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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**