SIMPLE STRUCTURES 



201 



may be taken as 400 Ib. per linear foot, and live load as 3500 Ib. per linear foot. 

 Calculate analytically the stresses in all the members. 



NOTE. Each truss carries one half the total load. In the present case, therefore, the 

 total load per linear foot per truss is 



1100 + 400 + 3500 



= 250011). 



307. In the saw-tooth type of roof truss shown in Fig. 149, obtain graphically 

 the stresses in all the members, the dimensions being as follows : span = 25 ft., dis- 

 tance apart of trusses = 15 ft., and pitch of roof = |, making the inclination of the 

 longer leg to the horizontal = 21 48'. 



FIG. 149 



As the span is short and the roof comparatively flat, it is sufficiently accurate 

 to assume that the combined action of wind and snow is equivalent to a uniform 

 vertical load, which in the present case may be assumed as 25 Ib./f t. 2 of roof. The 

 weight of this type of truss will be taken as 1.5 lb./ft. 2 of roof, and the weight of 

 roof covering as 7 lb./ft. 2 of roof. As the top-chord panel length is 8 ft., each panel 

 load will be 8 x 15 x 33.5 = 4020 Ib. 



308. Analyze graphically for both 

 dead and snow loads the French type 

 of roof truss shown in Fig. 151, the 

 dimensions being as follows : span 



BJ 



I N I \ p \ \ R k 



- 1601 -f 



I =100 ft., rise h = 30 ft., <Z = 5ft., 

 and distance apart of trusses b = 20 ft. 



The weight of truss in pounds for FIG. 150 



this type is given by the formula 



W = ^ bl 2 , where 6 and I are expressed in feet. The weight of the roof covering 

 may be assumed as 15 lb./ft. 2 of roof surface, and the snow load as 20 lb./ft. 2 of 

 horizontal projection. 



First calculate the dead load carried at each joint, due to weight of truss and 

 roof covering, and draw the diagram for this system of loads. The diagram for 



