28 STANDARD SPECIFICATIONS 



the structure is either fully loaded or loaded on either track with 

 empty cars, assumed to weigh 1200 Ibs. per lin.ft., whichever 

 gives the larger stress. 



115. Traction Force. Viaduct towers and similar structures 

 shall be designed for a longitudinal force, applied to the rail, of 

 20 per cent, of the live load. 



1 1 6. Centrifugal Force. Any structure located on a curve 

 shall be designed to resist a lateral force of 10 per cent of the 

 equivalent live load per lin.ft., applied 6 ft. above the top of 

 rails. (Par. 112.) 



UNIT STRESSES 



117. Structural Steel. All parts of structures shall be so 

 proportioned that the sum of the maximum stresses shall not 

 exceed for structural steel and rivet steel the following values 

 in Ibs. per sq.in., except as modified in Paragraphs 122 to 124. 



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



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



shapes, built sections, and plate girders 16,000 " 



Pins, on extreme fibers 24,000 " 



(c) Axial compression, gross section, 



S= 16000-70^' 



where S= allowable unit stress, 



I = unsupported length of member in inches, 

 r= Least radius of gyration of member in inches. (Par. 

 143.) 



(d) Shear on: Turned bolts and field rivets, when 



hand driven 9,000 Ibs. 



Field rivets, when power driven 11,000 " 



Shop-driven rivets and pins 12,000 " 



Plate girder webs, gross section 10,000 " 



(e) Bearing on: Turned bolts and field rivets, when 



hand driven 18,000 " 



Field rivets, when power driven 22,000 " 



Shop-driven rivets and pins 24,000 " 



Expansion rollers, per lin.in 600d 



where d= Diameter of roller in inches. 



