Art. 136. 



STRESSES IN A BALTIMORE TRUSS. 



creased by half of this panel load, and increased by the increase 

 in RI, which is always less than one-half a panel load. Thus 

 for panel 6-7, with loads 1-6 inclusive acting, the shear is f - 

 with load 7 added; the coefficient for D' 7 is ft~T 8 6 = ni- I n 

 short, the loading for maximum negative shear follows our 

 adopted rule (129). 



In combining the dead and live load stresses in the counter 

 ties to find the resultant maximum, it must be remembered that 

 the total shear in a panel is resisted by the algebraic sum of the 

 vertical components of the diagonals acting in the panel. In panel 

 7-8, the diagonal D 8 ' is the only one acting for the partial loading 

 (1 to 7 inclusive), and therefore D 8 ' resists the entire resultant 

 shear in the panel, just as for diagonals Z> 3 , D r> and Z> 7 . In panel 

 6-7, D/ is not the only diagonal acting, as the dead load panel load 

 at 7 is divided between the two diagonals just as shown for the 

 left half of the truss, and therefore Z> 7 will always carry at least 

 % panel load of dead load. This makes the resultant stress in 

 J>/=f 21 L.L.P.L.+ (7y 2 -9+y 2 )DLPL] sec 0=[H L. L. P. L 

 D.L.P.L.] sec 6. 



For maximum positive shear (to the left of the center), the 

 coefficients in the panels having but one diagonal are the same as 

 for a Pratt truss. For J9 5 , for example, loads 1 to 11 inclusive 

 should act ; the left hand reaction is f f and this is the coefficient. 



In the panels having two diagonal members, the coefficient 

 for the main diagonal may be greater when the sub-panel ahead 

 is on than when the loading follows the rule for maximum shear 

 in the panel. Trying both loadings and remembering that the 

 upper diagonal carries one-half of the sub-panel load only, 



For D 8 , loads 1 to 8. Shear= 



Ito 9. 



l+2+3+4+5+6+7+8_36 

 16 ~"16 



36+9 1 458 37 



For Z) 6 , loads 1 to 10. Shear= = - 



" Ito 11. 



For Z> loads 1 to 12. Shear= 



1 to 13. Shear= 



66+12 

 78+13 



16 



l 668 58 

 = -^=55 



91_8 _ 83 

 16 "16 



