tJSEFtTL DATA 



have value in themselves as columns. The general effect of closely spaced hooping is 

 to greatly increase the toughness of the column and to add to its ultimate strength, 

 but hooping has little effect on its behavior within the limit of elasticity. It thus 

 renders the concrete a safer and more reliable material, and should permit the use of 

 a somewhat higher working stress. The beneficial effects of toughening are adequately 

 provided by a moderate amount of hooping, a larger amount serving mainly to increase 

 the ultimate strength and the deformation possible before ultimate failure. 



Composite columns of structural steel and concrete in which the steel forms a column 

 by itself should be designed with caution. To classify this type as a concrete column 

 reinforced with structural steel is hardly permissible, as the steel, generally, will take 

 the greater part of the load. When this type of column is used, the concrete should 

 be adequately tied together by tie plates or lattice bars, which, together with other 

 details, such as splices, etc., should be designed in conformity with standard practice 

 for structural steel. The concrete may exert a beneficial effect in restraining the steel 

 from lateral deflection and also in increasing the carrying capacity of the column. The 

 proportion of load to be carried by the concrete will depend on the form of the column 

 and the method of construction. Generally, for high percentages of steel, the concrete 

 will develop relatively low unit stresses, and caution should be used in placing depend- 

 ence on the concrete. 



The following recommendations are made for the relative working stresses in the 

 concrete for the several types of columns : 



(a) Columns with longitudinal reinforcement to the extent of not less than 1 per 

 cent and not more than 4 per cent, and with lateral ties of not less than 3^ inch in 

 diameter 12 inches apart, nor more than 16 diameters of the longitudinal bar: the 

 unit stress recommended for axial compression, on concrete piers having a length not 

 more than four diameters, in section on "Working Stresses." 



(6) Columns reinforced with not less than 1 per cent and not more than 4 per cent 

 of longitudinal bars and with circular hoops or spirals not less than 1 per cent of the 

 volume of the concrete and as hereinafter specified: a unit stress 55 per cent higher 

 than given for (a), provided the ratio of unsupported length of column to diameter of 

 the hooped core is not more than 10. 



The foregoing recommendations are based on the following conditions : 



It is recommended that the minimum size of columns to which the working stresses 

 may be applied be 12 inches out to out. 



In all cases longitudinal reinforcement is assumed to carry its proportion of stress 

 in accordance with Section (c) 6, page 195. The hoops or bands are not to be counted 

 on directly as adding to the strength of the column. 



Longitudinal reinforcement bars should be maintained straight, and should 

 have sufficient lateral support to be securely held in place until the concrete 

 has set. 



Where hooping is used, the total amount of such reinforcement shall be not less than 

 one per cent of the volume of the column, enclosed. The clear spacing of such hooping 

 shall not be greater than one-sixth the diameter of the enclosed column and preferably 

 not greater than one-tenth, and in no case more than 23^in. Hooping is to be circular 

 and the ends of bands must be united in such a way as to develop their full strength. 

 Adequate means must be provided to hold bands or hoops in place so as to form a 



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