11 



thickness of the material countersunk to the diameter of the 

 rivet. 



In the case of field riveting (and for bolts as per 65) the 

 above-allowed shearing strains and pressures shall be re- 

 duced one-third. 



Rivets and bolts must not be used in direct tension. 



41. Pins shall be proportioned so that the shearing strain Pins. 

 shall not exceed 9,000 pounds per square inch ; nor the pres- 

 sure on the bearing surface of any member connected to 

 the pin be greater per square inch than 12,500 pounds for 

 the live load and 25,000 pounds for the dead load, (68) ; 

 nor the bending strain exceed 18,000 pounds, when the 

 applied forces are considered as uniformly distributed over 

 the middle half of the bearing of each member. 



42. When any member is subjected to the action of both combined 

 axial and bending strains, as in the case of end posts of S< 

 through bridges ( 39), or of chords carrying distributed 



floor loads, it must be proportioned so that the greatest fibre 

 strain will not exceed the allowed limits of tension or com- 

 pression on that member. 



43. If the fibre strain resulting from the weight only, of 

 any member, exceeds ten per cent, of the allowed unit strain 

 on such member, such excess must be considered in propor- 

 tioning the areas. 



44. In beams and plate girders the compression flanges compression 

 shall be made of same^r^wsection as the tension flanges. 



45. Riveted longitudinal girders shall have, preferably, a Depth oi 

 depth not less than j\ of the span. 



Rolled beams used as longitudinal girders shall have, 

 preferably, a depth not less than T f y of the span. 



46. Plate girders shall be proportioned upon the supposi- 

 tion that the bending or chord strains are resisted entirely 

 bv the upper and lower flanges, and that the shearing or 

 web strains are resisted entirely by the web- plate ; no part 

 of the web plate shall be estimated as flange area. 



The distance between centres of gravity of the flange 

 areas will be considered as the effective depth of all girders. 



