242 



LIVE LOAD FOR MAXIMUM WEB STRESSES. Art. 129 



If, in Fig. 178, panel 4-5 is considered, the maximum shea- 

 in it will occur when 

 there are no loads to 

 the left of panel 4-5, 

 and when all the 

 loads on the right act. 

 If R! is the reaction 

 for loads 1, 2, 3, and 

 4, then R^+ f P is 

 the reaction when 

 load 6 is added. In 

 the first case, 



3 \ \2 

 7 Equal Pone/s 



-s* 



-s 



max 



Fig. 178. 

 Shear in panel 4^-5=R 1 

 In the second case, 



Shear in panel 4r-o=R i +*-PP=R l -% P 



A load on the left increases the reaction less than it decreases 

 the shear ; the result is a decrease of shear. If the load be all on 

 the left of the panel the shear is evidently of opposite kind (con- 

 sider Z? 2 ) and the same reasoning applies. Fig. 178 shows, dia- 

 grammatically, the positions of the loads for both maxima. In 

 symmetrical trusses, it is customary to consider the panel 2-3 in 

 place of 4-5 for maximum negative shear (127) ; the shear is the 

 same for both. The positive shear is found on the left and the 

 negative shear on the right of the center of the truss. The above 

 conclusions are confirmed by the results of the stresses given in 

 Art. 127. 



For maximum positive shear in any pariel of a truss, the live 

 load should, extend from that panel to the farther support, and 

 for maximum negative shear to the nearer support. The live load 

 shear in a panel may reverse but the dead load shear is always 

 positive. Whether or not there will be a resultant reversal of 

 shear depends upon the relative amounts of the live load and dead 

 load. 



When there is a reversal of shear in a panel, there will be a 

 reversal of stress in the web members of that panel (F 2 and D 3 , 

 Fig. 178) if the chords are parallel, for then the web member 

 alone must resist the shear (sections 2-2 and 3-3). When the 

 chords are not parallel, the wicb member does not carry all of the 



