GIRDERS AND TRUSSES 



117 



bottom and set off first the amount of moment carried by the 

 web (J, T V or fa, according to specifications) and above this set 

 off the amount carried by the flange angles. The number of cover 

 plates having been determined, set off in succession the amount 

 of moment carried by each. Through the points fixing the amount 

 of moment carried by the web and the angles draw horizontal 

 lines to the ends of 

 the span. Through 

 the points fixing the 

 amount of moment 

 carried by each cover 

 plate draw horizontal 

 lines beyond the mo- 

 ment curve 2 or 3 

 feet. This projection 

 allows length in which 

 to place a few rivets 

 so the plates begin 

 to be effective when 

 needed. 



The lengths of the 

 cover plates are scaled 

 from the diagram. 

 When the plate girder 



Fig. 74 



carries a moving load on top it is usual to have the top cover 

 plate next the angles extend the full length of the girder. This 

 protects the angles against the entry of moisture in the joints and 

 stiffens them near the ends against the effects of deflection of the 

 frame carrying the load. 



A similar diagram may be used for spacing rivets. The vertical 

 scale represents the total maximum tensile (or compressive) stress 

 in the girder, instead of the maximum moment. The number 

 of rivets necessary to resist this stress is determined by dividing 

 the stress by the safe allowable stress on each rivet. The vertical 

 line is divided into as many parts as there are rivets required. 

 Through the division points draw horizontal lines, Fig. 75, to an 

 intersection with the boundary curve. From the points of inter- 

 section with the boundary curve drop vertical lines to the base. 

 A rivet will be placed at each intersection thus determined. A 

 similar method may be used for spacing stirrups in beams of 



