THE MIDDLEBORO REINFORCED CONCRETE TOWER TANK
The town of Middleboro is situated in Plymouth County, Massachusetts, and has no favorable site for a storage reservoir or standpipe for gravity fire supply. The highest point of land in Middleboro, or within fifteen miles roundabout, is Barden Hill, with an elevation of only 35 feet above a considerable part of the residential district. This fact, together with a brief description of the Middleboro Water Works, will aid in an understanding of the conditions which led up to the construction of the present concrete tower tank. The works were built in 1885 by the Middleboro Fire District. The source of supply is a large dug well 26 feet in diameter and 21! feet deep, located about a mile southeast of the center of the town and 00 feet from the East bank of the Nemasket Kiver. and ted tor the most part from the gravelly hills and table land to the cast. A brick pumping station was erected nearby, equipped with two Deane compound, duplex pumping engines, each of 600 gallons per minute caoaeitv. against a head of 200 teet. discharging through a 10-inch force main into the distribution system supplying the District. A wrought iron standpipe 20 feet in diameter by 103 feet in total height, with a capacity of about 235.0(H) gallons, w’as erected in a residential section of the town, the highest available location on the west side of the river. Pressure was sufficient for domestic uses hut inadequate for fire protection, especially in view of the increased pressure standards generally recognized as desirable. To relieve this condition, an electrically operated valve, controlled by a switch at the pumping station, was installed in the pressure main at the base of the standpipe; and a relief valve was placed on the discharge from the pumps set to blowoff at about 315 pounds at the station, which corresponds to a static pressure of 95 pounds in the business section. Upon the sounding of a fire alarm, the electric valve was closed and direct pumping into the mains with both pumps depended upon for the requisite quantity of water and the desired pressure. At the same time, the capacity of the two pumps was increased by boring out the pump cylinders, from 1,200 to 1,000 gallons per minute, which, with an allowance tor ordinary domestic uses, furnished from four to six good fire streams. Then came the 8-tiour laws in .Massachusetts which necessitated the employment of six men at the pumping station, three of whom must be licensed engineers. Add that the standpipe had been in service for thirty years; that a careful inspection in 1911 revealed an altogether too narrow margin of safety and another in 1913 showed a still more urgent need of its immediate discontinuance, and you can imagine the conditions that recently confronted the Water Commissioners of whom Mr. Alvin C. Howes, a member of this Association, is Chairman and Superintendent of the Water Works. It was voted, in February last, to construct on Barden Hill a standpipe of 500,000 gallons capacity, and the Commissioners engaged the writer to design and supervise the construction. The problem presented was the storing of half a million gallons of water at an elevation sufficient to meet the combined demands of domestic and fire services. The most economical type of structure fulfilling the requirements was a tower tank, an elevated tank supported on a tower, sometimes called a water tower, and the only practicable materials for building such a structure were steel and reinforced concrete. A steel tower tank was estimated to cost about $2,000 less than one of concrete, but the cost and inconvenience of frequent painting inside and outside and other considerations made it apparent that if a satisfactorily watertight concrete tank could he constructed, it was greatly to be preferred. The difficulties, however, of designing and building such a structure, especially of the proportions required, were many and vital. With due regard for the difficulties involved, it was nevertheless decided in favor of a concrete structure. The lowest of the bidders was the Henncbiqtie Construction Co., of New York City, to whom the contract was awarded on April 12 for the sum of $23,140. Mr. Thomas F. Dorsey was engaged by the writer as Resident Engineer in direct charge of the work.
•Abstract of I’apt-r read at December. 1915. meeting of the New Knglaml Water Works Association, Boston, Mass.
The foundation extends 7 feet below the finished grade and rests upon a fine, well compacted sand, the safe bearing power of which was determined experimentally by loading a timber set at the elevation of the bottom of the foundation. At a loading of from 1 to 3½ tons per square foot, the settlement was uniform and slight, being only 1-32 of an inch for each additional 1/2 ton; hut 4 tons per square foot produced a relatively large settlement— 5-32 of an inch for the last ½ ton. The footing area is such that with the total dead weight of 4.485 tons the soil is compressed to the xfnt of 2.28 tons per square foot. The wind load was computed from an assumed velocity of 70 miles per hour, the equivalent of 30 pounds per square foot on a vertical projection of the structure, which with a full tank adds a maximum of 0.68 tons, thereby giving a total maximum pressure of 2.96 tons per square foot.
A cylindrical tower was adopted. The plain aspect of a siniole cylinder is relieved by the addition of twelve 4 x 24 inch pilasters, which •• assumed to take their proportion of the load. The apnearance of solidity at the bottom is accomplished by a concrete seat extending around the base of the tower. A balcony of reinforced concrete with bracket supports, panelled posts and railing, the Moor of which is 108 feet above the ground, furnishes a suitable point of vantage for inspecting the outside of the tank and provides a visual starting point for the tank proper. The thickness of the tower wall between pilasters is 10 inches and the greatest compression due to the dead load is 483 pounds per square inch, increased to 631 pounds by a 70-mile gale. Entrance to the tower is through an iron door. Light and ventilation are provided for by means of twelve windows, three of which are arranged to open. An iron ladder within the tower leads from the ground to the balcony through an opening in the wall of the tower at the elevation of the balcony floor. Another iron ladder on the outside of the tank connects the balcony with the roof above. A 16-inch cast-iron flanged pipe with expansion joint, rises from the ground and enters the tank at its center. An 8-inch overflow and drain discharges at a safe distance from the tower.
