Page:Popular Science Monthly Volume 36.djvu/494

478 in the construction of the "bridge we have an open, clear span in the center three times the width that would be possible with the ordinary framed truss, and we have what is known and has lately become famous as the cantilever bridge. The spans C A and B D are called the cantilever spans, and A B the suspended span.

The variations in the cantilever bridge are almost infinite, although the principles in all of them are the same. In the place of using the upright truss, for example, this truss can be turned over, and the ties then become braces, the floor comes upon the



top, the shore ends being firmly anchored in place, and, the suspended span held in place, we have a cantilever of the type that has just been erected across the Hudson at Poughkeepsie (Fig. 32), while the first example given you is the type of cantilever that crosses the St. John River at St. John, New Brunswick (Fig. 31).

Then, when a greater length is desired and increased strength, as in the case of the bridge across the FrithFirth [sic] of Forth, in Scotland, we simply combine the two, putting the two king-post trusses base to base, and hanging the suspended truss between (Fig. 33).

In that way we have the strongest form for the cantilever bridge, and there is hardly any limit to the length of span that can be made by this method.



The advantages gained by the use of the cantilever for long-span bridges are as follow:

As a substitute for the suspension bridge it is, up to a certain length of span, less expensive, and it can be given great rigidity and stability which are impossible in the suspension.

As a substitute for the ordinary framed truss it has the advantages of not requiring any false works for its erection. In the erection of the ordinary bridge there must be built first a timber frame or staging between the piers to sustain the weight of the