336 



PORTAL WITH SIMPLE X BRACING. Art. 169. 



z 



Fig. 219. (Case No. 1.) 

 (Stresses given are for Case No. la.) 



The connection at B takes the stress in^BB' and the vertical 

 component of B'F=V, but these do not act simultaneously. When 

 the wind is reversed, this connection takes the horizontal compo- 

 nent of BF', an amount equal to the stress in BB' going to BB' 



and an amount equal to y 2 P+%(R-i-P) - going to the post. 







The connection at F takes y 2 of the horizontal component of 

 B'F into FF' and the other half into the post. At the same time 

 the vertical component of B'V=V goes into the post. 



If aB is inclined as shown in Fig. 215, the stress in the bot- 

 tom chord of the truss from the top chord wind forces will be 

 V sin e. 



Case No. Ib. Posts Fixed at the Top and Partially Fixed 

 at the Bottom. In order that a post may be fixed at the top we 

 must have from equation (121) y 2 k 1 D=M l ==^eR 1 =y 2 U(ax 1 ) 



taking n=y 2 (R+P), and x^= 

 satisfied w have Case No. la. 





if equation (122) is. also 



