Forth Bridge

The Forth Bridge is a cantilever railway bridge across the Firth of Forth in the east of Scotland, 9 miles (14 kilometres) west of central Edinburgh. Completed in 1890, it is considered a symbol of Scotland (having been voted Scotland's greatest man-made wonder in 2016), and is a UNESCO World Heritage Site. It was designed by English engineers Sir John Fowler and Sir Benjamin Baker. It is sometimes referred to as the Forth Rail Bridge (to distinguish it from the adjacent Forth Road Bridge), although this is not its official name.

Construction of the bridge began in 1882 and it was opened on 4 March 1890 by the Duke of Rothesay, the future Edward VII. The bridge carries the Edinburgh–Aberdeen line across the Forth between the villages of South Queensferry and North Queensferry and has a total length of 2,467 metres (8,094 ft). When it opened it had the longest single cantilever bridge span in the world, until 1919 when the single 1,801 ft (549 m) span Quebec Bridge in Canada was completed. It continues to be the world's second-longest single cantilever span, with two spans of 1,709 feet (521 m).

The bridge and its associated railway infrastructure are owned by Network Rail.

Before the construction of the bridge, ferries were used to cross the Firth. In 1806, a pair of tunnels, one for each direction, was proposed, and in 1818 James Anderson produced a design for a three-span suspension bridge close to the site of the present one. Calling for approximately 2,500 tonnes (2,500 long tons; 2,800 short tons) of iron, Wilhelm Westhofen said of it "and this quantity [of iron] distributed over the length would have given it a very light and slender appearance, so light indeed that on a dull day it would hardly have been visible, and after a heavy gale probably no longer to be seen on a clear day either".

For the railway age, Thomas Bouch designed for the Edinburgh and Northern Railway a roll-on/roll-off ferry between Granton and Burntisland that opened in 1850, which proved so successful that another was ordered for the Tay. In late 1863, a joint project between the North British Railway and Edinburgh and Glasgow Railway, which would merge in 1865, appointed Stephenson and Toner to design a bridge for the Forth, but the commission was given to Bouch around six months later.

It had proven difficult to engineer a suspension bridge that was able to carry railway traffic, and Thomas Bouch, engineer to the North British Railway (NBR) and Edinburgh and Glasgow Railway, was in 1863–1864 working on a single-track girder bridge crossing the Forth near Charlestown, where the river is around 2 miles (3.2 km) wide, but mostly relatively shallow. The promoters, however, were concerned about the ability to set foundations in the silty river bottom, as borings had gone as deep as 231 feet (70 m) into the mud without finding rock, but Bouch conducted experiments to demonstrate that it was possible for the silt to support considerable weight. Experiments in late 1864 with weighted caissons achieved a pressure of 5 t/sq ft (4.9 long ton/sq ft; 5.5 short ton/sq ft) on the silt, encouraging Bouch to continue with the design. In August 1865, Richard Hodgson, chairman of the NBR, proposed that the company invest £18,000 to try a different kind of foundation, as the weighted caissons had not been successful. Bouch proposed using a large pine platform underneath the piers, 80 by 60 by 7 feet (24.4 m × 18.3 m × 2.1 m) (the original design called for a 114 by 80 by 9 feet (34.7 m × 24.4 m × 2.7 m) platform of green beech) weighed down with 10,000 tonnes (9,800 long tons; 11,000 short tons) of pig iron which would sink the wooden platform to the level of the silt. The platform was launched on 14 June 1866 after some difficulty in getting it to move down the greased planks it rested on, and then moored in the harbour for six weeks pending completion. The bridge project was aborted just before the platform was sunk as the NBR expected to lose "through traffic" following the amalgamation of the Caledonian Railway and the Scottish North Eastern Railway. In September 1866, a committee of shareholders investigating rumours of financial difficulties found that accounts had been falsified, and the chairman and the entire board had resigned by November. By mid-1867 the NBR was nearly bankrupt, and all work on the Forth and Tay bridges was stopped.

The North British Railway took over the ferry at Queensferry in 1867, and completed a rail link from Ratho in 1868, establishing a contiguous link with Fife. Interest in bridging the Forth increased again, and in 1871 Bouch proposed a stiffened steel suspension bridge on roughly the same line as taken by the present rail bridge. This design was examined and pronounced acceptable by W. H. Barlow and William Pole, both "eminent" civil engineers, and Parliament passed in August 1873 an act authorising its construction. Work started in September 1878, in the form of a brick pier at the western end of the mid-Forth island of Inchgarvie.

After the Tay Bridge collapsed in 1879, confidence in Bouch dried up and the work stopped. The public inquiry into the disaster, chaired by Henry Cadogan Rothery, found the Tay Bridge to be "badly designed, badly constructed and badly maintained", with Bouch being "mainly to blame" for the defects in construction and maintenance and "entirely responsible" for the defects in design. In particular, Bouch had failed to properly account for the effect that high winds would have on the bridge, and in response to this finding the Board of Trade imposed a requirement that all bridges be designed to accept a lateral wind loading of 56 lb/sq ft (270 kg/m²).