Chepstow Railway Bridge

works.]] Brunel had to take the two tracks of the South Wales Railway across the River Wye. The Admiralty had insisted on a 300 ft clear span over the river, with the bridge a minimum of 50 ft 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…

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…