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Britannia Bridge (1974)
Menai Strait, Bangor, Gwynedd, Wales, UK
associated engineer
Robert Stephenson
Husband & Co
date  1970 - 1974 (stone piers 1850), October 1977 - 11th July 1980
UK era  Modern  |  category  Bridge  |  reference  SH541709
ICE reference number  HEW 110b
The present Britannia Bridge combines the stone substructure of the Victorian-era bridge by Robert Stephenson with a modern dual-use superstructure. The lower rail deck is carried on steel arches and concrete frames, and the upper road deck is of reinforced concrete. The bridge’s original iron railway tubes had to be removed after a devastating fire in 1970. Nevertheless, it retains its Grade II listing and is the major transport link between north Wales and Anglesey.
The original bridge was constructed in 1846-50, and designed by Robert Stephenson (1803-59), with input from ironmaster William Fairbairn (1789-1874), for the Chester & Holyhead Railway. It was 830m long and consisted of twin rectangular wrought iron plate tubes spanning between masonry abutments and three intermediate masonry towers, all of which still stand.
The pair of tubes, which were 4.5m wide and up to 9.1m deep, passed through the towers, where they were supported on cast iron beams. A timber roof was erected over the tubes and covered with hessian and layers of tar. The masonry abutments and towers take the form of Egyptian-style pylons, rectangular in plan. They are made of Anglesey limestone and Runcorn sandstone. The towers are up to 67.5m high, with one on each bank of the Menai Strait, and a central one on Britannia Rock (SH541710).
On the evening of 23rd May 1970, a fire started in one of the tubes, close to the stonework of the entrance portal in the south abutment, where there was timber jointing. The flames quickly spread to the timber roof, almost a metre above the tubes. The 3m gap between the pair of tubes acted as a flue and, fanned by the breeze, what began as a small fire soon became "a major conflagration". Rail crossings ceased immediately.
The fire was finally extinguished on 25th May, by which time it had completely destroyed the tarred roof, and had weakened the wrought iron tubes so much that the two main 140m spans over the strait were in danger of collapsing. The tubes had split open at the three towers and began to sag. The long spans deflected 490mm and 710mm. The sides of the tubes had also developed vertical corrugations.
On 29th May, the chief civil engineer of British Railways' London Midland region, W.F. Beatty, sought structural advice from consulting engineer Husband & Co. of Sheffield. Husband’s investigation revealed that the cast iron beams inside the towers, which carried the tubes and acted as guides for the tube ends over the bearings in the towers on the banks, had tilted and cracked. The proximity of the beams to the major cracks in the tubes themselves meant that the tubes required immediate support at all three towers.
To underpin the tubes as quickly as possible, the Royal Engineers were brought in. They used vertical Bailey bridge units to fill the original jacking slots in the masonry towers. By the end of July, eight Bailey bridge steel towers (two at each end of the main spans) had been erected from the slots. Each was able to bear a vertical load of around 200 tonnes. The remaining dead load of the tubes, plus live loads imposed during reconstruction, was borne by the existing beams and masonry.
Further analysis showed that the wrought iron tubes had been too badly damaged to be retained. They were eventually dismantled and replaced by a new deck constructed in stages, at the same level as the original tracks. One section of tube (SH542708) has been preserved as a monument to Stephenson’s innovation and is located on the Bangor side of the Menai Strait near the bridge.
The two long spans are now supported on arches, which had not been an option for Stephenson owing to the clearance needed for tall masted vessels. Modern navigational requirements called for less headroom.
The two arches each span 134.1m with a 27.4m rise. They consist of triple-rib steel trusses with N-braced spandrels, and eight panels of bracing in each half arch. They were prefabricated at Port Dinorwic, 3.2km away, and floated down the strait in sections to be jacked up the towers. The 70m side spans, located between the abutments and the bank towers, are supported by two intermediate reinforced concrete portal frames.
While the bridge was being rebuilt, Anglesey passenger trains ran between Holyhead and a temporary platform at the former Llanfair Pwllgwyngyll Station, with a bus link over the Menai Bridge to Bangor in Gwynedd. Anglesey freight trains operated between Holyhead and Gaerwen with ongoing road transport to north Wales.
