51. Lengths of Beams and Columns. The span length for beams and slabs shall be taken as the distance from center to center of supports, but shall not be taken to exceed the clear span plus the depth of beam or slab. Brackets shall not be considered as reducing the clear span in the sense here intended.
52. The length of columns shall be taken as the maxi mum unsupported length.
54. Working Stresses. The following working stresses are recommended for static loads. Proper allow ances for vibration and impact are to be added to live loads where necessary to produce an equivalent static load before applying the unit-stresses in proportioning parts.
55. In selecting the permissible working stress to be allowed on concrete, we should be guided by the working stresses usually allowed for other materials of construc tion, so that all structures of the same class, but com posed of different materials, may have approximately the same degree of safety.
56. The stresses for concrete are proposed for con crete composed of one part Portland cement and six parts aggregate, capable of developing an average com pressive strength of 2,000 pounds per square inch at 28 days, when tested in cylinders 8 inches in diameter and 16 inches long, under laboratory conditions of manufac ture and storage, using the same consistency as is used in the field. In considering the factors recommended with relation to this strength, it is to be borne in mind that the strength at 28 days is by no means the ultimate which will be developed at a longer period, and therefore they do not correspond with the real factor of safety. On concretes in which the material of the aggregate is inferior, all stresses should be proportionately reduced, and similar reduction should be made when leaner mixes are to be employed. On the other hand, if, with the best quality of aggregates, the richness is increased, an in crease may be made in all working stresses proportional to the increase in compressive strength at 28 days, but this increase shall not exceed 25 per cent.
57. Bearing.'' For compression on surface of concrete larger than loaded area, 32.5 per cent of compressive strength at 28 days, or 650 pounds per square inch on 2,000-pound concrete.
58. Plain columns. Plain columns or piers whose length does not exceed twelve diameters, per cent of compressive strength at 28 days, or 450 pounds per square inch on 2,000-pound concrete.
59. Reinforced Columns. See paragraphs 96 to 104, below, on "Columns." 60. Compression in Extreme Fiber. For extreme fiber stress of beams calculated for constant modulus of elas ticity, 32.5 per cent of the compressive strength at 28 days, or 650 pounds per square inch for 2,000-pound concrete.
61. Adjacent to the support of continuous beams, stresses 15 per cent greater may be allowed.
62. Shear. Pure shearing stresses uneombined with compression or tension, 6 per cent of compressive strength For beams and girders built Into pockets in concrete walls, the Lower compressive stress of 450 pounds per square inch should not be exceeded.
at 28 days, or 120 pounds per square inch for 2,000 pound concrete.
6:3. Diagonal Tension. In beams where diagonal ten sion is taken by concrete, the vertical shearing stresses should not exceed 2 per cent of compressive strength at 28 days, or 40 pounds per square inch for 2,000-pound concrete. See also paragraph 88, below, on "Shear and Diagonal Tension." 64. Bond for Plain Bars. Bonding stress between concrete and plain reinforcing bars, 4 per cent of com pressive strength at 28 days, or 80 pounds per square inch for 2,000-pound concrete.
For drawn wire, 2 per cent, or 40 pounds on 2,000 pound concrete.
65. Bond for Deformed Bars. Bonding stress be tween concrete and deformed bars may be assumed to vary with the character of the bar from 5 per cent to per cent of the compressive strength of the concrete at 28 days, or from 100 to 150 pounds per square inch for 2,000-pound concrete.
66. Reinforcement. The tensile stress in steel should not exceed 16,000 pounds per square inch. The compres sive stress in reinforcing steel should not exceed 16,000 pounds per square inch, or fifteen times the working com pressive stress in the concrete.
67. Modulus of Elasticity. It is recommended that in all computations the modulus be assumed as that of steel.