Page:Popular Science Monthly Volume 36.djvu/493

Rh the center of the river may be less than that required by navigation, so that, from what we now have, our only remedy would be the use of the suspension bridge.

In order to do away with this, and make a wider opening in the center of the river, suppose we take away the center pier and replace it by two piers directly under the king-posts of the truss (Fig. 30). In this way we see we have left a large, clear span in



the center of the river, and we have in no way increased the amount of material necessary for the building of the bridge, and the two spans that we are using are now balanced, each upon the top of its respective pier. These two spans are fastened together in the center, and the shore ends of both are anchored firmly, in order to keep them from tipping up whenever a load comes upon the river end. We thus see that we have doubled the clear span in the center of the river, and we have what is called a cantilever bridge; that is, a truss supported at one end, and extending out over an opening, there being no support under the other end. Now, suppose it was desired to make this center opening still larger, we have simply to move the piers apart (Fig. 31). We



have our two cantilever spans. The shore end of each is firmly anchored down, and the two other ends, A B, project simply into space. If we build thus, the two ends, A B, are firmly fixed and can not in any way yield to the load that may come upon them. If we now construct an ordinary framed truss, of either the Howe, Pratt, or any other type, and instead of putting this truss upon two piers or abutments we simply hang it between the two ends of the cantilever spans, A B, which are projecting over the river, the weight of this truss will be sustained by the tie-rods from the king-posts, or in the case of the cantilever that run over the tower and are anchored down upon the other side. We thus see that by increasing very slightly the amount of material used