Art. 123. 



MODERN BRIDGE TRUSSES. 



227 



QUADRUPLE INTERSECTION WARREN TRUSS 



Fig. 169. 



together at their intersections it is usually assumed that the web 

 members, cut by any cross section, take equal amounts of the 

 shear. 



The Town lattice truss bridge was patented in 1820 and was 

 built entirely of wood. Bridges of this type are still to be seen 

 in this country. 



Very few Warren trusses are built with pin connections, and 

 few Pratt trusses with riveted connections. Riveted Pratt trusses 

 are used for low trusses in which the number of panels exceeds 

 from six to eight. Baltimore and Pettit trusses are nearly 

 always built with pin connections. In general, pin connections 

 are used in bridges of long span in which the dead load is large 

 compared with the live load, and riveted connections in bridges 

 of short span. 



124. Special Types of Bridge Trusses. The cantilever 

 truss is used in bridges of long span, and particularly for those 

 which it is not practical to erect on falsework. Bridges of this 

 type are combinations of simple trusses supported at their ends 

 and of cantilever trusses, and these may have any style of web 

 bracing. Fig. 170 shows a common arrangement for a deck struc- 

 ture. The shore spans are usually erected on falsework and the 

 rest of the bridge is built out from the two ends (by means of 

 over-hanging travelers) in two cantilevers to meet at the middle 

 of the suspended span. The members of the lower chord, shown 

 dotted, are so made that the stress in them may be relieved. The 

 final arrangement is such that the suspended span DE is freely 

 supported by the cantilever arms CD anj. 'EF. 



The bridge is, of course, made symmetrical when possible. 

 Two ,other arrangements of spans are indicated diagramatically 



