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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's railway bridge over the Wye at Chepstow. Photograph by kind
   permission of John Morris
   Enlarge
   Brunel's railway bridge over the Wye at Chepstow. Photograph by kind
   permission of John Morris

   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.
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