PITCH OF TRUSS 



the weight of the purlins and girts per square foot of area increases as 

 the spacing increases. The economic spacing of the trusses is a func- 

 tion of the weight per square foot of floor area of the truss, the pur- 

 lins, the side girts and the columns, and also of the relative cost of each 

 kind of material. For any given conditions the spacing which makes 

 the sum of these quantities a minimum will be the economic spacing. 

 It is desirable to use simple rolled sections for purlins and girts, and 

 under these conditions the economic spacing will usually be between 16 

 and 25 feet. The smaller value being about right for spans up to, say, 

 60 feet, designed for moderate loads, while the greater value is about 

 right for long spans, designed for heavy loads. 



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 Part FraminqPlan 



Section A-B 

 FlG. 87. STEEL ROOF COVERED WITH LUDOWICI TII<E. 



Calculations of a series of simplt Fink trusses resting on walls 

 and having a uniform span of 60 feet, and different spacings gave the 

 least weight per square foot of horizontal projection of the roof for 

 a spacing of 18 feet, and the least weight of trusses and purlins com- 

 bined for a spacing of 10 feet. The weight of trusses per square foot 

 was, however, more for the lo-ft. spacing than for the i8-ft. spacing, 

 so that the actual cost of the steel in the roof was a minimum for a 



