RAILROAD BRIDGES 29 



118. Pressure on Foundations. The pressure on masonry 

 foundations shall not exceed the following values in Ibs. per 

 square inch: 



Portland cement concrete 1:2:4 and first-class 

 sandstone or limestone masonry, including 

 impact 400 Ibs. 



Portland cement concrete 1 :2 :0 (granitoid) and 



first-class granite masonry, including impact 600 " 



119. Pressure on Soils. For allowable pressures see Par. 20. 



1 20. Timber. For allowable stresses see Part VII. 



121. Cast Steel. For allowable stresses see Par. 70. 



122. Alternate Stresses. Members, subject to alternate 

 stresses of tension and compression shall be proportioned for 

 the stress giving the largest section. 



If alternate stresses occur in succession during the passage 

 of one train, as in stiff counters, each stress shall be increased 

 by 50 per cent of the smaller. The connections shall in all cases 

 be proportioned for the sum of the stresses. 



123. Counter-Stresses. Wherever the live- and dead-load 

 stresses are of opposite character, only two-thirds of the dead 

 load stress shall be considered as effective in counteracting the 

 live-load stress. 



124. Combined Stresses. Members subject to both axial 

 and bending stresses shall be proportioned so that the combined 

 fiber stresses will not exceed the specified axial stresses. (Par. 

 117.) 



For stresses produced by longitudinal and lateral or wind 

 forces combined with those from live and dead load and cen- 

 trifugal forces, the specified unit stresses (Par. 117) may be 

 increased 25 per cent, provided that this combination gives a 

 greater sectional area. 



DESIGN OF STRUCTURE. 



125. General Proportions. The width between centers of 

 trusses shall in no case be less than one-twentieth (--$) of the 

 span, nor less than is necessary to prevent overturning under 

 the assumed lateral loading. 



Trusses shall preferably have a depth of not less than one- 



