Civil Engineering heritage: Scotland - Lowlands and Borders
Date 2007
Event ID 602844
Category Descriptive Accounts
Type Publication Account
Permalink http://canmore.org.uk/event/602844
The Forth Bridge, often dubbed ‘the eighth wonder of the world’, carrying the east coast main line railway across the Forth at Queensferry was designed between 1880–82 in the aftermath of the Tay Bridge disaster. This event engendered a major loss of confidence in Victorian engineering, but resulted in a gigantic leap forward in bridge design and construction with the achievement of the Forth Bridge built from 1883–90. The bridge was to provide a shorter rail route to the north in place of the time-consuming ferry crossings. In order to cross two deep channels, two main spans of 1710 ft utilising the isle of Inchgarvie were required, a daunting and unprecedented project.
Initially Thomas Bouch proposed a stiffened suspension bridge design with two spans of 1600 ft and construction began in 1878, but, following the fall of the Tay Bridge, confidence in his judgement and design waned, although it had been checked and was being implemented by William Arrol. The base of one of the brick piers of his structure still remains at Inchgarvie, now carrying a beacon.
An innovative design involving the balanced cantilever principle and the use of steel was devised by consulting engineers Sir John Fowler & Benjamin Baker of Westminster. The Board of Trade laid down the stringent requirement of 56 lb sq ft to be allowed for in the design for wind pressure. Extensive use was made of tubular members in compression, many of unprecedented size. The design was subjected to intense scrutiny as it developed. Wind pressures were recorded and endless tests made on the quality of the steel. Nothing was left to chance.
The contract for constructing the bridge was awarded to Tancred, Arrol & Co. William Arrol took personal charge of the operations, both at his works in Glasgow and on site, Andrew Biggart playing a key role. He showed great ingenuity in the design of plant, including hydraulic riveting machines, cranes and drilling methods, and provided many safety devices for his workers. The carefully devised design enabled the bridge to be constructed with the minimum of temporary works and staging.Much of the labour employed on the bridge was recruited from shipyards of the Clyde and Forth, and steelworkers from Lanarkshire. The superstructure is basically three towers with cantilever arms on each side. The towers are 330 ft high above the granite pier foundations, and the cantilever arms are each 680 ft long, projecting outwards from the towers.The ends of the cantilevers over the river are linked by suspended spans of 350 ft. Other dimensions are:
Main spans 1710 ft
South approach ten spans of 168 ft
North approach five spans of 168 ft
Overall length 8296 ft
About 54 160 tons of steel were used in the bridge’s construction, secured by 6 500 000 rivets. At the peak of construction 4600 men were employed. For more than a century there were believed to have been 57 fatalities during construction but this number has increased into the 60s following recent research. Electric light was in its infancy, but primitive lighting was employed here for the first time on a major construction site. The final, so called, ‘Golden Rivet’ was ceremoniously driven by the Prince of Wales at the opening of the bridge on 4 March 1890. The bridge is now (2007) undergoing a major long-term refurbishment. Its traditional Craig & Rose paint is being replaced by a paint regime developed in the offshore oil industry consisting, after blast cleaning to bare metal, of an application of a zinc based primer (35 microns), a glass flake epoxy paint barrier (400 microns) and a polyurethane gloss top coat (35 microns).
R Paxton and J Shipway 2007
Reproduced from 'Civil Engineering heritage: Scotland - Lowlands and Borders' with kind permission of Thomas Telford Publishers.
