184 REINFORCED CONCRETE CONSTRUCTION 



70. Working Stresses. Considering the sudden manner of 

 failure of plain concrete columns, the factor of safety should be 

 relatively large. The Joint Committee recommend a working 

 stress of 22 1/2 per cent of the compressive strength at 28 days, 

 or 450 Ib. per square inch on 2000 Ib. concrete tested in cylin- 

 drical form. For richer and stronger mixtures the working 

 stress may be increased accordingly. 



In a column reinforced with longitudinal steel only, the con- 

 crete fails in a manner similar to the failure of an unreinforced 

 column. Failure takes place suddenly at approximately the same 

 stress per square inch on the concrete as in a plain concrete 

 column. Thus, the factor of safety employed for unreinforced 

 columns might well be used with respect to the concrete in 

 columns of this type. The value of 15 for n is also recommended. 

 (See Art. 25.) With n = 15, the corresponding stress in the steel, 

 for a working stress of 450 Ib. per square inch on the concrete, 

 is 6750 Ib. per square inch. 



The proper working stress for hooped columns should be 

 selected mainly with reference to the elastic limit, but the 

 greater toughness of the hooped column as compared with other 

 types, insures a much larger and more certain margin of safety; 

 in other words, the hooped column should permit the use of a 

 somewhat higher working stress in the concrete than for similar 

 columns without hoops. The Joint Committee recommends a 

 working stress 20 per cent higher than for a plain concrete column, 

 or 540 Ib. per square inch on 2000 Ib. concrete at the age of 28 

 days, when the amount of band or spiral reinforcement is not 

 less than 1 per cent of the volume of the column enclosed. It 

 is also recommended that the clear spacing of such bands or 

 hoops shall not be greater than one-fourth the diameter of the 

 enclosed column. The working stress allowed is to be applied to 

 the concrete alone, and the hooping is not to be taken directly 

 into account. 



For hooped columns containing an ordinary amount of longi- 

 tudinal reinforcement, the elastic limit of the column tends to 

 approach a point corresponding to the elastic limit of the longi- 

 tudinal steel. One per cent of hooping appears from the results of 

 tests to be sufficient to make possible very high working stresses. 

 The Joint Committee recommends that columns reinforced with 

 not less than 1 per cent and not more than 4 per cent of longi- 



