Jackfield Bridge
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Corus Construction Centre
Bridged in steel
J ackfield bridge
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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
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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.
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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
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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
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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.
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