32 I-l\ !.-!.< >.\1) SIHE88E8 



needed in writing the formula- for the live-loud stresses in 

 a through Pratt truss with non-parailel or parallel chord.-. 

 The influence ordinate at any salient point is the calcu- 

 lated <tre due to a one-pound load on the bridge at the 

 panel point above this salient point. By easily discovered 

 relations between similar triangles, the algebraic value of 

 each stress, or influence ordinate, is expressed in terms that 

 are most readily evaluated in any numerical problem. 



The derivation of any one formula for a live-load stress 

 is typical. Refer to Fig. 7. The stress in the lower chord 

 member 5 6 is found by taking moments about C. The 

 influence line for & is straight over each of the two inter- 

 vals kp and mn. The ordinates at the salient points 1 and 

 4 are zero. The ordinate at salient point 3 must be found 

 by placing a one-pound load at the lower panel point of 

 the truss above this salient point and calculating the value 

 of S. For the unit load so placed. 



kp k 



Reaction at .1 = - 



np n 



By moments about C, 



l n mp) = ,S (i 



Therefore. 



S 6 = + - Influence ordinatc at 3, 



UV 



The slopes of the segment- of this influence line follow. 



mien 1: 



Slope of ab = -jjf + ,, = - - 



The coefficients C for use in the general formula S = 

 -<'M are now found. 



