34 



INFLUENCE DIAGRAMS 



two simple trusses and also a part of the auxiliary frame 

 L^M^LQ. Neglecting the auxiliary truss LMzL, the 

 influence diagram is drawn in the manner outlined for a 

 top chord member. This diagram is DED"B"Fn f , etc., and 

 for a load of unity anywhere between 1/4 and LQ the moment 

 is equal to the ordinate immediately below the load between 

 the lines T>"E" and DB, and the stress in L^L& equals 

 this moment divided by the depth of the truss. The 

 stress in L^Ls as a part of the auxiliary truss L^MsLe can 



FIG. 24. 



be found by drawing the influence diagram for L^Ls and 

 dividing the ordinate directly below the load by the length 

 L 5 M 3 . If the scale of this diagram is properly taken 

 the ordinates may be added directly to those of the large 

 diagram. 



Following the general rule for constructing influence 

 diagrams, with D"C", Fig. 24, equal to s"=L 4 L 5 multi- 

 plied by LJ3 i-^LsM^ the diagram >"A"B" is obtained. 

 The ordinates are increased in the same ratio as the ratio 

 of the lever arms of L 4 Z/6. The shaded area is the influence 

 diagram for L^Le. This is simply the proper combination 



