Jackfield Bridge

7/29/2019 Jackfield Bridge

http://slidepdf.com/reader/full/jackfield-bridge 1/7

Corus Construction Centre

Bridged in steel

J ackfield bridge



J ackfield Bridge 32 J ackfield Bridge

Iron Bridge

This illustrious structure, the first

bridge to be constructed of I ron,

was built as a toll bridge by Thomas

Pritchard and Abraham Darby III in

1779, to serve the extensive

industrial development on both

sides of the valley. It influences the

surrounding area through its

designation as a scheduled Ancient

Monument and consequently

attracts large numbers of visitors

throughout the year. It is recognised

as the forerunner to all modern steel

bridges, although it used techniques

more akin to a wooden structure.

New Free Bridge

In the early 1900’s the local Mayor,

Councillor Maddox, raised funds to

build a toll-free alternative over the

Severn at J ackfield as a rival to the

famous Iron Bridge, then 130 years

old.

In 1909, the alternative was erected

less than one kilometre downstream

of the Iron Bridge at a cost of £1600

and was opened to traffic on 26

J une of that year. It was of

reinforced concrete construction

and known as the Free Bridge in

view of the fact that it was toll-free.

The load testing of the structure

was carried out by a 14 ton steam

roller.

The bridge itself earned recognition

as an important structure and was

eventually designated a Grade II

listed structure of architectural and

historic interest in 1985. The narrow

single carriageway three span open

spandrel arch had a central span of

80 feet and side spans of 56 feet. It

was an early example of reinforced

concrete, designed by L G Mouchel

and Partners and constructed in six

months by the Liverpool

Hennebique Company.

Decaying concrete and rusting steel

reinforcement was first noticed on

the structure in 1937 and heralded a

programme of extensive repairs

throughout the next 50 years initially

mainly due to carbonation of the

concrete and latterly also due to

chloride attack from de-ic ing salts.

A 12 ton weight limit was imposed

upon the structure, reducing to 10

tons in 1969 following major repairs

to one of the spandrel columns.

Further decay and damage was

identified and a survey in 1985

revealed that the bridge had

become seriously under strength to

cope with even the lightest modern

car traffic.

In the interest of public safety a 3

ton limit was imposed in April 1986,

with traffic being restricted to a

single lane, controlled by traffic

signals.

A structural assessment carried out

in the same year concluded that the

bridge was beyond repair to carry

vehicles. The C ounty Council

immediately erected a temporary

steel bridge to relieve the old bridge

of all traffic loading.

Client:

Shropshire County Council

Designer:

Gifford and Partners

Architect:

Percy Thomas Partnership

Main Contractor:

Alfred McAlpine Construction Ltd.

Steel Sub-Contractor:

Westbury Tubular Structures

Fabrication commenced:

October 1993

Fabrication completed:

J une 1994

Bridge open to traffic:

August 1994

Official opening:

18th October 1994

J ackfield BridgeSteel tonnage:

190 tonnes

Steel quality:

BS EN 10 025-grade FE 510D

Surface treatment:

Blast clean to 1st quality

Aluminium metal spray 100µ

1st Coat Aluminium epoxy sealer

2nd Coat 2. Pack high build epoxy MIO, 125µ

3rd Coat Recoatable polyurethane finish 50µ

4th Coat 2. Pack polyurethane finish 50µ,

colour - metallic silver

Design load:

Full HA loading - Special environmental

weight limit of 17T imposed

Design Code:

BS5400

Site history

Site historySome 8km south of Telford, the fast flowing watersof the River Severn pass through the picturesqueIronbridge Gorge and beneath the famous 18thCentury Iron Bridge itself. The area is designatedas a World Heritage Site.

Facts and figures



4 J ackfield Bridge

Conception Conception

The brief recognised the unique

location and demanded a bridge of

special quality and required wide

consultation to establish the

preferred type and form the

structure should take.Particular

attention was to be given to the

question as to whether it should be

a bold and visually striking

structure, or an unimposing and

purely functional one.

