Chepstow Bridge
2007 Schools Wikipedia Selection. Related subjects: Engineering
The Chepstow railway bridge was built by Isambard Kingdom Brunel in 1852.
Background
No one could accuse the “Great Tubular Bridge” bridge over the River Wye at Chepstow of being aesthetically pleasing. But it was a unique and elegant engineering achievement, and economic in its use of materials. It would also prove to be the design prototype for the great bridge at Saltash.
Brunel had to take the two tracks of the South Wales Railway across the river Wye. The Admiralty had insisted on a 300-foot clear span over the river, with the bridge a minimum of 50 feet above high tide. The span would have to be self supporting, since although the Gloucestershire side of the river consists of a limestone cliff, the Monmouthshire side is low-lying sedimentary deposit subject to regular flooding. Thus on that side, there was nowhere for an abutment capable of either resisting the outward push of an arch bridge, or the inward pull of a conventional suspension bridge. In any case, neither could be used: an arch bridge would not have met the height and width restrictions imposed by the Admiralty, and suspension bridges were notoriously unfit for carrying railway trains. The concentrated weight caused the chains to deflect, allowing the bridge-deck to ride dangerously up and down. A self-supporting truss bridge was the only option.
Robert Stephenson had bridged the River Conwy ( 1848) and the Menai Straits ( 1850) with spans of 400 and 450 feet respectively, using large box-girder sections of riveted wrought iron. Conwy-like box-girders would have been very expensive to use at Chepstow as well as being heavy (problematic, since the spans had to be lifted much higher than at Conwy). Brunel, characteristically, sought a radical solution. He had already built a bowstring or tied-arch bridge at Windsor ( 1849) consisting of three triangular cross-section cellular arch ribs “strung” by wrought iron deck girders supported by vertical hangers from the arches. This was the same year as Stephenson's tied-arch high level bridge at Newcastle upon Tyne, which was supposed to have influenced Brunel at Chepstow. However, Brunel's solution for the latter was to make a leap forward, based, nevertheless, on sound engineering principles and a variation of the tied-arch theme.
The experiments of William Fairbairn, and the mathematical analysis of Eaton Hodgkinson had shown by a series of experiments that an enclosed “box-section” girder, made of riveted wrought iron, combined relative lightness with great strength. The tubular wrought-iron girder – be the cross-section rectangular, triangular or circular – formed a most efficient truss component. If the cross-section was large enough it could be self-supporting. It was Fairbairn's experiments that led to the design of the Menai ( Britannia) and Conwy bridges. Stephenson had originally proposed using a box-girder section suspended from chains. The box section would, he argued, be stiff enough to overcome the conventional problems of the bridge-decks of suspension bridges. In the event, Fairbairn showed that a properly constructed box girder would be strong enough so that the chains could be dispensed with. Nevertheless, the decision (not to use chains) was taken late in the project, so the Britannia bridge support towers were still built with holes for the chains. Stephenson's box-girders were a great innovation, and using steel or pre-stressed concrete instead of wrought iron, box-girder construction is the standard today for large bridges.
But as Berridge has observed, “Brunel was never one to follow fashion for fashion's sake... (at Chepstow)... Here was the real engineer at work, designing the bridge to suit the site and the best way of getting it into position”.
The Chepstow Bridge design
Brunel recognised that a circular cross-section tubular girder — a shallow “bow”, excellent in compression and tension — could be strung by suspension chains to form a stiff, self-supporting structure very much lighter (thus less expensive) than a Stephenson-type box girder. Instead of hanging the chains from towers and suspending the bridge deck from them, Brunel used the chains to stress and slightly bow the tubes, which were braced against the chains using struts. The bridge deck was rigid, because it was effectively clamped against the tubes by the chains. Brunel solved the problem in his own way, and for more than 100 years, the Chepstow and subsequently the Royal Albert Bridge were the only suspension bridges on the British railway system.
In spite of their apparent rivalry, Brunel and Stephenson were great personal friends, to the extent that they supported each other professionally. When Stephenson was under pressure during the enquiry following the collapse of his cast-iron girder bridge over the River Dee killing several people, Brunel did not desert him. In spite of his extreme distrust of the use of cast-iron girders for such purposes, Brunel refused to condemn them when cross-examined as an expert witness. He was also present to provide Stephenson moral support, when the great Britannia box-girders were floated across the river prior to being jacked up to their final positions. So when it came to the revolutionary design at Chepstow, The Times of 24 February 1852, reported that “Mr Stephenson, the eminent engineer, has examined the (great railway) bridge (at Chepstow) and concurred in the plan adopted by Mr Brunel...”.
The bridge was a triumph of the application of a radical design to a specific problem using available materials. The total cost (£77,000) was half what the Conwy bridge cost (£145,190 18s 0d) — admittedly with a main span of only 300 feet compared with Conwy's 400 feet, but there were no deep-water foundations needed at Conwy, and at Chepstow, the cost included a further 300 feet of land spans.
Epilogue
However, even Brunel was not infallible, and his foresight in allowing for slight movement of the suspension chains against supports on the bridge-deck to relieve stress, led to a weakening of the structure requiring its replacement in the 1960s. Nevertheless, Brunel's Chepstow bridge was a watershed, leading to a final refinement of the design in his great masterpiece, the Royal Albert Bridge over the River Tamar at Saltash.
Of the bridges mentioned here, the Windsor and Conwy bridges are still standing and in use, although the Conwy spans have been shortened using intermediate supports. The Britannia bridge sadly had to be replaced when some boys, bird-nesting, managed to set fire to the bridge lining. The Chepstow bridge also had to be replaced. But Brunel's brilliant and economical design concept lives on in the Royal Albert Bridge, which continues to carry the former Cornwall Railway main line into Cornwall.