification studies to determine cost-

effective rehabilitation/ strengthening scheme where instrumentation

analysis indicates abnormality in the structure. This item has been

kept as an optional item for execution.

7.7 Continued monitoring for two years. This item has also been kept

as an optional item for execution.

8.0 INSTRUMENTATION AND DATA RECORDING OF BRIDGE NO. 105

IN DURG – DALLIRAJHARA SECTION

For Bridge no. 105 in Durg-Dallirajhara section testing/ data recording

with test trains for the first quarter was done on 25.05.2006 and 26.05.2006

and on 17.11.2006 and 18.11.2006 for the second quarter. Following are the

salient features of these recordings:

(i) The section involved is a non-electrified section. For the first

quarter recording Double headed WDM2 locomotives were used.

For the second quarter recording Double headed WDG3A

locomotives were used.

(ii) For the recording of vertical forces test train consisting of 10

numbers of BOBSNM1 wagons loaded with 100 ton (25 ton

axle load) was used. For the recording of longitudinal forces full

length test train (running in this section) of 48 numbers of

BOBSNM1 wagons loaded with 100 ton (25 ton axle load) was

used. In addition data recording was done for the normal revenue

and passenger running trains for 3 days.

(iii) Weighment of the test wagons was done in advance with the

help of On line weigh bridge at Marauda. 25 ton axle load on

each wagon was esured.

(iv) Instrumentation was commenced 4 days prior to the test runs.

(v) Special sanction of CCM and General Manager was obtained

for overloading of the wagons without any punitive charges.

Sanction of COM was taken in advance for arrangement of special

test trains and for arrangement of traffic blocks.

(vi) For helping the test trains to run at proper speeds and to carry

out proper braking, Sectional PWI and Loco Inspectors were

associated with and deputed with the test runs.

(vii) RDSO has been associated with the recording for the second

quarter on this bridge and onwards for further testing and

analysis.

(viii) For measurement of the vertical forced the test train was run at

following speeds:

(a) 5 Kmph (considered as static loading)

(b) 15 Kmph

(c) 30 Kmph

(d) 45 Kmph

(e) 60 Kmph

(f) 75 Kmph

(ix) For measuring the longitudinal forces i.e. Braking forces and

Tractive Efforts, the test train was run with Double Headed

Locomotives with 48 numbers of BOBSNM1 wagons loaded

with 25 ton axle load. The test was done as per the guidelines

issued by RDSO.

(x) Summary of Instrumentation: Instrumentation was done for

both type of Plate Girders i.e. for 30.5 m. span and 12.2 m.

span. A summary of the instrumentation done for the first quarter

recording is as follows:

166 167


9.0 DISCUSSION OF RESULTS

Detailed results in tabular and graphical form for the data recorded for

the first quarter recording have been received in the report. However, for easy

understanding a comparison of the recorded stresses with respect to the

result of numerical modeling and with theoretical and permissible stresses

has been shown in the following tables:

(b) Shear Stresses :

(a) Bending Stresses at Mid Span :

In the testing for the second quarter bearings were also instrumented for

measurement of longitudinal forces.

168 169


10.0 SUMMARY OF FINDINGS AS PER THE FIRST QUARTER

RECORDING FOR BRIDGE NO. 105:

As per the first quarter recordings of the Bridge No. 105, following

conclusions have been submitted by the specialized agency:

� Bridge 105 was instrumented, tested and analyzed, to evaluate

its suitability for carrying heavier axle loads.

� Numerical analysis of the bridge under MBG loading

indicates that even considering full CDA, the bridge has reserve

capacities for both the 30.5 m and the 12.2 m span.

10.1 SUPERSTRUCTURE BEHAVIOR

� Initial review indicates that the bridge spans are performing

as expected under the test train. Dynamic behavior is

consistent with that of similar bridges.

� Review of primary bending behavior of the 30.5 m spans indicates

that the measured values of bottom flange strain represent

about 63 - 68 % of the theoretical value of strain, and that

the shape of the time history is consistent with the expected

theoretical time history.

� For the 12.2 m spans, the measured values of bottom flange

bending strain represent about 82 . 85 % of the theoretical

value of strain, and the shape of the time histories was consistent

with the expected theoretical history.

� Similarly, for shear strain at .d. from the support, the measured

shapes were consistent with the expected theoretical shapes,

and the measurements were about 62% and 79% of the

theoretical value for the 12.2 m and 30.5 m girders, respectively.

� At higher speeds, increases in strain and deflection levels were

noted. These dynamic augment percentages are significantly

lower than the design values.

� The peak stresses measured under the test train (25 ton axle

loads) were significantly lower than the allowable stresses for

the superstructure.

10.2 PIER/SUBSTRUCTURE BEHAVIOR

� It was indicated that the piers and corresponding substructure

are in good condition and experienced no significant movement

under train passage.

� Vertical pier deflections, pier strains and pier tilt were all very

small and no significant movement was noted.

� The piers and substructure performed well under the test train

with 25-ton axle loads.

10.3 LONGITUDINAL FORCE EFFECTS

� No notable changes in pier behavior were observed during the

braking and acceleration runs also, implying that no significant

longitudinal forces were being transferred to the substructure.

170 171


� Review of forces measured on the rail indicate that peak forces

of about 15 tons were generated under normal rail operations

and peak forces of about 29 tons were generated during the

traction and braking runs.

� It was also observed that the forces generated were being

transferred by rail off the bridge and that the proportion transferred

through the bearings was very small, as expected.

11.0 CONCLUSION

(1) Results of first quarter report of the instrumentation and Bridge

monitoring for Bridge No. 105 are encouraging. The recorded

actual stresses are in general lower than the theoretical and

permissible stresses. It appears at this stage that with further

analysis after getting second quarter report and after getting

the report for residual life analysis, this bridge, which was

proposed and sanctioned for replacement, may not require

replacement and should be able to give service for many decades

even with increased axle load.

(2) Due to the changing scenario in the Indian Railways and in

order to optimum utilisation of assets, adoption of higher axle

loads is a beneficial preposition. To analyse the existing assets,

carrying out such scientific studies is very useful and essential.

Such rational studies will save huge assets from avoidable and

premature condemnations and replacements of assets. This

will not only save a lot of unnecessary expense of efforts and

money but, will also help Indian Railways in mopping up

additional revenue by effective and optimum utilisation of existing

resources.

ASSESSMENT OF LOAD CARRYING CAPACITY

OF MASONRY ARCH BRIDGES

CHAHATEY RAM*, SANJAY BANERJEE**

SYNOPSIS

Indian Railway has recently gone for an increase in the axle load of

freight trains on nominated routes to enhance throughput which has become

critical in view of fast growing economy of the country. It is therefore essential

that all existing bridges are assessed for their load carrying capacity. Masonry

arch bridges form a substantial percentage (about 20%) of all the bridges in

Indian Railways. There are no analytical methods available in Indian Railways

on date for assessment of these bridges. This paper brings out briefly the problem

which differentiates masonry arch bridges from other bridges, lists various

methods of analysis and describes two methods in details viz. RING 1.5 & Modified

MEXE Method.

INTRODUCTION

Indian Railways is one of the oldest Railway systems in the world, its

origin dating back to 1850s. Over the years, Indian Railways has undergone

tremendous changes in geographical and spatial spread, quantum of

passenger and freight traffic, trailing load density and speed.

