Art. 120. 



CONSTRUCTION. 



217 



through the panel, and if there are no other stresses in the diago- 

 nals than the initial stresses, from 3 vertical Components=0 we 

 have: 



The Vert. Comp. of the initial stress in cD= 



The Vert. Comp. of the initial stress in Cd ; therefore as the 

 inclinations of the members are equal, the initial stresses in them 

 are equal. 



Now suppose that a load is put on the bridge in such a posi- 

 tion that it would produce a tensile stress in cD. This stress would 

 elongate cD which would cause the dotted parallelogram to ap- 

 proach its original shape of zero stress and thus relieve the 

 initial stress in Cd. 



Let the initial stress in the diagonals be represented by T. 

 From Hooke's Law (11) that stress is proportional to deforma- 

 tion, it will require a total stress of 2 1 7 in cD to cause the parallel- 

 ogram to resume its original rectangular shape. But when the 

 figure is a rectangle, the stress in Cd is zero and, taking the sec- 

 tion win again and S Vert. Comp.=(9. 



Vert. Comp. of stress in cD= shear in the panel. 



Therefore when the shear in the panel is sufficient to pro- 

 duce a tensile stress in cD equal to twice the initial stress, the ini- 

 tial stress in Cd is reduced to zero and all the initial stresses dis- 

 appear. Any further increase of stress in cD will further elon- 

 gate that diagonal, and consequently buckle the diagonal Cd, as 

 neither of the diagonals are capable of taking compression. If 

 the stress in cD, due to the load, is less than 2T, by Hooke's Law, 

 one half is taken up in relieving stress in Cd and the other half 

 is added to the initial stress in cD. 



A post or brace is a web member in compression when the 

 bridge is fully loaded. When the chords are not parallel, there 

 may be tension in some of the intermediate posts and a reversal 

 of stress when the live load comes on. Such reversals are un- 

 desirable. 



121. Classification of Trusses. Trusses may be classified 

 in many different ways. So far as supports are concerned, they 

 may be classified just as solid beams are classified (66). The 

 material of which they are mado is usually structural steel. Some- 



