WORKING STRESS FOR COLUMNS. Compression on Concrete. A comparison of the results in Table 45, page 244, with those in Tables 29 (page 195), 30 (page 196) and 31 (page 197) shows that con crete in the form of a column is not as strong as in a cube doubtless because of the less restraint;—and a comparison of Table 45 with the results of tests of beams shows that the compressive strength of concrete is greater in a beam than in a column. In consideration of the above facts it is not safe to assume that the ultimate strength of a 1 : 2 4 concrete column 30 days old is more than 1,600 lb. per sq. in. To determine the strength of other proportions or other ages apply the proper ratio from Tables 32 (page 198) and 33 (page 199), respectively. In consideration of the effect of impact and of repeti tion of the load and also of the danger of improper proportions or of poor mixing, it is not safe to use a less factor of safety than four; and therefore the safe unit working stress for a 1 : 2 : 4 concrete 30 days old should not be taken at more than 400 lb. per sq. in.
The recommendation of the Joint Committee is: "For concentric compression on a plain concrete column or pier, the length of which does not exceed 12 diameters, 22.5 per cent of the compressive strength at 28 days, or 450 lb. per sq. in. on 2000-lb. concrete, may be allowed. For columns with the several types of reinforcement the following working stresses are recommended: "a. Columns with longitudinal reinforcement only, the unit stress as in the paragraph above Bars composing longitudinal reinforcement shall be straight, and shall have sufficient lateral support to be securely held in place until the concrete has set. In all cases, longitudinal steel is assumed to carry its proportion of stress.
"b. Columns with reinforcement of bands or hoops, stresses 20 per cent higher than given for a. Where bands or hoops are used, the total amount of such reinforcement shall not be less than I per cent of the volume of the column inclosed. The hoops or bands are not to be counted upon directly as adding to the strength of the columns. The clear spacing of such bands or hoops shall not be
greater than one fourth of the diameter of the inclosed column. Adequate means must be provided to hold the bands or hoops in place. so as to form a column the core of which shall be straight and well centered.
"d. Columns reinforced with structural steel-column units which thoroughly encase the concrete core, stresses 45 per cent higher than given for a." Tension in Steel. Since, for ordinary working stresses in the concrete, the stress in the steel with either longitudinal or circum ferential reinforcement is much less than the ordinary working stress of even low steel, no consideration need be given here to the unit working stress of that material.
Factor of Safety of Column. The unit stress in the steel of a longitudinally reinforced column is n times the unit stress in the concrete (see § 486), and since n increases as the stress in the con crete, it follows that at the ultimate load the steel takes a greater proportionate stress than at working loads; and consequently the ultimate strength of the column is greater than the working load multiplied by the factor of safety of the concrete, that is, the factor of safety of the column will be greater than the factor of safety of the concrete. A further result of the fact that as the load increases the stress taken by the steel increases, is that the factor of safety increases with the percentage of steel.