Engineering Wonders of the World
Volume I

THE STORY OF THE FORTH BRIDGE. 323 in diameter, braced together diagonally in all directions, and connected top and bottom by enormously strong horizontal members. Viewed from the side, these tubes appear vertical, but they are really parallel only in the direction of the axis of the bridge, as a uniform “ batter ” of 1 in 7 j diminishes the base distance of 120 feet between the two pairs of columns, centre to centre, to 33 feet at the top. The north and south towers are each 145 feet long between centres of columns, while the central, or Inchgarvie, tower has a length of 260 feet, for a reason to be explained later. Turning next to the six 680-foot brackets, or cantilevers, we may note that each has a roughly triangular shape. At the columns it is 330 feet high, but gradu- The ally tapers to 34 feet at the Cantilevers, end posts. It is made -up of tubular bottom compression booms, tapering from 12 feet in diameter at the piers to 3 feet square at the outer ex- tremities ; of a massive lattice-work top tension member ; and of six tubular struts and as many lattice ties to the side, making six “ bays.” These various parts are, like the towers, stiffened laterally by a number of diagonal and cross ties ; and, as the sides of a cantilever have the same batter as the towers, they are much narrower at the top than at the bottom. In short, from what- ever point they be viewed—sideways, end- ways, or vertically—the influence of the tri- angle is obvious. In addition to the six cantilevers there are the two components, which may be con- sidered the counterparts of the plank con- necting our two balconies. Suspended These, known as “ suspended Girders. girders,” are 350 feet long each, and rest on the two pairs of cantilevers which they join. Though small as compared with the rest of the bridge, they would, if placed elsewhere, attract con- siderable attention, and their individual weight of 820 tons certainly commands respect. The two main spans have each a length of 1,710 feet, and at present are the longest bridge spans in the world. Adding to them the two end cantilevers and the three towers, the length The of the main steelwork comes Approaches, out at 5,330 feet. The bridge proper also includes a north and a south approach, 1,989 and 968 feet long respectively, made up of arches and girders based on tall masonry piers, which carry the track into the cantilevers at an elevation of 150 feet above Why the Present Site was chosen. high tide. The grand total is therefore about 8,300 feet between abutments. As in the case of the Britannia Tubular Bridge, described in a previous chapter, the engineers were aided by a rock situated in the middle of the channel, or, to be more correct, separating the deeper waters of the Forth into two channels of almost equal width. Inchgarvie is its name. Another reason for selecting the site chosen was the triangular tongue of land projecting southwards from the Fife shore, and here reducing the Firth to about one-half of its average width, as reckoned over several miles. It was also fortunate that the general level of the land on both shores was such as to suit a rail level on the bridge sufficient to give ample headway for all shipping. The northernmost, or Fife, tower rises on the edge of the north channel ; or Inchgarvie, tower stands on end of the island of that name, referred to above ; while the south, or Queensferry, tower is situated at the south- ern edge of the deep water of the south, channel. The eight piers for the Fife and Inchgarvie towers had a rock foundation ; whereas the Queensferry piers had to be based on a hard boulder clay underlying a stratum of gravel, the central, the western The Three Towers.