Forth Bridge - Construction

Construction

The bridge is, even today, regarded as an engineering marvel. It is 2.5 kilometres (1.6 mi) in length, and the double track is elevated 46 metres (151 ft) above the water level at high tide. It consists of two main spans of 521.3 metres (1,710 ft), two side spans of 207.3 metres (680 ft), and 15 approach spans of 51.2 metres (168 ft). Each main span comprises two 207.3 metres (680 ft) cantilever arms supporting a central 106.7 metres (350 ft) span truss. The weight of the bridge superstructure was 51,324 tonnes (50,513 long tons), including the 6.5 million rivets used. The bridge also used 18,122 cubic metres (640,000 cu ft) of granite.

The three great four-tower cantilever structures are 100.6 metres (330 ft) tall, each tower resting on a separate granite pier. These were constructed using 21 metres (70 ft) diameter caissons, those for the north cantilever and two on Inchgarvie acting as coffer dams while the remaining two on Inchgarvie and those for the south cantilever, where the river bed was 28 m (91 ft) below high-water level used compressed air to keep water out of the working chamber at the base.

At its peak, approximately 4,600 workers were employed in its construction. Initially, it was recorded that 57 lives were lost; however, after extensive research by local historians, the figure was increased to 63. Eight men were saved from drowning by boats positioned in the river under the working areas. Hundreds of workers were left crippled by serious accidents, and one log book of accidents and sickness had 26,000 entries. In 2005, a project was set up by the Queensferry History Group to establish a memorial to those workers who died during the bridge's construction. In North Queensferry, a decision was also made to set up memorial benches to commemorate those who died during the construction of both the rail and the road bridges, and to seek support for this project from Fife Council.

Work at the site began at the end of 1882, with the construction at South Queensferry of the extensive workshops where the steelwork was to be fabricated. These eventually occupied more than 50 acres. Work on the foundations of the bridge began in February 1883, and the first of the caissons was launched on 26 May 1884. The bridge was completed in December 1889, and load testing of the completed bridge was carried out on 21 January 1890. Two trains, each consisting of three heavy locomotives and 50 wagons loaded with coal, totalling 1,880 tons in weight, were driven slowly from South Queensferry to the middle of the north cantilever, stopping frequently to measure the deflection of the bridge. This represented more than twice the design load of the bridge: the deflection under load was as expected. A few days previously there had been a violent storm, producing the highest wind pressure recorded to date at Inchgarvie, and the deflection of the cantilevers had been less than 25 mm (1 in). The first complete crossing took place on 24 February, when a train consisting of two carriages carrying the chairmen of the various railway companies involved made several crossings. The bridge was opened on 4 March 1890 by the Prince of Wales, later King Edward VII, who drove home the last rivet, which was gold plated and suitably inscribed. The key for the official opening was made by Edinburgh silversmith John Finlayson Bain. There is a plaque on the bridge commemorating this.

The use of a cantilever in bridge design was not a new idea, but the scale of Baker's undertaking was unprecedented. Much of the work done was without precedent, including calculations for incidence of erection stresses (the internal forces exerted on structural members during construction), provisions made for reducing future maintenance costs, calculations for wind pressures made evident by the Tay Bridge disaster and the effect of temperature stresses on the structure.

Where possible, the bridge used natural features such as Inchgarvie, an island, the promontories on either side of the firth at this point, and also the high banks on either side.

The bridge has a speed limit of 50 miles per hour (80 km/h) for passenger trains and 20 miles per hour (32 km/h) for freight trains. The weight limit for any train on the bridge is 1,422 tonnes (1,400 long tons; 1,567 short tons) although this was waived for the frequent coal trains which used the bridge prior to the reopening of the Stirling-Alloa-Kincardine railway, provided two such trains did not simultaneously occupy the bridge. The route availability code is RA8, meaning any current UK locomotive can use the bridge, which was designed to accommodate heavier steam locomotives.

Up to 190–200 trains per day crossed the bridge in 2006.