Page:Popular Science Monthly Volume 36.djvu/489

Rh is simplicity and a fewer number of parts than any other form of bridge truss; but by thus reducing the number of parts we have increased the size of the parts that are used, and thus, to some extent, done away with the advantage. Each of the tie-braces, as they are called, crosses one panel, and the bridge is thus divided into bays two panels long. The vertical rods, as shown in the drawing,



are not in any way necessary to the theoretically proper construction of the truss, but are simply put in to support the chord between the panel-points and make it able to bear the cross-strain that comes upon it from the floor system, provided the bridge is a through bridge. When the bridge is used as a deck bridge and the floor system laid upon the top chord, there is the same necessity for vertical posts.

In countries where it is possible to procure good timber of large size, the Warren triangular bridge is as economical and convenient a form of truss as can be built. To use it in its most economical manner the lower chord is usually made of iron, as that simply has to withstand tension; but the tie-braces are made of wood, and also the top chord. One point which is to be studied carefully in the Warren triangular truss is the fastening of these braces, as they must be fastened in such a manner that they not only will resist compression, but also that they will act as ties and resist tension. This necessitates a rather more complicated method of fastening.

Another great advantage connected with the use of the triangular truss is the ease with which, when necessary, any piece can be removed and replaced by a new piece without in any way impeding the passage of trains over the bridge during the operation. In the case of wooden or combination bridges this becomes a matter of great importance, as the timber in these bridges is exposed to alternate dryness and moisture, and thus, in a comparatively short time, decays, and there soon is the necessity of replacing the bridge piece by piece; therefore any bridge that is constructed in such a manner as to make this possible, without impeding the traffic on the road, possesses a great advantage over other forms of bridges. The triangular truss is a favorite method of construction on all railroads in the southern part of this country running through that belt where it is possible to obtain, at comparatively slight cost, yellow pine for the requisite timber.

We come next to the last type of bridge that has been used, to