CHAPTER X. 

 STRESSES IN BRIDGE TRUSSES. 



Method of Loading. The loads on highway bridges, and in 

 many cases on railway bridges as well, are assumed to be concentrated 

 at the joints of the loaded chord, and if the panels of the truss are equal 

 the joint loads are equal. The assumption of joint loads simplifies the 

 solution and gives values for the stresses that are on the safe side. Equal 

 joint loads will be assumed in this discussion. 



Algebraic Resolution.* Let the Warren truss in Fig. 42 have 

 dead loads applied at the joints as shown. From the fundamental 

 equations for equilibrium for translation, reaction R = R 2 = 3 W. 



WtonG 



WSec8 jA 



Dead Load Coefficients 



FIG. 42. 



The stresses in the members are calculated as follows : Resolving 

 at the left reaction, stress in \-x + 3 W sec 0, and stress in i-y = 



3 W tan 6. Resolving at first joint in upper chord, stress in 1-2 = 



3 W sec , and stress in 2,-x = + 6 W tan . Resolving at second 

 joint in lower chord, stress 2-3 = + 2 W seed, and stress 3-3; = 

 8 W tan 0. And in like manner the stresses in the remaining members 

 are found as shown. The coefficients shown in Fig. 42 for the chords 

 are to be multiplied by W tan 0; while those for the webs are to be 

 multiplied by W sec . 



*Also called "Method of Sections." 



