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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