Economy of Circular Reinforced Concrete Reservoir Construction.*
The economy in constructing service reservoirs for waterworks purposes, circular in shape, is not, in the opinion of the writer, appreciated as fully as it should be. The circular shape for small reservoirs is not only the safest type of construction from a structural standpoint, but permits also a more economical use of the structural materials. Among the many, and unfortunately only 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 workmanship and poor design, or both. One of the greatest advantages possessed by the circular section and not possessed by any other, is the ability to in increase economically the capacity of such a reservoir by simply increasing its depth. The ability thus to increase the capacity of a reservoir is of great importance in the design of waterworks improvements. It enables the designer to keep down the first cost of construction by building a reservoir of a size sufficient for the immediate needs. As the water consumption increases, it is possible to increase economically the capacity of the reservoir, and at the same time raise the water level to counteract the increasing frictional losses in the distribution system due to increased consumption. The design of a circular reinforced concrete reservoir appears to be so very simple that the inexperienced designer, carried away by his enthusiasm, is apt to create a structure of larger diameter than the application of the simple formula of tank design would seem to warrant. To him there appears to be no ostensible reason why a structure twice the size of one already built should not offer every evidence of strength and stability if designed in accordance with the formula for ring tension— a very misleading deduction. The secondary stresses, which in small structures are insignificant, and consequently deemed of too little importance to have attention called to them, increase rapidly with the size of the structure, and only too often limit the size to which any particular type of construction can be adopted. In a circular reinforced concrete tank, the writer has in mind the varying 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, as its effect upon the resultant stresses in the steel reinforcement is so slight. To make this point clear, some computations have been made by the writer based on the assumption that in the construction of this type, even with the best of care taken in the field, a variation of a half-inch in the middle ordinate of a 10-foot chord is likely to occur.
This table is not made to accurately show the variation in the tensile stresses of the steel reinforcement for the various dimensions given; it does, however, give a fair idea of what may be expected in the variation of the ring tension in a circular structure. It points out the danger resulting from carelessness in constructing a circular reservoir more than 100 feet in diameter. For reservoirs of large diameter, however, the economy resulting from the use of a circular section does not obtain to the same extent, and consequently recourse to this type is not so frequent. The writer’s experience would tend to limit the working tension in the steel reinforcement to 14,000 pounds per square inch in a small tank, and to 12,000 pounds per square inch for comparatively large tanks. A reduction in the allowable steel tension for large tanks is recommended because of the greater range in the ring tension present in the larger structure. It may even be advisable to reduce the allowable unit stresses below 12,000 pounds per square inch to keep within safe limits the excessive local stresses which cannot be avoided. The variation in the tension of the steel reinforcement from point to point due to the varying curvature of the shell, makes the use of a reinforcing bar with mechanical bond advisable. The reinforcing bars for this reason should also be of as small a size as it is possible to handle economically in the field. A high carbon steel with an elastic limit of 50,000 pounds per square inch, can be used to great advantage. Another difficulty to be considered in the design of a circular reservoir is the tendency to rupture along the line between the inside wall of the reservoir and the base, due to the expansion of the walls by internal water pressure and the consequent drawing away, as it were, from the base of the tank. A good example of increasing the capacity of a circular reservoir is the enlargement of the distribution reservoir for the Village of Suffern, N. Y. This village takes its water supply from Anthrim lake, formed by impounding a branch of the Ramapo river. The water is pumped from this lake to a distribution reservoir located on the side of a mountain to the north of the village, about 180 feet above the average village datum. This distribution reservoir, built a number of years ago, is a circular tank 70 feet in diameter and 10 feet 6 inches deep, sunk entirely into the ground. The walls forming the sides of the tank are 2 feet thick, and both bottom and sides are constructed of plain concrete. This reservoir had a storage capacity of 266,000 gallons, and cost approximately $4,000. The recent growth of the village has made it advisable 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 that the sides were being raised. The reservoir 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 on the 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-eighth inch square corrugated bars, possessing an elastic limit of 50,000 pounds per square inch. These bars are so spaced that the average unit tensile stress in them does not exceed 14,000 pounds per square inch. In designing the lining for the old reservoir, it was assumed that the reinforcement would only have to take care of the increased tension due to the additional depth of 10 feet 6 inches. This is a constant quantity with a full reservoir; consequently the spacing and size of the steel in the lining of the old reservoir is uniform. The existing wall is still relied upon to resist the hydrostatic pressure that it formerly did. ft is not very likely, because of the great daily fluctuation in the water level in this reservoir, that ice pressure will develop to such an extent as to overstress seriously the reinforced concrete shell, and consequently no provision has been made for such pressure. The bottom lining is reinforced with three-eighth-inch square corrugated bars, which have their ends hooked over the lowest reinforcing ring. Vertical five-eighth-inch square bars, spaced 8 feet centers, were used as vertical distributers. Each reinforcing ring is made up of six sections lapped 90 inches and wired. The rings were also wired to the vertical reinforcement at every intersection. The forms consisted on the inside of vertical sheathing extending the full height of the reservoir, and of horizontal sheathing on the outside. The thickness of the bottom lining varies from 3 inches to 6 inches, so arranged as to offer better drainage than was obtained in the old tank. The reservoir was completed on October 12, 1911. and filled for the first time to its full depth on November 11, 1911. No leaks whatever have thus far appeared. The only precaution to render the reservoir water-tight, other than that of using a fairly wet concrete which was mixed in the proportion of one part of cement to two parts of sand and four parts of three-quarter-inch broken trap rock, was to wash the inside of the tank with a semi-liquid cement. The writer wishes to call attention to the comparatively low cost of this work. An increase in the storage capacity of 294.000 gallons was obtained at a cost of $2,500, the contract price for this work. The location of the reservoir on a steep mountain slope about 180 feet above the street level, added considerable to the cost of hauling the structural material to the site, and consequently to the contract price.
*Read at the recent meeting of the New England Water works Association.