One of the oldest civil engineering assets of Indian Railways are its

bridges which vary widely in span, type, material of construction and age.

Although a large number of the bridges have been built after independence,

a vast majority are more than 100 years old. Among the bridges which are

more than a 100 years old, stone and masonry arch bridges are the most

common. This paper brings out the results of the analysis carried out over a

variety of stone / masonry arch bridges and compares the results with their

physical condition based on actual field inspections.

PURPOSE O F THE ASSESSMENT

The present loading standard of Indian Railways is Modified Broad

Gauge(MBG),1987, which stipulates a design axle load of 25 tonnes.

* Chief Bridge Engineer, Eastern Railway

** Deputy Chief Engineer/Bridge, Eastern Railway172


However, the actual axle load of BOXN being run at present is 20.32 tonnes.

The analysis was prompted by a recent Railway Board decision to permit

certain degree of overloading over specified routes, which means an axle

load of 22.83 tonnes. Also in the very near future, an actual axle load of 25

tonnes is to be introduced.

WHAT IS ASSESSMENT

Assessment is the procedure by which a Civil Engineering structure

is subjected to systematic and scientific study by appropriate method so as

to quantitatively determine the safe load carrying capacity. This also includes

adequate understanding about the behavior of the structure and its material

of construction. It also involves judging the effects of the assumptions made

prior to the analysis so that the end result is as free from any bias as

possible. Assessment also involves correlating the physical condition of the

structure with the results of the analysis in order to achieve a realistic

prediction of the load carrying capacity. Assessment is also a tool which is

used to diagnose the structural inadequacies to arrive at the extent, nature,

technique and quantum of repair / rehabilitation required.

WHY ASSESSMENT OF ARCH BRIDGES ARE SO DIFFICUL AN D

DEBATABLE ?

The first step of assessment of any civil engineering structure is analysis

of the structure. Depending on whether the structure is determinate or

indeterminate, several rational methods are available for analysis. Determinate

structure is easy to analyze as they offer easy solution by simple application

of the equation of statics alone. However, indeterminate structures pose a

greater level of difficulty as they cannot be analyzed by application of the

equation of statics alone. They are required to be analyzed by other methods

like double integration methods, strain energy methods, elastic centre

method, column analogy methods etc. All these above rational methods are

based on certain assumptions e.g.

a) The material of the structure is homogeneous and isotropic.

b) Elastic theory is applicable.

c) Plain sections remain plain before and after bending.

Masonry arch bridges do not conform to the above assumptions. Firstly,

masonry arch bridges are neither homogeneous nor isotropic. Secondly,

being non- homogeneous, the properties of the masonry arches depend on

the properties of the masonry unit (i.e. brick / stone), the mortar (i.e. lime/

surkhi/cement) and most importantly the properties of the bond at the interface

of the mortar and the masonry unit. It is extremely difficult to apply the

principles of elastic theory on such non- homogeneous material. Moreover,

arch bridges are indeterminate structures to third degree. Besides, the shape

of the arch also plays a crucial role in determining its final load carrying

capacity. In addition to the above, presence and depth of backing and the

depth of fill considerably modifies the load dispersion over the arch barrel.

Since this load dispersion finally determines the actual load intensity coming

on the arch, the more accurate is the assumed load dispersion mechanism

in the analysis, the more realistic will be the final output. However, there is

no consensus world wide regarding the actual load dispersion mechanism

of the arch. Finally, the effect of parapet walls and track, which significantly

increases the strength of the arch do not figure in any of the analyses.

Because of the above mentioned facts the results of analysis of masonry

arch bridges are very much difficult and debatable.

DIFFERENT LEVELS OF ASSESSMENTS OF MASONRY ARCH

BRIDGES

The different levels of assessment of masonry arch bridges along with

the methods used and the use the results are put to are given in the table

below :-

174 175


STAGES

The project commenced with the detailed field inspections of all the

bridges falling over the routes specified for this Railway by the Board. The

requisite data for the analysis were either obtained from the available

completion plans or were accurately obtained from field inspections. In all

40 arch bridges were analyzed.

176 177


abutments are fed in. The number of blocks mentioned herein

is the number of layers of brick/stone masonry which constitutes

the pier/abutment along its height. The span tab is used to

enter the values of clear span, rise of the arch above springing

and the number and thickness of arch rings.

The next tab pertains to material properties through which data

relating to material properties are fed in. This involves data related

to soil in the cushion, the properties of the stone/ brick masonry

i.e. the structural component of the arch bridge and the properties

related to interaction between soil and brick/stone masonry.

The next tab relates to loading. MBG loading as defined in IRS

Bridge Rules has been used for the purpose. More details have

been provided in the subsequent paragraphs.

The next part involves the analysis based on the data fed in so

far. By clicking on the analysis tab, the software performs

iterations for the various load cases and gives load position at

failure and the minimum load factor at that condition. There is

an option of viewing the report on the analysis, which when

clicked generates the report after analysis about the Bridge in

MS Word. This report also contains the diagram of the condition

of the bridge at failure. The load factor indicates the factor of

safety against such failure for that particular load case.

B) Modified MEXE Method – The first engineering method for the

assessment of masonry arch bridges was developed by Pippard

and Ashby (1939)and Pippard (1948) and was used extensively

during the second world war for military purposes for quick

assessment of load carrying capacity of these bridges. Full

scale tests were conducted in 1950s and the resulting knowledge

was consolidated. As a result of this research, MEXE method

was established. A computerized version of this method finds

mention in UIC Code 778-3R, Appendix – 4. Details of this

method are described in subsequent paragraphs.

ASSUMPTIONS

The various assumptions made in the analysis are given below:-

RING Software Version 1.5. – The input file consists of four

parameters viz. i) Geometry, ii) Material, iii) Loading and iv) Advanced.

SOFTWARE

The analysis were carried out by two methods

A) RING Software version 1.5.

B) Modified MEXE Method.

DISCUSSION ABOUT THE SOFTWARES

A) RING Software Version 1.5 - This Software has been developed

by Sheffield University, U.K. The same is available in

downloadable form through internet at www.shef.ac.uk/ring. The

software is available for use free-of-cost and may be downloaded

and used directly by any user having access to an internet

connection. The higher version of this software i.e. RING version

2.0 is presently under development as part of an on-going UIC

project on masonry arch bridges and is likely to be available for

use to users by 2007.

Downloading of the software automatically creates a link on the

desktop of the PC in which the software is downloaded. By

double clicking on the said link, one window opens which

prompts the user to choose from among the following three

options:

1. Create a new Bridge Project.

2. Open the existing Bridge project.

3. Open the recently access Bridge project.

Choosing the first option creates a new window, which is basically

used for entry of data regarding the bridge, loading etc. The

other options are meant for opening existing bridge projects

already analyzed by this software as the name suggests.

