COLUMNS 185 



tudinal steel and with bands or spirals spaced not greater than 

 one-fourth the diameter of the enclosed column, shall have an 

 allowable stress on the concrete 45 per cent higher than given 

 for a plain concrete column, or 650 Ib. per square inch on 2000 

 Ib. concrete at the age of 28 days. With n = 15, the corresponding 

 stress in the steel is 9750 Ib. per square inch. 



When columns reinforced with structural steel are designed in 

 accordance with the principles already stated, and the steel and 

 concrete receive their load simultaneously, the working stresses 

 may be taken the same as for hooped columns with vertical steel 

 rods. This is also in accordance with the Joint Committee's 

 recommendations. 



71. Value of Longitudinal Reinforcement in Columns. When a 

 column is of any considerable length something more than plain 

 concrete is desired in practice. Bending moments in such col- 

 umns are apt to occur from unforeseen conditions, and tensile 

 stresses may be produced which would rupture a column if 

 without reinforcement. In very large and relatively short 

 columns, little is to be feared from bending stresses, as in 

 such a case no resultant stress is likely to occur, but in small 

 sections where the danger of imperfect spots in the concrete is 

 greatest, steel reinforcement is of great value in producing a 

 more reliable structure. 



It should be understood, however, that from a theoretical 

 standpoint the use of steel in columns is not economical. From 

 equation (1) previously given, we see that with a value of n = 15, 

 the use of each 1 per cent of longitudinal steel adds 14 per cent to 

 the strength of a column, while with 50 as the ratio of cost of 

 steel to cost of concrete per unit volume, the increased cost of a 

 column with 1 per cent of steel will be 50 X 1 = 50 per cent. That 

 is, the relative economy of the reinforced column is ordinarily 



114 

 only = 76 per cent that of the concrete column without 



10U 



reinforcement. 



Illustrative Problem. The effective area of a column is 144 sq. in. ; load 

 to be carried is 80,000 Ib.; and working stress on the concrete is 450 Ib. 

 per square inch. What percentage of longitudinal bars without hooping 

 will be required? Take n = 15. 



The safe strength of a plain concrete column would be 

 144 X 450 = 64,800 Ib. 



