WEIGHT OF ROOF TRUSSES 5 



dead load to assume for computing stresses, and the approximate weight 

 of metal to use as a basis for preliminary estimates. 



Weight of Roof Trusses. The weight of roof trusses varies 

 with the span, the distance between trusses, the load carried or capacity 

 of the truss, and the pitch. 



The empirical formula 



, 0) 



where 



PF:=weight of steel roof truss in pounds; 



P=capacity of truss in pounds per square foot of horizontal pro- 

 jection of roof (30 to So Ibs.) ; 



A= distance center to center of trusses in feet (8 to 30 feet) ; 



Z,=span of truss in feet ; 



was deduced by the author from the computed and shipping weights 

 of mill building trusses. The trusses were riveted Fink trusses with 

 purlins placed at panel points, and were made up of angles with con- 

 necting plates ; minimum size of angles 2" x 2" x J4", minimum thick- 

 ness of plates l /4 rt . 



The trusses whose weights were used in deducing this formula had 

 a pitch of y^. (6" in 12"), but the formula gives quite accurate results 

 for trusses having a pitch of % to ^3. The trusses were designed 

 for a tensile stress of 15000 Ibs. per square inch and a compressive 



stress of 15000 55 Ibs. per square inch, where / = length and r 



the radius of gyration of the member, both in inches. 



The weight of steel roof trusses for a capacity, P t of 40 Ibs. per 

 square foot for different spacings is given in Fig. 2. The weights of 

 trusses for other capacities can be obtained by multiplying the tabular 

 values by the ratio of the capacities. 



Dividing (i) by A L we have the weight of roof truss, W B , per 

 square foot of horizontal projection of the roof 



