Forth Bridge graphic

Classic Projects: Forth Bridge

Widely recognised as one of Scotland's defining landmarks, this Victorian engineering icon was the world's longest single cantilever construction when it was built 125 years ago.

Widely recognised as one of Scotland's defining landmarks, this 125-year-old Victorian engineering icon was the world's longest single cantilever construction until 1917, when it was surpassed by the Quebec Bridge. Today, the Forth Bridge that spans the Firth of Forth nine miles west of Edinburgh is often (and incorrectly) known as the 'Forth Rail Bridge', to distinguish it from the Forth Road Bridge that opened in 1964.

Although the crossing was not to materialise until 1890, there had been plans to build a bridge across the Firth as early as the beginning of the 19th century.

Thomas Bouch was one of the engineering pioneers who put together proposals to replace the existing ferry service. Bouch conducted various experiments to determine what type of construction was needed, but mounting geological and financial problems, coupled with the collapse of his Tay Bridge in 1879, meant that confidence in Bouch's original proposals tailed off, paving the way for the design we have today.

Brainchild of Sir John Fowler and Sir Benjamin Baker, the bridge took eight years to build, employing the civil engineering skills of Sir Thomas Tancred, T H Falkiner and Joseph Philips, with the main contractor Sir William Arrol & Co. Several restrictions were placed upon the construction by both the Admiralty and the Board of Trade which, understandably nervous after the Tay Bridge disaster, wanted to make sure that not only was the bridge capable of carrying the heaviest freight trains, but would also leave the Firth of Forth navigable to sea traffic.

With these caveats very much in mind, the designers came up with new plans that abandoned the original idea for a suspension bridge in favour of a cantilever format. The result was a design incorporating three cantilevers with two 1700ft (520m) suspended spans between them - a principle that is illustrated in a famous photograph, where Japanese engineer Kaichi Watanabe is shown supported on a cantilever beam between Fowler and Baker, sitting on chairs with their arms in tension and supporting sticks under compression.

Baker and Fowler were responsible for the first major civil engineering project in Britain to be made from steel. Calculations were made to allow for a thermal expansion of 420mm over the 1630m central steel structure, which included 6.5 million rivets. Construction was actually finished in November 1889, but due to the extreme cold of the Scottish winter, engineers had to wait for the weather to warm up before the steel could expand enough for the final rivets to be put in place. The Prince of Wales, who later became King Edward VII, hammered the final golden rivet home in a ceremony on 4 March 1890.

One of the enduring myths to have entered the language is that painting the Forth Bridge is a never-ending task (engineering maintenance, however, definitely is). As part of the 2001 refurbishment project the original paint was stripped back to the bare metal and the bridge was completely repainted in three coats, both by airless spray and by hand in the less accessible areas. Thanks to the most modern of paint techniques, it is unlikely that the bridge will need another coat for two decades.

The Forth Bridge has made its way into popular culture. In 2004 it appeared on a British pound coin and latterly on Bank of Scotland banknotes. It has featured in the 1935 Alfred Hitchcock film 'The 39 Steps', Irn-Bru adverts, Alan Turing's paper on artificial intelligence and even in the video game Grand Theft Auto.

Next month: Flying Scotsman steam locomotive

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