Department of Construction Engineering JADAVPUR UNIVERSITY

Block – LB, Plot – 8, Sector – III, Salt Lake

Kolkata – 700 098, W.B.Ph. No. +91-98300 27204 / 033-2335 5211 18.03.14

6.0 Past Performance Verification of Tallah Tank

Performance of the structure has been satisfactory till now. The structure has been exposed to various actions which are categorized as below.

6.1 Actions on Structure

6.1 Permanent Actions6.1.1 Dead Load : The dead load or self load of the structure and material on the structures comprise pre-cast concrete roof slab, tank-wall plate, wooden planks & purlin members.

6.2 Variable Actions6.2.1 Live Load : Water load upto full depth of 4.7m is considered as live load as a lumped mass on top. This load varies with the water level in the tank.

6.2.2 Wind Load : The basic wind speed for Kolkata is 50m/sec and the tank is is 110 feet (33.33m ) height. The wind load history of Kolkata Is shown here and the Tank structure is qualitatively verified for past load history and structural response.

Bengal cyclone, Calcutta 1942 SevereDeaths 40,000

Bengal cyclone, Calcutta May’ 11th& June’ 1st 1965 Severe

Bengal cyclone, Calcutta 5th December’ 1973 Severe

Bhola cyclone routed through Kolkata 8th November’ 1970 Moderate

Aila cyclone 25th May’ 2009 Severe; wind speed 116km/hr

A total of 31 severe cyclones were reported from the Indian Sundarbans during 1961-2000 of which at least in 5 cases, the

wind speed exceeded 200km/hr. This affected the city of Kolkata in general and in particular the tallah tank structure with a

little lesser speed and impact than the coastal area

6.2.3 Earthquake: Available records indicating the major occurrences of earthquake in Kolkata over the last century are shown below.

CONDITION ASSESSESMENT TALLAH TANK

Record of Number of cyclonic storms in West

Bengal 1891-2006

6.3.3 Fatigue Action :

Due to the filing up and emptying of the tank twice on an average daily basis for the last 105 years i.e. 76650 times , there is a fatigue action on the structure due to long cyclic loading action over time.

6.3.4 Corrosion Fatigue:

Due to the very humid climate of Kolkata, along with the regular emptying and filling of tank as described above, it has resulted in corrosion of the steel members of the Tallah tank which has caused distress to some local members.

MAJOR

MEDIUM

MINOR

Historical Graph of Seismic activity in Kolkata

6.3.2 Earthquake Load contd.

CONDITION ASSESSESMENT TALLAH TANK

Absence of major earthquakeActivity between 1910-2006

Srimangal

Earthquake

Calcutta

8th July’ 1918 Ominous cracks appeared in many new and old buildings of

Calcutta

Bihar-Nepal

Earthquake

15th January’ 1934 at 2-

40pm in Calcutta lasted

for 5min.

Considerable movement in lake water, Damage in St. Paul's

Cathedral and other buildings

Calcutta

Earthquake

15th April’ 1964 Was more damaging with the development of serious

gapping cracks and fall of plaster in many old and new

buildings. Almost all the people heard sounds like distant

thunder preceding the shaking and a N-S movement was

reported.

Sikkim

Earthquake

18th September 2011 Felt across east and northeast india. Damage and loss of life

reported in West Bengal. Tremors felt in Kolkata.

Available records indicate that Calcutta and its surroundings experienced about 30 strongly felt earthquakes

during the last 350 years. While many of these events are of ‘far source’ a few are of ‘near source’ origin.

6.3.5 Environmental Action :

a) Temperature and Ultraviolet Radiation : Kolkata is classified as tropical climate . While The temperature does not affect the degradation of steel directly, But it is an important indirect factor in the corrosion of the tank. Prolonged exposure to Ultraviolet radiation causes deterioration of the external finishes to steel, thus making it more susceptible to environmental attack.

