Art. 129. LIVE LOAD FOR MAXIMUM WEB STRESSES. 



245 



sect betiveen the supports, its stress will not reverse and it will 

 be a maximum for full live load. For appearance sake, simple 



n 



Fig. 181. 



trusses are occasionally made as shown in Fig. 181. Rollers are 

 placed under one end of the truss so that the reactions will be 

 vertical. Considering Z> 2 , its center of moments is at n. For 

 loads on the right of section 2-2, R : is the only external force 

 acting on the left-hand part of the truss ; to balance its moment 

 about n t D ? must be in tension. For loads on the left of the sec- 

 tion, R 2 is the only external force acting on the right-hand part ; 

 to balance its moment about n, D 2 must be in tension. Hence D 2 

 is in tension for any load and its maximum stress occurs with 

 full load. 



Similar reasoning will show that V 3 is in tension for loads 

 on the left of section 3-3, and in compression for loads on the 

 right ; its center of moments lies beyond the supports. 



In this truss, then, some of the web members will not suffer 

 a reversal of stress, and their maximum stresses occur for full 

 load, while the others follow the regular rule for maximum shear 

 in a panel. Since the chords are always a maximum for full load, 

 the general principle may be laid down that, when the center of 

 moments for a member lies betiveen the supports, its stress is a 

 maximum for full load. 



A roof truss with curved lower chord, and one arm of a 

 swing bridge with curved upper chord, when treated as a simple 

 span, are other cases in which the center of moments for a web 

 member may come between the supports. 



Another exception to the rule will be found in Art. 137 for 

 maximum tension in the posts of a truss having a curved chord 

 and counter ties. 



