Corus Construction Centre

Jackfield bridge

Bridged in steel Site history

Facts and figures Site history Some 8km south of , the fast flowing waters of the pass through the picturesque Gorge and beneath the famous 18th Bridge Century Iron Bridge itself. The area is designated

Client: Steel tonnage: as a World Heritage Site. County Council 190 tonnes Designer: Steel quality: Gifford and Partners BS EN 10 025-grade FE 510D Iron Bridge view of the fact that it was toll-free. identified and a survey in 1985 Architect: Surface treatment: This illustrious structure, the first The load testing of the structure revealed that the bridge had Percy Thomas Partnership Blast clean to 1st quality bridge to be constructed of Iron, was carried out by a 14 ton steam become seriously under strength to Main Contractor: Aluminium metal spray 100µ was built as a toll bridge by Thomas roller. cope with even the lightest modern Alfred McAlpine Construction Ltd. 1st Coat Aluminium epoxy sealer Pritchard and Abraham Darby III in car traffic. Steel Sub-Contractor: 2nd Coat 2. Pack high build epoxy MIO, 125µ 1779, to serve the extensive The bridge itself earned recognition Westbury Tubular Structures 3rd Coat Recoatable polyurethane finish 50µ industrial development on both as an important structure and was In the interest of public safety a 3 Fabrication commenced: 4th Coat 2. Pack polyurethane finish 50µ, sides of the valley. It influences the eventually designated a Grade II ton limit was imposed in April 1986, October 1993 colour - metallic silver surrounding area through its listed structure of architectural and with traffic being restricted to a Fabrication completed: Design load: designation as a scheduled Ancient historic interest in 1985. The narrow single lane, controlled by traffic June 1994 Full HA loading - Special environmental Monument and consequently single carriageway three span open signals. Bridge open to traffic: weight limit of 17T imposed attracts large numbers of visitors spandrel arch had a central span of August 1994 Design Code: throughout the year. It is recognised 80 feet and side spans of 56 feet. It A structural assessment carried out Official opening: BS5400 as the forerunner to all modern steel was an early example of reinforced in the same year concluded that the 18th October 1994 bridges, although it used techniques concrete, designed by L G Mouchel bridge was beyond repair to carry more akin to a wooden structure. and Partners and constructed in six vehicles. The County Council months by the Liverpool immediately erected a temporary New Free Bridge Hennebique Company. steel bridge to relieve the old bridge In the early 1900’s the local Mayor, of all traffic loading. Councillor Maddox, raised funds to Decaying concrete and rusting steel build a toll-free alternative over the reinforcement was first noticed on Severn at Jackfield as a rival to the the structure in 1937 and heralded a famous Iron Bridge, then 130 years programme of extensive repairs old. throughout the next 50 years initially mainly due to carbonation of the In 1909, the alternative was erected concrete and latterly also due to less than one kilometre downstream chloride attack from de-icing salts. of at a cost of £1600 A 12 ton weight limit was imposed and was opened to traffic on 26 upon the structure, reducing to 10 June of that year. It was of tons in 1969 following major repairs reinforced concrete construction to one of the spandrel columns. and known as the Free Bridge in Further decay and damage was

