278 



STRESSES IN A PETTIT TRUSS. 



Art. 138. 



and the sub-diagonal takes a half of a panel load as is proven in 

 Art. 136. 



The position of the live load for maximum stress in a diag- 

 onal where there is but one in action in the panel, as D 3 and 

 Z> 5 , follows the regular rule of loading to the section. Where 

 the section cuts a sub-diagonal as well as the member whose 

 stress is desired, the moment of the stress in the sub-diagonal 

 enters into the equation of stress. Again referring to the double 

 panel 12-14, 



/ \ d 3 di n do \d 



a = (n+ra-f x)p cos 3 . x=- . cos #3= . 



2d 3 



n+w + 2=- 



n + m + 1 = 



d 3 



pp 



(6) 



This equation will be useful in getting the stresses in the bal- 

 ance of the diagonals and in the vertical posts. 



As with the Baltimore truss, in some cases the stress in 

 the lower half of a main diagonal is greater when the sub-panel 

 load ahead is on. This may be determined by trial. 



For Z) 4 , the increase in the reaction due to the addition of 

 the sub-panel load at 13 'is yfP. The change in the stress in 

 D 4 due to this load is 



lo 



d* \ 

 d 3 -dj 



pP 



// 



j 



arm of Z> 4 

 13 



arm of D^ 



is greater than n the loading follows the regular rule, 

 16 



for maximum stress in D. 



The same process is necessary with regard to the main 



verticals. For V^, the regular rule would require loading up 



to joint 11 for maximum compression; if two more loads be added 



the increase in the reaction is f-fP. The change in the stress 

 in F 3 due to this addition is 



16 



16 



(n+m+2)p 



n+m+2 



ip. 



