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As
an illustration of this formula, if the eccentricity on a 12-inch concrete
column were 2 inches, we should have b = 12, and e = 2. Substituting these
values in Equation 45, we should have f = 2 f, which means that the maximum
pressure would equal twice the average pressure. In the extreme case, where the
line of pressure came to the outside of the concrete column, or when e =12 b,
we should have that the maximum pressure on the edge of the concrete column
would equal four times the average pressure. Any refinements in such a
calculation, however, are frequently overshadowed by the uncertainty of the
actual location of the center of pressure. A concrete column which supports two
equally loaded beams on each side is probably loaded more symmetrically than a concrete
column which supports merely the end of a beam on one side of it.

The
best that can be done is arbitrarily to lower the unit-stress on a concrete
column which is probably loaded somewhat eccentrically. The extreme durability
of reinforced concrete tanks, and their immunity from deterioration by rust,
which so quickly destroys steel concrete tanks, has resulted in the
construction of a large and increasing number of concrete tanks in reinforced
concrete. Such concrete tanks must be designed to withstand the bursting
pressure of the water. If they are very high compared with their diameter, it
is even possible that failure might result from excessive wind pressure. The
method of designing one of these concrete tanks may best be considered from an
example. Suppose that it is required to design a reinforced-concrete concrete
tank with a capacity of 50,000 gallons, which shall have an inside diameter of
18 feet. At 7.48 gallons per cubic foot, a capacity of 50,000 gallons will
require 6,684 cubic feet. If the inside diameter of the concrete tank is to be
18 feet, then the 18-foot circle will contain an area of 254.5 square feet. The
depth of the water in the concrete tank will therefore be 26.26 feet. The
lowest foot of the concrete tank will therefore be subjected to a bursting
pressure due to 25.76 vertical feet of water. Since the water pressure per
square foot increases 62 pounds for each foot of depth, we shall have a total
pressure of 1,610 pounds per square foot on the lowest foot of the concrete
tank. Since the diameter is 18 feet, the bursting pressure it must resist on
each side is one-half of 18 X 1,610 = X 28,980 = 14,490 pounds.

If
we allow a working stress of 15,000 pounds per square inch, this will require
.966 square inch of metal in the lower foot. Since the bursting pressure is
strictly proportional to the depth of the water, we need only divide this
number proportionally to the depth to obtain the bursting pressure at other
depths. For example, the ring one foot high,, at one-half the depth of the concrete
tank, should have .483 square inch of metal; and that at one-third of the
depth, should have .322 square inch of metal.

The
actual bars required for the lowest foot may be figured as follows: .966 square
inch per foot equals .0805 square inch per inch; i-inch
square bars, having an area .5625 square inch, will furnish the required
strength when spaced 7 inches apart. - At one-half the height, the required
metal per linear inch of height is half of the above, or .040. This could be
provided by using 1- inch bars spaced 14 inches apart; but this is not so good
a distribution of metal as to use 1-inch square bars having an area of .39
square inch, and to space the bars nearly 10 inches apart. It would give a
still better distribution of metal, to use-inch bars spaced 6 inches apart at
this point, although the is--inch bars are a little more expensive per pound,
and, if they are spaced very closely, will add slightly to the cost of placing
the steel. The size and spacing of bars for other points in the height can be
similarly determined. A circle 18 feet in diameter has a circumference of
somewhat over 56 feet.

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

**We Are Your Local
Concrete Cutter**

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

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