56 INFLUENCE DIAGRAMS 



for H will be a triangle between the support and the 

 center hinge. Placing a unit load at the center hinge, 



Lay off this distance above DB directly below the 

 center hinge and thus locate the point K. Draw KD 

 and KB] then the shaded area is the influence diagram 



M 

 for * corresponding to the diagonal C/2^3- Fig. 366 



is a similar diagram for the diagonal 



For chord members the same method is employed. 

 The triangle DKB is constant for all influence diagrams. 



The position of wheel loads producing the maximum 

 moment is best found by trial as some of the criterions 

 become too complex for easy application. 



To illustrate the shape of one criterion, consider the 

 influence diagram in Fig. 366 in deriving a criterion for 

 the diagonal 



- s = -TJY(-tan 05-ftan 2 )+W 2 '(+tan 5 -tan 2 ) 



oX 



+ W 3 ' tan 05 + WY tan 4 = 0, . (88) 



or 



f ) tan 2 

 tan 04=0. . . (89) 



Substituting the values of the tangents 



' (90) 



