5 2 



STRESSES IN ROOF TRUSSES 



Wind Load 

 Rollers Leevva 



FIG. 33. 



The stress diagram is constructed in the same way as the stress 

 diagram for dead loads. It will be seen that the load P has no effect 

 on the stresses in the truss in this case. Heavy lines in truss and stress 

 diagram indicate compression and light lines indicate tension. 



Rollers Windward. The wind load stresses in the same trian- 

 gular Pratt truss as shown in Fig. 33, with rollers under the windward 

 side of the truss are calculated by graphic resolution in Fig. 34. 



The true reactions were determined directly by means of force and 

 equilibrium polygons as in Fig. 16. The direction of the reaction R l 

 is known to be vertical, but the direction of the reaction R 2 is unknown, 

 the only known point in its line of action being the right abutment. The 

 equilibrium polygon is drawn to pass through the right abutment and 

 the direction of the right reaction is determined by connecting the 



