particles of which the beam is composed are forced together. It is the same strain obtained if you take a vertical post and put a weight on top of it; that weight tends to force together the particles of the post, and the post is said to be in compression.
It is well to get a thorough understanding of these two kinds of strains, as they are the principal strains that have to be considered in all bridge-building. There is a point between the top and bot-tom of the beam at which the character of the strain changes from compression into tension, where there is no strain at all, and the amount of strain in the beam decreases from the outside toward the center until this zero-point, or neutral plane, is reached; and, as the greater part of the strain comes upon that portion of the beam farthest from the center, you will at once see the economy and necessity of placing as much of the material as far from the center as possible—that is, placing the material where it is going to do the most work, and this is what has led to the adoption of the flanged girder, or I-beam, as a favorite method of construction. The principal part of the material is placed at the two outside edges of the beam where the strain is the greatest, and the amount of material between these two outside flanges is simply enough to keep the flanges apart.
As the size of the opening to be crossed increases, the size of the flanged girder necessary to hold up a given load increases, so that in a very short time the piece of iron or steel necessary becomes so large as to make it almost impossible to handle if it is all in one piece, and also a great deal of the material in the flanged girder is absolutely of no use—that is, a great deal of it can be cut away and used to more advantage in other places.
This leads us at once to the framed truss or framed girder. There is one thing in connection with framed trusses to which I wish to call your attention, and that is, the whole foundation of the framed truss is based upon a triangle. You will readily see the object of this. Suppose four pieces of timber are framed together,
as shown in the drawing (Fig. 7), in the form of a square or rectangle. Then any strain coming upon one side of this rectangle tends to change the form of the figure, and, unless the joints are made perfectly stiff, the rectangle is changed to the shape shown in Fig. 8, where every piece is of its original length, and
