Forth Bridge
The Forth Bridge is a cantilever railway bridge across the Firth of Forth in the east of Scotland, west of central Edinburgh. Completed in 1890, it is considered a symbol of Scotland, 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, 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. When it opened it had the longest single cantilever bridge span in the world, until 1919 when the single span Quebec Bridge in Canada was completed. It continues to be the world's second-longest single cantilever span, with two spans of.
The bridge and its associated railway infrastructure are owned by Network Rail.
Background
Earlier proposals
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 of iron, Wilhelm Westhofen said of it "and this quantity 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 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 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 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 on the silt, encouraging Bouch to continue with the design. In August 1865, Richard Hodgson, chairman of the NBR, proposed that the company invest 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, weighed down with 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.
Bouch's 1871 design for the Forth Bridge fell significantly short of this figure, ason the advice of the Astronomer Royalhe had assumed a wind loading of only. This had been accepted by Barlow and Pole in their 1873 assessment of the design, though they qualified in their report that " we raise no object to Mr. Bouch's system, we do not commit ourselves to an opinion that it is the best possible".
Bouch's design was formally abandoned on 13 January 1881, and Sir John Fowler, W. H. Barlow, and T. E. Harrison, consulting engineers to the project, were invited to propose new designs. Bouch's Inchgarvie pier was left in place, protruding approximately from the water at high tide. It lies directly under the present bridge and was equipped with a small navigational light.
Design
Dimensions
The bridge spans the Forth between the villages of South Queensferry and North Queensferry and has a total length of with the double track elevated above the water level at high tide. It consists of two main spans of, two side spans of, and 15 approach spans of. Each main span consists of two cantilever arms supporting a central span truss. The weight of the bridge superstructure was, including the 6.5 million rivets used. The bridge also used of granite.The three great four-tower cantilever structures are tall, each tower resting on a separate granite pier. These were constructed using diameter caissons; those for the north cantilever and two on the small uninhabited island of Inchgarvie acted as cofferdams, while the remaining two on Inchgarvie and those for the south cantilever, where the river bed was below high-water level, used compressed air to keep water out of the working chamber at the base.
Engineering principles
The bridge is built on the principle of the cantilever bridge, where a cantilever beam supports a light central girder, a principle that has been used for thousands of years in the construction of bridges. In order to illustrate the use of tension and compression in the bridge, a demonstration in 1887 had the Japanese engineer Kaichi Watanabe supported between Fowler and Baker sitting in chairs. Fowler and Baker represent the cantilevers, with their arms in tension and the sticks under compression, and the bricks the cantilever end piers which are weighted with cast iron.Materials
The bridge was the first major structure in Britain to be constructed of steel; its French contemporary, the Eiffel Tower, was built of wrought iron. Large amounts of steel became available after the invention of the Bessemer process, patented in 1856. In 1859, the Board of Trade imposed a limit of for the maximum design stress in railway bridges; this was revised as technology progressed.The original design required for the cantilevers only, of which was to come from Siemens' steel works in Landore, Wales and the remainder from the Steel Company of Scotland's works near Glasgow. When modifications to the design necessitated a further, about half of this was supplied by the Steel Company of Scotland Ltd. and half by Dalzell's Iron and Steel Works in Motherwell. About of rivets came from the Clyde Rivet Company of Glasgow. Around three or four thousand tons of steel was scrapped, some of which was used for temporary purposes, resulting in the discrepancy between the quantity delivered and the quantity erected.
Approaches
After Dalmeny railway station, the track curves slightly to the east before coming to the southern approach viaduct. After the railway crosses the bridge, it passes through North Queensferry railway station, before curving to the west, and then back to the east over the Jamestown Viaduct.The approaches were built under separate contract and were to the design of the engineer James Carswell. The supports of the approach viaducts are tapered to prevent the impression of the columns widening as they approach the top, and an evaluation of the aesthetics of the Bridge in 2007, by A. D. Magee of the University of Bath, identified that order was present throughout, and this included in the approach viaducts. Magee points out that the masonry was carefully planned, and has neat block work even in areas not immediately visible from the ground.
Construction
The Bill for the construction of the bridge was passed on 19 May 1882 after an eight-day enquiry, the only objections being from rival railway companies. On 21 December, the contract was let to Sir Thomas Tancred, T. H. Falkiner and Joseph Philips, civil engineers and contractor, and Sir William Arrol & Co. Arrol was a self-made man, who had been apprenticed to a blacksmith at the age of thirteen before going on to have a highly successful business. Tancred was a professional engineer who had worked with Arrol before, but he would leave the partnership during the course of construction.The steel was produced by Frederick and William Siemens and Pierre and Emile Martin. Following advances in furnace design by the Siemens brothers and improvements by the Martin brothers, the process of manufacture enabled high quality steel to be produced quickly.