About Forth Bridge

Construction of an earlier bridge, designed by Sir Thomas Bouch, got as far as the laying of the foundation stone, but was stopped after the failure of another of his works, the Tay Bridge. Bouch had proposed a suspension bridge but the public inquiry into the Tay bridge disaster showed that he had under-designed the structure and mistakenly used cast iron, which weakened the entire structure.

On Bouch's death the project was handed over to SirJohn Fowler and Sir Benjamin Baker, who designed a structure that was built by Sir William Arrol & Co.between 1883 and 1890. It was built in steel alone, the first bridge to use that material. Baker "one of the most remarkable civil engineers Britain ever produced" and his colleague Allan Stewart received the major credit for design and overseeing construction work.

The bridge is, even today, regarded as an engineering marvel. It is 2.5 km (1.5 miles) in length, and the double track is elevated 46 m (approx. 150 ft) above high tide. It consists of two main spans of 1,710 ft (520 m), two side spans of 675 ft, 15 approach spans of 168 ft (51 m), and five of 25 ft (7.6 m). Each main span comprises two 680 ft (210 m) cantilever arms supporting a central 350 ft (110 m) span girder bridge.

The three great four-tower cantilever structures are 340 ft (104 m) tall, each 70 ft (21 m) diameter foot resting on a separate foundation. The southern group of foundations had to be constructed as caissons under compressed air, to a depth of 90 ft (27 m). At its peak, approximately 4,600 workers were employed in its construction. Initially, it was recorded that 57 lives were lost;however, after extensive research by local historians, the figure has been revised upwards to 98.

More than 55,000 tons of steel were used, as well as 18,122 m³ of granite and over eight million rivets. The bridge was opened on 4 March 1890 by the Prince of Wales, later King Edward VII, who drove home the last rivet, which was gold plated and suitably inscribed. A contemporary materials analysis of the bridge in 2002 found that the steel in the bridge was of good quality, with little variation.

Each of the 110 m. high double-cantilevers is supported on well-founded granite faced piers. The bridge's construction involved the employment of 4,000 men at times, the use of 54,000 tonnes of steel and the driving of 6,5 million rivets. Its total cost was £3,200,000, a phenomenal sum in its day.

The use of a cantilever in bridge design was not a new idea, but the scale of Baker's undertaking was a pioneering effort, later followed in different parts of the world. Much of the work done was without precedent, including calculations for incidence of erection stresses, provisions made for reducing future maintenance costs, calculations for wind pressures made evident by the Tay Bridge disaster, the effect of temperature stresses on the structure, and so on. Where possible, the bridge used natural features such as Inchgarvie, an island, the promontories on either side of the firth at this point, and also the high banks on either side.

In 1996 Railtrack (now Network Rail) began a structural and maintenance assessment of the structure at the request of the Health & Safety Executive. The resulting £40m refurbishment package began in 1998, comprising steelwork repairs, surface coating, access improvements and an upgrade of the floodlighting. The contractor undertaking this work left site in 2002 due to financial problems. Balfour Beatty Civil Engineering gained the maintenance contract in 2002 (£10M per annum until March 2009). The coating system employed for the steelwork requires blast cleaning to bare metal; an application of zinc based primer to prevent corrosion (35 microns); a glass flake epoxy intermediate coat providing a barrier (400 microns); and, a polyurethane gloss top coat to give an attractive "Forth Bridge Red" finish (35 microns) on all of its estimated 400,000m². This system which has been tried and tested in an offshore environment is designed to give a 20­year life which means the bridge may be free of its legendary painters after 2009 for a short while!

The bridge has a speed limit of 50 mph (80 km/h) for passenger trains and 20 mph (32 km/h) for freight trains. The weight limit for any train on the bridge is 1,422 tonnes (1,442,000 kg) although this is waived for the frequent coal trains, provided two such trains do not simultaneously occupy the bridge.

Up to 190-200 trains per day crossed the bridge in 2006.