43 PROBLEMS 



PROBLEM 9. DEAD LOAD STRESSES IN A TRIANGULAR TRUSS BY 

 GRAPHIC RESOLUTION. 



(a) Problem. Given a triangular truss, span 6o'-o" ; pitch y$ ; 

 camber of the bottom chord 3'-o" ; trusses spaced i4'-o" ; load 40 Ibs. 

 per sq. ft. of horizontal projection. Calculate the reactions by means 

 of force and equilibrium polygons. Calculate the stresses by graphic 

 resolution. Scale of truss, i" = lo'-o". Scale of loads, i" = 5000 Ibs. 



(b) Methods. Start the truss at (075", 7.0"). Start the stress 

 diagram at (675", 6.25"). Calculate the stresses beginning at R it 

 as described in Fig. 27, using care to analyze each joint before pro- 

 ceeding to the next. Check at R 2 . 



(c) Results. The upper chord is in compression while the lower 

 chord is in tension. The vertical web members are in compression while 

 the inclined web members are in tension for dead loads. As a check 

 the points 2, 4, 6, should be in a straight line. The partial loads coming 

 on the reactions (not shown) are not considered as they have no effect 

 on the stresses in the truss. This truss is a triangular Pratt and is 

 quite economical, but is somewhat more expensive in material and 

 labor than the Fink truss. This type of truss is much used for com- 

 bination trusses, in which the tension members are made of iron or 

 steel, while the compression members are made of timber. The dead 

 joint load will be equal to the horizontal projection of the area sup- 

 ported by a panel point, multiplied by the dead load per square foot, 

 is equal toi2XioX4O = 4800 Ibs. 



PROBLEM 9a. DEAD LOAD STRESSES IN A TRIANGULAR TRUSS BY 



GRAPHIC RESOLUTION. 



(a) Problem. Given a triangular truss, span 6o'-o" ; pitch y$ ; 

 camber of the bottom chord 2'-o"; trusses spaced i6'-o"; load 40 Ibs. 

 per sq. ft. of horizontal projection. Calculate the reactions by means 

 of force and equilibrium polygons. Calculate the stresses by graphic 

 resolution. Scale of truss, i" = lo'-o". Scale of loads, i" = 5000 Ibs. 



