16 STANDARD SPECIFICATIONS 



roadway and sidewalks for spans of 100 ft. or less, 60 

 Ibs. for spans of 200 ft. or over, and proportionally 

 for intermediate spans. 



(d) Electric Railways. Any bridge carrying electric railway 

 tracks (excepting those for exclusive railroad use) shall 

 in addition to one of the above loadings be designed to 

 carry on each track a series of cars, each weighing 

 100,000 Ibs., unless otherwise specified. This load to be 

 distributed equally on two trucks 20 ft. centers, each 

 having two axles 5 ft. centers and 5 ft. gauge (occupy- 

 ing a length of 40 ft. and a width of 10 ft. ) 



61. Impact. An impact allowance shall be added to the 

 computed maximum live load stresses, as follows : 



For bridges of all classes carrying highway } T / 100 \ 



J Impact = AS ^- ?,) 

 traffic only ........................ J \L + 300/ 



For bridges, or part of bridges, carrying j T _ ~ / 200 \ 



electric railway traffic ........... "... I \L + 3w) ' 



where S= computed maximum live load stress, moment or shear; 

 L = loaded length of span in feet. 



No impact allowance shall be added to stresses produced by 

 wind, centrifugal or traction forces. 



62. Wind Pressure. The wind bracing shall be designed to 

 resist one of the following lateral loadings, whichever produces 

 the greater stress: 



(a) Structure unloaded. 50 Ibs. per sq.ft. on the exposed 



surface of all trusses and the floor as seen in eleva- 

 tion, or 



(b) Structure loaded. (Bridges of all classes carrying highway 



traffic only), 30 Ibs. per sq.ft. on the exposed surface 

 of all trusses and the floor as seen in elevation in addi- 

 tion to a uniform load of 150 Ibs. per lin.ft. of structure 

 applied on the " loaded " chord, or 



(c) Structure loaded. (Bridges of all classes carrying electric 



railway traffic) the same loading as under (6), except 

 that the additional uniform load is 300 Ibs. per lin.ft. 

 of structure and is applied 7 ft. above the base of raiL 



