Art. 163. THROUGH BRIDGE LATERAL SYSTEMS. 



321 



of moments should be taken at the panel point of the truss 

 instead of at the rail, in figuring the increase of load on the truss. 

 This will increase the lever arms a and b of Fig. 20. The in- 

 crease of stress in the truss members due to wind is usually 

 small with the exception of the chords; these receive stress 

 both from the overturning moment of the wind on the train and 

 from their action as chords of the lateral systems also. 



The wind load also tends to overturn the bridge and suf- 

 ficient anchorage must be provided at the shoes to take the 

 difference between the upward pull of the wind and the downward 

 reaction of the load acting at the same time. 



163. Through Bridge Lateral Systems. In a through 

 bridge with vertical or inclined end posts, the top lateral wind 



a' I? c 1 d' c' b' &' 



Fig. 115. 



load is carried by the lateral system to the tops of the end posts 

 and must be transferred through portal bracing and the end 

 posts, to the supports. The end bracing cannot extend to the 

 bottom of the posts as in a deck bridge, on account of clearance 

 for traffic; therefore the end posts must act as beams and will 

 be subjected to both bending and direct stresses. (115.) 



There are transverse frames placed at the intermediate 

 panel points also, to stiffen the bridge and reduce the vibration. 

 These will transfer some of the top lateral load down to the 

 bottom lateral system at each panel point. The amount of 

 load so transferred will depend upon the relative stiffness of 

 the two systems and of the portal bracing. It is frequently 

 assumed that from one-third to one-half of the top lateral load 

 is so transferred, the balance being carried to the portals. 



