Friday, August 15, 2008
Royal Albert Bridge
The Royal Albert Bridge at Saltash, completed in 1859, was Isambard Kingdom Brunel’s last bridge and probably his finest work. Certainly, it was one of the great engineering feats of the nineteenth century, because (it is widely agreed) of its size, its economy of design, its revolutionary superstructure, and not least because of the way in which Brunel solved difficult logistical problems. It was one of the first bridge projects on which compressed air was used to allow underwater foundation work to proceed.
Dividing Cornwall from the rest of England, the tidal reaches of the River Tamar were once a major maritime thoroughfare. The twelfth-century port of Saltash lies on the west shore of the Tamar Estuary near the English Channel coast, nearly facing Plymouth on the opposite side. A railroad into Cornwall, the county in the extreme southwest of England, was first proposed in 1844. The Cornwall Railway Company was formed in 1845, and it successfully applied for the necessary act of Parliament to provide either a steam ferry to transport trains across the 1,100-foot-wide (336-meter) 85-foot-deep (26-meter) river or to build a bridge. The project was delayed because the Admiralty was concerned about restricted access to the Devonport naval base, close to Saltash. Finally, in 1852 Brunel’s proposal for a bridge with two main spans was adopted because a single pier in the river would offer least hindrance to water traffic. During the construction the plans were changed for financial reasons; Brunel designed the bridge for a single-track railroad. The authorities demanded a clearance of 100 feet (31 meters) under the bridge at high tide.
The Royal Albert Bridge is 2,240 feet (683 meters) long. Each of the two main spans is 455 feet (140 meters), and the 17 side spans of the long, curving approach viaducts vary between 70 and 90 feet (21 and 28 meters). Brunel first proposed a single-span bridge but because of difficult ground conditions changed the design.
Brunel found a firm base on rock in the middle of the river for the center pier, at a depth of more than 87 feet (27 meters) below high-water mark. Debris had to be cleared to expose a good foundation. To that end, a. 95-foot-tall (29-meter) iron cylinder, 35 feet (11 meters) in diameter, was fabricated onshore. A dome was constructed about 20 feet (6 meters) above its lower end, and a 4-foot-wide (1.2-meter) gallery, divided into 11 compartments, was built around the cylinder below the dome. The cylinder was floated into position and sunk to the riverbed in June 1854. Compressed air was fed into only those compartments where men were working, obviating the need to supply it to the whole space under the dome all the time. The foundation was cleared, and the rock was leveled with a 16-foot-thick (5-meter) base layer. By the end of 1856 the circular granite center pier was completed to a height of 12 feet (3.7 meters) above river level.
Four hollow octagonal cast-iron columns, 10 feet (3 meters) in diameter and stiffened by cross-bracing, rise from the center pier to the same height as the tapering masonry piers at the ends of the approach viaducts. Two columns support each of the huge main trusses. Those trusses were fabricated on the riverbank. Each comprises a curved, wrought-iron elliptical tube 16.75 feet (5.1 meters) wide—constrained by the single-track railroad—and 12.25 feet (3.7 meters) high, forming a flat arch that carries the weight of the superstructure. The arch is connected to massive catenary iron chains at eleven equidistant points by pairs of vertical standards, braced by diagonal bars; the chains support the girders under the railroad deck, 110 feet (34 meters) above high-water mark.
Beginning on 1 September 1857, the first 1,200-ton (1,016-tonne) truss was floated into position on four pontoons. Through the combined efforts of 500 men on shore and on five vessels at strategic points in the river, it was put into place with great accuracy. As the masonry pier progressed, the truss was raised a little at a time by hydraulic jacks. By July 1858 it had reached its full height, and the second was ready for floating into position. The process was repeated for the second truss. At their landward ends, the trusses are carried by piers, with arched openings through which the railroad passes.
The bridge was opened by Queen Victoria’s consort, Prince Albert—hence the name—on 3 May 1859, just a few months before its creator, Brunel, died. Its construction made possible a continuous rail journey between London and Truro. A branch line to Falmouth opened in 1863 and was later extended to the new docks then being built. A neighboring suspension bridge carrying the A38 road over the Tamar was completed in 1961. Early in 1998 the Royal Albert Bridge was refurbished. The £1.2 million (U.S.$1.75 million) project involved cleaning back the paintwork to bare metal and repainting and replacing much of the timber deck, all without unduly disrupting the rail services.