The first tab pertains to geometry of the bridge through which

the dimensional details of the bridge is entered. By clicking on

the geometry tab, the first window opens which is used to enter

the global parameters like number of spans, whether abutments

and piers shall be considered in the analysis, whether effects of

backing would be considered, width of the bridge to be

analyzed and the depth of fill. The parameter “width of the

bridge to be analyzed” is described more elaborately in the

subsequent paragraphs. Once these values are entered, the

next tabs are clicked thereafter one-by-one and data related to

height, bottom and top width and number of blocks of piers and

178 179


• In the Geometry, one very important parameter is the “width of

the bridge to be analyzed”. Since the load imposed over the fill

of the arch ring shall be distributed under the sleepers, it has

been assumed that there is a 1:1 dispersion in the transverse

direction, i.e. perpendicular to the track and the width at top of

arch ring extrados has been assumed to be the width for which

the bridge is to be analyzed. The other input data are quite

evident. However, all the analyses have been done without

considering the effect of backing which the software caters to in

sufficient details.

• In case of Materials, the input parameter seeks to input the

data required for the masonry and the backfill material. For

masonry, the default values of unit weight, coefficient of friction

(both tangential and radial) have been adopted. For arch bridges

which appeared to be physically sound upon inspection the

default value of crushing strength of masonry of 10 kn/m2 have

been assumed but for those which inspection revealed some

sort of physical distress, a value of 5 KN/m2 have been assumed.

The default values of solution convergence tolerance and

maximum number of iterations have also been retained without

change. For backfill, the default values of unit weight, limiting fill

friction along with Boussinesq type load distribution with a limiting

angle of 0.524 radians have been retained. A classical horizontal

pressure distribution has been assumed with a coefficient of

earth pressure of 0.33 with the option of automatic selection of

passive pressure zones.

• In case of loading, a load vehicle has been created as per MBG-

1987, as per the details of axle load and its distribution as in

Annexure – I. This load vehicle has been assumed to cross

the bridge and several load cases depending on the span of the

bridge have been created by incrementing the load vehicle

position by 300 mm. There is very critical parameter to be

considered while creating the load case, i.e. the width of the

load. After careful consideration, since the load will act on the

fill through the sleepers, it was decided to adopt the value of the

width of the sleepers across the track i.e. in the transverse

direction as the width of the load.

• The default values of advanced properties regarding constraints

and block weights/pressures have not been touched.

A) There is no scope for any assumptions in the Modified MEXE

Method as all the input parameters are well defined. The input

parameters are as follows:-

d = thickness of arch

h = fill from top of arch

L = clear span

rc = rise of arch above springing

rq = rise of arch above springing at quarter point.

Fsr = Span/Rise factor. Graphs are available for calculating Fsr

values for Span/Rise ratios more than 4. For Span/Rise ratios

less than and equal to 4, Fsr = 1.0.

Fp = Profile factor. If rq/ r

c value is less than equal to 0.75, then

Fp = 1.0. If rq/ r

c value is more than 0.75, then Fp = 2.3{(r

c - r

q)/ r

c}0.6.

Fb = Barrel factor. This can be obtained from a given table which

gives a value depending on the material of construction of the

arch and also the physical condition.

Ff = Fill Factor. This can be obtained from a given table which

gives a value depending on the material of the fill.

Fw = Width factor. This can be obtained from a given table

which gives a value depending on the width of joints in masonry.

Fmo = Mortar factor. This can be obtained from a given table

which gives a value depending on the condition of the mortar in

masonry.

Fd = Depth factor. This can be obtained from a given table which

gives a value depending on the condition of joints in masonry.

Fj = Joint factor. This factor is a product of the above three

factors specifying the effect of joints. Fj = Fw x Fmo x Fd.

Fcm = Condition factor. This can be taken between 0.0 and 1.0

(lower limiting value excluded) at the discretion of the engineer

depending on the condition of the bridge as a whole, the highest

value to be adopted for a sound arch.

A provisional value of permissible axle load over the bridge PAL

is calculated from the following formula, PAL = 740 {(d+h)2/L1.3}

or 70 tonnes whichever is less.

The modified value of the axle load PALm is calculated as follows,

PALm = PAL x Fsr x Fp x Ff x Fj x Fcm.

The tables mentioned above are given below for ready reference:

180 181


# Interpolation between these values is permitted, depending upon the extent

and position of the joint deficiency.

182 183


DISCUSSION & COMMENTS

A) RING Software Version 1.5 – After analysis of 35 arch bridges

by this software, it is seen that total 4 nos. bridges are having

load factor less than 1.0. The failure mechanism after analysis

of three of these four bridges are enclosed as Annexure – III (a

to c). It is seen that the failure mechanism in all these cases

pertains to tilting failure of abutments/piers while the actual arch

ring maintains continuity even at failure. The values of the critical

parameters of these four bridges are given in tabular form below:-

substructure making the slenderness effects more pronounced.

Decrease in depth of fill and width of bridge shall increase the

intensity of applied load over the arch leading to decrease in

load factor.

The calculations have been performed without taking into account the

effect of backing. Also, RING software does not take into consideration

the effects of parapet walls and track, all of which significantly increase

the strength of the arches to the extent of nearly 30 to 40%. Moreover,

the tilting failure of abutments is extremely unlikely in actual scenario

and at the present moment, none of these bridges show any visual

signs of distress. It is felt that these bridges shall offer sufficient advance

warning before failure which can be safely detected in the periodicity

of inspections of these bridges. Hence these bridges are considered

as safe.

From the above discussions it may be concluded that the program in

its present version suffers from the following limitations:

• Modeling of piers and abutments – While data feeding, provision

of feeding batter angles must be available to ensure that the

exact profile of the abutment / pier both at the front and back is

ensured. This is not possible at the present moment as the

resultant diagram always gives uniform batter angles to both

ends which is not always so for the actual bridges.

• All material properties should be available in tabular from to

ensure that the user makes a judicious choice on the basis of

field observations and avoids using the default values arbitrarily.

For example, soil properties may be given in tabular form against

the various types of soil to enable the user to make an informed

choice about the engineering properties from the basic input

from field i.e. soil type.

• The effects of multiple tracks over a single arch and parapet

walls needs to be added with their corresponding input

parameters to make the result more realistic and reliable. This

is so as considering the effects parapet walls will serve to

increase the strength of the arch, multiple tracks over the same

arch shall increase the intensity of loading due to load coming

from the adjacent track and shall considerably reduce the load

factor. In fact this is one of the most serious limitations of the

software.

From the above, it is seen that these bridges are all having

span equal to or greater than 3.66 m. Bridge Nos. 3 is having

quite high abutments which is the principal cause of low load

factor. Although other three bridges are not having very high

abutments, their principal cause of failure are low depth of fill

and low value of the width of the bridge where the load is acting

or a combination of these two factors. Hence it may be concluded

that this software is extremely sensitive to the three values of

height of abutments, depth of fill and width of the bridge over

which the load acts. This is of course most consistent with the

general expectations of actual behavior of arch bridges. Increase

in height of abutments will adversely affect the stability of the

ANALYSIS RESULTS

The results of the analysis is available at Annexure – II.

The summary of the results are given in tabular form below:-

184 185


• If the height of the fill is large and if the number of blocks in a

ring exceeds the limit set in the program, it is seen that no

convergent solution can be found. This places a severe restriction

on the range of applicability of the program and hence these

input parameters need considerable upward revision to ensure

that all arch bridges irrespective of input parameters may be

analyzed by this software.

• A database is required to be developed containing the data of

the bridges analyzed supported by full scale load tests over the

same bridges to check and verify the reliability of assessment

by this software.

