RAILROAD BRIDGES 27 



to Cooper's series, or a system of loading giving practically 

 equivalent stresses. The minimum loading shall be Cooper's 

 E.40, as shown in the following diagrams: 



n o n n ^o QQOQ n n o o 



FIG. 1. 



g 



f=\ ,f=\ 4,000 Ibs. per llll. ft. 



or 



FIG. 2. 



The diagram that gives the larger stresses shall be used. 



in. Heavier Loading. Heavier loadings shall be propor- 

 tional to the above diagrams on the same spacing. 

 ^ '* 112. Impact. An impact allowance shall be added to the com- 

 puted maximum live load stresses, as follows: 



c,/ 300 \ 

 Im P act=S lLT300> 



where S= Computed max. live-load stress, moment or shear. 



L= Loaded length of track in feet producing the maximum 

 stress in the member. For bridges carrying more 

 than one track the aggregate length of all tracks 

 producing the stress shall be used. 



No impact allowance shall be added to stresses produced by 

 traction, centrifugal and lateral or wind forces. 



113. Lateral Force. All spans shall be designed for a lateral 

 force on the " loaded " chord of 200 Ibs. per lin.ft. plus 10 per 

 cent of the specified train load on one track, and 200 Ibs. per 

 lin.ft. on the " unloaded " chord; these forces being considered 

 as moving. 



114. Wind Pressure. Viaduct towers shall be designed for 

 a force of 50 Ibs. per sq.ft. on one and one-half (1J) times the 

 vertical projection of the structure unloaded; or 30 Ibs. per 

 sq.ft. on the same surface plus 400 Ibs. per lin.ft. of structure 

 applied 7 ft. above the rail for assumed wind load on train when 



