suppose we take one of the planks and stand it up edgewise, and then place the other plank upon its flat side upon the top of this, as shown in Fig. 3, and nail the planks firmly together.
We now have a bridge twelve inches wide, as we originally had, but fifteen inches deep, or what is known as the T-bar or girder, and the only difficulty about this bridge is the trouble in making it stand up; being so much higher than it is wide, it has a great tendency to tip over. But supposing the planks are made to stand in this shape, which is a simple matter, we then have a bridge fifteen inches deep, which will hold about seven times as much load as the original plank. Of course, if the bridge were made fifteen inches deep and the same width, that is, twelve inches wide, it would hold twenty-five times as much as the original bridge; but by turning one of the planks upon the edge we have increased the depth and decreased the breadth, so that the breadth of the bridge under the top plank is only one fourth of what it was before, and the total strength of the bridge is from seven to eight times that of the original plank. Now, in order to obviate some of the difficulty in making this bridge stand up, suppose we take the plank that is upon the edge and make two planks, each of them twelve inches wide but only an inch and a half thick, and then nail the floor plank upon the edges of this, making an inverted box, as shown in Fig. 4. We then have a bridge that there is no trouble in making stand, as it has twelve inches of bearing surface, and we have the same amount of strength as when it was in the shape of the T-bar, and we have what is technically known as the U-bar or channel-bar.
In this U-bar there is this trouble: that, having the sides only an inch and a half thick and twelve inches deep, there is a tendency to bend in the sides that is, a tendency to give sidewise; and in order to obviate this we take the top plank and split it in two, making two planks twelve inches wide and an inch and a half thick. Nail one on the top and the other on the bottom: we get what is called a box girder (Fig. 5), and which has about nineteen times the strength of the original three-inch plank and only double the amount of material.
So far we have considered our bridge as being only twelve
