Monday, November 5, 2007

Forth Bridge

The Forth Bridge is a railway bridge over the Firth of Forth in the east of Scotland, 14 km (9 miles) west of central Edinburgh. It is often called the "Forth Rail Bridge" to distinguish it from the Forth Road Bridge. The bridge connects Scotland's capital Edinburgh with the Kingdom of Fife, and acts as a major artery connecting the north-east and south-east of the country. Described as "the one internationally recognized Scottish landmark", it may be nominated by the British government as a UNESCO World Heritage Site.


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History

Construction of an earlier bridge, designed by Sir Thomas Bouch, got as far as the laying of the foundation stone, but was stopped after the failure of another of his works, the Tay Bridge. On Bouch's death the project was handed over to Sir John Fowler and Sir Benjamin Baker, who designed a structure that was built by Sir William Arrol's company between 1883 and 1890. Baker.


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Construction

The bridge is, even today, regarded as an engineering marvel. It is 2.5 km (1.5 miles) in length, and the double track is elevated 46 m (approx. 150 ft) above high tide. It consists of two main spans of 1,710 ft, two side spans of 675 ft, 15 approach spans of 168 ft, and five of 25 ft. Each main span comprises two 680 ft cantilever arms supporting a central 350 ft span girder bridge). The three great four-tower cantilever structures are 340 ft (104 m) tall, each 70 ft diameter foot resting on a separate foundation. At its peak, approximately 4,600 workers were employed in its construction.

More than 55,000 tons of steel were used, as well as 18,122 m³ of granite and over eight million rivets. The bridge was opened on March 4, 1890 by the Prince of Wales, later King Edward VII. A contemporary materials analysis of the bridge, circa 2002, found that the steel in the bridge is of good quality, with little variation.

The use of a cantilever in bridge design was not a new idea, but the scale of Baker's undertaking was a real pioneering effort, afterwards extensively followed in different parts of the world. Much of the work done was without precedent, including calculations for incidence of erection stresses, provisions made for reducing future maintenance costs, calculations for wind pressures made evident by the Tay Bridge disaster, the effect of temperature stresses on the structure, and so on.


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Maintaining the Forth Bridge

Although modern trains put fewer stresses on the bridge than the earlier steam trains, the bridge needs constant maintenance.

"Painting the Forth Bridge" is a colloquial term for a never-ending task (a modern rendering of the myth of Sisyphus), coined on the erroneous belief that, at one time in the history of the bridge, repainting was required and commenced immediately upon completion of the previous repaint. According to a 2004 New Civil Engineer report on contemporary maintenance, such a practice never existed, although under British Rail management, and before, the bridge had a permanent maintenance crew.

A contemporary repainting of the bridge commenced with a contract award in 2002, for a schedule of work expected to continue until March 2009, involving the application of 20,000 m² of paint at a cost estimate of £10M a year. This new coat of paint is expected to have a life of 20 years.


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All photographs used are under Creative Commons license.