268 



STRESSES IN A CAMEL-BACK TRUSS. 



Art. 137. 



Live Load =2000 Ibs. per ft. of bridge. 



Dead load panel load = J X3GOOX21 =37,800 Ibs. 



Live load panel load = J X2000 X21 =21,000 Ibs. 



^X1600^2 

 q ~ 3600 ~V 



Diagonal length =63.8 ft. 



sec = 



63.9 



48 ' 



D' fl = (UX21,000- 1X37,800) sec =no reversal. There- 

 fore, D' G is not required and only panel 6-7-8 needs counters. 

 D 7 =}JX21,000 sec 0=41,900 Ibs. (-) 

 D' 7 = (iX21,000+iX37,800) sec 0=39,100 Ibs. (+) 



Max. comp. 

 D' 7 = (2^ X 2l,000-| X 37,800 sec/? = 16,800 Ibs. (-). 



Max. tens. 



Vi = (2 X 2 1,000 + 1| X37,800) = 109,200 Ibs. (-) 

 F 2 = (ff X21,000+2f X37,800) =141,000 Ibs. 



137. Stresses in a Camel -back Truss. For trusses 

 with inclined chords, as in Figs. 162 and 164, coefficients of 

 stress cannot be written by inspection as in trusses with parallel 

 chords. The simplest way is to get the stresses directly by 

 the method of sections, .using the moment equation. The cal- 

 culation of the lever arms for the upper chords and diagonals is 

 somewhat laborious and should be checked by scaling from an 

 accurate diagram to avoid large errors. 



The end and middle panels of Fig. 194 are quite similar to 

 those considered in Art. 130. Since D' 4 is zero under full load, 

 it is evident, at joint 10, that V 3 will be in tension to resist the 

 vertical component of C/ 3 . At joint 9 there are four vertical 

 components, but it will be found that V 2 will also be in tension 

 under certain loads the posts are subject to reversals of stress. 



As was pointed out in Art. 129, no reversal of shear in a 

 panel does not necessarily mean that a counter tie is not re- 

 quired, or that there is no reversal of stress in the diagonal. 

 Nor will the compressive stress in a diagonal, when the counter 

 tie is not present, be equal to the tensile stress in the counter 

 tie, because their inclinations are different; their horizontal 

 components are equal, however, for the same loading. 



This fact furnishes a ready means of finding a counter stress 



