178 GENERAL DESIGN 



Heating in large buildings is generally done by the hot blast sys- 

 tem in which fans draw the air across heated coils, which are heated 

 by exhaust steam, and the heated air is conveyed by ducts suspended 

 from the roof or placed under the ground. In smaller buildings, direct 

 radiation from steam or hot water pipes is commonly used. 



The proper unit stresses, minimum size of sections and thickness 

 of metal will depend upon whether the building, is to be permanent or 

 temporary, and upon whether or not the metal is liable to be subjected 

 to the action of corrosive gases. For permanent buildings the author 

 would recommend 16,000 Ibs. per square inch for allowable tensile, and 



16,000 70 Ibs. per square inch for allowable compressive stress for 



direct dead, snow and wind stresses in trusses and columns ; / being 

 the center to center length and r the radius of gyration of the member, 

 both in inches. For wind bracing and flexural stresses in columns due 

 to wind, add 25 per cent to the allowable stresses for dead, snow and 

 wind loads. For temporary structures the above allowable stresses may 

 be increased 20 to 25 per cent. 



The minimum size of angles should be 2" x 2" x J4", and the 

 minimum thickness of plates J4", f r both permanent and temporary 

 structures. Where the metal will be subjected to corrosive gases as in 

 smelters and train sheds, the allowable stresses should be decreased 20 

 to 25 per cent, and the minimum thickness of metal increased 25 per cent, 

 unless the metal is fully -protected by an acid-proof coating (at present 

 the best paints do little more in any case than delay and retard the 

 corrosion) . 



The minimum thickness of corrugated steel should be No. 20 

 gage for the roof and No. 22 for the sides; where there is certain to 

 be no corrosion Nos. 22 and 24 may be used for the roof and sides 

 respectively. 



The different parts of mill buildings will be taken up and discussed 

 at some length in the following chapters. 



