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THE FORTH RAIL BRIDGE

The 2.5 km. (1.5 mile) Forth Railway Bridge, the world’s first major steel bridge, with its gigantic girder spans of 521 m. (1710 ft.) ranks as one of the great feats of civilization. It was begun in 1883 and formally completed on 4 March 1890 when HRH Edward Prince of Wales tapped into place a ‘golden’ rivet.

 

Tancred–Arrol, constructed the bridge, robustly designed in the aftermath of the Tay Bridge disaster by civil engineers Sir John Fowler and Benjamin Baker. The balanced cantilever principle was adopted. The main crossing comprises tubular struts and lattice-girder ties in three double-cantilevers each connected by 105 m. (345 ft.) ‘suspended’ girder spans resting on the cantilever ends and secured by man-sized pins. The outside double-cantilever shoreward ends carry weights of about 1000 tonnes to counter-balance half the weight of the suspended span and live load.

The Forth Rail Bridge
 

The Trust has its own copy of the human cantilver model

This concept is readily understood from Baker’s ‘human cantilever’ model with his assistant Kaichi Watanabe representing the live load. The pull in his supporters arms indicates the tension in the ties and the push in the lower struts the compression in the tubes.

Each of the 110 m. (361 ft.) 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 driving 6,500,000 rivets. Its total cost was £3,200,000 (~£235,000,000 today).

During operations, rescue boats were stationed under each cantilever saving at least 8 lives, but still 57 men lost their lives. The continuous painting of its 18 ha. (45 acre) surface, for the first 100 years using paint supplied by Craig & Rose, is now being done using a state-of-the-art paint regime with at least a 20-year life.

In 1996 Railtrack (now Network Rail), at the request of the Health & Safety executive, began a structural and maintenance assessment of the structure. The result of this was the £40m refurbishment package which began in 1998; this comprised 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!

Today, the bridge, Scotland’s biggest ‘listed’ building, continues to form a vital artery in Network Rail's East Coast railway system; it carries 180 - 200 train movements per day.

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