44 2 PROBLEMS 



PROBLEM 15. DEAD LOAD STRESSES IN A TRIANGULAR TRUSS BY 

 GRAPHIC AND ALGEBRAIC MOMENTS. 



(a) Problem. Given a triangular truss, span 6o'-o", pitch *4> 

 trusses spaced 15' o", dead load 40 Ibs. per sq. ft. of horizontal pro- 

 jection. Calculate the stresses by graphic moments. Check by calcu- 

 lating the stresses by algebraic moments, giving all equations. Scale 

 of truss, i"= lo'-o". Scale of loads, i" = 10,000 Ibs. Use pole dis- 

 tance H = 3000 Ibs. 



(b) Methods. Use the methods for algebraic and graphic mo- 

 ments described in Fig. 28 and Fig. 29, respectively. Calculate all 

 moment arms and check by scaling from the diagram. The pole dis- 

 tance is measured in pounds, while the intercepts are measured to the 

 same scale as the truss. Take the section and choose the center of mo- 

 ments so that but one unknown force will produce moments. Take 

 the unknown external force as acting from the outside toward the 

 cut section, the sign of the result if plus will indicate compression, 

 if minus tension. Be careful to take forces on one side of the cut 

 section only. 



(c) Results. The kinds of stress in the members are the same as 

 in Problem 9. The center of moments used in calculating each stress 

 can be easily determined from the equations. The method of alge- 

 braic moments is much used for calculating stresses in bridges, and 

 other frameworks which carry moving loads. The method of graphic 

 moments is used principally as an explanatory method. 



PROBLEM i5a. DEAD LOAD STRESSES IN A TRIANGULAR TRUSS BY 



GRAPHIC AND ALGEBRAIC MOMENTS. 



(a) Problem. Given a triangular truss, span 6o'-o", pitch J^, 

 trusses spaced i6'-o", dead lead 40 Ibs. per sq. ft. of horizontal pro- 

 jection. Calculate the stresses by graphic moments. Check by calcu- 

 lating the stresses by algebraic moments, giving all equations. Scale 

 of truss, i"= lo'-o". Scale of loads, i" = 10,000 Ibs. Use pole dis- 

 tance H = 3000 Ibs. 



