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Conwy Tubular Bridge
Conwy, Wales, UK
Conwy Tubular Bridge
associated engineer
Robert Stephenson
Sir William Fairbairn
date  May 1846 - 18th April 1848, opened 1st May 1848, 1899
UK era  Victorian  |  category  Bridge  |  reference  SH785774
ICE reference number  HEW 108
photo  courtesy PHEW, ICE
For his railway bridge over the River Conwy in north Wales, Robert Stephenson used a design similar to that which he was developing for the Britannia Bridge over the Menai Strait. In his novel use of wrought iron tubes, he pioneered the modern box girder. Conwy Tubular Bridge was the first and is now the only surviving example of its type. It remains in use. Its Grade I listing and scheduled ancient monument status reflect its importance to engineering heritage.
The Chester & Holyhead Railway did much to improve communications between London and Ireland as part of the wider national rail network under development in the Victorian era. Its engineer, Robert Stephenson (1803-59), designed the bridge at this spot to carry the railway across the River Conwy between stations at Conwy town and Llandudno Junction.
By April 1845, he had conceived the idea of a tunnel-like tube through which the railway might run above the river. He knew that a box section would give the structure rigidity, enabling it to be self-supporting over a long span.
Working concurrently on ideas for the Conwy and Britannia bridges, Stephenson consulted ironmaster William Fairbairn (1789-1874) and mathematician Professor Eaton Hodgkinson (1789-1861). The three men reported their findings to the railway company in February 1846.
After experimenting with different cross sections, an easy-to-build rectangular-section tube was tested in a one-sixteenth scale model of Britannia Bridge (23.8m long), which had maximum spans of 140m. The testing was innovative in that it included the effects of wind and temperature variations. Stephenson was further aided in his work by Edwin Clark (1814-94), his resident engineer, and the eventual designs for both bridges were similar.
At Conwy, a pair of parallel iron tubes bridge the river, 5.5m above the water, in single 122m spans between two masonry abutment towers. But before the bridge was commissioned, the tubes were load tested with up to 305 tonnes of iron kentledge (ballast weights), resulting in a 76mm deflection at mid span. By comparison, an ordinary train is said to produce a deflection of only 3mm.
Groundwork commenced in May 1846. The underlying bedrock was stepped and levelled close to low water for the tower foundations. In addition, timber piles were driven at the south east corner of Conwy Tower, where the rock dips, and the masonry here is founded on a timber platform some 600mm below low water.
The two limestone-faced towers, with their crenelated turrets, were designed by architect Francis Thompson (1808-95) in a style that harmonises with the adjacent 13th century Conwy Castle and the elegant 1826 Conwy Suspension Bridge by Thomas Telford (1757-1834). The first stone of the towers was laid on 15th June 1846. Entrance portals with twin arches lead the rail tracks into the tubes.
The construction of the tubes owes much to shipbuilding technology. They are made of 16mm riveted wrought iron plates, with cellular roofs and bases, and sheeted sides. Each weighs 1,320 tonnes. As a decorative feature, the tubes were intended to have durable casings with machicolated cornices, stringcourses and loopholes to echo the castle walls. The idea was abandoned as too expensive and it would have added extra weight to the structures.
The tubes are 129.2m long and 4.4m wide, a maximum 7.8m high in the centre reducing to 6.9m at the ends. Once complete, they were lashed to pontoons, floated onto the river and jacked into position between the abutments. The first tube was floated on 6th March 1848. They were lifted individually using hydraulic pumps and the process took nine days.
On 18th April 1848, the first tube was ready for service. Stephenson was aboard the first locomotive to cross the bridge, and it opened to rail traffic on 1st May that year. The second tube was completed between October 1848 and January 1849.
Two further piers were added in 1899, reducing the clear span to 94.5m. This compensated for the increased weight of the trains then in service. The piers support the underside of the tubes and consist of cast iron cylinders filled with concrete. Each tube is also fitted with a travelling gantry crane for inspection and maintenance (date of installation unknown).
In September 1950, Conwy Tubular Bridge became a Grade I listed structure. It is also a scheduled ancient monument (CN167).
In 1958, a new road bridge (SH784775) carrying the A547 was constructed just to the north of Telford’s suspension bridge, so that the three Conwy bridges sit side by side. In 1991, a further road crossing was added downstream in the form of the Conwy Immersed Tube Tunnel, which carries the North Wales Expressway.
In 2009, the two inspection gantries installed in 1899 were removed from the Conwy Bridge tubes. The work was undertaken by contractors working from floating plant. The bridge is now owned and maintained by Network Rail.
Architect (towers): Francis Thompson
Resident engineer: Edwin Clark
Contractor: William Evans
Masonry contractor: Nowell, Hemingway & Pearson
Ironwork: Ditchburn & Mare
Riveting machine: Garforth of Dukinfield, Manchester
Research: ECPK
"An Account of the Construction of the Britannia and Conway Tubular Bridges" by William Fairbairn, John Weale, London, 1849
reference sources   CEH WalesBDCE2

Conwy Tubular Bridge