296 



CONCRETE DESIGN 



To design such a beam, assume values for p, p', d', d, 

 and n. Then find k by the formula 



After k is found, assume a value for b, and find F c by the 

 formula 



Also, find F s by the formula 



and F s ' by the formula 



*-! 



The values of .F .Fy, and F s ' should all be within safe 

 limits. F s ' will usually be low, which shows that the steel 

 is not used economically. On account of this defect, double 

 reinforced beams are seldom used except in places where 



FIG. 1 



the size of the beam is limited by the conditions of the 

 problem. 



As an example, design a girder, as shown in Fig. 1, to 

 carry besides its own weight 450 Ib. per ft. The section of 



