Art. 124. SPECIAL TYPES OF BRIDGE TRUSSES. 



229 



The Wabash Railroad bridge over the Monongahela River, at 

 Pittsburgh, is a cantilever bridge having a channel span of 

 812 ft. 



The longest span of the cantilever bridge at Memphis, Tenn., 

 is 790 ft. There are numerous other examples of this type of 

 construction in this country. 



Many different styles of swing-bridge trusses have been 

 built; the type now in common use is shown in Fig. 172. In long 

 spans, some of the panels may be sub-trussed. The form is 

 that of two simple spans hung from a central tower. When 

 the draw is open, the truss is simply a double cantilever. When 

 the draw is closed and the ends are raised so that there will 

 be no stress in the top chord at the tower, the truss consists of 

 two simple spans. For any intermediate end conditions, the 



SWING &R/DGE TRUSS 

 Fig. 172. 



truss is continuous over four supports. It is evident that there 

 will be many members subject to reversals of stress. 



This style of truss is usually made partially continuous by 

 putting bracing in the tower panel so light that it carries practi- 

 cally no shear. 



In place of the tower a single post is used for short spans, 

 making the truss a two-span continuous truss. 



The stresses in continuous trusses are statically indeter- 

 minate, but the assumption that the moment of inertia of the 

 truss is constant (97) is usually made in calculating the stresses 

 in swing bridges. This assumption of course is not strictly 

 true. 



Swing bridge trusses are very seldom built with pin connec- 

 tions at all joints, but are either riveted or partly riveted and 

 partly pin-connected. 



Arches differ from the trusses described above in that they 

 have inclined reactions for vertical loads (Fig. 40). The abut- 



