218 



CLASSIFICATION OF TRUSSES. 



Art. 121, 



times they are made of timber or a combination of timber and 

 iron. Otherwise they may be classified as follows: 

 AB to uses. 



Bridge trusses. 

 Roof trusses. 

 Cranes. 



As to connections at the joints. 

 Pin-connected trusses. 

 Riveted trusses. 

 As to chords. 



Trusses with parallel chords. 

 Trusses with inclined chords. 

 As to web bracing. 



Trusses with triangular web bracing. 

 Trusses with quadrangular web bracing. 

 Trusses with sub-trussed web bracing. 

 Trusses with sub-divided web bracing. 

 Trusses with multiple systems of web bracing. 

 Trusses with combined systems of web bracing. 

 When a bridge truss is deep enough to permit overhead brac- 

 ing between trusses, it is called a high truss. A low truss is not 

 deep enough to permit overhead bracing and is often called a 

 pony truss. (Figs. 165 and 166.) 



When the trusses of a bridge support the floor near the lower 

 chord, the bridge is a through bridge; if the floor is supported 

 near the upper chord, it is a deck bridge-, and if between the 

 chords, it is a half-deck bridge. 



Figs. 155 to 174 show bridge trusses of various types. 

 These are explained below in detail. In these figures, heavy 

 lines denote compression members; light lines, tension mem- 

 bers; heavy dotted lines, counter braces; and light dotted 

 lines, counter ties. 



Any type of truss may, in general, be modified so as to be 

 supported at the top or bottom chord, and may carry loads at 

 both chords. 



122. Bridge Trusses Now Out of Date. Some notable 

 wooden bridges were built in this country late in the eighteenth 

 and early in the nineteenth century. The building of iron 

 bridges began in 1840, the compression members being of cast 

 iron and the tension members of wrought iron. About 1863 the 



