228 



SPECIAL TYPES OF BRIDGE TRUSSES. 



Art. 124. 



in Fig. 170. A fixed panel may take the place of a fixed span 

 when conditions demand. There may be either an anchor 

 span or a simple span at the end of the bridge, but it is usually 

 not wise to have a cantilever arm at the end, since it must be 



B c 



F G 



Anchor Arm ^ J^, Cantilever _L Suspended K 



4 



Arm I Span 



t 1 



t 



Fig. 170. 



free to deflect upward and downward and this may interfere with 

 the traffic. 



The cantilever bridge now being built over the St. Lawrence 

 River near Quebec, Canada, will have a channel span of 1800 

 ft., which will be the longest span in the world. The suspended 

 span of this bridge is 640 ft. long and when completed will be 

 one of the longest simple spans yet built. The Municipal Bridge 



Fig. 171. The Quebec Bridge. 



over the Mississippi River at St. Louis contains simple trusses 

 with a span of 668 ft., and the bridge over the Ohio River at 

 Metropolis, 111., for the C. B. & Q. R. R., which is now under 

 construction, will contain a simple span of 723 ft. All of these 

 long simple spans have trusses of the Pettit type. Fig. 171 

 shows the outline of the trusses of the Quebec bridge. The web 

 bracing of the cantilever and anchor arms is called the " K " 

 type. It is statically determinate. 



The cantilever bridge over the Firth of Forth, in Scotland, 

 has two spans of 1710 ft. each; these are the longest now in 

 existence. 