Work progressed rapidly. The new arches initially supported the original wrought iron tubes, which enabled railway services to resume on 30th January 1972, temporarily running through the 'up' line tube. The ‘down’ line tube was then removed and a new concrete deck with outer steel parapet constructed in its place.
Rail services were then transferred to the new deck. Then the 'up' line tube was removed and the deck extended to the full width of the bridge. Though Stephenson’s structure had carried two railway tracks, the new deck has only one, on the west side. The rail line reverts to dual tracks at each end of the bridge. The east side of the deck is used as a maintenance access road and for water and gas pipelines.
The new rail crossing was completed in 1974, with the installation of a two-lane road deck above the rail line planned for the future. Traffic volumes on the A5 over Thomas Telford’s (1757-1834) nearby Menai Suspension Bridge were causing serious congestion. At the time it was the only road crossing of the strait.
Some preparatory work had been done by 1974, such as the enlargement of the rectangular openings through the towers and their strengthening with concrete lintels. The abutment tube portals had been opened up so that only the side walls remained. Photographs show that in January 1977, foundations for the road deck were in place. Construction began in October 1977, with the bulk of the project carried out 1978-80. Steel portals were inserted inside the abutment side walls to carry the new deck.
The completed roadway forms part of the A55 North Wales Expressway from Chester to Holyhead. It consists of one 5m wide lane in each direction, with no pedestrian footway. Large concrete blocks positioned around the deck supports protect them from potential damage by train derailment.
The road deck was opened by HRH Prince Charles on 11th July 1980, more than 10 years after the fire. The four Victorian-era stone lions, which once guarded the ends of the tubes at rail level, are now hidden from view by the new structure.
In 2011, the bridge underwent strengthening works costing £4m. The project was a collaboration between Network Rail, the Welsh Assembly Government and the UK Highway Agency. Using a barge, a movable platform and abseiling techniques, engineers replaced eroded steelwork, bridge parapets and stonework and improved the highway drainage system on the bridge.
Debate about the increasing traffic congestion on Britannia Bridge and the possibility of a third road crossing of the Menai Strait is ongoing.
The bridge retains the Grade II listed status it received in March 1966. It is recognised as "one of the most audacious and exciting monuments of the great age of engineering", a notion undiminished by the modern reconstruction works.
Similarly listed as Grade II is the Britannia Bridge Memorial (SH537711) in the churchyard of St Mary at Llanfair Pwllgwyngyll, Anglesey. It commemorates the 18 people who died during the bridge's original construction in 1846-50, along with the two men who lost their lives during the reconstruction (1972 and 1973).
Masonry contractor (1848-50): Nowell, Hemingway & Pearson
Resident engineer: Richard Husband
Marine superintendent: D. Stuart Tod
Contractor: Cleveland Bridge & Engineering Co. Ltd
Contractor: Cementation Construction Ltd
Steelwork: Fairfield-Mabey Ltd
Contractor: Fairclough Civil Engineering Ltd
Research: ECPK
"A Critical Analysis of the Britannia Bridge, Wales" by S.M. Collingwood, in Proceedings of Bridge Engineering 2 Conference, University of Bath, April 2010
"Understanding Bridge Collapses" by Bjorn Akesson, Taylor & Francis, Abingdon, May 2008
"Discussion: Reconstruction of Britannia Bridge" by H.C. Husband, R.W. Husband et al, ICE Proceedings, Vol.58, Issue 4, pp.639-655, London, November 1975
"Reconstruction of the Britannia Bridge" by H.C. Husband and R.W. Husband, ICE Proceedings, Vol.58, Issue 1, pp.25-66, London, February 1975
"Joint report on a fire in the Britannia Tubular Bridge, Menai Straits, on Saturday - May 23rd, 1970" by F.W. Hutchinson, Chief Fire Officer, Caernarvonshire County Fire Brigade, and W.J. Coates, Chief Fire Officer, Anglesey County Fire Service, 1970
reference sources   CEH Wales

Britannia Bridge (1974)