It's one of Alfred Hitchcock's most memorable scenes from The 39 Steps (1935); Robert Donat as Richard Hannay escapes pursuit by stopping a train on the Forth Bridge and hiding perilously behind one of the girders, clinging to the steelwork 150 feet above the cold waters. This scene introduced viewers all over the world to one of the highlights of British engineering history, a marvellous example of what Victorian engineers were capable of. It is considered one of the strongest and most stable bridges in the world.

History

When the railways came to Scotland it was obvious that an east coast line linking Edinburgh, Dundee and Aberdeen would be extremely profitable. But the shortest route faced the difficulties of crossing the estuaries of the rivers Forth and Tay.

To provide a railway link between both shores of the Firth¹ of Forth, the initial solution proposed was that of a tunnel. Several studies were made, but finding no practical solution to the many problems, this approach was abandoned in 1806 and the idea of building a bridge was then put forward.

In 1865, a parliamentary resolution authorised the North British Railway, and its engineer Thomas Bouch, to construct a bridge over the Forth. A ferry crossing had been in use across the Forth at Queensferry since the 12th Century, and this was selected as the site for the bridge. The proposal envisioned a suspended bridge with twin aprons of 1600 feet each. However, Bouch was also responsible for the construction of a bridge over the River Tay. On the night of 28 December, 1879, the Tay Bridge collapsed during a severe storm and 75 passengers from the Edinburgh train were killed.

In the aftermath of this disaster, the requirements for a bridge over the Forth were reassessed. The railway needed to cross the Forth, but had to do so in a way that would be acceptable to a public still shocked by what was then the worst bridge disaster in history. What was produced by designers Sir John Fowler and Benjamin Baker wasn't just another cantilever-truss bridge. What resulted was a marvel of Victorian engineering, and a span so massively overbuilt that it barely moves in even the heaviest wind - although most large structures are designed to accommodate a certain amount of movement. This was to be achieved by using steel plate tubes with internal braces, the stoutest of these members to be 4m in diameter².

Construction

The contract for the bridge's construction was given out on 21 December, 1882, and work began on the erection of piers intended to support the three cantilevers. Each double-cantilever tower is founded on four piers consisting of 27.4m tall steel caissons filled with concrete up to the topmost 11m which consists of a core of stone faced granite.

By 1887, the cantilever arms had reached their full height of 100.6m and work began to attach them together. In September 1889, the cantilever arms were almost touching one another and workers were able to reach across, 48m above the river. On 6 November, 1889, the centre aprons were ready to be attached, but it was necessary to wait for the proper weather conditions to permit the steel to expand to enable the fixing of the aprons to the cantilevers.

The bridge was formally completed on 4 March, 1890 when HRH Edward Prince of Wales tapped into place the traditional 'golden'³ rivet.

It was the largest civil engineering structure achieved during the 19th Century and, although poet and artist William Morris described it as 'the supremest specimen of all ugliness', it remains today as one of the industrial wonders of the world. When completed, it was the longest span in the world. Even today it is still the second longest of its kind⁴.

The bridge (including approaches) extends just over 2.5km and required 54,000 tons of steel, 20,950 cubic metres of granite, 6780 cubic metres of stone, 49,200 cubic metres of concrete and about seven million rivets. 57 workers died while building the bridge, and 4600 worked on the bridge at peak period. Its total cost was £3,200,000⁵. Engineer and Chief Designer Benjamin Baker received a knighthood in 1890. He was also involved in the building of the London underground, the Aswan Dam and the transportation of Cleopatra's Needle from Egypt to England.

Present Day

The Forth Bridge, Scotland's biggest 'listed'⁶ building, is still very much in use today. Though owned by Network Rail, ScotRail and other Intercity services use the bridge to carry the main east coast line over to Fife and eventually onwards to Dundee and Aberdeen. On average more than 200 trains cross the bridge each day, but due to lighter modern rolling-stock the stresses placed on the bridge are much less than that caused by the heavier steam-powered trains.

While the expression 'like painting the Forth Bridge' was once used for a never-ending task, modern technology may have rendered it obsolete as the coating now used is glass flake epoxy paint which is usually used on oil rigs.

When the bridge was completed one of the engineers was asked how long it would last. He replied, 'forever if you look after it!'. Looking at the bridge today, it would be very hard to argue with that assessment.