318 



DECK PLATE GIRDER LATERAL SYSTEMS. Art. 160. 



If we assume the live load applied equally to the two 

 girders (Fig. 210), dd' will carry 'a half panel of dead load wind, 

 and a quarter panel of live load wind, ce being a panel length. 

 dd is the top member of a cross frame. (Shown in Fig. 210 (a).) 

 The intermediate frames are usually made heavier than would 

 be necessary simply to take the wind stress. Similar cross- 

 frames are also placed at aa' and gg' . These are figured to 

 carry all the horizontal forces to the supports. 



Since those diagonals which are in tension when the wind 

 blows from one side are in compression when it blows from the 

 other side, and, since it requires more material for a compression 

 member than for a tension member, the diagonals are all propor- 

 tioned for compression. Most specifications allow the chord 

 stresses for wind to be neglected unless they exceed from 25% to 

 50% of the total live and dead load stresses from vertical loads. l 

 The calculation of the stresses is a simple matter. (131.) 



161. Through Plate Girder and Pony Truss Lateral Sys- 

 tems. For a Pony truss or through plate girder bridge, the later- 

 als are usually calculated as tension members, even if they are 



made up of angles. Fig. 

 211 shows the lateral 

 system of a railway 

 bridge. It is figured as 

 a simple Pratt truss 

 one system acting when 



G/rder 



\ Strtf* i*r 



/Stri 



\ 





\ 







Girder 



Fig. 211. 



the wind blows from one side, and the other when it blows from 

 the other side. 



The laterals are riveted to the stringers at their intersections 

 to keep them from rattling and saging, but this is not supposed 

 to have any influence on the stresses in them. The plane of the 

 laterals is usually near the lower flange of the floor beams. The 

 wind stress in the latter, therefore, need not be taken into account 

 as it reduces the tension already there. In case there is no end 

 floor beam, the end strut is proportioned for y 2 the component 

 parallel to it, from the end lateral, the other half being assumed 

 to go direct to the support. 



162. Deck Bridge Lateral Systems. Fig. 212 shows a deck 

 truss bridge with top and bottom lateral systems and sway brae- 



^ee Am. Ry. Eng. Assoc. Spec., 4th Edition, Art. 25. 



