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 Inch | Steel Pounds | Wrought Iron 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
| N = | 5.16 × 144 × 16 | = 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