Page:Popular Science Monthly Volume 36.djvu/483

 iron rod, and all the weight upon these ends of the beams passes up the rod, and then comes down the two diagonal beams to the abutments. The one thing necessary in this is that the lower ends of the two diagonal beams shall be so fixed as to make it impossible for them to slip out in the direction of the arrows, and this object is usually attained by making the floor stringers serve as a tie to hold them together. In the drawings, the full black lines are in compression and the dotted lines are in tension. Thus, you see the vertical rod or rope in the center is in tension—that is, a weight being at W, all of that weight comes directly upon the rod and is carried to the apex, A; then half of it passes to each side down the inclined braces, and they are in compression. The tendency at the foot of these braces is for them to slip out in the direction of the arrows. They are held together by the tie-rod or floor stringers, which are in tension. In regard to tension and compression, you may get a better comprehension of them if you understand that a cord or rope can be used for any member of a bridge that is in tension, while a post or some stiff piece of timber or iron is necessary for anything in compression—that is, in all these diagrams the dotted lines could be replaced by ropes or cords, while the full black lines are obliged to be iron or wooden posts or braces.

You thus see that we have the simplest form of a framed truss. This form of truss is called the king-post truss. Now, as the width of the opening increases, the height of the posts would also have to increase, and in a very short time would get so high, and make the inclined braces so long, as to become unwieldy. In order to overcome this, after a certain height has been reached, instead of continuing the king-post higher, we simply cut it off and substitute two posts or rods in its place (Fig. 12). In this the



length of span that can be covered with the same sized material is one half larger, and the bridge is divided into three panels, as they are called. A panel is one of any number of equal parts into which the truss of a bridge is divided by means of the posts or rods. This second truss is called the queen-post truss; here also the full black lines are in compression and the dotted lines are in tension. As you will notice in this truss, which is also the case in