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.
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).