CONDITION ASSESSESMENT TALLAH TANK

(b)Relative Humidity : Kolkata is in a high humidity zone and experiences rainfall for 4 months in a year. This has caused corrosion of steel members of the structure over the years. Maximum and minimum Relative humidity in Kolkata area over one typical year is given below. As is evident from the graph, maximum relative humidity is greater than 90 % throughout the year.

CONDITION ASSESSESMENT TALLAH TANK

c) Atmospheric Pollution : The presence of atmospheric pollutants such as SO2, CO2, nitrogen oxides and suspended particulate matter in the air causes the deterioration of steel structure such as Tallah tank. Over time, the vehicular load on the Barrackpore Trunk road has increased exponentially causing more pollution and contributing to the corrosion of Tallah tank.

d) Lack of Tree Cover : Over time, the spread of the metropolis in all directions has led to a reduction in the tree cover and added to the pollution load on the structure.

e) Stagnation of Water : The water leaking from the underside of the tanks, along with inadequate drainage provisions have led to water stagnating at the base of the columns constantly. This may be detrimental to the health of the structure.

Stagnation of Water at Base Of Columns

CONDITION ASSESSESMENT TALLAH TANK

Extreme Event history of Tallah Tank

The structure was built at a time when there was a lack of proper standards. The concept of wind load was known, but not properly.The performance of any structure is dependent on the actions it is subject to in its design life and beyond. Please refer section 6.0 for more load details. A list of the extreme events acted on the Tallah tank till date is listed as follows

Sr.No

Event Year Category

1 Srimangal Earthquake 1918 severe

2 Bihar Nepal Earthquake 1934 severe

3 Bengal Cyclone 1942 Severe

4 Calcutta Earthquake 1964 Severe

5 Bengal Cyclone 1965 Severe

6 Cyclone Aila 2009 Severe

7 Sikkim Earthquake 2011 Moderate

CONDITION ASSESSESMENT TALLAH TANK

7.1.1 Structural Features of the Century old Tallah Tank

a) Material of Construction :The structure of the Tallah tank comprises mainly of low carbon (mild) steel resting on ferroconcrete foundations. Despite being situated in a high humidity area i.e. Kolkata, the mild steel has not shown any significant loss of metal even after 105 years of continued service.

b) Foundation : The foundation system is robust and comprises of grillage type foundation arrangement encased with concrete, finally supported on soil stabilized by Sal bullahs. These foundations show no signs of distress.

c) Structural Arrangement of Supporting trestles : The structure of Tallah tank is of a robust nature. The tank is supported by 49 nos. 4 legged trestles, 14 nos 6 legged trestles, 1 no 9 legged trestle & 1 no 7 legged trestle supporting the riser pipe. The original connections are mostly riveted though bolted connections were observed at some places.

d) Tank Proper : The overall tank dimensions are 96m x 96m x 6m height. The roof is overlaid with Precast concrete slabs which replaced the steel plate roof. Bottom plate is also simple plate. The tank bottom is rstiffened by series of steel angles to prevent local failure.

e) Riveted Joints : The entire structure is erected using rivets, though some bolted connections were also observed. Some of the rivets are flawed in the form of distorted head, loosened head and missing rivets.

Despite the intensity of these events, the Tallah tank has executed the desired level of performance without apparent structural distress. Hence, it may be said that the Ultimate limit state of the structure was not violated over this period of time.

However, The structure has undergone some visible deformations in the side plate portion. Leakages have been observed at bottom corner regions. Some Riveted connections are distorted, loosened or missing. Some deformations were observed in supporting beams. The serviceability limit state has therefore been violated.

7.1 Resistance Model of Tallah Tank

CONDITION ASSESSESMENT TALLAH TANK

7.1.1

f) Timber Sleepers on Steel Purlins : The tank is freely supported on timber sleepers arranged above purlin beams at the base of tank. The purlin beams section is 375X150 as measured on site.

