“Yes. A terrible disaster. I should think they would give up their scheme of bridging the Firth of Forth after that.”
“Not they! The scheme may be altered, but bridge it they will. Engineers never give in.”
The comments of these newspaper readers were right. The Tay Bridge, the longest in the world, had been blown down one wild December night in 1879, and girders, towers, and the train which was rushing over it, were suddenly hurled into the surging flood.
At that time a scheme was in hand to bridge the Forth for the North British Railway system, and Sir Thomas Bouch had proposed two suspension bridges hung by steel chains. But ultimately a new design altogether was adopted, the plan being by Sir Benjamin Baker and Sir John Fowler.
It was the new principle—or, rather, a remarkable development of an old principle—for which the bridge-making world was waiting: the principle, namely, of the cantilever.
A cantilever is, in fact, a bracket; and Sir Benjamin Baker has described it as such. It is a strong support, built out from a firm base, and is like a powerful and magnified bracket upholding a shelf.
In the Forth Bridge there are two huge spans, 1700 feet wide, crossed by these cantilevers; bridging channels of some 200 feet deep.
The longest spans on the Tay Bridge were 245 feet; it was over two miles long, and had ninety spans. It was an iron girder bridge, and was opened on the 31st of May, 1878. Not to be beaten, however, after the panic had subsided, another and more stable bridge was constructed, also a girder, but not so high in elevation, and sixty feet further up the river. It was opened in 1887, and is 10,779 feet long, with 85 piers, the navigable channel being under four of the spans, the centre spans being 245 feet wide.
It will be seen at once that the cantilevers at the Forth Bridge cover very much wider spans; and the channel being so deep, the impossibility of building piers will also be obvious. The best place for the bridge was marked by the projection of the Inverkeithing peninsula on the north shore, and also the Inchgarvie rock in the channel itself. The peninsula brought the two shores together, reducing the space to be bridged, and the rock gave firm support for a pier. Still there were the two immense spans of 1700 feet to be crossed, and the engineers decided on the cantilever principle. Thus, though the Tay Bridge was the longest in the world, the Forth presented by far the greatest spans—viz., the two main spans of 1700 feet each, in addition to which there are two of 675 feet each, and fifteen of 168 feet each.