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Assuming
as a preliminary figure that the concrete tank is to be 10 inches thick at the
bottom, the mean diameter of the base ring would be 18.83 feet, which would
give a circumference of over 59 feet. Allowing a lap of 3 feet on the bars,
this would require that the bars should be about 62 feet long. Although it is
possible to have bars rolled of this length, they are very difficult to handle,
and require to be transported on the railroads on two fiat cars. It is
therefore preferable to use bars of slightly more than half this length, and to
make two joints in each band. The bands which are used for ordinary wooden concrete
tanks are usually fastened at the ends by screw-bolts. Some such method is
necessary for the bands of concrete tanks, provided the bands are made of plain
bars. Deformed bars have a great advantage in such work, owing to the fact
that, if the bars are overlapped from 18 inches to 3 feet, according to their
size, and are then wired together, it will require a greater force than the
strength of the bar to pull the joints apart after they are once thoroughly
incased in the concrete and the concrete has hardened. Since the computed depth
of the water is over 26 feet, we must calculate that the concrete tank will be,
say, 28 feet high.

Its outer diameter will be approximately 20 feet. The total
area exposed to the surface of the wind, will be 560 square feet. We may assume
that the wind has an average pressure of 50 pounds per square foot; but owing
to the circular form of the concrete tank, we shall assume that it’s effective
pressure is only one-half of this; and therefore we may figure that the total
overturning pressure of the wind equals 560 X 25 = 14,000 pounds. If this is
considered to be applied at a point 14 feet above the ground, we have an
overturning moment of 196,000 foot-pounds, or 2,352,000 inch-pounds. Although
it is not strictly accurate to consider the moment of inertia of this circular
section of the concrete tank as it would be done if it were a strictly
homogeneous material, since the neutral axis, instead of being at the center of
the section, will be nearer to the compression side of the section, our simplest
method of making such a calculation is to assume that the simple theory
applies, and then to use a generous factor of safety. The effect of shifting
the neutral axis from the center toward the compression side, will be to
increase the unit-compression on the concrete, and reduce the unit-tension in
the steel; but, as will be seen, it is generally necessary to make the concrete
so thick that its unit compressive stress is at a very safe figure, while the
reduction of the unit-tension in the steel is merely on the side of safety.

Applying
the usual theory, we have, for the moment of inertia of a ring section, .049
(d14 - d4). Let us assume as a preliminary figure, that the concrete wall of
the concrete tank is 10 inches thick at the bottom. Its outside diameter is
therefore 18 feet + twice 10 inches, or 236 inches. The moment of inertia I =
.049 (236 - 216) = 45,337,842 biquadrate inches. Calling c the
unit-compression, we have, as the ultimate moment due to wind pressure. Solving
the above equation for c, we have c equals a fraction less than 6 pounds per
square inch. This pressure is so utterly insignificant, that, even if we double
or treble it to allow for the shifting of the neutral axis from the center, and
also double or treble the allowance made for wind pressure, although the
pressure chosen is usually considered ample, we shall still find that there is
practically no danger that the concrete tank will fail owing to a crushing of
the concrete due to wind pressure. The above method of computation has its
value in estimating the amount of steel required for vertical reinforcement. On
the basis of 6 pounds per square inch, a sector with an average width of 1 inch
and a diametric thickness of 10 inches would sustain a compression of about 60
pounds.

**Are You in ****Fairhaven Massachusetts****? Do You
Need Concrete Cutting?**

**We Are Your Local
Concrete Cutter**

**Call ****781-519-2456**

**We Service Fairhaven
MA and all surrounding Cities & Towns**