=68. Sprinkler Tanks.=—In the large cities, where fire risks are great,
and where nearly all the buildings and their contents are protected
by insurance, the owners of the buildings are subjected to the rules
and regulations of the Underwriters, or Associations of Insurance
Companies. These Underwriters from time to time pass regulations
insisting on certain further precautions and protection against fire,
such as the installation of sprinkler systems, stand pipes for hose
attachment for each floor, etc.
As the available city pressure or water supply of the municipality may
be limited, or uncertain, or the pressure too low for a high building,
it is sometimes necessary to place water tanks of from 10,000 to 30,000
gallons capacity in towers on the roofs of factories, and in the design
of new factories provision is usually made for three tanks.
In designing a building, these tanks are located at such a point that
their support is insured by the walls beneath, and the most convenient
place is found to be over the stair tower or adjacent to it. As 1
gallon of water, together with the tank containing it, has a unit
weight of 8 pounds, a 30,000-gallon tank complete will weigh in the
neighborhood of 240,000 pounds, which must be supported on the walls
and by means of iron beams.
The architect, besides providing adequate support for these tanks,
must so design the tanks as to secure them against bursting, which
would lead to serious consequences. For durability, sprinkler or
fire-protection tanks are made of either cypress or cedar from 2 to
3 inches in thickness. They are usually in the shape of a truncated
cone, and the bottom of the tank is required to be at least 20 feet
above the highest point of the top story.
The important feature in the design of such tanks is to see that they
are properly braced with hoops, and it is usual to specify that no hoop
shall be subjected to a unit stress of more than 12,000 pounds for
iron and 16,000 pounds for steel. These hoops are made from ¾-inch to
1-inch round iron, not less than the former, and the required strength
is obtained by spacing them closer together at the bottom and farther
apart toward the top. They are held together with adjustable clamps,
as indicated in Fig. 33, and by the use of such clamps they may be
readily tightened. The bottom hoops of the tank are subjected to great
stress, and it is good practice for these hoops to bear against a flat
iron hoop, as indicated in Fig. 34. By this construction much greater
bearing is provided on the wood, and the round iron is prevented from
cutting into the staves of the tank. In some instances flat iron hoops
are used altogether, but it is considered better to use round iron
hoops, from the fact that they are not likely to corrode through as
rapidly as the thin flat iron.
[Illustration: FIG. 33]
[Illustration: FIG. 34]
=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.