Page:Popular Science Monthly Volume 36.djvu/477

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BRIDGE is a structure over a river, ravine, or other opening, for the purpose of sustaining a moving load. This, in the case of a railroad bridge, consists of a heavy locomotive and train coming on at one end, rushing rapidly over the bridge, and off at the other end. This fact, that the load to which bridges are subjected is a moving load applied for only a short period of time and then removed, is a most important factor to be considered in calculating the necessary strength of the various members, as the strain produced in any piece of material by the application of a load is nearly doubled when the load is applied quickly as compared with that produced by the same load when applied gradually.

Bridges may be divided into the following classes: 1. The beam or girder. 2. The framed truss. 3. The arch. 4. The suspension bridge.

The most ordinary form which we see in this country, and the one most generally used for the purpose of railway bridges, is the framed truss, and that is the one the development of which it is our purpose to show.

The one point to be carefully studied in all bridge construction is economy—that is, to get as much strength with as little material as possible; in other words, the maximum amount of strength with the minimum amount of material.

The simplest method of crossing any opening where the dimensions of the opening are not so great, or the load so heavy, as to forbid its use, is by means of a plank placed from one side to the other, making the plank of such a length that the ends may have sufficient bearing upon each side of the opening (Fig. 1).



In crossing an opening by means of a simple plank or beam, supposing we make the beam large enough, it answers every purpose and will hold up the required load. But in this there is great waste of material. We will take, for example, a plank twelve inches wide, and three inches deep, over an eighteen-foot opening—that is, the plank would have to be about twenty-one or twenty-two feet long, in order to allow the ends sufficient bearing surface upon the masonry on each side. This plank would hold up a certain amount of weight, but, as the weight is increased, in