=69. Proportioning the Hoops.=—The principal element of engineering entering into the design of large wooden water tanks consists in the proportioning of the hoops, and Table II will be found convenient in determining the hoops required for any size of tank. =70.= In order to determine the number of hoops of a certain size required for any span of 12 inches at a point any distance from the water-line, the following formula may be used: 5.16 _d H_ _N_ = ----------, _S_ in which _N_ = number of hoops required in 1 foot of height of tank; _d_ = diameter of tank, in inches; _H_ = height of water-line from center of space under consideration, in feet; _S_ = actual safe strength, in pounds, of hoops assumed to be used. This last value may be found from Table II. TABLE II SAFE STRENGTH OF ROUND TANK HOOPS =========+=========+============= Diameter | Steel | Wrought Iron Inch | Pounds | Pounds ---------+---------+------------- ⅝ | 3,232 | 2,424 ¾ | 4,832 | 3,624 ⅞ | 6,720 | 5,040 1 | 8,800 | 6,600 =========+=========+============= =71.= To illustrate the foregoing, assume that it is desired to find what will be the spacing of ⅞-inch steel hoops at the bottom of a tank 12 feet in diameter, in which the water-line is 16 feet from the middle of the section under consideration. Applying the formula in Art. =70=, using in conjunction therewith Table II, it is found that 5.16 × 144 × 16 _N_ = --------------- = 1.77. 6,720 This result, 1.77, is the number of hoops required in 12 inches of height from the bottom of the tank, and would indicate that the hoops should be spaced about 7 inches from center to center, for 12 inches divided by 1.77 gives approximately 7 inches, the pitch of the hoops. This process should be repeated for different points throughout the height of the tank, and from the results the tank may be designed. =72.= In the installation of sprinkler tanks, it must be observed that they are placed some distance above the highest point of the top floor, the distance usually required by the Underwriters being 20 feet, if it is possible of attainment. The tank should always be roofed, have a ladder from the roof of the building to its top, and a steam pipe inside to prevent the water from freezing in winter. This pipe is furnished with a check-valve to prevent the water in the tank from siphoning. EXAMPLE FOR PRACTICE What should be the spacing of the ¾-inch round wrought-iron hoops on a tank 10 feet in diameter and 12 feet high at a distance of 6 feet from the water-line? Ans. 12 in.