Location

Traffic movements from Broseley to

Telford relied heavily on the old FreeBridge, and with increasing tourism

due to the growing popularity of the

World Heritage Site it was therefore

essential that a replacement bridge

be built close to the existing one as

the only alternative route for heavy

vehicles involved a detour of 14

miles. After much consultation, the

construction of a new structure was

proposed between Iron Bridge and

J ackfield, some 400m downstream

of the former.

The design put forward met the

practical requirements of the

crossing, including the easing of

access from the approach roads by

eliminating a notorious hairpin bend

on the south side. The existing

concrete bridge would have been

retained and converted into apedestrian crossing following

restoration. However in 1990,

following a Public Inquiry, the

Secretary of State for the

Environment rejected the proposal

on the grounds that it was felt to be

too close to the Iron Bridge, mainly

because it was judged that the sight

and sound of traffic cross ing the

new bridge would damage the

setting of the Historic Monument.

The pressing need for a new bridge

was recognised and following

discussions with the Royal Fine Art

Commission and English Heritage,

the Secretary of State in 1993

granted listed building consent for

the demolition of the old Free

Bridge and for a new structure to be

built on the site of the old bridge.With the location of the proposed

structure agreed, attention was

switched to selecting the right type

of crossing to suit the environment.

Site constraints on bridge

form and design

Practical considerations helped

dictate the choice of structure, with

visual implications also being of

great importance considering the

sensitive location of the structure.

The sides of the Severn Gorge are

unstable due to the cutting action of

the river creating oversteep side

slopes and evidence can be seen of

landslips and damage to buildings

caused by subsidence. The Iron

Bridge itself has a history of

remedial measures and repairs

throughout its life, the most recent

being the installation of a strut

under the river in 1974 to prevent

the lateral movements pushing the

abutments together.

The old Free Bridge was in one of

the more stable parts of the Gorge,

but nevertheless ground treatment

was decided upon prior to

commencement of the new

structure. Injection grouting of coal

seams in the underlying strata was

carried out to fill any voids and

fissures which might influence local

stability and construction of the

bored pile foundations.

The River Severn is also notoriously

prone to flooding and combined

with the turbulent nature of flow,

imposed two constraints upon the

design which greatly influenced the

choice of structure. No

intermediate piers were to be

erected within the water and the

deck was to be pos itioned well

above flood level. A solution to the

flood level was complicated further

by the requirement that the low

levels of the approach roads were to

be retained. A structure was

therefore required that had its feet

out of the water, except in extreme

flood conditions, had minimum deck

depth and could be erected from

the embankments.

Another practical consideration was

that materials should be delivered to

site in small sections as the narrow

winding roads, such a notable

feature of the gorge, would prevent

large sections being moved.

The status of the surrounding area

influenced the final decision.

Residing within the gorge is an

interesting sequence of bridges.

The most impressive being the Iron

Bridge with its pioneering status.

However, other structures on the

site also hold significant roles,

namely the old Free Bridge at

J ackfield which used the then newly

discovered reinforced concrete

technique, the delicate tracery of

cast iron at Coalport Bridge

replaced a timber structure lost in

the 1795 flood and the medieval

stone bridge at Buildwas which was

also lost in the same flood and

replaced by a cast iron bridge

designed by Telford.

It was considered that the

innovative tradition of bridge

building in the gorge should be

continued and that the vigorous,

challenging approach of the original

iron masters be matched.

Additionally, it was agreed that the

new bridge should provide a striking

counterpoint to the older gorge

architecture.

The combination of considerable

practical implications and strong

visual requirements made it clear

that a high quality design was

required.

Conception of J ackfield Bridge

Gifford and Partners were commissioned in July1986 to carry out a study into the provision of areplacement bridge.

J ackfield Bridge 5



Design solutionDesign solution

J ackfield Bridge 76 J ackfield Bridge

corrosion protection led to the

tubes being filled with concrete to

form composite members.

The tower leg cross-member is

required to resist lateral forces and

in-plan twisting due to eccentric

loading on the deck. Several

different forms of cross member

were considered. The circular ring

of steel box construction was

chosen as having the best overall

visual and structural performance

characteristics for the multitude of

different force combinations

imposed on it. It also echoes the

use of circular forms which appear

on both the Iron Bridge and theCoalport Bridge.

Design solutionIn order to comply with the strict requirementsand limiting constraints previously described,the proposed solution took the form of anasymmetrical cable stayed structure with aslender deck supported by a single pylon withits base out of the water.

