IO2 



STATICS. 



[171. 



example illustrates a simple method for finding these stresses 

 when the external forces are given. 



Let the frame-work represented in Fig. 48 be cut in two along 

 any line aft ; the portion on either side of this line must be in 

 equilibrium under the action of its external forces and the 



Fig. 48. 



stresses in the members intersected by aft. Thus, in the figure, 

 the forces A, W^ F v F 2 , F z form a system in equilibrium ; 

 hence, the sum of the moments of these forces with respect to 

 any point must vanish. 



To determine 'F v take moments about the intersection of F 2 

 and F B ; thus F% and F B are eliminated from the equation of 

 moments, and F 1 is found. Similarly F 2 is obtained by taking 

 moments about the intersection of F 3 and F r The arms of the 

 moments are best taken from a correctly drawn diagram of 

 the frame-work. 



If only two members be intersected by aft, the origin for 

 the moments is taken first on one, then on the other, of the two 

 members intersected. 



By beginning at one of the supports and taking ^sections 



