Forth Rail Bridge facts
constructed ... 1882 - 1890
overall length ... 2,465m
maximum span ... 521m
height of towers ... 104m
height above river (high tide) ... 46m
steel in superstructure ... 50,000 tons
number of rivets ... 6,500,000
maximum workforce ... 4,600
cost ... £2.5m
The Forth Rail Bridge
The cantilever principle
New proposals emerged from Sir John Fowler, W.H. Barlow and T.E. Harrison. The design was based on the 'continuous girder principle' essentially a continuous girder with definitive breaks at chosen points of contraflexure.
The original proposal was modified by Fowler and his junior partner, Benjamin Baker
, to the form we know today
. Speaking to the Royal Institution in 1887, Baker described the cantilever principle:
'Two men sitting on chairs extend their arms, and support the same by grasping sticks which are butted against the chairs. There are thus two complete piers, as represented in the drawing above their heads. The centre girder is represented by a stick suspended or slung from the two inner hands of the men, while the anchorage provided by the counterpoise in the cantilever end piers is represented here by a pile of bricks at each end.
'When a load is put on the central girder by a person sitting on it, the men's arms and the anchorage ropes come into tension.
'The chairs are representative of the circular granite piers. Imagine the chairs one-third of a mile apart and the men's heads as high as the cross of St Paul's, their arms represented by huge lattice steel girders and the sticks by tubes 12 feet in diameter at the base, and a very good notion of the structure is obtained.'
The first modern cantilever had been built in 1867 by Heinrich Gerber across the River Main at Hassfurt in Germany, with a central span of 130m (425ft). Named after him, cantilevers were originally called Gerber bridges. In the United States, the earliest bridge of the type was built in 1876 by Charles Shaler Smith over the Kentucky River, and in 1883 a notable cantilever bridge was built over the Niagara River by C.C. Schneider. This was the first to be called a cantilever ..... more >
Photo courtesy the Civil and Environmental Engineering Department Library,
Imperial College, London