B) Modified MEXE Method - After analysis of 35 arch bridges by

this software, it is seen that 10 Nos. bridges are having

permissible axle load less than 25 tons. “Modified MEXE Method”

being a totally empirical method, no failure mechanism is

obtainable and the method merely gives a rough assessment of

the permissible axle load over the arch bridge. The values of the

critical parameters of these ten bridges are given in tabular form

below:-

This method do not take into consideration the effects of 3D, track,

backing and parapet wall which significantly increases the strength of

arches. Moreover, 8 out of 10 of these bridges do not show any sign of

distress and are hence considered safe. It is also mentioned that all

these 10 bridges are having load factor much greater than 1.0 by the

RING software. Only two bridges out of these 10 have some visual

signs of distress.

Apart from the above, the method has the following serious limitations:

• The method completely ignores the effects of piers and

abutments in its calculations. This makes the results obtained

by the method doubtful as the effects of the piers and abutments

on the overall assessment of the load carrying capacity of arch

bridges are significantly large to ignore.

• The method is applicable only to single span arch bridges. The

effect of multi span which considerably modifies the behavior of

the arch bridges being completely ignored again renders the

assessment of results liable to grave doubts. So the results

obtained by this method for Bridge Nos. 515, 501, 491, 482 &

480 are most doubtful.

• This method is also silent about the arch bridges carrying

multiple parallel tracks. In fact, width of the bridge which is

important in view if the fact that it gives the intensity of load is

completely ignored. Here in case, results obtained for Bridge

Nos. 515 to 480 are also most doubtful.

SUMMING UP

1. Although these methods provide an excellent tool for first level

assessment of arch bridges to the extent that the load carrying

capacity is quickly assessed, too much importance should not

be given on the results obtained. The results need to be

interpreted taking into account the assumptions made in the

analysis. They should also be correlated with the physical

condition of the bridge found on inspection. The assessment

should in NO WAY be an alternative to VISUAL INSPECTION

which will remain to be of primary importance to the bridge

engineer. These methods in their present form may be used as

an aid to decision making regarding adoption of repair techniques

and their extent and decision for the frequency of inspections.

It is quite evident from the above table, that the combination of

these four factors i.e. span, arch ring thickness, rise of arch

and permissible axle load are the most critical in this method.

This is in line of what is expected. Although these input

parameters are exact, other input parameters are the condition

factors which involve heavy subjectivity and may vary

considerably from user to user leading to doubts about reliability.

186 187


2. Doubts resulting from discrepancy between results obtained by

these methods and inspection should be cleared after close

study with or without adequate instrumentation supplemented

by non-destructive methods which are available as tools for the

bridge engineers of today.

3. After analyzing a large number of masonry arch bridges the

following important facts emerge –

• A fill of about 1 metre above crown of extrados upto bottom

of sleeper is a must for satisfactory performance of arch

bridges. If the fill is about 60 to 70 cm, no amount of repairs

can hold good.

• Clean ballast cushion is very important over arch bridges.

There should be no fish plated joint over such bridges. If it

is unavoidable than it should be located just over the piers.

Deep screening over masonry arch bridges is required to

be done more frequently say once in 3/5 years.

• As all the masonry arch bridges are more than 100 years

old and hardly any maintenance, repairs have been carried

out, it is desirable to take up the work of thorough pointing

and cement grouting of all masonry arch bridges from one

end of the section to the other.

• RCC jacketing of arch bridges should not be thought of

unless other measures have been exhausted. The quality

of RCC jacketing which is normally done manually leaves

much to be desired and in real terms does not improve

performance of arch bridges. At best it gives only a

psychological satisfaction. If RCC jacketing is required, the

use of batching plant and concrete pump must be made

compulsory for making its use effective.

These measures are expected to enhance life of masonry arch

bridges substantially.

188 189


ANNEXURE – III(a)

Bridge No. 3, DME – KPK

Span 1 X 3.66 m

Analysis result

Critical load factor = -0.83 (load case 6)

190 191

ANNEXURE – II


ANNEXURE – III(b)

Bridge No. 4, DME – KPK

Span 1 X 4.57 m

Analysis result

Critical load factor = -0.86 (load case 5)

ANNEXURE – III(c)

Bridge No. 14, BQT – DSEY

Span 1 X 3.66 m

Analysis result

Critical load factor = 0.12 (load case 5)

192 193


FATIGUE ASSESSMENT CRITERIA FOR

DESIGN AND ANALYSIS OF STEEL GIRDER

BRIDGES FOR HEAVY AXLE LOAD

OPERATIONS

PIYUSH AGARWAL* , R.K. GOEL**

1.0 INTRODUCTION

1.1 Railway bridges are subjected to heavy fluctuating dynamic loads.

These fluctuations cause fatigue failure of members or connections at

lower stresses than those at which would otherwise fail under static

load. IRS provisions, which are based on stress ratio concept does

not taken into account the phenomenon of fatigue adequately. World

over, such effects are taken into account following the Palmgren Miner

cumulative damage rule based on stress range concept. British Code

BS-5400 part 10 and more recently Euro Code EN-1993-1-9: 2002

address the fatigue effects in a rational manner by taking design

parameters such as route GMT, type of traffic, design life and detailed

category of connection. However, these provisions are specific to loading

conditions prevailing in their countries and the design parameters as

given in these codes can not be straightaway applied for design of

bridges in traffic conditions prevailing in India.

1.2 Recently, Indian Railways has taken a leap forward to cope up with

the increased demand of freight transportation by deciding to have a

dedicated freight corridor (DFC) for which a new set of standard designs

would have to be prepared. This would be a big challenge for Bridge

Designers to develop the new designs, based on rational criteria for

fatigue in accordance with established international practices.

Development of a fatigue load model for such a dedicated freight route

is the first design input required by the designers. In this paper an

attempt has been made to develop such a load model based on the

anticipated axle load, type of locos and the train lengths. This model

with modifications can be subsequently used for analyzing the existing

bridges on feeder routes.

* Executive Director/Bridges & Structures /RDSO/Lucknow,

** Director/Bridges & Structures /RDSO/Lucknow.

2.0 PLAMGREN-MINER LINEAR DAMAGE RULE

2.1 Palmgren has proposed a damage model on the basis of constant

energy absorption per cycle. The energy absorption per cycle leads

to linear summation of damage. Miner has subsequently, represented

this concept in mathematical form. Palmgren-Miner rule states that

the fatigue damage contribution by each individual load spectrum at a

given stress level is proportional to the number of cycles applied at a

stress interval, ni, divided by the total number of cycles to failure at

the same stress level, Ni. It is obvious that each ratio can be equal to

unity if the fatigue cycles at the same stress level would continue

until failure occurs. The total damage, in terms of partial cycle ratios

or damage, can be written as –

2.2 The Palmgren-Miner rule, described above is considered a simplified

and versatile tool for determining the total life of the structure under

study. It is apparent that the detail under consideration is said to have

failed if the Total Damage becomes unity (1.0). The rule does not

account for the effect of load sequence and load interaction on damage

accrued, and have an over simplified assumption of linear summation.

However, Palmgren-Miner rule is still widely used to estimate life of a

structure, on account of its ease of application.

