74 PRACTICAL STRUCTURAL DESIGN 



,. v fjc f c k k 1 k 1 k 

 p(steelratl )=: T = 2A = 2 X 2 = 2 X m = ^ 



/. fc 



Another expression for p is found as follows : The unit moment 

 of resistance is expressed by a Resistance Factor, R. For the steel 

 this is R t and for the concrete this is R c . 



Re = - kj and R a = pjf t . 



The moment of resistance of a concrete beam = Rbd?, in which 

 R is the resistance factor for the " balanced " beam, that is, a 

 beam in which the tensile force exactly equals the compressive 

 force. Fig. 59 is a chart for obtaining R and p for different 

 stresses in steel and concrete. The stresses recommended by 

 the Joint Committee on Concrete and Reinforced Concrete are 

 16,000 Ibs. per square inch for the steel and 650 Ibs. per square 

 inch for 1:2:4 concrete. 



For rectangular reinforced-concrete beams (and for slabs with 

 width 6, of 12 ins.) the following formulas are used, the moment 

 being in inch pounds and all beam dimensions in inches. 



M = 

 h M 



b = m 



d =\Rb' 



When the moment is in foot pounds, 6 will be hi feet and d 

 in niches, or 6 and d will be in inches and R will be divided 

 by 12. 



The Portland Cement Association, Chicago, 111., is an organiza- 

 tion supported by the cement manufacturers of the United States 

 and Canada for the purpose of disseminating information about 

 cement and concrete. Every man in the building business should 

 have his name and address on the mailing list, for some of the 

 Association bulletins deal with questions of design, while all the 

 bulletins should be on file in the office of every one who has anything 

 to do with construction work. 



