When the concrete is used for fireproofing, the steel should he covered by at least 2 inches of concrete, and about 12 inches of con crete on the exterior of the column should not be considered in deter mining the strength of the column.
121. Columns with Hooped Reinforcement. As shown in Section 95, the failure of concrete under compression commonly occurs through shearing due to lateral expansion. If the concrete in the column be held by band or spiral steel (see Fig. 58 (b)) from yielding to lateral expansion, the resistance to crushing will be materially in creased. Such reinforcement is either formed of steel bars bent to form a spiral or bands of steel spaced at a uniform distance apart, but in either case, the bands should not be spaced more than about one-sixth of the diameter of the column apart, and must be held in place by longitudinal spacing bars.
Experiments upon columns with hooped reinforcement indicate that the deflections under working loads are not decreased by the reinforcement, but the ultimate strength is considerably increased, as compared with columns without such reinforcement. When hooped reinforcement is used, it is usual to allow a larger unit stress than for plain columns, or those with longitudinal reinforcement only. The effective area of the column is that inside the reinforcement. The concrete outside the hooping is stripped off when a stress is reached at which plain concrete would fail.
Hooped reinforcement prevents crushing of the concrete until a load is reached which stresses the steel to its yield point, but does not stiffen the column longitudinally, and columns so reinforced fre quently fail by bending. This reinforcement is commonly combined with longitudinal steel as shown in Fig. 58 (c). The longitudinal steel serves to stiffen the against bending, and makes the hooping more effective. In general, it is not advisable to use hooped reinforcement without longitudinal steel, as the same amount of steel would be more effective in strengthening the column if used as longitudinal steel.
Experiments indicate that. about 1 per cent of steel in closely spaced pooping is sufficient to resist lateral expansion and give in creased strength in compression. Larger amounts of steel do not
materially increase the resistance. The Joint Committee makes the following recommendations: (b) 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 reComn1)endations are based on the following conditions: It is recommended that the minimum size of columns to which the working stresses may he applied he 12 inches out to out.
In all cases longitudinal reinforcement is assumed to carry its proportion of stress. The hoops or bands are not to be counted on directly as adding to the strength of the calmnnn.
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 1 per cent of the volume of the column, enclosed. The clear spacing of such pooping shall be not greater than one-sixth the diameter of the enclosed column and preferably not greater than one-tenth, and in no case inure than 2 inches. Ilooping is to be circular and the ends of bands must he 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 column, the core of which shall be straight and well centered. The strength of hooped columns depends very much upon the ratio of length to diameter of hooped core, and the strength due to hooping decreases rapidly as this ratio increases beyond five. The work ing stresses recommended are for hooped columns with a length of not more than ten diameters of the hooped core.
The Committee has no recommendation to make for a formula for working stresses for columns longer than ten diameters.