44 INFLUENCE DIAGRAMS 



This shows that the lines DB and CB of the base dia- 

 gram are extended until they cut the vertical through the 

 right end of the cantilever. The influence diagram is 

 shown by the shaded area BKL. 



For loads in the suspended span, 



M L = 0, M R = WB j 1 2 , m a = 0, 

 and hence 



M s = Wz(h a ^T~'T~ (^1) 



This is the equation for the straight line KM when 

 tan 65 = T- j~ = -j . The shaded area in Fig. 30 is the 



complete moment influence diagram for the chord 



The criterion for wheel loads which produce the max- 

 imum negative moment is 



_ 



1 2 " = Is ' 



Top Chord of Cantilever Span. The chord member, 

 Ui U 2 , Fig. 31 will be considered. For loads in this span 

 M L W 2 a 2) M R = and hence, from equation (d), 



(72) 



The base diagram is first constructed for the term m, as 

 shown by the heavy lined triangle in Fig. 31. Prolong 

 the line DA to K\ then from the triangle DKB, 



b'c' :KB::a 2 : 1 2 , 



