SIMPLE TRUSSES 23 



If the line DA is prolonged until it cuts the vertical through 

 K the ordinate cut-off above K equals AK and hence it 

 cuts the vertical at E, and EK in the small diagram equals 

 EK of the large diagram. This shows, if the horizontal 

 projection of VL*> is taken as the length of the cut stringer 

 and an influence diagram constructed according to the 

 general rule, that this is the true influence diagram for VZ/s. 

 The criterion for wheel loads producing maximum 

 moment is found as follows: 



-tan 0i+tan 3 )+W 2 (-tan t +tan 3 ) 



ox 



+TfY(tan 2 + tan 3 ) +TF 3 tan 3 =0; (53) 

 then 



W tan 3 = (Wi +W 2 ) tan X - W 2 ' tan 2 . . (54) 



Substituting the values of the tangents and dividing 

 through by s 



-- ' ' ' (55) 



If b$=d, then 



W_W 1 +W 2 -W 2 ' ' 



I s 



Top Chord of Sub-strut Truss. In Fig. 15 the top chord 

 UU' has no double duty to perform as it is simply a chord 

 member of the main truss. The influence diagram is 

 drawn according to the general rule using the horizontal 

 projection of the chord UU' as the length of the cut stringer. 



