45 2 PROBLEMS 



PROBLEM 20. DEAD LOAD STRESSES IN A PRATT TRUSS BY GRAPHIC 



RESOLUTION. 



(a) Problem. Given a Pratt truss, span I4o'-o", panel length 

 2o'-o", depth 24'-o", dead load 800 Ibs. per lineal foot per truss. Cal- 

 culate the dead load stresses by graphic resolution. Scale of truss 

 i" = 2o'-o". Scale of loads, i" = 16,000 Ibs. 



(b) Methods. The loads beginning with the first load on the 

 left are laid off from the bottom upwards. Calculate the stresses by 

 graphic resolution, beginning at R 1 and checking at R 2 , following the 

 order shown in the stress diagram. 



(c) Results. The top chord is in compression and the bottom 

 chord is in tension as in the Warren and Howe trusses. The inclined 

 web members are in tension, while the vertical posts are in compression. 

 Member 1-2 is simply a hanger. There is no stress due to dead loads 

 in the diagonal members in the middle panel. The stresses in the 

 posts are equal to the vertical components of the diagonal members 

 meeting them on the unloaded chord. Stresses in certain panels in 

 the top and bottom chord are equal. The Pratt truss is quite generally 

 used for steel bridges, and is also used for combination bridges, where 

 the tension members are made of iron or steel and the compression 

 members are made of timber. 



PROBLEM 2oa. DEAD LOAD STRESSES IN A PRATT TRUSS BY GRAPHIC 



RESOLUTION. 



(a) Problem. Given a Pratt truss, span i6o'-o", panel length 

 2o'-o", depth 24'-o", dead load 800 Ibs. per lineal feet per truss. Cal- 

 culate the dead load stresses .by graphic resolution. Scale of truss 

 i" = 25'-o". Scale of loads, i" = 20,000 Ibs. 



