46 STRESSES IN FRAMED STRUCTURES 



taking moments about joint L as a center. The stresses in ^-3 and 

 y-i are found by taking moments about joints U 2 and U 1} respectively; 

 and the stresses in x-2, and x-i are found by taking moments about 

 joint Z, . 



The method of algebraic moments is the most common method 

 used for calculating the stresses in bridge trusses with inclined chords 

 and similar frameworks which carry moving loads. 



Graphic Moments. The bending moment at any point in a 

 truss may be found by means of a force and equilibrium polygon as 



IR, 



(b) 



FIG. 29. 



in (b) and (a) Fig. 29. To determine the stress in %-x t cut section 

 /i A and take moments about joint L 2 as in Fig. 28. The moment of 

 the external forces on the left of L 2 will be M 2 = // y 2 , and stress 



To obtain stress in 4-5 take center of moments at joint Z, , and 

 stress 



To obtain stress in 5-3; take center of moments at joint U 3 , and 

 stress 



The method of graphic moments is principally used to explain 

 other methods and is little used as a direct method of calculation. 



