322 



PORTAL BRACING. 



Art. 164. 



Fig. 216. 



The top and bottom lateral systems are figured exactly as in 

 a deck bridge. (162.) Fig. 215 shows the lateral systems of a 

 through bridge. Each portal gets a stress R equal to the trans- 

 verse component of the end diagonal EC' or B'C, and half the 

 wind loads on Ba, Bb, Be, and BC, which may be called a panel 

 load=P. 



164. Portal Bracing. The stresses in the portal brac- 

 ing depend upon the condition of the ends of the posts. In 

 practice, one or both ends may be partially or wholly fixed, as 

 far as elastic supports will permit fixity. (112.) The end con- 

 ditions are due mainly to the direct stress 

 in the post, which always exists to some 

 extent. This direct stress is due to the 

 dead and live loads on the structure, and 

 to the wind. 



End posts are nearly always made 

 with two webs, each one of which trans- 

 mits half of any direct stress in the post. 

 This direct stress D offers a resistance 

 to the turning of the end of the post equal to %kD. See Fig. 

 216. If %kD is equal to or greater than the moment required 

 to hold the end of the post in the line of its axis the ends may 

 be considered fixed. (112.) 



The maximum resultant of all stresses in the post will occur 

 when the live- load is on the bridge and the wind forces are acting. 

 The maximum stresses in the portal bracing will occur, in general, 

 when there is no live load on the bridge. 



The problem of finding the exact stresses in the portal 

 bracing is very complicated. The main factors which render 

 it so are the following. (See Fig. 215 (a)). 



1st. The condition of partial fixity of the posts at a and B. 

 2d. The distribution of the wind load reaction between 

 the supports a and a' '. 



3d. The deflection of the point F compared with that of B. 

 4th. The stiffness of the connections at B and F. 

 The wind loads will cause the portal to deflect somewhat 

 as shown in Fig. 217. This figure represents the general case 

 in which the lengths of the posts are different. This could 

 scarcely occur in a bridge, but might be met with in viaduct or 

 elevated railway work, where the same analysis of stresses will 

 hold good. 



