Page:Popular Science Monthly Volume 36.djvu/480

464 inches wide. We now wish a bridge wider than this, but with no more material in it except what may be needed in order to make the floor. Suppose we take the top and bottom plank of this box girder, cut them in two lengthwise, making out of each two planks six inches wide instead of twelve, and we fasten these



upon each of the vertical planks, as shown in the drawing. We then have what is technically known as an I-bar, or flanged girder (Fig. 6). Provided these flanged girders are so braced as to prevent their bending sideways, the two flanged girders are of exactly the same strength as the box girder, and, as you see, can be placed at any distance apart, and the floor simply placed on top of them or on the lower flange, and we have a bridge as wide as we wish, with the strength of the box girder.

This I-bar, or flanged girder, is one of the most generally used forms of construction for bridges of short spans.

So far we have considered the material used to be simply wood, but I-bars are now made of iron or steel, and within the last few



years entirely of steel, owing to the fact that the improved method of making steel has rendered it even cheaper than wrought iron.

Let Fig. 6 represent a side-view of the flanged girder, or I-bar, of which Fig. 6 A is an end-view.

Suppose this beam to be supported at each end and a load placed upon the center. Then the tendency of that load would be to bend the beam down in the shape of the dotted line, and, in case the load is sufficient, it would break in that way.

Before the breaking-point is reached the top and bottom of this beam are subjected to totally opposite classes of strain, as you will see. If you bend the beam, the tendency in the bottom of it is to pull the beam apart, or, in technical language, the bottom flange of the beam is in tension—tension being the force which tends to pull apart the particles of the beam. Thus, if you take a string fastened at one end, and hang a weight on the other end, the string is in tension, the action of the weight being to pull the particles of the string apart.

The top flange of the beam is in compression—that is, the