2 INFLUENCE DIAGRAMS 



s= the horizontal distance from the left support of the 

 truss to the center of moments, s will be considered 

 positive when measured to the right. 



h = the vertical distance, at the left end of the cut stringer, 

 between the two chord members which are cut by 

 the section. 

 h' = the vertical distance between the same chords at the 



right end of the cut stringer. 



The General Rule for drawing the influence line is as 

 follows: 



(a) Through any point A (refer to Fig. 1; the other 

 figures are lettered correspondingly) in a vertical line pass- 

 ing through the center of moments, draw a horizontal 

 line cutting the vertical line through the left support of 

 the truss at C and the vertical line through the right support 

 at B. 



(b) From C, at any convenient scale, lay off vertically 

 downward the distance CD=AC=s, and connect B and D 

 by a straight line. 



(c) Through A and D draw a straight line and prolong 

 it until it cuts the vertical line drawn through the left end 

 of the cut stringer at E. 



(d) Draw a vertical line through the right end of the 

 cut stringer, intersecting the line ACB at F, and connect 

 E and F by a right line. 



(e) The polygon DEFBD contains the influence line 

 sought; the ordinates between the line DEFB and the 

 line DB are the respective moments for unit loads on the 

 truss vertically above them, i.e., the lines ef=Zi, rc=z 2 , 

 gk=z 3 , are proportional to the stresses produced by loads 

 of one pound at Wi, W 2 and W 3 , respectively. The 

 ordinates are measured at the scale used in laying off the 

 distance CD = s. 



The application of the above rule to several forms of 

 trusses will now be considered. 



