300 STRESSES IN A PRATT TRUSS FROM WHEEL LOADS. Art. 153. 



=215.2 



00 



5X29.15X^ = 114.5 (+) . L. 



Jo 



329.7 



For joint 4, 



Wheel 10 at joint 4, M 4 =7166.1 

 Wheel 11 at joint 4 ; A/ 4 =7191.3 



=256.8 



(+) Total 



= 6X29.15X^ = 137.4 (+) D. L. 



394.2 (+) Total. 



If the shear in panel 3-4 were zero or such as to produce 

 tension in diagonal 10-3, the stress in L 4 would be the same as 

 that just found for U 3 ; but for this position of the load the 

 shear in the panel is negative and 4-11 is in action. This will 

 make the tension in L 4 slightly less than the compression in 

 U 3 . The maximum stresses in the members U 3 , f/ 4 , and L 4 

 are usually assumed to be equal. 



The stresses in the web members are obtained from the 

 shears. The shear criterion, equation (89) is best applied by 

 finding the fields of shear (150). The shear criterion may be 

 written thus: 



>NG 2 



G 



<N(G 2 +P). 



Any wheel will satisfy this criterion at any point so long as 

 the total load on the bridge lies between these two limits, with 

 the load entering from the right. 



Wheel 1 governs from =7X0 until (7 = 7x10=70, or 

 from the time it (wheel 1} enters the bridge until wheel 5 is 

 ready to come on. As soon as wheel 5 enters G becomes 90, 

 and the criterion is not satisfied for wheel 1 . 



Wheel 2 governs from G = 7X10 =70 until =7X30=210, 

 or from the time wheel 4 enters the bridge until wheel 13 is 

 ready to enter. 



