EVOLUTION OF THE MODERN RAILWAY BRIDGE. 465 



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 parti- 

 cles 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 to- 



J L 



gether, as shown in the drawing (Fig. 7), in the form of a square 

 or rectangle. Then any strain coming upon one side of this rect- 

 angle 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 



VOL. XXXVI. 30 