It was accepted that this solution

was the most practical and elegant

to relate with the environment of the

gorge.

The bridge has a single overall span

of 57.6m which is supported

intermediately by the cable stays.

The overall width is 11.6m in order

to accommodate the rural all

purpose single carriageway.

Considering the constraints on

access, erection and appearance, a

steel solution was chosen which

met the strict needs of the site.

The deck structure comprises two700mm x 450mm longitudinal steel

edge plate girders with composite

transverse beams at 2.4m centres,

each having two lines of shear

studs. The 200mm reinforced

concrete deck slab was cast in s itu

using permanent precast formwork

planks. The twin legged tower is

inclined and formed from tubular

steel filled with concrete and linked

at the top cable anchorages by a

fabricated rectangular steel box

section of circular form. The tower

supports the deck via 8 locked co il

rope cables of 96mm diameter, with

a further 8 cables anchoring it to the

abutment.

Design parameters were agreed at

an early stage, since they would

have a major influence on the

appearance of the bridge. Design

for abnormal vehicles being

excluded enabled a reduction in

member sizes. Allowance was made

within the design for accidental

severance of any one cable, and for

cable replacement without full

closure of the bridge.

The detailed design included two

and three-dimensional computer

analysis of the structure, and local

finite element analyses of highly

stressed cable anchorages and

bearing zones.

The minimum deck cross-section

depth was achieved by placing the

main longitudinal beams at the

outside edge and designing them

to carry the entire global

longitudinal compression and

bending, such that the deck slab

could be at the same level as the

beam top flange. The concrete deck

slab was designed simply to span

longitudinally between the

transverse steel cross girders. This

arrangement allowed the deck

parapet railings to be inside the

longitudinal beams and helped to

maintain headroom to the inward

inclined cables by placing their

attachments to the deck well

outside the parapets. The deck

depth was further minimised by

designing the footway drainage as a

separate system and by having the

footway crossfall away from thecarriageway.

The longitudinal compression from

the deck, carried by the main steel

girders, is restrained at the main

southern abutment by two thrust

bearings designed as pin joints,

each having a capacity of 4MN.

The tower was initially envisaged as

hollow steel tube but constraints on

design for resistance to impact from

flood-borne debris and internal

6 J ackfield Bridge



Demolition of the existing bridge

commenced, revealing some

surprising facts in the process.

During the structure’s life several

weight restrictions had beenimposed, culminating in the erection

of a temporary steel bridge in order

to relieve all loading. However, as

the structure was demolished, it

became clear just how weak the

bridge was. Instead of the planned

programme of demolition utilising

explosives and divers, the piles for

the bridge were simply pulled out by

a crane, revealing a length of a mere

3 metres. Unique screw threaded

reinforcement turnbuckle couplers

were discovered on the main

reinforcing bars in the arch ribs,

revealing their use several years

earlier than had previously been

known.

Demolition was carried out by

conventional ball-and-chain

techniques with a barge mounted

crane in the river which also served

to collect debris and prevent it

falling into the river.

Foundations

Following demolition, construction

of a braced sheet-pile cofferdam on

the south bank enabled the

contractor to dig down some 8m to

the cut-off level of the 42 No. 1.2m

diameter bored piles which had

been sunk 12m through two seamsof coal beneath the river. The piles

were then capped and a cellular

abutment of reinforced concrete

and ballast built up. This abutment

supports the entire weight of the

bridge which is transferred through

the pylon and cables. The North

abutment is principally a bored pile

retaining wall.

Fabrication

The cable stay pylon, deck and

parapets were prefabricated 180

miles away by Westbury Tubular

Structures. This involved very

intricate welding, especially at the

pylon cable anchorages where

internal strengthening stiffeners

were placed to transfer the cable

loads through the pylon legs.

For the top anchorage zone the

900mm diameter tubes were sliced

longitudinally into several segments

to enable welding of the internal

stiffeners and subsequently

reassembled into their circular

shape. The deck and abutment

anchorages also involved complex

geometry and careful planning of

the sequence of welding.