3.0 TRAIN LOADS AND TRAFFIC MODEL

3.1 Railway Board vide its letter No.2006/CE-II/TS/2 dated 26-10-06 has

advised the axle load and Track Loading Density (TLD) for design of

foundation and bridges. As per this letter axle load of 32.5t and TLD

of 12 t/m has to be adopted for design of bridges. Based on this

information, the freight train compositions including their length, weight

and GMT etc. have been developed for light, medium and heavy traffic

classifications.

3.2 The wagon details for DFC are yet to be finalised, therefore the wagon

details (axle spacings) of IRS HM loading have been taken in the

above analysis. No passenger trains have been considered and the

195


existing combinations of HM loading have been modified to develop

the above load model for fatigue assessment. The traffic classification

and load model developed have been shown in Table–1 & Table-2

respectively. The salient features of the load model are described as

under:

3.2.1 LOADING

For 32.5 t, none of the existing loadings given in IRS Bridge Rules is

suitable. Therefore, the train formations have been tentatively

developed on the basis of the details of locomotives and wagons for

HM loading as available in Bridge Rules with following modifications-

i) The number of train formations, have been reduced to 12 from

17 by removing the similar type of formations.

ii) Trailing Load density: The Gondola wagon as taken in HM loading

has been adopted with same axle spacing and increased axle

load of 32.5 t. It gives a trailing load density of 12.79 t/m. Railway

board vide its letter no.2006/CE-II/TS/2 dated 26-10-06 has

instructed to design the bridges for TLD of 12 t/m. During

discussion with Wagon Directorate of RDSO, it has been learnt

that wagons of BOXN type are being modified for heavy axle

load and these would be giving a TLD of 12.33 t/m. The proposed

TLD is therefore, slightly on higher side and the designs of

standard spans shall be safer. The locomotives of HM loading

have been adopted with their axle spacings unchanged. However,

the axle loads of all the locomotives have been proposed as

32.5 t for stress analysis. This will take care of any possible

increase in axle loads of the locomotives.

iii) Tractive Effort: In the HM loading the tractive effort of locomotives

has been observed as 60t, 45t and 30.5t depending upon the

number of locomotives coupled together. The maximum tractive

effort of HM loading is 135 t with three locomotives of WAG6C &

WAG6B. It has been learnt that increasing the axle load

increases the tractive effort of locomotive. As discussed with

concerned directorates there is no possibilities of increasing

the tractive effort of an electric loco beyond 75t. In such a case

only 2 locomotives would be normally sufficient for heavy haul.

For heavier traction, three or four locomotives of lesser tractive

effort can be coupled. In the proposed loading the maximum

tractive effort of 180 t has been proposed with a combination of

3 locomotives. The comparison of locomotives and their tractive

efforts for double, triple or quadruple traction has been shown in

table given below:

iv) Braking Forces: The braking force of the locomotive has been

indicated against the train formations in Table-2. The braking

force of the train load has been taken as 13.4% of train load as

has been done in the case of HM loading. Therefore it is in

accordance with prevailing practice.

3.2.2 SPEED

It has been advised by Railway Board that the maximum permissible

speed of freight trains of heavy axle load would be 100 kmph. It was

also learnt that speed trials are required to be done with 10% extra

speed. Further keeping in view the possibilities of 10% increase in

future, design speed of 125 kmph has been proposed. It is also in

conformity with the Coefficient of Dynamic Augment (CDA), given in

IRS Bridge Rules which has been developed for a speed of 125

kmph.

196 197


Ta

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Sh

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Sh

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202 203


Sh

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Figure-1 : Fatigue strength curves for director stress ranges

4.0 RELEVANCE OF LOAD MODEL WITH DESIGN LIFE OF BRIDGE

4.1 Following factors primarily affect the fatigue strength of a typical

connection-

i) Type of joint details

ii) Stress range at the location under consideration.

iii) No. of cycles of stress range

4.2 The type of joint detailing is decided keeping in view the methodology

of fabrication to be adopted. Once the type of joint detail is finalised,

the allowable stress-range, to which the connection can be subjected

to, can be obtained from relevant S-N curve for ‘N’ number of cycles.

‘N’ is usually taken as 2 million. S-N curves developed for the purpose

are shown in Fig. 1.

204 205


Figure-2 : Typical stress range histogram

can be adequately assessed over a period of time. The load model

will have to specify the distribution of train types and their frequencies

with respect to their cumulative GMT and traffic volume. These

parameters in turn will have to be taken as design input for assessing

the fatigue strength of connections.

5.0 FATIGUE ASSESSMENT CRITERIA

5.1 It is well known that the fatigue provisions of IRS Steel Bridge Code

are based on stress ratio concept, which is quite obsolete. Revision

of fatigue provisions is already being done with active support of IIT/

Roorkee. The new provisions are to be based on stress range concept

using the Palmgren- Miner cumulative damage rule. It has been

observed that the provisions of British Code BS-5400 are based on

stress range concept and are well understood by practicing design

consultants. Therefore, the fatigue assessment criterion has been

framed based on BS-5400. Accordingly, the fatigue assessment to

be done in accordance with Palmgren-Miner summation rule by

assessing accumulated damage as described below.

5.2 From the train configurations given in fatigue load model (Table-2)

stress histories shall be determined at the structural detail (including

such secondary effects as would be relevant) and stress-ranges and

their numbers shall be evaluated by rain flow method or reservoir

method. Fatigue damage analysis shall be done by applying

appropriate partial safety factors to stress ranges and characteristic

S-N curves in accordance with relevant provisions of BS:5400.

5.3 The damage summation shall be performed as per Clause 8.4, 9.2

and 11.1 of BS: 5400 Part-10 as under:

Where

n is the number of cycles associated with stress range modified

with appropriate partial safety factor for load

N is the number of cycles corresponding to the design S-N curve

modified with appropriate partial safety factor for material.

5.4 Failure shall assumed when Dd≥1.0

4.3 It implies that the connection of a particular category is able to safely

withstand 2 million cycles of the allowable stress-range. As practically

observed, the different components of the structure undergoes different

no. of cycles of different stress-ranges. Therefore, every connection

detail, over a period of time, is subjected to a stress-range histogram

consisting of number of stress-ranges and corresponding number of

cycles. A typical stress-range histogram is shown in Fig. 2.

4.4 The actual damage to the connection detail is the cumulative effect of

all such stress-ranges that are included in the stress-range histogram.

The concept of design life comes into picture at this stage, as the

cumulative damage should be equal to unity, at the end of design life.

The stress-range histogram, to which the detail is subject to, is a

function of type of trains, frequency of trains, speed and the GMT etc.

In practical scenario, it is a complex phenomenon of cumulative fatigue

damage, which will be very difficult to model unless some kind of

standard of load-frequency distribution is assumed. Therefore, In order

to standardize the stress-range histogram, it is necessary to

standardize the load models so that the cumulative fatigue damage

206 207


7.0 REFERENCES

7.1 Revision of Fatigue Provisions in IRS Steel Bridge Code (2004), First

Interim Project Report submitted by Department of Earthquake

Engineering, Indian Institute of Technology, Roorkee to Research

Designs Standards Organisation, Ministry of Railways, Lucknow (UP)-

226011.

7.2 Stress Spectra for Fatigue Design of Railway Bridges (1991), Project

report submitted by Department of Civil Engineering, Indian Institute

of Technology, Kanpur to Research Designs Standards Organisation,

Ministry of Railways, Lucknow (UP)-226011.

