Art. 138. 



STRESSES IN A PETTIT TRUSS. 



279 



If -is greater than n, the loading follows the regular 



(is U2 16 



rule, for maximum compression in V 3 . 



For trusses with inclined chords, the loading for compression 

 in the posts usually follows the regular rule. 



Tension in the posts. On account of the action of the sub- 

 diagonals, the tension in the main posts will seldom be very 

 large. In general there are two possible positions which must 

 be tested. For V 3 the load may extend from the left end up 

 to joint 14, so that the sub-diagonal at the top of V 3 will have 

 no live load stress, or it may extend beyond joint 12, thus reduc- 

 ing the tension in D 5 to a minimum. 



A few of the stresses will now be calculated to illustrate 

 the methods. The following loads will be assumed : 



Weight of trusses and bracing =2200 Ibs. per ft. 

 Weight of floor system =1100 Ibs. per ft. 



Total dead load =3300 Ibs. of ft. of bridge. 



Live load =4000 Ibs. per ft. of bridge. 



From these the following constants are obtained : 

 Dead load panel load =33,000 Ibs. 

 Live load panel load =40,000 Ibs. 



_aX2200_'l 

 q ~ 3300 3* 



Diagonal D 3 . Loading from right to joint 13. 



Center of moments at A, Arm of D 3 = 118.6 ft. 

 118.6> 3 =7iPXlOO-P(120 + 140) =490P for dead load. 



Q1 



= ^ipxl00 for live load. 



J>3 



4?OX 33,000 

 118.6 



= 136,300 Ibs. 



= 9100X40,000 = 

 16X118.6 



D.L. 

 L. L. 

 Total. 



328,100 Ibs. 



Diagonal D 4 . Applying the criterion developed for the 

 position of the live load : 



d 2 35 13 13X5 .1 



13 



n 

 16 



16 



