Great Western Railway Bridge Chepstow (1849)
At Chepstow, during the building of the Great Western Railway, Isambard Kingdom Brunel was faced with the task of taking two tracks of 7 ft gauge across a 600 ft gap. The bridge cost £77000 - Brunel had to sink foundations far below the water level of the river Wye which has the second highest tidal range in the world. His first thoughts were on a timber trussed arches type, but the Admiralty insisted on one with a headroom of 50 ft above water over a 300 ft width, as sailing ships used to go up to Rastrick’s iron road bridge less than half a mile away, this being the location of the customs house. River navigation also had to be considered as traffic on the Wye still went to Brockweir, Redbrook and Monmouth.
The two sides of the river are very different. On the Gloucester side, there is a 120 ft limestone cliff whereas on the Monmouth side, there is level wash land. Therefore the bridge is not symmetrical - unusual for Brunel. He planned a single high level span of 300 ft long and 100 ft above high water and across the wash land by three approach spans each 100 ft long. The cliff forms a natural end one side, the railway emerging from a cutting. The other end of the bridge is supported by cast iron piers which were sunk through soft ground until they found a solid foundation and then filled with concrete. The railway then continues on a high embankment into Chepstow station.
Brunel decided to use tubular construction as tubes are ideal to carry the stresses and weigh much less. The tube would hold apart the ends of “suspension chains” which would look after the tensile forces in a truss carrying the plate girder railway bridge below. Brunel argued that the tube would be stiff, and with the railway deck “strutted” and “ trussed” to the tube, the excessive distortions, to which a suspension bridge is liable under the passage of concentrated load, would be avoided. Each 300 ft span depended upon a 9 ft tube of riveted wrought iron plates to resist the inward strain of the chains. Brunel built his tubes on the flat land behind the shore of the right bank of the river; they had to be raised more than 100 ft. and lifted clear of shipping within 12 hours. Brunel kept the weight as low as possible. The tubes were 9 ft in diameter, 312 ft long and weighed a mere 138 tons apiece, but were not self-supporting. Without the inward pull from the chains at each end, and the upward support of the posts or struts of the “trusses”, the tubes could not be expected to carry their own deadweight. Temporary trusses were therefore used and tested. The tube was built on low ground and floated by manoeuvring into launching position at right angles to the bank. A pontoon of 6 barges was brought under the overhanging tube. The tube was launched end on across the river. On 8th April 1852 launch began at 9am. The tube was lifted during the course of the day well clear of ship masts. Shortly afterwards the tube reached tower top level, built of cast-iron above the pier cylinders which Brunel had sunk by “pneumatic method”, the upper end being sealed and compressed air introduced to keep water out.
The down line spans were opened for single line traffic on 14 July 1852. The up line spans were dealt with in the same way and completed the following April.
The secondary stresses caused by deforming of the truss were alarmingly high and in 1944 one of the plate girders in the land spans partially collapsed. For many years, all trains crossing Chepstow bridge were restricted to speed limits of 15 mph.. The right-hand main girder of the down line broke on 4th April 1944. Temporary repairs were made. Gradually, more extensive repairs were made, and by 1962, the bridge had changed completely, being rebuilt into an inverted box girder form.
After Chepstow bridge, Brunel built his last great railway work, the Royal Albert bridge at Saltash.