7.3 Technical Documents on Traffic Details for Revision of Fatigue

Provisions of IRS Steel Bridge Code (1989), file No.CBS/PSB, Research

Designs Standards Organisation, Ministry of Railways, Lucknow (UP)-

226011.

7.4 IRS Bridge Rules (1986), Research Designs & Standards Organisation,

Ministry of Railways, Lucknow (U.P.).

7.5 British Standards BS 5400 : Steel, Concrete and Composite Bridges,

Part-1 General Statement, British Standards Institution, 1980.

7.6 British Standards BS 5400: Steel, Concrete and Composite Bridges,

Part-3 Code of Practice for Steel Bridges, British Standards Institution,

1980.

7.7 British Standards BS 5400 : Steel, Concrete and Composite Bridges,

Part-10 Code of Practice for Fatigue, British Standards Institution,

1980.

7.8 EN 1990:2002 (Eurocode – Basis of Structural Design) – (For safety,

comfort, deformation including twist and deflection)

7.9 EN 1991-2:2003 (Eurocode I – Action on Structures, Part 2 – Traffic

Loads on Bridges) – (Natural frequency range and Loading for fatigue

estimation)

7.10 EN 1992-1:2004 (Eurocode 2 – Design of Concrete Structures, Part –

I – General Rules and Rules for Buildings)

7.11 EN 1992-1-1:2004 (Eurocode 3 – Design of Steel Structures, Part I –

1 – General Rules) – (Classification of cross sections)

5.5 Partial safety factor for load, γfL & partial safety factor for material

strength, γm shall be taken as per BS:5400 Part-1 & Part-3.

5.6 Following parameters shall be considered in assessment of fatigue

damage:

5.6.1 Fatigue load Model and traffic classification as per Table-2. Bridges

shall be designed for Medium traffic for annual GMT of 100.

5.6.2 S-N curves for Direct stress range as per BS: 5400 Part -10.

5.6.3 Joint detail classification as per Table-17 of BS: 5400 Part -10. The

design shall be in conformity with the description and requirements

of the connection detail chosen.

5.6.4 Appropriate Correction factors for stress concentration as given in

Appendix-H of BS: 5400 Part-10 for the detailed classification shall

be applied.

5.6.5 Design life of 120 years shall be considered.

5.6.6 Maximum design speed 125 kmph shall be considered.

6.0 CONCLUSIONS

6.1 A rational approach, in accordance with latest international practice

has been suggested for design of new steel girder bridges for heavy

axle load operations. The approach follows stress-range concept and

Palmgren-Miner cumulative damage rule, which forms the basis of

fatigue provisions of British Standards and Euro Codes. The approach

can also be applied for fatigue assessment of existing bridges for

heavy axle load operations.

6.2 A fatigue load model has been developed keeping in view the

requirements of heavy haul on Dedicated Freight Corridor, which is to

come in near future. The load model takes into account the parameters

of locomotives and wagons that are necessary inputs required for

design of steel girder bridges.

6.3 Fatigue assessment criteria has been proposed in accordance with

British Standard BS: 5400, keeping in view the fact that the provisions

are well understood and practiced by leading design consultants in

India. It is expected that a rational fatigue assessment procedure

would be followed in developing the future designs of steel bridges on

Indian Railways.

208 209


7.12 EN 1993-1-8:2002 (Eurocode 3 – Design of Steel Structures, Part 1-8

– Design of Joints) – (Classification of HSFG Bolts)

7.13 EN 1993-1-9:2002 (Eurocode 3 – Design of Steel Structures, Part 1-9

– Fatigue Strength of Steel Structures)

7.14 EN 1993-2:2004 (Eurocode 3 – Design of Steel Structures, Part 2 –

Steel Bridges) – (Requirements for fatigue assessment, Road and

Rail Bridges)

7.15 EN 1994-2:2003 (Eurocode 4 – Design of Composite Steel and Concrete

Structures, Part 2 – Rules for Bridges) – (Width of effective flange,

shear connectors)

SUITABILITY OF BGML AND RBG STANDARD

BRIDGES FOR HIGHER AXLE LOADS

RAMA KANT GUPTA*

Most of the bridges of Indian Railways are still of RBG, BGML or even

prior to that standard of loading. While permitting Higher Axle Load, it is

necessary to ascertain the safety of the bridges. In this paper, Author first tries to

share the input required for upgrading the bridges to MBG standard of loading.

Simultaneously, it has been tried to suggest the via media, if found safe, to permit

the powerful locomotive/rolling stocks even before up gradation of the existing

old bridges.

* This paper has been written based on the past experience of the

author as Executive Director, Bridge & Structures at RDSO, Lucknow

1. INTRODUCTION

For survival of the Indian Railways in the competitive market, it is

necessary to reduce the transportation cost. Since major share of the

earnings of Indian Railways is from goods traffic, hence it is but natural that

first consideration goes to the freight stocks, particularly regarding how to

operate them with higher axle load, so that with the same configuration and

same frequency, more load can be transported. (As far as coaching stocks

are concerned, these are having much lighter TLD as compared to freight

stocks and as such, don’t pose any problem.). In this paper, it has been

tried to explain the modalities how to upgrade the existing bridges of lighter

loading standard with bare minimum input and as a interim measure even

before upgrading the bridges, how to check the adequacy of operation of

any particular locomotive/rolling stocks on a particular section.

2. PERCENTAGE POPULATION OF THE BRIDGES WITH DIFFERENT

STANDARDS OF LOADING

Before discussing anything more, first let us have an idea bout the

population of bridges conforming to different standards of loading along

* General Manager/Bridges, IRCON, New Delhi – 110066.210


with load intensity of various standard loadings. The same is given in

Table No. -1:

eras still exist. But, the capacity of Arch Bridges to take the load is much

more than its designed load. As such, Arch Bridges are also not posing any

problem provided the same are in sound condition.

From the above Table, it is also clear that there was no consideration

of longitudinal forces in olden days. Actually, longitudinal forces were made

part of BRIDGE RULES in 1923. It does not mean that prior to 1923,

Engineers were not aware about the longitudinal forces, particularly Tractive

Efforts and Braking Forces. But, its magnitude was so small and majority

of the bridge foundations being of gravity type of masonry structures, there

was no any problem to the Tractive Efforts/Braking Force of the locomotives

of that period. Problems only started after increase in axle load over a

period of time, warranting requirement of powerful locomotives to haul the

heavier trains not only having comparatively higher axle loads but also having

more length of the trains as well It is worthwhile to mention that for bridges,

longitudinal forces are creating more severity than the vertical forces, and

most of the bridges are becoming unsafe on account of increased longitudinal

forces of present day locomotives.

For further appreciation, comparative statement of forces for Bending

Moment and Shear Force for different standard of loading has been given in

Annexure-I. Similarly, for Tractive Effort and Braking Forces for different loading

standard, the same is given in Annexure-II.

4. ACTION TAKEN BY RDSO

After revision of loading standards, RDSO came into action to check

the adequacy of the existing bridges whether the same are safe for revised

loading standard or not? In most of the cases, it was found that the input

required to upgrade the bridges of BGML and RBG standard to MBG standard

of loading is not much. The same is discussed as below:

4.1 INPUT REQUIRED FOR UPGRADATION OF SUPERSTRUCTURES

OF THE BRIDGES TO MBG

LOADING STANDARD

After thorough checking, it was found that majority of the BGML

standard bridges are safe, except in most of the cases, bearings

require strengthening. Wherever such requirements were felt,

supplementary drawings were prepared and issued to all concerned.

