LOADS IN STRUCTURES 57 



is often desirable to know the approximate weight of roof 

 trusses before the actual size of the members has been cal- 

 culated. In such cases, the following empirical formula 

 may be used: 



in which W is the approximate weight of the truss, in pounds; 

 a, a constant for wood .5, for steel .75; D, the distance, in 

 feet, from center to center of trusses; and L, the span of the 

 truss, in feet. 



It is sometimes desirable to know the approximate weight 

 of roof truss per square foot of roof surface. It will be noted 

 that this is not the same thing as per "square foot of hori- 

 zontal surface, since the roof slants. If x is the angle 

 between the rafter and the chord, L the span of the truss 

 in feet, and w the approximate weight of the truss per square 

 foot of roof surface, then 



o(10 + L) cos* 



~To~ 



in which a is .5 for wooden trusses and .75 for steel trusses. 



EXAMPLE. Determine the weight, per square foot, that 

 must be added to the weight of a roof covering to provide for 

 the weight of the principals, the steel trusses in this case 

 having a span of 72 ft. and a rise of 18 ft. 



SOLUTION. The angle between the rafter and the chord 



may be found from the formula tan # = -^^- = i = .5, corre- 



span 



2 



spending to an angle x of 26 34'. Substituting the values 

 of a, L, and cos * in the preceding formula, 



.75 (10 + 72) cos 26 34' .75X82X.8944 



The following table has been calculated from the preceding 

 formula. This table gives the weight that must be added 

 to a square foot of roof covering to provide for the weight of 

 the principals, or trusses. The slope of the roof is in each 

 case given in terms of its "pitch," which is here taken as 

 the ratio of rise to span. Thus, a $ pitch is such a slope that 

 the rise of the truss is one-third the span of the truss. 



