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Chepstow Railway Bridge was built to the instructions of Isambard Kingdom Brunel in 1852. The "Great Tubular Bridge" over the River Wye at Chepstow, which at that point forms the boundary between Wales and England, is considered one of Brunel's major achievements, despite its appearance. It was economical in its use of materials, and would prove to be the design prototype for Brunel's Royal Albert Bridge at Saltash. == Background == Brunel had to take the two tracks of the South Wales Railway across the River Wye. The Admiralty had insisted on a clear span over the river, with the bridge a minimum of 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 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 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 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 bridge filled a gap in the main rail line between Gloucester and Swansea. The line between Chepstow railway station and Swansea was opened on 18 June 1850, and on 19 September 1851 the line was completed between Gloucester and a station east of the river, known as Chepstow East. Until the bridge was completed and opened, through passengers were carried from one station to the other by coach, using the 1816 road bridge across the River Wye. The railway bridge was opened to public use for the first time on 19 July 1852; Chepstow East station closed at the same time as redundant. Originally there was only one line of railway over the bridge; a second was brought into use on 18 April 1853. The new railway line, and bridge, had the effect of reducing the journey time between London and Swansea from 15 hours, by rail, road and ferry, to 5 hours by rail.〔Chepstow Society, ''Brunel's Tubular Suspension Bridge over the River Wye'', 1970, SBN 900278 04 8〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chepstow Railway Bridge」の詳細全文を読む スポンサード リンク
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