68 



STRESSES IN BRIDGE TRUSSES 



the maximum stress in the panel to the left of the first load. A rule 

 for finding the coefficient of left reaction for any loading is as follows : 

 Multiply the number of loads on the truss by the number of loads plus 

 unity, and divide the product by twice the number of panels in the truss 

 and the result will be the coefficient of .the left reaction. 



PsecG 



-12 <O -|i <O -8 



Moximum in Webs Ptan 6 Minimum in Webs 



Live Load Coefficients 



FIG. 45. 



If the second differences of the maximum coefficients in the web 

 members are calculated, they will be found to be constant, which shows 

 that the coefficients are equal to the ordinates of a parabola. 



1 



3 



6 



10 



15 



21 



SECOND DIFFERENCES OF NUMERATORS OF WEB COEFFICIENTS. 



This relation gives an easy method for checking up th^ maximum web 

 coefficients, since the numerators of the coefficients are always the same 

 beginning with unity in the first panel on the right and progressing in 

 order i, 3, 6, 10, etc.; the denominators always being the number of 

 panels in the truss. 



It should be noted that in the Warren truss the members meeting 

 on the unloaded chord always have stresses equal in amount, but op- 

 posite in sign. 



Stresses in Pratt Truss. In the Pratt truss the diagonal members 

 are tension members and counters (see dotted members in (c) Fig. 46) 

 must be supplied where there is a reversal of stress. The coefficients for 

 the dead and live load stresses in the Pratt truss shown in (a) and (b) 



