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THE POPULAR SCIENCE MONTHLY. 



spans, and. it is considerable trouble to so brace them as to keep 

 them in a true vertical plane. These pony trusses, however, when 

 used as a deck bridge, that is, when the train runs on the top, can 

 be braced so as to form a very firm bridge, and practically it is 

 simply a box girder (Fig. 5). 



The members of a bridge at the top and bottom of each truss, 

 either horizontal, inclined, or curved, are called chords ; that at 

 the top is called the top chord (A B, Fig. 22), the bottom one, the 

 bottom chord (CD, Fig. 22). In the bridges we are considering, 

 they are usually parallel. 



The brace or strut is a compression member, and may be either 

 vertical or inclined (E F, Figs. 21 and 22), the object of which is to 

 keep the two chords apart. The tie is a tension member, and also 

 may be either vertical or inclined (G H, Figs. 21 and 22). The 

 lower chord being always in tension is sometimes called the strain- 

 ing piece. 



E G 



HOWE TRUSS 



Fig. 21 



PRATT TRUSS 



Fig. 22 



In some types of bridges which we will take up at once there 

 is no bottom chord (Fig. 19). We have what is called the Fink 

 truss. As will be readily seen, it is merely a combination of the 

 inverted king-post trusses, combined in such a way as to suit 

 any required span. In this bridge the bottom chord is not in 

 any way necessary to the proper construction of the truss, but in 

 case of a long span it is usually put in as shown by the dotted line, 

 not in any way to increase the strength of the truss, but simply to 

 add to its stiffness and stability. The Fink truss was invented by 

 Albert Fink, and manufactured for many years by the Louisville 

 Bridge and Iron Company. For short spans, or what are usually 

 called shore spans in many-span bridges, it is a most convenient 

 and economical method of construction, and has been very much 

 used. The top chord is in compression, as shown in the drawing, 

 and is usually made of wood, although this is not by any means 

 necessary. The posts, or vertical compression members, are usu- 

 ally of iron, and the tension members consist of round iron rods, 

 fastened by means of an eye and pin at the ends. 



