Art. 159. 



LATERAL SYSTEMS. 



317 



159. Lateral Systems. The horizontal forces are taken 

 care of by horizontal trusses between the main vertical trusses, 

 the chords of the latter acting also as chords for the wind trusses. 

 The horizontal trusses are spoken of as lateral systems, and in 

 high truss bridges we have a " top lateral system " and a " bottom 

 lateral system." At the " loaded chord " the floor beams are 

 usually made to act as members of the lateral system. 



We, therefore, have, in addition to stresses from dead and 

 live load, wind stresses in floor beams, and in the chords of the 

 main trusses. Pony truss bridges and through plate girder 

 bridges have but one lateral system. Deck plate girder bridges 

 may have one or two systems. 



The usual practice is to use adjustable rods for the diagonal 

 members of ordinary highway bridge lateral systems, and angles 

 with riveted connections for city bridges and railway bridges. 

 These are all usually considered as tension members and therefore 

 two diagonals are used in each panel. An exception is the deck 

 plate girder bridge, whose lateral systems are usually Warren 

 trusses. 



The lateral system at the "loaded chord" is assumed to carry 

 all the live load, due to the wind blowing against a moving train 

 or against vehicles moving across a bridge, as well as its propor- 

 tion of the dead load. The other system is assumed to carry dead 

 wind load only. In a bridge having but one lateral system, this 

 <is proportioned to take all the dead and live wind load. 



160. Deck Plate Girder Lateral Systems. Fig. 210 

 shows the top lateral system of a deck plate girder bridge. Fre- 



Fig. 210. 



quently no bottom lateral system is used, in which case the 

 wind loads from the bottom of the girder are transferred to the 

 top by cross frames placed at intervals, usually not exceeding 

 sixteen times the width of the girder flange. 



