58 



STEEL ROOF TRUSSES AND MILL BUILDINGS. 



CHAP. I. 



The eccentric stress caused by connecting angles by one leg when used as ties or struts shall 

 be calculated, or only one leg will be considered effective. 



40. Rivets. Rivets shall be so spaced that the shearing stress shall not exceed 11,000 Ib. 

 per sq. in.; nor the pressure on the bearing surface (diameter X thickness of piece) of the rivet 

 hole exceed 22,000 Ib. per sq. in. 



Rivets in lateral connections may have stresses 25 per cent in excess of the above. 



Field rivets shall be spaced for stresses two-thirds those allowed for shop rivets. 



Field bolts, when allowed, shall be spaced for stresses two-thirds those allowed for field 

 rivets. 



Rivets and field bolts must not be used in direct tension. Where it is necessary that con- 

 nections take tension turned bolts shall be used. 



41. Pins. Pins shall be proportioned so that the shearing stress shall not exceed 11,000 Ib. 

 per sq. in.; nor the pressure on the bearing surface (diameter X thickness of piece) of the pin 

 hole exceed 22,000 Ib. per sq. in.; nor the extreme fiber stress due to cross bending exceed 24,000 

 Ib. per sq. in. when the applied forces are assumed as acting at the center of the members. 



42. Plate Girders. Plate girders shall be proportioned by the moment of inertia of their 

 net section or on the assumption that | of the gross area of the web is available as flange area, 

 and the shear is resisted by the web. The distance between centers of gravity of the flange areas 

 shall be considered as the effective depth of the girder. 



43. Web Stiffeners. The web of plate girders shall have stiffeners at the ends and inner 

 edges of bearing plates, and at points of concentrated loads, and also at intermediate points where 

 the thickness of the web is less than ^fo.of the unsupported distance between flange angles, not 

 farther apart than the depth of the full web plate with a maximum limit of 5 ft. Stiffeners shall 

 be designed as columns for a length equal to one-half the depth of the girder. Stiffener angles 

 must have enough rivets to properly transmit the shear. 



44. Compression flanges of plate girders shall have at least the same sectional area as the 



tension flanges, and shall not have a stress per sq. in. on the gross area greater than 16,000 150 r , 



where / = unsupported distance, and b = width of flange, both in inches. Compression flanges 

 of plate girders shall be stayed transversely when their length is more than thirty times their 

 width. 



45. Rolled Beams. Rolled beams shall be proportioned by their moment of inertia. The 

 depth of rolled beams in floors shall not be less than -fa of the span. Where rolled beams or 

 channels are used as roof purlins the depths shall not be less than -fa of the span. 



46. Timber. The allowable stresses in timber purlins and other timber shall be taken from 

 the following table. 



ALLOWABLE WORKING UNIT STRESSES IN TIMBER, IN POUNDS PER SQUARE INCH. 



Columns may be used with a length not exceeding 45 times the least dimension. The unit 

 stress for lengths of more than 10 times the least dimension shall be reduced by the following 

 formula: 



p = c-i 



ioo a 



where C = unit stress, as given above for short columns; 

 P = allowable unit stress in Ib. per sq. in. ; 

 / = length of column in inches; 

 d = least side of column in inches. 





