[STRESSES IN TRUSSES. 



Art, 5S. 



RU the vertical component of Bc-~=R^ the vertical component of 

 dE = R l P, the vertical component of Ef = R 2 . These equa- 

 tions determine the diagonal stresses. Getting their horizontal 

 components and using the horizontal resolution equation, the 

 chord stresses may be determined by taking successive sections, 

 but these are more easily found from the moment equation. 



Taking the more general case of Fig. 56, to find L, the center 

 of moments is chosen at 0, the point of intersection of the two 

 other unknowns, so that their moments are zero. There being no 

 loads to the left of the section, in this case, the only forces left 

 are L and JB 1B 



For M c =0 



= LXd or L = KI~-. 

 For finding U, the center of moments is taken at E. 



M R or RiX%p = UXc U= RI~-. 



c 



For Z>, the center of moments is taken at the intersection A, 

 of 7 and L. 



ML = or R\a = Db or D = RI-^-. 



The directions of U. L, and D are obtained by a considera- 

 tion of the direction which the resisting moment must have. A 

 stress acting toward the part under consideration is compressivc 

 and one acting away from it is tensile. 



I 



6 A 



In Fig. 57, if a section is taken through three members only, 

 and the left hand portion considered, as shown, there are six 

 forces in equilibrium, namely, R^ P at B, P at C, CD, ND, and 

 NM. 



M D = or NM X d = RI X \ LP( J L + jj L) 



j|f A = or ND Xa = P(\L+^L) 



M v = or CDXb = RiX cP(c-\ L) + P( ^Lc) 



