EVOLUTION OF THE MODERN RAILWAY BRIDGE. 473 



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 in- 

 creased 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 draw- 



WARREN TRIANGULAR TRUSS 



Fig. 23 



ing, are not in any way necessary to the theoretically proper con- 

 struction 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 neces- 

 sity 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 trian- 

 gular truss is the ease with which, when necessary, any piece can 

 be removed and replaced by a new piece without in any way im- 

 peding 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 ex- 

 posed to alternate dryness and moisture, and thus, in a compara- 

 tively short time, decays, and there soon is the necessity of replac- 

 ing the bridge piece by piece ; therefore any bridge that is con- 

 structed in such a manner as to make this possible, without im- 

 peding 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 coun- 

 try 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 



