264 



STRESSES IN A BALTIMORE TRUSS. 



Art. 136. 



For D 2 , loads 1 to 14. Shear= 



1 to 15. Shear= 



91+14 _ 105 

 16 



105+15 



16 

 1 



16 



1208 

 16 



112 

 16 



An inspection of these figures shows that, for this style of 

 web bracing, the maximum stress in the main diagonal of a panel 

 having two diagonal members, occurs when the sub-panel load 

 ahead of the panel is on. It will be noted also that the shear car- 

 ried by these diagonals is always decreased by a half panel load, 

 when the sub-panel load acts, while it is always increased more 

 than this ; the smallest increase above is ^ for the first panel load 

 to the left of the center. 



Taking a section through a post and sub-diagonal, remember- 

 ing that there is no stress in the sub-diagonal when the sub-panel 

 load is not on, and that it carries a half panel load when the 

 sub-panel load is on, we have 



For y 4 , loads 1 to 7, at joint 24, fj+0 =f f 



" 1 to 9, tt lfr=H 



For y 3 , loads 1 to 9, at joint 22, f J+0 =^ 



" l to 11, ?* ij=tt 



For y 2 , loads 1 to 11, at joint 20, f f +0 =ff 

 " 1 to 13, tt lj=tt 



For the posts, the regular rule for loading applies in getting 

 aiaximum stress, except for the post V 2 , in which the stress will 

 be slightly larger if two more panel loads are added than the rule 

 requires. All of the post coefficients will be larger than thoe 

 given above if it is assumed reasonable to have a discontinuous 

 load ; if loads 1 to 11 inclusive, except load 10, acted, for example, 

 the coefficient for V 3 would be f f 



For the position of live load (loads 1 tc 7) which gives a 

 maximum stress in y 4 , (the center post) the acting members of 

 yhe truss are as shown in Fig. 191, and the total stress in V 4 is 

 equal to the sum of the vertical components of the diagonals 

 meeting at joint 24, plus whatever load is applied at joint 24, or 



y 4 = [f|L. L. P. L.-i/;. L. P. L.] +[!/>. L. P. L.]+[qD. L. P. L.] 

 =$%L.L.P.L.+ql).L.I'.L. 



For a symmetrical loading with live loads at joints 7, 8, 

 and 9, 



V 4 = (1 +q)D. L. P. L. + 1L. L. P. L. 



