GRAPHIC MOMENTS 

 X 



75 



ordinate Y multiplied by the pole distance H. But the truss is a sim- 

 ple truss and the moment of the right reaction will be equal to the 

 moment at the left abutment and 



and since H = L 



and 



Now, with the loads remaining stationary, move the truss one panel to 

 the right as shown by the dotted truss. With the same force polygon 

 draw a new equilibrium polygon as above. This equilibrium polygon 

 will be identical with a part of the first equilibrium polygon as shown. 

 As above, the bending moment at left reaction is F 3 H = F 3 L = 

 R s L, and F 3 = ^ 3 . In like manner Y 5 can be shown to be the right 

 reaction with three loads on, etc. Since the bridge is symmetrical with 

 reference to the center line, the ordinates to the shear polygon in Fig. 50, 

 are equal to the maximum shears in the panel to the right of the or- 

 dinate as the load moves off the bridge to the right. 



