Art. 138. 



STRESSES IN A PETTIT TRUSS. 



277 



These, in connection with the dead load stresses, give all of 

 the stresses with the exception of the tension in the posts. 



138. Stresses in a Pettit Truss. For long span bridges 

 the Pettit is by far the most common form of truss. The anal- 

 ysis of the stresses offers no particular difficulties which have 

 not already been explained in connection with the Baltimore 

 and camel-back Pratt. Fig. 196 shows a Pettit truss with the 

 sub-diagonals in tension. The two end diagonals extend over 

 only one panel in this case; in some cases they are run over two 

 panels, as in the Baltimore. 



It is evident that, as the inclinations of the two diagonals 

 in a panel differ, the vertical component of the stress in the 



FIG. 196. 



sub-diagonal no longer equals a half panel load. Its value may 

 be found by vertical and horizontal resolutions, as was done in 

 the case of the Baltimore truss (136), or by a moment equation. 

 In Fig. 196 consider the forces acting at joint 19. Let d 3 = 

 depth of truss at 7 3 , /-length of D' 4 and p=panel length, 

 then with a center of moments at 12, 



\ Xd 3 sin 3 =Pp. 



sin 3 =7- 

 i 



If the chords are parallel as in a Baltimore truss, -r= 



