Engineering Wonders of the World
Volume I

REINFORCED CONCRETE CONSTRUCTION. 423 between the supports is bent, but the maximum bending occurs at the middle of the span. All along the top surface the fibres of the wood are more or less crumpled up or compressed lengthways in consequence of the length, being reduced by curvature ; while all along the bottom surface the fibres of the wood are stretched, because the length is increased by curvature. The X-shaped area in Fig. 12 represents the effects produced at the middle of the beam. At the base of the inverted triangle the com- pressive stress (c) is greatest, its intensity decreasing progressively downwards to zero at the apex of the triangle. Similarly at the base of the upright triangle the tensile stress (/) is greatest, and its intensity uniformly decreases upwards to zero at the apex of the triangle. Therefore we find that there is a plane in the beam where no stress is caused by curva- ture. That plane is called the neutral axis, and as compres- sive and tensile stresses, distributed in the manner described, exist all along the beam, the neutral axis extends from end to end of the member. Little consideration of Fig. 12 is needed to bring home the fact that concrete is well adapted for resistance to compressive stress above the neutral axis, and badly adapted for resistance to tensile stress below the same axis. As a matter of fact, the strength of concrete in tension is only about one-tenth the strength of concrete in compression. This means that in a plain concrete beam nine-tenths of the strength possessed by the material in com- pression must be absolutely wasted ; or, con- versely stated, that by the addition of steel as reinforcement in the tension area the resist- ance of a plain concrete beam can be increased tenfold. Fig. 13 contains longitudinal and transverse sections of a reinforced concrete beam in its Fig. 13.—REINFORCED CONCRETE BEAM. COMPRES- SION AREA SHADED, TENSION AREA (WHITE) REIN- FORCED BY STEEL BARS (SOLID BLACK). most simple form, where the reinforcement consists of straight bars in the tension area. Although a beam so designed constitutes an apt illustration of the advantages gained by the combination of concrete and steel, it does not embody the most effective arrangement. The reason will be clear when we state that in a beam which is straight or virtually straight under load, the maximum tensile stresses are practically horizontal for a considerable dis- Fig. 14.—DIAGRAM ILLUSTRATING THE DIRECTION OF THE LINES OF MAXIMUM TENSION IN A BEAM. How Steel is distributed in a Beam. tance on each side of the middle. But the direction of the stresses is really curved, as shown in Fig. 14, a diagram which at once suggests that the ideal form of reinforcement would be provided by curved bars. For practical reasons, however, curved bars are undesirable, and engineers adopt in their stead various alternatives, one of the most effective being that of M. Hennebique, represented in Fig. 15, where the main rein- forcement comprises two bars—one horizontal, and the other bent up towards the ends— and a number of U-shaped loops. The bent- up ends of the main bars withstand part of the diagonal tensile stresses in the concrete, while the vertical loops serve the double pur- pose of taking a further part of the diagonal tensile stresses, and of providing a connecting link between the tension and compression areas of the beam.