2 Jackfield Bridge Jackfield Bridge 3 Conception Conception

Site constraints on bridge The River Severn is also notoriously However, other structures on the form and design prone to flooding and combined site also hold significant roles, Practical considerations helped with the turbulent nature of flow, namely the old Free Bridge at dictate the choice of structure, with imposed two constraints upon the Jackfield which used the then newly visual implications also being of design which greatly influenced the discovered reinforced concrete great importance considering the choice of structure. No technique, the delicate tracery of sensitive location of the structure. intermediate piers were to be at Bridge erected within the water and the replaced a timber structure lost in The sides of the Severn Gorge are deck was to be positioned well the 1795 flood and the medieval unstable due to the cutting action of above flood level. A solution to the stone bridge at Buildwas which was the river creating oversteep side flood level was complicated further also lost in the same flood and slopes and evidence can be seen of by the requirement that the low replaced by a cast iron bridge landslips and damage to buildings levels of the approach roads were to designed by Telford. caused by subsidence. The Iron be retained. A structure was Conception of Jackfield Bridge Bridge itself has a history of therefore required that had its feet It was considered that the remedial measures and repairs out of the water, except in extreme innovative tradition of bridge Gifford and Partners were commissioned in July throughout its life, the most recent flood conditions, had minimum deck building in the gorge should be 1986 to carry out a study into the provision of a being the installation of a strut depth and could be erected from continued and that the vigorous, under the river in 1974 to prevent the embankments. challenging approach of the original replacement bridge. the lateral movements pushing the iron masters be matched. abutments together. Another practical consideration was Additionally, it was agreed that the that materials should be delivered to new bridge should provide a striking The old Free Bridge was in one of site in small sections as the narrow counterpoint to the older gorge The brief recognised the unique vehicles involved a detour of 14 too close to the Iron Bridge, mainly the more stable parts of the Gorge, winding roads, such a notable architecture. location and demanded a bridge of miles. After much consultation, the because it was judged that the sight but nevertheless ground treatment feature of the gorge, would prevent special quality and required wide construction of a new structure was and sound of traffic crossing the was decided upon prior to large sections being moved. The combination of considerable consultation to establish the proposed between Iron Bridge and new bridge would damage the commencement of the new practical implications and strong preferred type and form the Jackfield, some 400m downstream setting of the Historic Monument. structure. Injection grouting of coal The status of the surrounding area visual requirements made it clear structure should take.Particular of the former. seams in the underlying strata was influenced the final decision. that a high quality design was attention was to be given to the The pressing need for a new bridge carried out to fill any voids and Residing within the gorge is an required. question as to whether it should be The design put forward met the was recognised and following fissures which might influence local interesting sequence of bridges. a bold and visually striking practical requirements of the discussions with the Royal Fine Art stability and construction of the The most impressive being the Iron structure, or an unimposing and crossing, including the easing of Commission and English Heritage, bored pile foundations. Bridge with its pioneering status. purely functional one. access from the approach roads by the Secretary of State in 1993 eliminating a notorious hairpin bend granted listed building consent for Location on the south side. The existing the demolition of the old Free Traffic movements from to concrete bridge would have been Bridge and for a new structure to be Telford relied heavily on the old Free retained and converted into a built on the site of the old bridge. Bridge, and with increasing tourism pedestrian crossing following With the location of the proposed due to the growing popularity of the restoration. However in 1990, structure agreed, attention was World Heritage Site it was therefore following a Public Inquiry, the switched to selecting the right type essential that a replacement bridge Secretary of State for the of crossing to suit the environment. be built close to the existing one as Environment rejected the proposal the only alternative route for heavy on the grounds that it was felt to be

4 Jackfield Bridge Jackfield Bridge 5 Design solution Design solution

Design solution In order to comply with the strict requirements and limiting constraints previously described, the proposed solution took the form of an asymmetrical cable stayed structure with a slender deck supported by a single pylon with its base out of the water.

It was accepted that this solution section of circular form. The tower longitudinal compression and corrosion protection led to the was the most practical and elegant supports the deck via 8 locked coil bending, such that the deck slab tubes being filled with concrete to to relate with the environment of the rope cables of 96mm diameter, with could be at the same level as the form composite members. gorge. a further 8 cables anchoring it to the beam top flange. The concrete deck abutment. slab was designed simply to span The tower leg cross-member is The bridge has a single overall span longitudinally between the required to resist lateral forces and of 57.6m which is supported Design parameters were agreed at transverse steel cross girders. This in-plan twisting due to eccentric intermediately by the cable stays. an early stage, since they would arrangement allowed the deck loading on the deck. Several The overall width is 11.6m in order have a major influence on the parapet railings to be inside the different forms of cross member to accommodate the rural all appearance of the bridge. Design longitudinal beams and helped to were considered. The circular ring purpose single carriageway. for abnormal vehicles being maintain headroom to the inward of steel box construction was excluded enabled a reduction in inclined cables by placing their chosen as having the best overall Considering the constraints on member sizes. Allowance was made attachments to the deck well visual and structural performance access, erection and appearance, a within the design for accidental outside the parapets. The deck characteristics for the multitude of steel solution was chosen which severance of any one cable, and for depth was further minimised by different force combinations met the strict needs of the site. cable replacement without full designing the footway drainage as a imposed on it. It also echoes the closure of the bridge. separate system and by having the use of circular forms which appear The deck structure comprises two footway crossfall away from the on both the Iron Bridge and the 700mm x 450mm longitudinal steel The detailed design included two carriageway. . edge plate girders with composite and three-dimensional computer transverse beams at 2.4m centres, analysis of the structure, and local The longitudinal compression from each having two lines of shear finite element analyses of highly the deck, carried by the main steel studs. The 200mm reinforced stressed cable anchorages and girders, is restrained at the main concrete deck slab was cast in situ bearing zones. southern abutment by two thrust using permanent precast formwork bearings designed as pin joints, planks. The twin legged tower is The minimum deck cross-section each having a capacity of 4MN. inclined and formed from tubular depth was achieved by placing the The tower was initially envisaged as steel filled with concrete and linked main longitudinal beams at the hollow steel tube but constraints on at the top cable anchorages by a outside edge and designing them design for resistance to impact from fabricated rectangular steel box to carry the entire global flood-borne debris and internal

