40 INFLUENCE DIAGEAMS 



the upper segment. Graphically determine the stress in 

 M si/a produced by a unit load at L 4 and lay off this stress 

 below F as Fg. Locate D by drawing a straight line through 

 B and g and then complete the influence diagram according 

 to the 'general rule. This gives the diagram DEFBD. 

 A unit load at L 3 produces a stress in MJL>z due to its 

 position in the principal truss and also a local stress con- 

 sidering MzLs as a piece which simply transfers a portion 

 of the load at L 3 to the main truss. While the stress in 

 MsL s produced by a unit load at L 3 is readily computed 

 yet it is usually more satisfactory to find this stress graph- 

 ically by means of an ordinary stress diagram. After 

 this stress is found, lay off mn equal to it and connect 

 / and n and F and n. The shaded area shown is the 

 stress influence diagram for the member MzL%. 



