TANKS 313. Design. The extreme durability of reinforced-concrete tanks, and their immunity from deterioration by rust, which so quickly destroys steel tanks, have resulted in the of a large and increasing number of tanks in reinforced concrete. Such 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 tanks may best be con sidered from an example. Suppose that it is required to design a reinforced-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 tank is to be 18 feet, then the 18-foot circle will con tain an area of 254.5 square feet. The depth of the water in the tank will therefore be 26.26 feet. The lowest foot of the 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 621 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 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 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 he figured as follows: .966 square inch per foot equals .0505 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 i-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 .30 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 --inch bars are a little more expensive per pound, and, if they are spaced very closely, will add slightly to the cost of placing tire 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. Assuming as a preliminary figure that the tank is to be 10 inches thick at the bottom, the mean diameter of the base ring would be 18.53 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 flat 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 tanks are usually fastened at tire 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.