262 



STRESSES IN A BALTIMORE TRUSS. 



Art. 136 



Sub-diagonal D' 8 in connection with D' 7 forms the counter 

 of panel 8-10 or 6-8. When these are in action, D 7 becomes 

 the sub-diagonal and D 8 goes out of action. The dead load 

 coefficients to be used in connection with the live load to find 

 the stresses in the counters, are given on the right half of Fig. 

 191. D'% and D r may get their maximum stress as a sub- 

 diagonal or as a counter, depending upon the relation of the 

 dead load to the live load. Both stresses must be calculated 

 to find out which is the greater. 



The coefficient for a post may be gotten by taking a section 

 through it and a sub-diagonal, or by adding the diagonal coef- 

 ficients at the upper joint to the load applied at the point. As 

 in the case of the Pratt truss with an even number of panels, 

 there are two conditions affecting the stress in the middle ver- 

 tical, V ' \. When the counter is in action as shown, the dead 

 load coefficient of F 4 will be q, but when the bridge is symmet- 

 rically loaded, the counter will not be in action and the coeffi- 

 cient for V 4 will be J+J+g = l+g. These must be used in 

 connection with their proper live load coefficients. 



Knowing the directions in which the horizontal components 

 of the diagonals act, the chord coefficients are gotten by algebraic 

 addition at each chord joint. t/ 3 = C/ 4 =7+5i = 12i. Uirt7 a 



As a check, with a section through U 7 , D 7 , and L 7 , 

 or 50L 7 =7J X6 X20 - 1 (1 +2 +3 +4 + 5)20. 



45X20-15X20 30X20 



50 



50 



15Xfg = 15 land. 



M s =0, or 



=7i X8 X20- 1(2 +3 +4+5+6+7)20. 



60X20-27X20 

 50 



33X20 

 50 



The live load coefficients for the counter ties (maximum neg- 

 ative shears) are gotten on the right of the center line just as 

 for a Pratt truss. This is evident for those panels having but 

 one diagonal. (The main tie is not in action when the counter 

 tie is.) In the other panels the stress in the upper member is 

 also zero for live load unless the sub-panel load is on; but when 

 the sub-panel load acts, the shear carried by the counter tie is de- 



