The three chief piers consist of groups of four columns of masonry, each gradually tapering from 55 feet in diameter to 49 feet at the top, and about 36 feet high. From these rise the huge cantilevers connected together by girders 350 feet in length.
The centre of these three main piers rests on the island of Inchgarvie; the two others are known as the Fife and the Queensferry piers respectively, and are placed on the side of the deep water channels. In addition to these three main piers are several others, some in shallow water and some on land. The part of the bridge which they carry is an ordinary girder of steel leading to the immense cantilevers. For founding the shallow water piers, cofferdams were used; the caissons with compressed air chambers being for the deep water structures.
They were put together on shore, launched, floated, steered to the desired position, and sunk. One proved cranky and turned over, and was only brought right after much expense and difficulty.
The cantilevers are bolted down to each pier by numbers of huge steel ties, 24 feet in length and 2½ inches in diameter, embedded in the masonry, there being 48 of these bolts or ties to each column. And now as to these cantilevers.
Four huge tubular shafts, two on each side, rise from the group of columns forming each pier, to the height of 350 feet. From these shafts, which slope slightly inward, project the cantilevers, the upper and lower parts being strongly braced together by diagonal ties. In shape the gigantic brackets taper towards a point, the width decreasing as much as from 120 feet at the commencement of the piers to 32 feet at the ends. The wind, it is believed, will be more effectually resisted by this means.
The cantilevers are hung back to back, one to some extent counter-weighing the other. The component parts consist of cylinders of steel or struts for resisting compression—these are the lower parts; and ties of lattice-work made of steel plates for resisting tension,—placed above.
Thus, then, from each of the three chief piers two pairs of gigantic brackets project, each pair placed side by side and braced together, and forming one composite cantilever jutting to the north and one to the south. The rails run on sleepers placed lengthwise and fixed in troughs of steel, so that should a train run off the line the wheels will be caught by these supports.
It is calculated that there are about 45,000 tons of steel in the bridge, and 120,000 cubic yards of masonry in the piers. The contract price was £1,600,000, which works out at about £215 per foot; and the contractors, who were able to obtain an admirable organisation of some 2000 men to carry out the magnificent design, were Messrs. Tancred, Arrol, & Co. Some special tools for use in the work were planned by Sir William Arrol. The bridge was opened by the Prince of Wales on the 4th of March, 1890.
The success of this magnificent structure has assured the wider adoption of the cantilever principle. Long-span bridges, in several cases, have since been built on this design. Its engineers may claim indeed to have widened the scope and possibilities of bridge-building.
Still, when another bridge was wanted over the Thames, at a busy spot, crowded with shipping and near the historic Tower of London, another kind of structure was adopted. What was it?