42 INFLUENCE DIAGRAMS 



m, =the moment about a center of moments distant s 

 from the left support of the truss, considering 

 the truss as a simple girder on two supports. 

 s=the distance of the center of moments from the left 

 end of the span. Positive when measured to the 

 right. 



Then 



s>a 



M s =M L +S L s-W(s-a), .... (a) 



but 



M B -M, 

 Si= ~7~ ~ +R *> ( fo ) 



therefore 



M =M MR ~ ML S+R s -W(s- a a\ 



s>a 



Now, Ris W(s a) = m t , 

 hence 



M 8 M L +M R T+m s (d) 



s I 



This expression shows that the influence diagram for 

 M t may be considered as being composed of two separate 

 diagrams combined algebraically. One of the diagrams 

 is that for the common moment m 8 or the base diagram. 

 It will be shown in the problems which follow that the 

 influence diagram for M a can be easily constructed upon 

 the base diagram and with a small amount of calculation. 



