Art. 164. 



PORTAL BRACING. 



323 



If the location of the points of contraflexure in the posts 

 are known, the stresses are very easily found. 



In our analysis we will make the following assumptions, 

 which are not strictly true, but which will give results abundantly 

 accurate for all practical purposes. 



1. The force acting at the top of the portal is equal to 

 the reaction from the top lateral system plus one panel load. 



2. Neglect the effect of the direct wind pressure on the 

 side of the end post. 



Fig. 217. 



3. The longitudinal deformations of the members of the 

 portal frame are neglected. This gives F and F' the same 

 amount of deflection as B and B' '. 



4. The deflections of the posts are neglected in getting 

 the lever arms of the vertical forces. 



There will be three general cases to consider : 



1. When the posts are fixed at both ends. 



2. When the posts are partially fixed at the bottoms and 

 fixed at the tops. 



3. When the posts are partially fixed at both ends. 

 These will be considered separately. 



