Art. 138. STRESSES IN A PETTIT TRUSS. 



Center of moments at A . 



281 



300P, for dead load, 

 =^PX 100, for live load. 

 300X33,000 



180 



6600X40,000 

 180X16 



= 55,000 Ibs. (+) D. L. 

 = 91,700 Ibs. (+) L, L. 



146,700 Ibs. (+) Total. 



Tension in F 3 . As D' 5 cannot come into action in this 

 case (see above) the position of the live load will be either 14 

 and 15, or 12, 13, 14, and 15. 



Loads 12, 13, 14, and 15 on: 



180 F 3 =P X420 -D'a = 172.5P. L. L. tension. 

 16 



Loads 14 and 15 on: 



1SOF 3 = . X420 =78.75P. L. L. tension. 

 16 



The first case gives the greater tension, so, 





Dead load from above =55,000 Ibs. ( +) 



No reversal. 



139. Skew Bridges. When for any reason the supports 

 for a bridge cannot be made perpendicular to the center line of 

 the bridge, the bridge is said to be skewed. This is illustrated 

 in Fig. 197. 



The span is usually divided into equal panel lengths along 

 the center line of the bridge, and the inclinations of the end , posts 

 are made equal. This causes the end suspenders to be inclined 

 and the end panels of the trusses to be unequal. 



The floor beams are usually placed normal to the trusses. 

 This is done because the connections are more easily made and 

 the construction is cheaper. 



The inclinations of the end posts are usually made the 

 same, as this will make the portal bracing lie in a plane, other- 



