Art. 147. CRITERION FOR MAXIMUM WEB STRESSES. 



293 



that no other loading will give a greater maximum. For example, 

 it may be a question whether adding a load at the left, or right 

 of those shown in Fig. 202 will give a greater moment in a span 

 longer than the limiting span for the 5 loads. It will usually be 

 found that the maximum moment occurs when the 3rd or 2nd 

 driver is near the center. 



147. Position of Load for Maximum Web Stresses in a 

 Truss. The general case is a truss with one or both chords in- 

 clined. Since in this case the web does not carry all the shear, 

 we resort to the moment equation to get the stresses in the web 

 members. 



Let us consider the diagonal 5-6 of Fig. 200. Taking the 

 section pq the center of moments will be at 7, and the stress in 5-6 

 will be a maximum when the moment about / is a maximum. 



Substituting for R t and differentiating we have, when M l is a 

 maximum 



dM, 

 dx 



=0=- 



(oc+Q 

 P 



or -=-=2- 



(87) 



Equation (87) may be written in a form more convenient in 

 application thus 



(88) 



G! is usually and some load near the head of the train, as 

 wheel 2, 3 or 4 at the right hand end of the panel in question (the 

 load extending to the right) will usually give a maximum. In 

 general there must be a load at 7 to satisfy (88) so that when it is 

 counted with G 2 the second member of equation (88) will be 





greater than -^and when it is counted with G 3 it will be less than 



N 



G 



-*f. If equation (88) is satisfied without a load at 7, the moment 

 about Z will not be changed if the loads are moved so that one 



