Engineering Wonders of the World
Volume I

REINFORCED CONCRETE CONSTRUCTION. 425 Fig.' 17.—DIAGRAMS ILLUSTRATING THE ADHESION BETWEEN CONCRETE AND STEEL. If a bar 1 inch square in section be embedded 10 inches in concrete, a pull of 16,000 lbs. will be required to withdraw it. The resistance varies proportionately with the area of steel in contact with the concrete. A 1-inch bar of average mild steel could not be withdrawn if embedded 40 inches, as the adhesion would be greater than thø tensile strength of the metal. will develop cracks long before there is any danger of rupture in the beam as a whole. Nevertheless, the cracks are so numerous and so exceedingly small that the concrete in tension does not fail as a whole, its useful- ness is not impaired, and the concrete in com- pression (see Fig. 19) does not suffer in any way. The general result is that, while a plain con- crete beam would suddenly break under loads causing the tensile stress of, say, 200 lbs. per square inch, a reinforced concrete beam would not begin to show any signs of impending failure until stretched to an extent equivalent to that produced by a 2,000 lbs. per square inch ; and even then the indications of threat- ened failure would simply be due to the fact that the steel had been stressed up to or beyond the limit of the concrete’s elasticity. Finally, it should be noted that, whereas plain concrete gives no warning of failure, reinforced concrete always does so by gradual widening of the hair cracks, and that in cases where ordinary loads are adopted the cracks are quite invisible, and close up again on removal of the load, and the beam resumes its original form. The result is that reinforced concrete beams behave exactly as if they were constructed of a perfectly elastic material. In fact, we are justified in considering rein- forced concrete to be a species of elastic stone, combining the compressive strength, solidity, and durability of rock with the tenacity and elasticity of steel. By its aid light and elegant structures can be designed which compare in these respects with the best examples of steel- work ; but, unlike the latter, they are immune from injury by the atmosphere, and, unlike structures built of any other material, they continue to increase in strength year by year. In one form or another beams enter into almost every type of construction, as the girders and platforms of bridges, the decking of wharves and piers, the floors and roofs of buildings, and the bottom of docks, reservoirs, and aqueducts. Beams are applied vertically of walls for buildings, docks, reservoirs, of retaining walls, and of various other struc- tures too numerous for individual mention. However, if reinforced concrete could be employed with advantage in the form of beams and kindred members only, the the new material would be seriously limited. The fact is that in constructive work beams have generally to be used in conjunction with members designed to with- stand forces applied parallel to their length, or in conjunction with members designed to withstand forces applied parallel to their length and other forces acting transversely. These two kinds of members are illustrated in Figs. 20 and 21. In th© former diagram we have a thick column loaded at the top in such manner that the lines of pressure due to the collective load P are parallel to the sides of the member. It will be seen from Fig. 20 Many Uses of Ferro- Concrete. in the design usefulness of Reinforced Concrete Columns.