6 Jackfield Bridge Jackfield Bridge 7 Construction Construction

Construction Erection capping plates at the top. As the adjusted again to give the correct The planned erection sequence for subsequent sections of deck geometry and then checked against Construction commenced with the erection of a the 70 tonne tower included use of steelwork were lifted into place, calculated cable loads. Following temporary bridge alongside the existing structure a winch located on the north side of first the 9.6m longitudinal beam this, the final paint coat was the river to assist with the lift. In a sections then the crossbeams, they applied to all the steelwork. Traffic enabling traffic to cross the River Severn relatively carefully planned operation, were anchored back to the tower by and services were switched to the unhindered during the contract period, and achieved in one day, the tower legs the next set of stay cables. This permanent structure, thus allowing were pivoted on temporary steel sequence was repeated until the the temporary bridge to be providing a route for existing services. trestling on top of the south steel grillage reached the north removed. abutment and hoisted on to a abutment. temporary cradle, then slowly Demolition of the existing bridge of a braced sheet-pile cofferdam on shape. The deck and abutment jacked down to their permanent Upon completion of the steel work commenced, revealing some the south bank enabled the anchorages also involved complex location upon stainless steel the lower thread anchorages were surprising facts in the process. contractor to dig down some 8m to geometry and careful planning of bearings. adjusted accordingly in order to the cut-off level of the 42 No. 1.2m the sequence of welding. give the proper alignment. Only During the structure’s life several diameter bored piles which had The first pair of cable stays then were the HSFG bolted splices weight restrictions had been been sunk 12m through two seams Many aspects of the design sought anchored the tower to the fully tensioned. imposed, culminating in the erection of coal beneath the river. The piles to harmonise the various parts, abutment, after which the crane of a temporary steel bridge in order were then capped and a cellular namely the pylon, the cables, could be released. Then the first 11 This procedure was followed to to relieve all loading. However, as abutment of reinforced concrete the webs on the longitudinal beams, metre lengths of main steel deck prevent any locking in of stresses. the structure was demolished, it and ballast built up. This abutment the parapets and the cable beams were lifted onto temporary Permanent formwork was placed to became clear just how weak the supports the entire weight of the anchorages which all have inward supports from the river bank while receive reinforcement and the deck bridge was. Instead of the planned bridge which is transferred through sloping geometry and added to the the cross beams were fixed into concrete which was poured in bays programme of demolition utilising the pylon and cables. The North challenges for the fabricators. place and the first pair of forward in a sequence designed to minimise explosives and divers, the piles for abutment is principally a bored pile cables installed to take the weight locked-in shrinkage stresses. the bridge were simply pulled out by retaining wall. At the feet of pylon legs of the steelwork. Following this the a crane, revealing a length of a mere concentration of load onto the 450 tower legs were filled with concrete Once waterproofing and surfacing 3 metres. Unique screw threaded Fabrication diameter bearing required special pumped through removable had been applied the stays were reinforcement turnbuckle couplers The cable stay pylon, deck and steel castings. were discovered on the main parapets were prefabricated 180 reinforcing bars in the arch ribs, miles away by Westbury Tubular Because of access and revealing their use several years Structures. This involved very transportation limitation the tower earlier than had previously been intricate welding, especially at the was delivered to site in 4 sections, known. pylon cable anchorages where laid on its back across the internal strengthening stiffeners abutment and welded together prior Demolition was carried out by were placed to transfer the cable to being rotated and hoisted into conventional ball-and-chain loads through the pylon legs. position. The 200 tonne crane techniques with a barge mounted required to hoist the tower was crane in the river which also served For the top anchorage zone the itself so large that it too was to collect debris and prevent it 900mm diameter tubes were sliced delivered to site in sections and falling into the river. longitudinally into several segments took 2 days to erect. The 67m jib to enable welding of the internal needed 4 separate low loaders to Foundations stiffeners and subsequently carry it, and an 80 tonne crane to Following demolition, construction reassembled into their circular lift it.

8 Jackfield Bridge Jackfield Bridge 9 The finished bridge

The finished bridge During the opening ceremony the load test first carried out on the original Free Bridge was duplicated with a 14 tonne steam roller.

The contract had taken 16 months Commission to comment that it is to complete at a cost of £1.8m. “worthy of the ” Despite the complex geometrical and “an overwhelming success”. challenges the prefabricated The Iron Bridge has a striking new steelwork had fitted at the first neighbour which is attracting attempt, with bolted splices considerable interest and deliberately chosen in favour of complements the historic sequence welding to express visually how the of bridges in the Gorge. deck had been erected. The County Council is naturally proud of the achievement which on completion prompted the Royal Fine Art

10 Jackfield Bridge www.corusgroup.com

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