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At the center of the height, there is neither tension nor compression. This is called the neutral axis (see Fig. 90). Let us consider for simplicity a very narrow portion of the concrete beam, having the full length and depth, but so narrow that it includes only one set of fibers, one above the other, as shown in Fig. 91. In the case of a plain, rectangular, homogeneous concrete beam, the stresses in the fibers would be as given in Fig. 90; the neutral axis would be at the center of the height, and the stress at the bottom and the top would be equal but opposite. If the section were at the center of the concrete beam with a uniformly distributed load the shear would be zero. A concrete beam may be constructed of plain concrete; but its strength will be very small, since the tensile strength of concrete is comparatively insignificant. Reinforced concrete utilizes the great tensile strength of steel, in combination with the compressive strength of concrete. It should be realized that the essential qualities are compression and tension, and that (other things being equal) the cheapest method of obtaining the necessary compression and tension is the most economical. The ultimate compressive strength of concrete is generally 2,000 pounds or over per square inch. With a factor of safety of four, a working stress of 500 pounds per square inch may be considered allowable. We may estimate that the concrete costs twenty cents per cubic foot, or $5.40 per cubic yard. On the other hand, we may estimate that the steel, placed in the work, costs about three cents per pound. It will weigh 480 pounds per cubic foot; therefore the steel costs $14.40 per cubic foot, or 72 times as much as an equal volume of concrete or an equal cross-section per unit of length. But the steel can safely withstand a compressive stress of 16,000 pounds per square inch, which is 32 times the safe working load on concrete. Since, however, a given volume of steel costs 72 times an equal volume of concrete, the cost of a given compressive resistance in steel is - (or 2.25) times the cost of that resistance in concrete. Of course, the above assumed unit- prices of concrete and steel will vary with circumstances. The advantage of concrete over steel for compression may be somewhat greater or less than the ratio given above, but the advantage is almost invariably with the concrete. There are many other advantages in addition, which will be discussed later. The ultimate tensile strength of ordinary concrete is rarely more than 200 pounds per square inch. With a factor of safety of four, this would allow a working stress of only 50 pounds per square inch. This is generally too small for practical use, and certainly too small for economical use. On the other hand, steel may be used with a working stress of 16,000 pounds per square inch, which is 320 times that allowable for concrete. Using the same unit-values for the cost of steel and concrete as given in the previous section, even if steel costs 72 times as much as an equal volume of concrete, its real tensile value economically is (or 4.44) times as great. Any reasonable variation from the above unit-values cannot alter the essential truths of the economy of steel for tension and of concrete for compression. In a reinforced concrete beam, the steel is placed in the tension side of the concrete beam. Usually it is placed from one to two inches from the outer face, with the double purpose of protecting the steel from corrosion or fire, and also to better insure the union of the concrete and the steel. But the concrete below the steel is not considered in the numerical calculations. Even the concrete which is between the steel and the neutral axis (whose position will be discussed later), is chiefly useful in transmitting the tension in the steel to the concrete.

Are You in Milford New Hampshire? Do You Need Concrete Cutting?

We Are Your Local Concrete Cutter

Call 603-622-4441

We Service Milford NH and all surrounding Cities & Towns