Chepstow Railway Bridge
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 great 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 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 girder, made of riveted wrought iron, combined relative lightness with great strength. The tubular wrought-iron girder – whether the cross-section was 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".
Construction
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.
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.
With regard to the appearance of the bridge, the Illustrated London News stated that "the peculiarity of the site did not permit any display of 'Art' - that is, of architectural embellishment; indeed, a pure taste rejects any attempt to decorate a large mechanical work with sham columns, pilasters, and small ornamental details."
The bridge was constructed on site for Brunel by Edward Finch of Liverpool as partner in the firm Finch and Willey. After it was completed, Finch remained in Chepstow, and developed a major engineering and, later, shipbuilding business on the site, beside the river (see Finch and Co).
An 1852 Report
'Raising of The Chepstow Railway Tube—Thursday Evening.—
After two postponements, the important operation of floating the tube to its position between the piers has been accomplished to-day with the most complete success. At a quarter to nine o'clock this morning the operation of floating commenced. Among those upon the tube, or who witnessed the experiment, were Lord Villiers, M.P.; Earl Cadogan, General Pasley, late government inspector of railways; Captain Claxton, R.N., engineer of the Great Britain; Mr. Grissell, Mr. C. Owen, and a large number of Mr. Brunel's staff. It should be explained that the bridge, when completed, will be 610 feet in length, and of four spans. On three of these, about 100 feet each, the simple principle of supporting the girders, on which the railway will rest on piers, will be adopted, but on the third span (300 feet),in addition to the pier supporting the girders at each end, the suspension principle is also introduced, and for this purpose this tube, 309 feet in length, and nine feet in diameter, has been constructed. The floating of this tube from the shore to its first position was the operation of to-day. The floating commenced a quarter to nine o'clock, and was completed, without hitch or hindrance, exactly in an hour. It was floated on iron pontoons, constructed under the supervision of Captain Claxton ; and on reaching its destination, a few feet above high-water mark, where it was made fast to a platform placed to receive it, the consummation of the experiment was announced by the firing of a cannon and the enthusiastic cheers of the workmen and the spectators, who had congregated from all parts on the cliffs and shores the river. At the same moment the bells of Chepstow church rung out a merry peal, and " Success to the South Wales Railway" was drunk by those on the top of the tube in bumpers of champagne. The experiment was most successful. The tube was gradually drawn and propelled upon the pontoons by powerful crabs, and the pontoons being kept in a direct line across the river by powerful cables attached to moorings both up and down the river, the tube was gradually drawn across to its first position, a few feet above high water mark. Here, the tide having ebbed out, it rested on the platform prepared for it, and the pontoons were removed, the tube remaining safely suspended. The second operation of lifting the tube, each end in its turn, to its second position, on a level with the railway, was commenced early in the afternoon, and was making satisfactory progress when our report was despatched by the last available train. For this purpose three sets of chain-lifting tackle are employed. The lower parts of these chains are of 3-inch iron, each tested bear a weight of 80 tons without injury. These chains extend from a timber framing at the top of the east pier, 180 feet above the railway level, and are worked by three double "crabs" of great power, twelve men to each crab. Great credit is given to the principal foreman, to whom much this difficult and important work has been entrusted. One accident only, and that not of a serious nature, occurred. A man fell from the top of some scaffolding into the river, but was rescued by some of the boatmen, who were waiting below for casualties.
[We are glad to be enabled to state a fact in connexion with this great work, which will be interesting to Irishmen, viz:- That the cast iron work for this important structure, including the cylinders which form the piers, was constructed, under the directions of Mr. Brunel, by the Irish Engineering Company at Seville Works, Dublin.'[1]
Subsequent Performance
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. Also, the bridge was subjected to frequent heavy loading by freight trains in WW2. In the 1950s, the speed of trains using the bridge was restricted to 15 miles per hour, because some of the girders had become distorted.
In 1962, a new structure to support the bridge beneath the main span was put in place. 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.
A good 'inside' account of the reconstruction of the bridge in 1962 was provided by P. S. A. Berridge [2]