Engineering Wonders of the World
Volume I

422 ENGINEERING WONDERS OF THE WORLD. times the stress that can be developed in con- crete (Fig. 8), because the resistance of the latter material to tension is only about one- tenth of its resistance to compression. How- ever, since steel costs fifty times as much as concrete (Fig. 6), the result is that for equal resistance the cost of steel is only one-sixth that of concrete. The economy of steel used in tension is as represented in Fig. 9. Fig. 9.—RELATIVE COST OF CONCRETE AND STEEL IN TENSION FOR EQUAL STRENGTH. The practical bearing of the foregoing com- parisons is explained by Fig. 10, where we have at the left hand a rolled steel beam, the cost of which is taken as the The Question s^anjarc]! as regards both com- of Cost. . , AT pression and tension. JNext we have a plain concrete beam, on the end of which is written the relative cost of the Fig. 10.—DIAGRAMS TO SHOW COST OF ALL-CONCRETE (centre) AND COMPOUND CONCRETE AND STEEL BEAMS (RIGHT) RELATIVELY TO THAT OF ALL- STEEL BEAM (LEFT). posite beam of concrete and steel can be made for four-fifths the cost of a steel beam, or less than one-fourth the cost of a plain concrete beam. Of course it must be understood that the third sketch, in Fig. 10 is purely diagrammatic, and does not in any way depict the manner in which concrete and steel are combined to form reinforced concrete. To make that matter plain, it will be necessary to consider briefly the effects produced by the application of external forces to elementary members such as are employed in all forms of engineering and architectural construction. We will first take the case of a beam, which Stresses in TT . , a Beam. Having arranged the reader can practically represent by a strip of moderately flexible wood, placed on two supports, as in Fig. 11. the beam thus, a weight W placed at the middle of the span will cause the beam to Fig. 11.—BEAM UNDER LOAD. bend downwards, the extent of the curvature depending upon the force exerted by the weight, and also upon the elastic strength of the wood. Let it be assumed that the conditions are such as to produce the amount of curvature indicated in Fig. 12. Every part of the beam material in compression and in tension. Fin- ally, we have the top half of a concrete beam and the bottom half of a steel beam, each element being marked with the relative cost of the material. From this series of sketches it is evident that a plain concrete beam must cost 3/^ times as much as a steel beam, but that a com- Fig. 12.—BENDING OF BEAM UNDER LOAD, SHOWING DISTRIBUTION OF COMPRESSIVE AND TENSILE STRESS AND THE NEUTRAL AXIS. Curvature greatly exaggerated.