The Forth Bridge is a celebrated Scottish landmark, and a milestone in the development of railway civil engineering.
It was the first major structure in Britain to be made of steel and its construction resulted in a continuous East Coast railway route from London to Aberdeen.
The first crossing is begun
The first crossing of the Forth by the railway came in 1850 when the Edinburgh, Leith and Granton Railway started the world’s first ‘train ferry’ – a ferry boat specially designed by Thomas Bouch to take railway coaches – between Granton and Burntisland.
In August 1873 the North British Railway obtained authority to build a railway bridge across the Firth of Forth and construction of a suspension bridge, also designed by Thomas Bouch, began in 1878. However when Bouch’s original Tay Bridge collapsed during a storm in December 1879, work on his bridge across the Forth stopped immediately pending a full inquiry.
Bouch’s suspension bridge plans were abandoned in 1881 and designs for a new bridge were invited by the newly formed Forth Bridge Railway Company which had been established jointly by those railway companies who had most to gain from a railway crossing the Forth: the North British Railway, the Midland Railway and the North Eastern and Great Northern railways.
New plans are made
The bridge was to cross the Forth between South Queensferry, now part of Edinburgh and North Queensferry in Fife, making use of the island of Inch Garvie a little way from the north shore. Its design had to conform to specifications from both the admiralty who stipulated that the Forth remained a navigable channel, and the Board of Trade, concerned by the recent Tay Bridge disaster, who stipulated that the bridge must be rigid and stiff and capable of carrying the heaviest freight trains.
John Fowler and his partner Benjamin Baker were engaged by the Forth Bridge Company to develop their cantilevered design for the bridge which took into account these restrictions. The contract for its construction was let to Messers Arrol & Co of Glasgow in 1882 and work on the bridge started in 1883.
Bridge building on a vast scale
The Forth Bridge has three double cantilevers with two 1700ft suspended spans between them, at the time the longest bridge spans in the world. As required by the Admiralty, the rail level is 150ft (46m) above high water. Each of the towers has four steel tubes 12ft (3.7m) in diameter and reach to a height of 361ft (110m) above high water. Their foundations extend 89ft below this into the river bed, making the total height from foundations to the top of the towers 137 metres. The total length of the bridge, including its approach viaducts is 2,467 metres. The main structure itself measures 1,630 metres portal to portal.
Baker and Fowler’s bridge was the first major construction in Britain to be made from steel; the bridge incorporates 53,000 tonnes of the material.
The design of the bridge was very carefully balanced, with allowance being made for a maximum thermal expansion of 16½ inches (420mm) over the 5350ft (1630m) steel central structure. It incorporated 6.5 million rivets, which aggregated 4,200 tons weight alone. It was designed to withstand a wind force of 56lb per square foot.
Foundations and steelwork
Building the foundations for the vast towers started with the construction of huge caissons which were built on site and sunk using compressed air. The first of the caissons was floated into position on 26 May 1884.
By 1886 all the foundations were in position ready to take the steelwork. Thanks to the organisation and inventiveness of William Arrol, the bridge was completed in November 1889, just 6 years after work started (although at the time the weather was particularly cold and Arrol had to wait for milder weather conditions before the enormous structure expanded sufficiently for the final rivets to be inserted).
Overall the bridge cost £3m to build and employed a workforce of 4,600 men at the height of construction.
After all testing and inspections of the bridge were completed, it was formally opened by the Prince of Wales (later King Edward VII), who drove home a final gold plated rivet, on 4 March 1890. At the same ceremony, he also knighted Benjamin Baker.
Painting the Forth Bridge
In 2001 a major refurbishment project on the Forth Bridge was announced. Over the following 10 years, sections of the bridge were covered with significant scaffold access systems with specially prepared screening to prevent debris from the sandblasting and paint affecting or contaminating the environment.
After removing the old paint back to the metal, any steelwork that required maintenance was repaired before the new paint was applied.
After thorough cleaning of the steel structure, paint was then applied in three protective layers, both by airless spray and by hand in areas particularly difficult to access, over an area of 230,000 square metres.
The techniques and paint used during the refurbishment means that the bridge will not require a full paint for at least 20 years, finally putting an end to the myth that ‘painting the Forth Bridge’ is a never-ending task.
Did you know?
All that was built of Thomas Bouch’s suspension bridge was a single pier on Inch Garvie. Today it has a small lighthouse; dwarfed by the vast scale of the Bridge itself.