Page:Popular Science Monthly Volume 36.djvu/486

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470
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 straining piece.

PSM V36 D486 Howe and pratt bridge trusses.jpg

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 usually of iron, and the tension members consist of round iron rods, fastened by means of an eye and pin at the ends.