332 FOUNDATIONS 



forms a considerable part of the section, which will happen 

 when the beam is large and the slab is shallow. In the latter 

 case, it is well to neglect the T effect and consider that the 

 beam carries the entire load. 



FORMULAS FOR COLUMNS 



Let, in addition to previous notation, a be the cross-sec- 

 tional area of the column, a s the cross-sectional area of the steel, 

 and a c the cross-sectional area of the concrete. Let, further, 

 s s and s c denote the unit stresses in steel and concrete, respec- 

 tively, and W the total load on column centrally loaded. Then 



s s = ns c (1) 



W=s e (fi s n+a e ) (2) 



As an example, let it be required to find W for a column 18 

 in. square and reinforced with eight rods $ in. square, using s c 

 = 450 and n=15. Applying formula 1, s.y = 450X15 = 6,750. 

 To apply formula 2, substitute for a s , 8XJX|=4.5, and 

 for a c , 18X18-4.5 = 319.5. Then, W= 450(4.5X15+319.5) 

 - 174,150 Ib. 



FOUNDATIONS 



SUBFOUNDATIONS 



The subfoundation of a structure is that part of the natural 

 surface of the earth on which the structure rests. The founda- 

 tion is the lower part of the structure, which connects it with 

 the subfoundation. 



Materials for Subfoundations. The materials usually 

 regarded as suitable for subfoundations are solid rock, loose 

 rock, earth, and sand. 



The supporting power of a rock subfoundation may be con- 

 sidered as approximately equal to the resistance to crushing of 

 the material of which the rock is composed, modified by a suit- 

 able factor of safety. The accompanying table is based on a 

 factor of safety of 10. 



