Systems in reinforced concrete, such as the Monier, Hennibique, Kahn, Joluison, etc., are named after the inventor of a special arrange ment of reinforcing to provide for the stresses, or for a specially formed bar, or unit assem blage. of bars. These are described in special treatises. Patents in reinforced concrete are many, and the designer .must have knowledge of some of them to anticipate payment of royalties. Recently there has been a tendency in the courts to invalidate patents that claim nghts to particular arrangements of bars in reinforced concrete on the ground that only mechanical skill, and not inventive faculty, is required to place steel where tensile stress is to be met or else to interpret the patent to narro.w claims covering only the particular thing descnbed in the patent.
Use of Steel.— Steel for reinforcing is in form of bars, usually round or square with ordi nary surfaces (plain bars) or deformed surfaces, such as corrugations, lugs, bent fins. The en deavor is to design the bar so that the cross section may be equal throughout Reticulated metal, such as expanded metal or woven mesh, is also used for short spans, and to resist volumetric change. Steel is usually of low carbon steel, especially when bars of greater diameter than one inch are to be used, but high carbon steel, both from billets and recoiled from steel rails, is extensively used. Usual thick nesses of steel rods are: for slabs A-inch to A-inch; for beams A-inch to 1A-inch; for girders up to 2A inches.
The amount of steel in a beam depends upon the unit working stress for which the structure is designed. For stresses of 16,000 pounds per square inch in the steel, and 750 in the concrete in compression, a beam or slab is balanced mechanically, i.e., will have the assumed stresses, when 0.87 of 1 per cent of steel is present. This statement is based upon the conventional methods of design. In col umns longitudinal steel is used up to 4 per cent and spiral steel from 1 to 2 per cent. In arches, 1 per cent of the cross-section at the crown both in the extrados and the intrados. Rein forcement for temperature is generally from 025 to 0.4 per cent of the cross-section which is subjected to temperature change.
Structural Forms in Reinforced Concrete. — Reinforced concrete construction is gen erally continuous and monolithk, all parts of the structure being tied together except for expansion joints. In this it is contrasted to wooden or steel construction where the parts act more as separate units. Concrete beams or girders are reinforced with about 0.9 per cent of steel at the lower side at the centre and an equal amount over the support when con tinuous. The latter amount is obtained by
bending one-half of the bars up from the bot tom around the quarter-point of the span, the remaining one-half coming from the other side of the support. The bars that are not bent continue horizontally to the support. Width of beams and girders is generally one-half to one-third the depth. The ends of beams and girders are strengthened against shear when necessary, by use of web reinforcement con sisting of stirrups either vertically or inclined upward toward the support, rigidly attached to the horizontal bars and running to the top of the beam. The bent bars also provide this strengthening, which is necessary when the unit shearing stress (that is the load carried to the support divided by the cross-section of the beam above the horizontal steel), exceeds 40 pounds per square inch. In no case should this shear stress exceeds 120 pounds per square inch in strengthened beam. The size of beams de pends upon loading. A moment of 1/12WL must be provided for in continuous beams. W ---.total load on beam and I.= span from clear to clear plus IA depth of beams. A non continuous span must carry a moment of AWL. This resisting moment is calculated by multi plying the area of the steel by the unit stress and by a leverage (about the compre.ssion centre) of approximately 7A the depth of the beam above the steel.
Slabs run between beams. and are designed upon the same principles as beams. Slabs have spans from 4 to 20 feet. For usual loads a span of 8 feet between beams is economical.
Columns may be reinforced by from 1 to 4 per cent, longitudinal rods well tied together by S and 34 wire at distances not to exceed 16 times the diameter of the rod. There should be a minimum of 2 inches of concrete outside the rods. The concrete of the core, i.e., inside the steel, may be loaded to 500 pounds per square inch plus the amount carried by the steel. The unit stress on the latter is 15 times the unit stress on the con crete, Preferably columns should be rein forced with longitudinal rods and spirals, the latter to at least 1 per cent. In this case the core can carry safely 750 pounds per square inch plus the amount on the longitudinal steel for 1:2:4 concrete. In case of ncher concrete an increased stress in proportion to the increased strength of the concrete may be used. Columns should not be too slender. Lengths of from 15 to 20 times the diameter are conserva tive. Practice in design is largely regulated by the Report of the Joint Committee on Concrete and Reinforced Concrete.