In few cases only, overstressing was found in BGML standard of

bridges but that too, were within the codal provisions of keeping such

Since, currency of RBG standard of loading was for a short time,

hence number of bridges conforming to RBG standard are less.

3. SEVERITY OF THE PROBLEMS OF OLD BRIDGES IN CONTEXT

TO PRESENT DAY LOADING

We are now familiar about BGML, RBG and MBG loading. Let us

have an idea about the loading standard even prior to introduction of BGML

i.e. prior to 1926. The same is given in Table No. – 2.

From the above Table, it is clear that over a period of time, axle load

has increased tremendously. Fortunately, Steel Bridges of those eras are

not existing at present. Those were of EARLY STEEL category and had

been replaced on account of designed for lighter loading and hence unsafe

or on account of its EARLY STEEL category. Only Arch Bridges of those

212 213


bridges under observation and as such, here as well, requirement of

regurdering avoided.

Similarly, in case of RBG standard of bridges, most of them were

found safe, except strengthening of bearings and keeping the few

bridge members under observation. Only one standard bridge span of

76.2m of RBG standard was found having overstressing of 6.5%, if

MBG standard of loading is to be introduced. It is further worthwhile

to mention that a few members that are having such an extent of

overstressing can either be strengthened or bridge span as a whole

can be replaced as per the situation to make such bridges safe.

Complete position about the input requirement for making old standard

bridges fit for MBG loading is given in Table No.-3

All other standard steel spans of BGML and RBG not included in

the above table are safe for MBG loading.

214 215


4.2 SUBSTRUCTURES

For substructures as well, RDSO did the remarkable work. Here,

required profiles of all standard spans were suggested. Many factors

are there for consideration, starting from different types of materials

used, piers and abutments including its shapes and overall, varieties

of standard spans existing on Indian Railways. The alternatives tried

is given as below:

� CONSTRUCTION MATERIALS

� Brick Masonry in Lime Mortar 1:2

� Brick Masonry in Cement Mortar 1:4

� Coarsed Rubble Masonry in Lime Mortar 1:2

� Coarsed Rubble Masonry in Cement Mortar 1:4

� Mass Concrete Substructures of M-10 Grade concrete

� TYPES OF CUT WATER

� Semi circular ends

� Triangular cut and ease water, angle included between the

faces being 90 degree.

� Arcs of cut and ease water intersecting at 90 degrees named

as ‘Standard Cut Water’

� GEO-TECHNICAL PARAMETERS

The following Geo-technical parameters have been considered:

� Angle of internal friction = 35 0

� Density of Backfill material = 1.76 t/m2

� Front slope of abutment = 1 in 15

� Length of abutment = 6.1 m

In addition to above, 100% overstressing in masonry structure as per

Sub-Structures Code of Indian Railways have also been considered. It

is worthwhile to point out that earlier, Factor of Safety for Masonry

Structures was taken as 6. Hence, even after allowing 100%

overstressing, there remains Factor of Safety of 3 which is adequate

as per Material Science knowledge of present day, provided the

masonry structure is in sound condition.

Based on the considerations/factors mentioned above, profiles for the

following Standard Spans have been developed:

� Profiles for abutments for spans 6.1 m (PSC slabs and Plate

Girders), 9.15 m (PG), 12.2 m (PG), 18.3 m (PG), 24.4 m (PG),

30.5 m (PG), 30.5 m (US), 30.5 m (OW), 45.7 m (OW), 61.0 m

(OW) and 76.2 m (OW)

� Profiles for piers for spans 6.1 m (PSC slabs and Plate Girders),

9.15 m (PG), 12.2 m (PG), 18.3 m (PG), 24.4 m (PG), 30.5 m

(PG), 30.5 m (US), 30.5 m (OW), 45.7 m (OW), 61.0 m (OW)

and 76.2 m (OW)

With the help of above-mentioned exercise of RDSO, one has to just

compare the profile of existing bridge substructure with the required

profile. If the required profile is less than the existing profile, then but

natural, the substructure is safe. Strengthening of the substructure

will only be required when the required profile is more than the existing

profile. This is subject to sound condition of the substructures

5. CHECKING ADEQUACY OF THE BRIDGES FOR MODERN

LOCOMOTIVES HAVING HIGHER TRACTIVE EFFORTS (TE) EVEN

WITHOUT CONVERTING TO MBG STANDARD

Till now, we have discussed the modalities required in upgradation of

old standard bridges to MBG standard. Actually, loading standard defines

the maximum loading which is likely to come based on which civil engineering

structures are deigned. Furthermore, future rolling stocks are designed in

such a way to keep its loading profile well within the envelop of that particular

loading standard.

In upgradation of the bridges, it is possible that the same may take

time. Now, we will discuss the action required in checking adequacy of the

bridges of modern locomotives having higher Tractive Efforts (TE)

Process start with the Speed Certificate issued by RDSO. RDSO

issue the Speed Certificates for standard span only with the condition that

condition of the bridge is sound. In addition to standard spans, there might

be so many non-standard spans existing on a particular Railways, whose

adequacy also need to be ascertained for safe operation by the concerned

Zonal Railways. Furthermore, process of checking for any new Locomotive/

Freight Stock is based on the EUDL generated by them and comparing the

same with that particular standard of loading for which the bridge was

designed. It is the most conservative way and inadequate in many aspects.

But, sitting in RDSO and without having appropriate field data, it is not

possible for RDSO to do better. Actual practice should be to work out the

216 217


actual Tractive Efforts required for operation of that particular load on particular

section. It is possible that bridges of BGML and RBG standard might be

safe even before strengthening them to MBG standard. Here, we are frequently

talking about MBG loading. This is so, since, it is the loading standard

which is enveloping almost all rolling stocks except a few aberrations.

As already briefed, firstly, one should go through the Speed Certificate

issued by RDSO to ascertain the restricted spans, if any and then after,

check the availability of that particular span on the section under

consideration. In case any span has been restricted and that is available on

the section under consideration, one has to calculate the actual Tractive

Effort required to haul the desired load on that particular section. Such exercise

is very much section dependent since gradients and curves demand extra

Tractive Efforts while negotiating them. Furthermore, locomotives are designed

with extreme upper limit requirement of Tractive Effort, so that the same can

work in most of the situations. It is further worthwhile to point out that

locomotive exerts that much Tractive Effort only that is required in haulage

of particular load in particular situation. Exertion of extra Tractive Effort may

cause slippage to the wheels and thus, may not be operative. To understand

the modalities, let us proceed step by step, starting from locomotive and

freight stock characteristics.

5.1 LOAD PARTICULARS OF LOCOMOTIVES

Load particulars of various locomotives are given in Table No. – 4

BRIDGE RULES permit operation of Coupled Locomotives. From the

above Table, we see that in case of Coupled Locomotives; total Tractive

Efforts generated by the locomotives in many cases will become more

than the designed value of Tractive Efforts taken in BGML and RBG

loading standards. Normally, worry to the Bridge Engineer from this

consideration. We will see in the subsequent paras about the reality

of such worry.

