Savings in the construction of circular reinforced concrete tanks.*

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Savings in the construction of circular reinforced concrete tanks.*

The economy of constructing service reservoirs for water supply purposes, circular in shape, is not, in the opinion of the author, appreciated as fully as it should be. The circular shape for small tanks is not only the safest type of construction from a structural point of view, but also allows a more economical use of structural materials. Among the many and unfortunately all too frequent failures in reinforced concrete construction, it is rare to note failures of circular reinforced concrete tanks, other than those of badly leaking tanks due either to poor execution and poor design, or to the of them. One of the greatest advantages that the circular section has, and that no other has, is the possibility of economically increasing the capacity of such a tank by simply increasing its depth. The possibility of increasing the capacity of a reservoir in this way is of great importance in the design of improvements to hydraulic structures. It allows the designer to limit the initial cost of construction by building a tank of sufficient size for immediate needs. As water consumption increases, it is possible to economically increase the capacity of the reservoir, and at the same time raise the water level to counter the increasing frictional losses in the distribution system due to to increased consumption. The design of a circular reinforced concrete tank seems so simple that the inexperienced designer, carried away by his enthusiasm, is likely to create a larger diameter structure than the application of the simple tank design formula would seem to justify. To him, there seems to be no ostensible reason why a structure twice the size of a structure already built should not offer all the evidence of strength and stability if designed according to the ring tension formula. – a very misleading deduction. Secondary stresses, insignificant in small structures, and therefore considered too unimportant to pay attention to, increase rapidly with the size of the structure, and too often limit the extent to which such a type can adapt. of construction. adopted. In a circular reinforced concrete tank, the author has in mind the variable tension from point to point in the steel reinforcement due to the difficulty of obtaining a true circle in the field. In a small tank this is not so serious, since its effect on the resultant stresses in the steel reinforcement is so small. To clarify this point, certain calculations have been made by the author on the assumption that in the construction of this type, even with the best care taken in the field, a variation of half an inch in the mid-axis of a 10-foot chord is likely to occur.

This table is not intended to accurately show the variation in tensile stresses of steel reinforcement for the various dimensions given; however, it does give a good idea of ​​what can be expected from the variation in ring tension in a circular structure. It points out the danger resulting from the negligence in the construction of a circular tank of more than 100 feet in diameter. For large diameter tanks, however, the economy resulting from the use of a circular section is not obtained to the same extent, and therefore the use of this type is not so frequent. The author’s experience would tend to limit the working stress in the steel frame to 14,000 psi in a small tank and 12,000 psi for relatively large tanks. A reduction in allowable steel tension for large tanks is recommended due to the greater range of ring tension present in the larger structure. It may even be advisable to reduce allowable unit stresses below 12,000 pounds per square inch to keep unavoidable local excessive stresses within safe limits. The variation in tension of the steel reinforcement from point to point due to the varying curvature of the shell makes the use of mechanically bonded rebar advisable. Reinforcement bars for this reason should also be of such a small size that it is possible to handle them economically in the field. A high carbon steel with a yield strength of 50,000 pounds per square inch can be used to great advantage. Another difficulty to consider in the design of a circular tank is the tendency for rupture along the line between the inside wall of the tank and the base, due to the expansion of the walls by the internal pressure of the tank. water and consequent shrinkage, as there were, from the base of the tank. A good example of increasing the capacity of a circular reservoir is the expansion of the distribution reservoir for the village of Suffern, NY. This village draws water from Lake Anthrim, formed by impounding an arm of the river Ramapo. Water is pumped from this lake to a distribution reservoir located on the side of a mountain north of the village, approximately 180 feet above the mean village datum. This distribution tank, built several years ago, is a circular tank 70 feet in diameter and 10 feet 6 inches deep, completely sunk into the ground. The walls forming the sides of the tank are 2 feet thick and the bottom and sides are constructed of plain concrete. This tank had a storage capacity of 266,000 gallons and cost around $4,000. The recent growth of the village has made it appropriate to double the capacity of this distribution reservoir. The old structure, although massive, nevertheless leaked to a considerable extent, especially in the bottom. It was therefore decided to line the bottom of the tank at the same time as the sides were raised. The tank as remodeled has an inside diameter of 60 feet and holds approximately 20 feet of water, giving a storage of 559,000 gallons. The side walls of the old tank are lined inside with 6 inches of reinforced concrete. Above the old work, the width of the new work is 12 inches, tapering to 8 inches at the top. The circumferential reinforcement consists of five-eighths inch square corrugated bars, having a yield strength of 50,000 pounds per square inch. These bars are spaced so far apart that the average unit tensile stress does not exceed 14,000 pounds per square inch. When designing the liner for the old tank, it was assumed that the reinforcement would only have to support the increased tension due to the additional depth of 10 feet 6 inches. This is a constant amount with a full tank; therefore, the spacing and size of the steel in the liner of the old tank is uniform. We still rely on the existing wall to resist the hydrostatic pressure that it did before. It is unlikely, due to the large daily fluctuation of the water level in this reservoir, that the ice pressure will develop to the point of seriously overloading the reinforced concrete shell, and therefore no provision has been made. for such pressure. The bottom liner is reinforced with three-eighths inch square corrugated bars, the ends of which hook over the lowest reinforcing ring. Five-eighths-inch vertical square bars, spaced on 8-foot centers, were used as the vertical distributors. Each reinforcement ring is made up of six 90-inch overlapped and wired sections. The rings were also wired to the vertical brace at each intersection. The formwork consisted of a vertical lining on the inside extending over the entire height of the tank and a horizontal lining on the outside. The thickness of the bottom liner varies from 3 inches to 6 inches, arranged to provide better drainage than that obtained in the old tank. The reservoir was completed on October 12, 1911 and first filled to its full depth on November 11, 1911. No leaks have appeared so far. The only precaution in making the tank watertight, other than using fairly wet concrete which was mixed in the proportion of one part cement to two parts sand and four parts three-quarters broken hatch inch, was washing the inside of the tub with a semi-liquid cement. The author wishes to draw attention to the relatively low cost of this work. An increase in storage capacity of 294,000 gallons was obtained at a cost of $2,500, the contract price for this work. The reservoir’s location on a steep mountain slope approximately 180 feet above street level significantly increased the cost of transporting structural materials to the site and, therefore, the contract price.

* Read at the recent meeting of the New England Water Works Association.

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