Art. 136. 



STRESSES IN A BALTIMORE TRUSS. 



267 



At joint 12, the horizontal components of D' 3 and D' 2 

 balance each other; the coefficient of L 3 must, therefore, be the 

 same as for L 2 . At joint 10, Z/r>=6^ +3J +i = 10. At joint 8, 

 1/7 = 104 + 14+4 = 124. At joint 16, 1/3=6+4 = 10. At joint 

 18, U 5 = 10 +2 = 12. At joint 20, U 7 = 12 +0 = 12. 



As a check M 8 =0, section through U 7 , D' 7 , and L 7 . 



481/7 = 6^X6X21 -(1+2 +3 +4 + 5)21. 

 (39-15)21 



48 



= 12x1-1 = 12 tanO. 



For the same section M 2 2 =0. 



48 L 7 =64X8X21 -(2 +3 +4 +5 +6 +7)21. 



(52-27)21 



L 7 = 



48 



TF 



124 tan 6. 



At joint 12, one-half of each of the loads 11 and 13 is added 

 to load 12, making Vi=2 q. 



The coefficients for the posts are evident by considering 

 joints 18 and 20. 



The live load coefficients are gotten as for a Pratt truss, ex- 

 cept that in the panels with two diagonals, the sub-diagonal 

 carries a half panel load of shear, and, that, as was shown for 

 the truss of Fig. 192, D 2 requires a full load, 



A question might arise as to the middle panel 6-7-8. The 

 compression sub-diagonal cannot hang slack and, under full 

 load it is a question whether D' 7 or D 7 is acting as the sub- 

 diagonal; but under partial loading for maximum counter stress 

 the distortion of the panel (120) throws the upper half of the 

 diagonal, from joint 21 to 22, out of action (being a tension 

 member) and the coefficients are as given in Fig. 193. This form 

 of sub-trussing is often used for riveted bridges. 



The maximum shear in panel 7-8 is -f-f , of which D' 7 carries 

 T 7 T , and D 7 , fj-. The maximum shear in panel 9-10 is ff, of 

 which D' 5 carries -f% and D 5 , ff . In like manner the coefficient 

 forD s isfl-A=H. 



As the calculation of stresses is now a simple matter, only 

 a few will be given. 



Dead Load. 



Trusses and bracing = 1600 Ibs. per ft. of bridge. 

 Floor system =2000 Ibs. per ft. of bridge. 



Total 



= 3600 Ibs. per ft. of bridge. 



