HIGHWAY BRIDGES 17 



The minimum value of the above pressures shall be 250 Ibs. 

 per lin.ft. for the " loaded" and 150 Ibs. for the " unloaded" 

 chord of the structure. 



Trestles shall in addition to one of the above wind loadings 

 be designed to resist a pressure of 200 Ibs. for each vertical foot 

 of bent in height. 



The above wind pressure shall in all cases be treated as 

 moving loads. (Par. 61.) 



63. Centrifugal Force. Any structure on a curve carrying 

 an electric railway shall be designed to resist a lateral force of 

 10 per cent of the equivalent live load per lin.ft., applied 5 ft. 

 above the top of rail. (Par. 61). 



64. Traction Force. Any structure carrying an electric 

 railway shall be designed to resist a longitudinal force of 20 

 per cent of the greatest live load placed upon the same. 



UNIT STRESSES 



65. Structural Steel. In designing the component parts of 

 any highway structure the maximum stresses due to the com- 

 bined effects from dead and live loads, including impact, or due 

 to wind pressure, centrifugal and traction forces, shall not exceed 

 for structural steel and rivet steel, the following values in Ibs. 

 per square inch, except as modified in Pars. 71 to 74. 



(a) Axial tension, net section 16,000 Ibs, 



(b) Bending, net section, on extreme fibers of rolled 



shapes (Par. 77), built sections and plate girders 16,000 " 

 Joists, under concentrated loads, when flooring 



is non-continuous (Par. 77) 20,000 " 



Pins, on extreme fibers 24,000 " 



(c) Axial compression, gross section, 



S= 16,000-70- 

 r 



where S= allowable unit stress, 



I unsupported length of member in inches, 



r least radius of gyration of member in inches (Par. 96). 



