Engineering Wonders of the World
Volume I

REINFORCED CONCRETE CONSTRUCTION. 421 Fig. 5.—THE TOTAL AREA REPRESENTS THE LOAD- BEARING CAPACITY OF STEEL IN COMPRESSION, AND THE BLACK AREA THAT OF CONCRETE IN COMPRESSION, FOR EQUAL SECTIONS. Fig. 6.—THE TOTAL AREA REPRESENTS THE COST OF STEEL, AND THE BLACK AREA THE COST OF CONCRETE, FOR EQUAL VOLUMES. Fig. 7.—RELATIVE COST OF STEEL AND CONCRETE IN COMPRESSION FOR EQUAL STRENGTH. taking down Smeaton’s Eddystone Lighthouse in 1881, he discovered a bundle of iron rods which, had been embedded in lime concrete ; and although that material had been exposed to sea air and sea water for a century and a half, “ the colour of these rods was just as if they had come from the mill, and there was no mark of rust whatever on them.” No question exists as to the value of steel in construction, so far as concerns tensile strength and elasticity, and there is none as to the compressive strength and permanent durability of concrete. Hence there is ample justification for the combination by which the physical strength of steel is supplemented and preserved by the constitutional strength of concrete. The happy union of the two materials has given to the world the product here termed “ reinforced concrete,” and which is also known by the designations “ armoured concrete,” “ béton armé,” “ concrete steel,” “ steel con- crete,” and “ ferro-concrete.” What Reinforced Concrete is. At this stag© we take the opportunity of explaining that a steel frame or other struc- ture embedded in concrete does not necessarily constitute genuine reinforced concrete. On the contrary, such an arrangement would more properly be called “ reinforced steel.” To create reinforced concrete pure and simple, the materials must be so disposed that all compressive stresses are resisted alone by the concrete, and all tensile stresses by the steel, with or without assistance from the concrete. In practice, how- ever, the tensile resistance of the concrete is entirely neglected because its value is too small for consideration. A few figures will enable us to demonstrate at once the economy of reinforced concrete. To begin with, a given section of steel will carry in compression thirty times the load that can be imposed on an equal section of concrete (as represented diagrammatically in Fig. 5) ; but, as a set-off, the cost of steel is about fifty times that of concrete (as shown in Fig. 6). Therefore for equal resistance the cost of concrete is only three-fifths that of steel (see Fig. 7) ; and so we see that concrete used in compression is far more economical than steel. Fig. 8.—THE TOTAL AREA REPRESENTS THE LOAD- BEARING CAPACITY OF STEEL IN TENSION, THE BLACK AREA THAT OF CONCRETE IN TENSION, FOR EQUAL SECTIONS.