244 LIVE LOAD FOR MAXIMUM WEB STRESSES Art. 129. 



any load on the right of the section 2-2, and in tension for any 

 load on the left. 



For maximum stress of one kind in any web member of a 

 truss, the live load should extend from its panel to one support, 

 and for maximum stress of the other kind, to the other support. 

 For a vertical member, the panel is determined by the section. 



Since Z> 3 (Fig. 180) has its maximum tension with loads 1, 

 2, and 3 acting, and its maximum compression with loads 4 and 

 5 acting, the algebraic sum of these stresses will be the full load 

 stress for D 3 . This will be a check on the stresses in D 3 if the 

 stress in Z> 3 for full load is determined; and this may easily be 

 done, because the ratio of live load stress to dead load stress is 

 equal to the ratio of live load to dead load. 



If a load on the right of a panel produces stress of one kind, 

 and a load on the left stress of the opposite kind, there must be 

 some position of a load in the panel which will produce zero 

 stress. In Fig. 180, for example, a load in the panel 3-4 may be 

 so located that it will produce such panel loads at 3 and 4 that 

 the tension produced by one will be just equal to the compression 

 produced by the other. There must also be a certain position of 

 a load whicj^ wift "produce zero shear in the panel. These posi- 

 tions are easily found and are convenient in placing a live load 

 for actual maximum stress or shear (127). 



The stress in a web member will also be zero for loads on 

 the left of its panel (for example) if the chords cut by the sec- 

 tion intersect on the right-hand reaction, for then the moment of 

 the external force is zero. A case of this kind is illustrated in the 

 roof truss of Fig. 51, Art. 54; there is no stress in any web mem- 

 ber of the right-hand half. A section through any web member 

 cuts off but one external force (R 2 ) on the right and it passes 

 through the center of moments, E. 



AYhen there are loads on both sides of the cutting section, 

 the web stress is zero, evidently, when the chords cut intersect on 

 the resultant of all external forces on one side of the section. A 

 graphic method of getting the location of this resultant is given 

 in Art. 61; its position may, however, be more quickly determined 

 algebraically. The moment of the resultant about the point of 

 application of the reaction must equal the sum of the moments of 

 the loads on one side of the section about the same point. 



// the center of moments of a web member lies between the 

 supports, that is, if the chords cut by a section through it inter- 