5.2 LOAD PARTICULARS OF FREIGHT AND COACHING STOCKS

Load particulars of freight and coaching stocks, which are plying on

Indian Railways, is given in Table No. – 5

* Often, Mechanical and Operating Departments ask the Engineering

Department that when BOBs and BOYs with 22.9t of axle loads are

plying since long on Indian Railways, why not the same axle load of

other stocks, particularly BOXN is being permitted? In this regard, it

is worthwhile to point out that in addition to axle load, it is equally

important to know the TLD that rolling stock is producing. From the

above table, it is clear that although BOBs and BOYs are having

22.9t of axle loads but, its TLDs are 7.9 and 7.68 t/m respectively i.e.

closer to the values prescribed for BGML and RBG loading. Even

permitting CC+10 for BOXN wagons having axle load of 22.82 t/m,

i.e. slightly less than that of BOBs and BOY, but it generates the TLD

of 8.52 t/m which is not even more than that of BOBs and BOYs but,

even more than the prescribed TLD of 8.25 t/m of MBG loading

standard.

218 219


5.3 FORMULA TO BE USED FOR CALCULATING TRACTIVE EFFORT

REQUIRED FOR HAULING A GIVEN LOAD

Tractive Effort (TE) required for hauling a load “T” tonnes on one in “G”

grade and “S” degree curve is give by:

TE (kg) = T1 + T

2 + T

3 + T

4

Where

T1 = R

V x Train load gives Train Resistance in kg

RV is running resistance of stock in kg/t

At start RV is taken as 4 kg/t for BOXN Wagon

For loaded BOXN in running condition,

RV = 0.6438797 + 0.01047218 V + 0.00007323 V2

For Empty BOX N in running condition,

RV = 1.333973 + 0.21983 V + 0.000242 V2

T2 = R

1 x Loco Weight gives locomotive resistance in kg

R1 is specific resistance of locomotive in kg/t

At start R1 taken as 6 kg/t

In running condition,

R1

= 0.647 + (13.17 / W) + 0.00933 V + (0.057 / WN) V2

Where

N is number of Axles.

W is axle load of the locomotive in tonnes

V is speed in Kmph

T3 is Grade Resistance for train and loco in kg.

T3

= (1 / G) x 1000 x (Train load in tonnes + loco wt. in tonnes).

T4 is curvature resistance for train and loco in kg.

T4= 0.4 x S Degree of curvature X (Train load in tonnes + loco

wt. in tonnes)

Based on the above formula, it was tried to calculate the total Tractive

Efforts required for different load combinations and for different degrees

of curvatures of the curves and different gradients. The same are

summarised in Table No. - 6

220 221


Similar exercise can be done with other locomotives as well. Difference

is not likely to be much since weight of the locomotives is the only

variable.

From Table No. – 6, it is clear that in most of the cases, it is likely that

starting Tractive Efforts are not very much in most of the cases. Here,

the Author tries to say that it is not an appropriate decision to go by

the capability of a particular locomotive regarding its Tractive Efforts.

Actual thing that matters is the requirement of Tractive Efforts for a

particular load on a particular Section having particular steepest

gradient and particular sharpest degree of curvature. After working out

the Tractive Effort requirement, the same should be compared with

the designed longitudinal force of that particular bridge to ascertain

whether the bridge is safe for that particular Tractive Effort or not?

5.4 FURTHER COMMENTS ABOUT THE ACTUAL TRACTIVE EFFORTS

REQUIRED FOR HAULING OF THE LOAD

On the advice of Railway Board, one exercise was done on Varanasi-

Aurihar Section of Varanasi Division of NE Railway in association with

Electrical, Mechanical and Operating Departments of NE Railway and

RDSO. That particular Section was chosen since the same was

having steepest gradient where operation of WDG-2 Locomotive was

planned for operation but the bridges were not coming safe with respect

to its designed longitudinal forces, when compared with the Tractive

Effort capability of WDG-2 locomotive. In the exercise, many

shortcomings crept in on account of some fellow Departments and as

such, actual conclusion could not be drawn. However, one finding,

which B&S Directorate of RDSO noticed that during the operation, it

has been tried not to accelerate the train to the extent so that it may

pick up that speed after covering the prescribed distance, as given in

Working Time Table. Not speeding up means not providing proper

acceleration to achieve a particular speed over a particular stretch.

From running point of view, it is necessary. This is as per requirement

of the Working Time Table of any particular Division of any railway.

However, this force can be worked out as suggested below:

Let v be the speed to be achieved after traveling a distance s,

then, as per Newton’s Law,

v2 = u2 + 2 a s

When the locomotive starts from rest, u = 0

Then, the equation modifies to v2 = 2 a s

Since v and s are known, we can calculate acceleration

required i.e. ‘a’

After knowing the required acceleration, force (TE) can be worked

with the following formula:

F(which actually in this case is TE) = ma

This force (TE) can be added to the TE required for hauling the

train at a particular speed to know the total TE required for

haulage of the proposed train on the proposed section having

its own gradients and curves .

If the total TE is less than the designed longitudinal force of the bridge,

then, it is safe; otherwise, its strengthening/rebuilding will be required

as the case may be.

CONCLUSIONS

It is not necessary to re-build each and every bridge to make it fit for

MBG standard. Even old bridges conforming to BGML and RBG standards

can be upgraded to MBG standard with the small input as suggested in the

paper.

In case any span is restricted for operation of any particular rolling

stock, actual Tractive Efforts required on that particular Section should be

worked out. If the same is less than the designed longitudinal force of that

particular bridge, then even without any strengthening input, that particular

rolling stock can safely be permitted for its operation.

REFERENCES

1. RDSO letter No. CBS/PBR/RLS dated 05.08.1988 regarding ‘MBG

Loading – 1987 – Position of Designs and Drawings of Bridges’

circulated to all the Zonal Railways.

2. RDSO letter No. CBS/PBR/RLS dated 24.04.1989 regarding

‘Guidelines for checking and strengthening of bridges for MBG loading

of 1987’ circulated to all the Zonal Railways.

3. RDSO letter No. CBS/DOW dated 28.10.1993 regarding ‘Suitability of

BGML Fixed End Bearings of Open Web Girders for MBG Loading –

1987 for Spans 30.5m (Through Type) and 45.7m up to Seismic Zone

– III, circulated to all the Zonal Railways.

4. RDSO letter No.CBS/DOW dated 31.08.1994 regarding ‘Suitability of

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BGML Fixed End Bearings of Open Web Girders for MBG Loading of

1987’ circulated to all the Zonal Railways.

5. RDSO letter No. CT/DG/LW/HAW dated 21.03.1998 addressed to

CRB regarding ‘Higher Axle Load Wagons’ with a copy to all the Zonal

Railways.

6. RDSO letter No. CBS/DPA dated 09.11.1998 regarding ‘Guidelines for

Checking Suitability of Substructures of Existing Bridges for MBG

Loading – 1987’ circulated to all the Zonal Railways.

7. RDSO letter No. CBS/Golden/Q/Strength dated 30.12.29\004 regarding

‘Strengthening of Golden Quadrilaterals, its Diagonals and other

identified Routes for 100/75 kmph Goods Trains Operation’ circulated

to all the Zonal Railways. This letter contains RDSO letters mentioned

from Sl.1 to Sl.6 as a part of Annexure.

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Volume - I Volume - I226 114

END

Volume - i - Final

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