Many aspects of the design soughtto harmonise the various parts,

namely the pylon, the cables,

the webs on the longitudinal beams,

the parapets and the cable

anchorages which all have inward

sloping geometry and added to the

challenges for the fabricators.

At the feet of pylon legs

concentration of load onto the 450

diameter bearing required special

steel castings.

Because of access and

transportation limitation the tower

was delivered to s ite in 4 sections,

laid on its back across the

abutment and welded together prior

to being rotated and hoisted into

position. The 200 tonne crane

required to hoist the tower was

itself so large that it too was

delivered to site in sections and

took 2 days to erect. The 67m jib

needed 4 separate low loaders to

carry it, and an 80 tonne crane to

lift it.

ConstructionConstruction commenced with the erection of atemporary bridge alongside the existing structureenabling traffic to cross the River Severn relativelyunhindered during the contract period, andproviding a route for existing services.

8 J ackfield Bridge J ackfield Bridge 9

Construction

Erection

The planned erection sequence for

the 70 tonne tower included use of

a winch located on the north side of

the river to assist with the lift. In a

carefully planned operation,

achieved in one day, the tower legs

were pivoted on temporary steel

trestling on top of the south

abutment and hoisted on to a

temporary cradle, then slowly

jacked down to their permanent

location upon stainless steel

bearings.

The first pair of cable stays

anchored the tower to the

abutment, after which the crane

could be released. Then the first 11

metre lengths of main steel deck

beams were lifted onto temporary

supports f rom the river bank while

the cross beams were fixed into

place and the first pair of forward

cables installed to take the weight

of the steelwork. Following this the

tower legs were filled with concrete

pumped through removable

capping plates at the top. As the

subsequent sections of deck

steelwork were lifted into place,

first the 9.6m longitudinal beam

sections then the crossbeams, they

were anchored back to the tower by

the next set of stay cables. This

sequence was repeated until the

steel grillage reached the north

abutment.

Upon completion of the steel work

the lower thread anchorages were

adjusted accordingly in order to

give the proper alignment. Only

then were the HSFG bolted splices

fully tensioned.

This procedure was followed to

prevent any locking in of stresses.

Permanent formwork was placed to

receive reinforcement and the deck

concrete which was poured in bays

in a sequence designed to minimise

locked-in shrinkage stresses.

Once waterproofing and surfacing

had been applied the stays were

adjusted again to give the correct

geometry and then checked against

calculated cable loads. Following

this, the final paint coat was

applied to all the steelwork. Traffic

and services were switched to the

permanent structure, thus allowing

the temporary bridge to be

removed.

Construction



The finished bridge

The contract had taken 16 months

to complete at a cost of £1.8m.

Despite the complex geometrical

challenges the prefabricated

steelwork had fitted at the first

attempt, with bolted splices

deliberately chosen in favour of

welding to express visually how the

deck had been erected. The County

Council is naturally proud of the

achievement which on completion

prompted the Royal Fine Art

Commission to comment that it is

“worthy of the Ironbridge Gorge”

and “an overwhelming success”.

The Iron Bridge has a striking new

neighbour which is attracting

considerable interest and

complements the historic sequence

of bridges in the Gorge.

10 J ackfield Bridge

The finished bridgeDuring the opening ceremony the load test firstcarried out on the original Free Bridge wasduplicated with a 14 tonne steam roller.



www.corusgroup.com

Designed and produced by

Orchard Corporate Ltd.

Care has been taken to ensure that the contents of

this publication are accurate, but Corus UK

Limited and its subsidiary companies do notaccept responsibility for errors or for information

which is found to be misleading. Suggestions for

or descriptions of the end use or application of

products or methods of working are for

information only and Corus UK Limited and its

subsidiaries accept no liability in respect thereof.

Before using products supplied or manufactured

by Corus UK Limited and its subsidiaries the

customer should satisfy himself of their suitability.

Copyright 2000

Corus

CC&I:J B02:2000:UK:09/2000

Corus Construction Centre

PO Box 1

Brigg Road

Scunthorpe

North Lincolnshire

DN16 1BP

Tel: +44 (0) 1724 405060

Fax: +44 (0) 1724 404224

email: [email protected]

website: www.corusconstruction.com

Jackfield Bridge

Documents

Transcript of Jackfield Bridge