304 CONCRETE DESIGN 



If a column is 12 in. square and has 3 sq. in. of steel, 

 determine the safe load that it will carry. Deducting 1 in. 

 from each surface, the column will be 81 sq. in. But 3 sq. in. 

 of this area is steel. Therefore, the effective area of the 

 column is 81-3 = 78 sq. in. By the formula, W = 450 

 (78 + 15X3) = 55,350 Ib. The stress in the steel is there- 

 fore 450 X 15 = 6,750 Ib. per sq. in., which is safe. 



The use of steel in a column has two advantages. In the 

 first place, the formulas do not take into account the length 

 of the column; the longer a column is, the more it is apt to 

 bend or bulge, and the steel helps materially to resist this 

 tendency. Then again, the introduction of steel permits 

 the column to be made much smaller in size, and this is often 

 of great advantage. 



Having shown how to investigate a column, the method 

 of designing one to carry a certain load will now be con- 

 sidered. Thus, assume that it is desired to design a column 

 to carry a load of 40 T. The percentage of reinforcement 

 does not have to be a definite amount as is the case with 

 beams, but may be any amount. Suppose that it is desired 

 not to have the column too large; therefore, assume that the 

 reinforcement will occupy 4% as much area as the concrete. 

 Let A c = area of concrete. Then, A s = 4% of A c = TsA c . 

 Substituting the correct values in the formula it will be found 

 that 80,000 = 450(A C + NA S )=450(A C + 15 X^4<:)= 720 A . 

 .4, = 111 sq. in/ and A s = -- s A c = 4A+ sq. in. Therefore, the 

 total area = A C + A S = 116 sq. in. 



If the column is square, it will be 10.77 in. on a side; 

 say 11 in., to allow for chamfered corners. When 1 in. 

 of fireproofing is put on, the column will be 14 in. square 

 and will contain a little over 4.4 sq. in. of steel. 



Empirical Rules for Straight Reinforcement. One or two 

 more or less empirical formulas are used in designing concrete 

 columns with straight reinforcement. These formulas 

 mostly originate from the building laws of various cities. 

 The building laws of one large city, for instance, furnish 

 good examples of such formulas. They stipulate that the 

 safe allowable load shall be 500 Ib. per sq. in. of column 

 section, counting both concrete and steel the same. If 