The Tallah tank was envisaged as an experimental design due to the following reasons:a. Structural steel was a relatively new engineering material at that time and full

knowledge of its properties was not available.b. Lack of adequate knowledge of various combinations of loads acting

concurrently on the structure. c. Lack of standardisation in the method of steel production leading to variability

in the response to loading combinations.

However, over the 105 years of its operations, the Tallah tank has proved itself as a successful experiment in structural design

by withstanding all the adverse load combinations over time.

7.1 Assessment of Serviceabilityd. The structure has undergone some visible deformations in the side plate

portion.e. Leakages have been observed at bottom corner regions.f. Some Riveted connections are distorted, loosened or missing.g. Some deformations were observed in supporting beams.

7.2 Structural System Evaluation

However, overall assessment of the structure reveals some flaws which are due to both prolonged use over time as well as design deficiencies, and they include

h. The bracing system and trestles for the staging portion of the tank is weak against horizontal loading, such as wind and earthquakes.

i. Since the trestles are supporting the tank individually and not integrally with the other trestles, the structural integrity of this system is low especially when exposed to horizontal forces.

j. The portal effect in orthogonal vertical planes is not achieved as the trestles are acting individually.

k. The tank roof and bottom plate is weak under horizontal loads such as wind and seismic loads due to its inherent weakness along the plane.

Agree on brief with client, study documentation recd. From client, carry out initial

inspection

Are any parts visibly

in distress

?

Simple calculations of suspect parts, using

estimated or actual loading

Are suspect

parts safe?

Take preventive action before further appraisal

Check again at site if necessary

Do site observations confirm

paper evidence?

Can other

documents be

found?

Study further documents

Yes No

Yes

Carry out Further survey

No

Is information

now adequate

for assessment

?

No

Does “New use” impose

heavier loads than designed

for

Uncover Samples of hithertho Hidden details

Are all structural details in

good condition?

Is robustness established

?

Are defects easy to mend?

Yes

Propose repairs

YesYes

No

Structure adequate for unchanged use, subject to

repairs

No Yes

Consider the need to carry out full assessment

Yes

Flowchart of assessment of existing Structures

Is it a mechanism as defined ?

Will removal of single critical

elements endanger

robustness of whole

structure or parts?

Define the Structure

Can the structure be redefined?

Has it collapsed?

Yes No

Yes

Consider further support

Investigate other

possible causes of collapse

Assess forces on the structure as a whole and

check stability

Is robustness adequate ?

Can safety factors be reconsider

ed?

Yes

No

Can critical elements

resist accidental

forces

Yes

Protect or strengthen

critical elements

No

No

Assess actual loads on each element and (in situ) testing

of materials

Yes

Conventional check calculation

Check Satified?

Do in situ conditions satisfy all assumptio

ns?

YesElemen

t is adequa

te

Is deficiency of the element drastic?

Are real conditio

ns as bad as

assumed?

No

Can element shed load to other ?

Can other

elements carry their

share?

Element is

inadequat

Yes

Reasses strengths, sizes or assumptions

Yes Yes No

Yes

NoWill Early yield of element affect

robustness of other parts of

structureRecheck robustness of

structure or parts

Robustness still

adequate?

Robustness is

adequate

No

Yes

Are past assumption

s too conservativ

e?

Go to : Define the structure

Robustness is

inadequate

Recheck Element

No

No

Yes

Yes

NoNo

Yes

No

Is Mathematic

al model too crude, as so far defined

Refine Mathematical Model, reconsider

assumptionsmake accurate analysis of redefined

structure

Make accurate analysis of elements and sections

Is Icalculated

capacity adequate?

Have material

strengths been

checked in situ

Make accurate analysis of elements and sections

Are loads or

strengths known more

accurately than at design?

Reconsider safety factors where appropriate

Final ULS analysis

Is clculated capacity

adequate?

Reject, test load or continually monitor element

where appropriate

Element (s) are adequat

e

Yes

No

Yes

Yes

No

No

YesNo

Yes

No