Monday, August 4, 2008

Firth of Forth Railway Bridge

Nine miles west of Edinburgh, Scotland, the mouth of the River Forth is spanned by Europe’s first all-steel, long-span bridge. Completed in 1890 it was then the longest bridge in the world. Until 1917 it was also the largest metal cantilever, and at the beginning of the twenty-first century it remains the second largest ever built. It was a major accomplishment of Victorian engineering.

The extension of the railroad along Scotland’s east coast, to complete the direct route between Edinburgh and Aberdeen, was hampered for most of the nineteenth century by two broad inlets of the North Sea: the Firth (mouth) of Tay and the Firth of Forth. The River Forth rises near Aberfoyle and widens into its firth about 50 miles (80 kilometers) from the ocean.
Vessels up to about 300 tons (270 tonnes) could navigate as far as Alloa, about 16 miles (26 kilometers) inland; those up to about 100 tons (91 tonnes) could reach Stirling, a little further on.

After earlier aborted proposals—a tunnel in 1806 and a bridge in 1818—for crossing the firth, little more was attempted for fifty years. In 1865 an act of Parliament sanctioned a bridge across the Queens-ferry Narrows, where the river passes between steep banks at the neck of the firth. Four railroad companies—North British, North Eastern, Midland, and Great Northern—formed a consortium in 1873 and commissioned Thomas Bouch, engineer for North British, to design the bridge. He proposed a suspension structure with twin spans of 1,600 feet (480 meters). The project was delayed for five years because of lack of funds; by spring 1879 only one pier had been started.

When the much-vaunted Tay Railway Bridge, also designed by Bouch and less than two years old, collapsed in a gale on 28 December 1879 with the loss of seventy-five lives, work on the Forth bridge was immediately suspended by another act of Parliament. In January 1881 a British Board of Trade inquiry found that the Tay disaster was caused by inadequate design and poor supervision. Bouch’s Firth of Forth scheme was abandoned. Within months the engineer died, a broken man. The engineers of the Forth consortium’s member railways, Thomas Harrison, William Barlow, John Fowler, and Benjamin Baker, had to develop a new design. In May 1881 Fowler and Baker submitted a plan for a continuous girder, or balanced cantilever, structure. In July 1882 yet another act authorized construction. The Tay bridge affair had so undermined public confidence in railroads that the legislation insisted that the Forth bridge should “enjoy a reputation of being not only the biggest and strongest, but also the stiffest bridge in the world.” There was to be no vibration, even as trains passed over it. Consequently, it was greatly over-engineered.

Before 1877 steel bridges had been banned by the Board of Trade because the Bessemer conversion process produced steel of unpredictable strength. The Siemens-Martin open-hearth process, developed by 1875, bad changed that, yielding material of consistent quality. That kind of steel was used in the Forth bridge, heralding the transition from cast and wrought iron in such structures. A smaller steel cantilever bridge had been built in Germany, but the Scottish project was on a larger scale than had been seen before. There is little doubt that its designers owed much to a U.S. model of several years earlier. Between 1869 and 1874 James B. Eads had designed and built the world’s first steel bridge, over the Mississippi at St. Louis, Missouri. Its three-arch superstructure, with a center span of 520 feet (156 meters) and side spans of 502 feet (150 meters), supported by four massive limestone piers, carried a railroad and a road for other traffic on two levels. Other pioneering features of Eads’s bridge were adopted by the British: the use of pneumatic caissons (large diving bells fed with compressed air) to excavate the foundation, tubular steel structural members, and a balanced cantilever design that allowed construction to proceed without temporary supports that would have obstructed the waterway.

In December 1882 the contract for the Forth bridge was awarded to a consortium led by Tancred Arrol, an experienced and respected company headed by William Arrol, which already had contracts for the Caledonian Railway Bridge over the Clyde and the replacement Tay bridge. At the height of building activity, there would be 4,600 Britons, Italians, Germans, and Austrians working shifts around the clock. The construction of the foundations and piers took until the end of 1885. Each of the bridge’s three cantilever towers stands on four 70-foot-diameter (21-meter) granite piers, founded on the bedrock. Eight of the piers are in water, and their foundations were excavated by men working in wrought-iron pneumatic caissons, sunk up to 90 feet (27 meters) below the river surface. The massive cylinders were prefabricated in Glasgow, then dismantled and taken to Queensferry, where they were reassembled. Once excavation was complete, the air shafts and the working spaces were filled with concrete, and the granite piers rose above them.

Work on the superstructure began in 1886 using 64,800 tons (54,860 tonnes) of steel from two steelworks in Scotland and another in Wales, fixed with rivets from a Glasgow foundry. All the structural members were fabricated in on-site workshops, pre-drilled, test-assembled—exact dimensions were needed in a riveted structure—and then dismantled to be painted and carried to the site for erection. Each of the 331-foot-high (99.3-meter) cantilevers consists of two inward-sloping trusses fabricated from huge, internally stiffened tubular members up to 12 feet (3.6 meters) in diameter. They support 680-foot-long (204-meter) cantilever arms that are linked midspan by suspended girders of about half that length, making the distances between the towers about 1,700 feet (540 meters). The length of the bridge between the end piers is about 5,300 feet (1,600 meters). Together with the approach viaducts and arches at each end, the bridge carries the double-track railroad for 2,765 yards (2,490 meters), 150 feet (45 meters) above the surface of the Firth of Forth. The central gap was closed on 14 November 1889, and the Prince of Wales ceremonially opened the bridge on 4 March 1890.

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