Page:Popular Science Monthly Volume 36.djvu/495

Rh different parts of the bridge while it is being put together. This is a great expense; and in some cases, where the bridge is far above the water, the current very rapid, or an existing necessity of not obstructing the water-way, becomes impossible. In the erection of a cantilever, each cantilever span is balanced on its own pier and built out each side from the tower in such a manner as to preserve this balance until the shore ends are anchored firmly, after which the river ends can be extended as far as desired. The suspended span, which is never of extraordinary length, can usually be built directly from the ends of the cantilever spans, and the necessity of false work entirely done away with.

In all bridges of long span that the weight of the bridge itself is by far the greatest load that the bridge has to bear. In bridges of short spans, the weight of the locomotive and train coming upon them suddenly constitutes the greatest load, as the weight of the train is greater than the weight of the bridge itself; but as the size of the bridge increases, its weight increases very rapidly, and the weight of the locomotive and train becomes almost nothing as compared with the weight of the bridge itself; that is, if any of these long bridges are built strong enough to hold themselves up, with a very slight margin of safety above that, there is scarcely any danger of their ever falling from any weight that could come upon them from an outside load. For this reason, in building short-span bridges, the amount of economy that can be exercised in the use of material is very small, as the bridge must be built stiff and rigid, even if this necessitates the using of much more material than the absolute weight of the tender and locomotive that come upon it would demand. For this reason plate girders or flanged girders have many advantages connected with their use for short-span bridges, because the whole amount of material used is comparatively of little value, and extreme stiffness and rigidity are the result; while in the case of long-span bridges, such as cantilever, or any remarkably long bridge, every calculation must be made with the greatest care in order to reduce the amount of necessary material as much as possible, because by reducing the amount of material used the weight is reduced, and that again reduces the amount of material. The factor of safety to be used can be only two or three in long-span bridges, while in short spans it should run up even to ten.

Although cantilever bridges are of rather recent use in this country and in Europe, and much has been written claiming them as an invention of modern times, still, the same can be said of them that was said of suspension bridges—that there at present exists in Japan, built by the order of the Mikado two hundred and fifty years ago, as perfect and scientific a cantilever bridge as any that