The tank proper, 41 feet in inside diameter and having a depth of water at its center of 59 feet, has a capacity of slightly over half a million gallons. The bottom of the tank is a hemispherical bowl hung at its rim from the wall of the tower, and marks the first application of this type of bottom to an elevated •concrete tank, although for many years it has been used with success in elevated steel tanks. The bowl itself has a capacity of 125.000 gallons. The vertical walls of the tank varies in thickness from 10 inches at the top to 16 inches at the bottom and the hemispherical bowl from 18 inches at its connection with the tank wall to 14 inches at its center. The thickness of concrete is such that without any assistance from the steel reinforcement its stress in tension is about 250 pounds per square inch. The tensile stress in the steel acting independently is approximately 14,000 pounds per square inch; and with both materials acting in conjunction the computed stresses are 215 and 2,150 pounds. The tank is covered to guard against the growth of algae, and to prevent freezing. The roof is a concrete dome 4 inches thick. 41 feet in diameter and with a rise of 4 feet at the center.
Whitehall cement from the Whitehall Portland Cement Company’s Mill at Cementon. Pa., was used throughout the work. The sand and coarse aggregate were obtained from a gravel pit about 1⅞ miles distant, screened by hand at the pit to give the sizes required. The sand was tested for tensile strength as compared with standard Ottawa sand and gave an average of 112½ per cent thereof. The gravel stones were washed to remove a slight coating of clay and cemented sand. The proportions of the concrete for the foundation are 1 of cement, 2½ of sand and 5 of gravel stone; for the tower 1:2:4, for the hemispherical bottom and wall of the tank to elevation 279. 1:1:3; from elevation 279 to 293, 1:1½:3; and above elevation 293, including the dome roof, 1:2:4. The relative amounts of sand and stone were somewhat varied to provide the densest practical mix, the percentage of cement to the sum of the aggregates, however, remaining as stated. The amount of concrete used in the work was 1,120 cubic yards.
“Havemeyer.” round, deformed, open hearth, hard grade reinforcing rods furnished by the Concrete Steel Company of New York, and bent at the rolling mills, were used in the structure. These were tested by the New England Bureau of Tests, a large proportion of which were 1¾-⅛⅛ and developed an ultimate tensile strength of 120,000 pounds per rod. Laps were 40 diameters and in addition each joint in the tank was secured by two cable clips. The horizontal rods of the tank were firmly secured in their true position to 16 vertical 3-inch channels drilled to the exact spacing of the rods. 160,000 pounds of steel reinforcement were embedded in the concrete.
Steel forms made by the Blaw Steel Construction Co., of New York were used for the outside surface of the tower and tank. Two complete rings were provided, each 4 feet in height. Two 4-foot rings of wooden forms were used on the inside. The hemispherical bottom was cast between forms consisting of wooden ribs covered with sheet iron.
Joint Between Old and New Concrete.
For the tower, in order to secure a bond between the successive rings of concrete, the forms were overfilled about ¼ inch, which surplus containing the laitance was screeded previous to the initial set of the concrete, following which as soon as the final set occurred the surface was wire-brushed to slightly expose the stones and immediately before placing fresh concrete, washed with a hose stream and coated witu a neat cement grout. For the tank, in addition to the above precautions, there was cast in the old concrete a continuous triangular groove, about 1½ inch in depth, running around the wall near its center and subsequently filled with grout and concrete; to still further minimize the chances of leakage at each horizontal joint an uncoated steel plate or dam of No. t4 gauge metal 10 inches wide, turned over 1 inch at each edge to form a channel and bolted together to form a continuous watertight stop, was embedded 4 inches into the old work, thus leaving 4 inches extending up for a bond with the new concrete.
Progress of Work.
Ground for the foundation was broken on April 26. The average rate of progress on the tower was a.4-foot ring each alternate day, with a marked increase in efficiency as the work advanced. The actual time required for concreting was about 4 hours to each 4-foot section. The bottom of the tank, including 4 feet of the wall immediately above the tower was poured continuously. Concreting began at 5 a. m. on September 9 and the last batch was deposited at 4 a. m. the following day, during which time about 130 cubic yards of 1:1:2 concrete were placed. Five days elapsed before the following section was in place and two more days before the succeeding ring was concreted. Four days were then spent in preparing for future operations, so that when concreting began again on the fifth day, the following eight successive lifts were made in nine (lavs and it was only due to a violent gale of about 70 miles per hour that operations had to ‘be suspended for one day.
As soon as the forms were removed from the inside of the tank and the staging from around the outside, the surfaces were cleaned. After the tank has been fully tested and accepted as satisfactorily watertight, ths question of waterproofiing the inside and dampproofiing the outside surface will be considered. The static hydrant pressure with a full tank will be 84 pounds in the higher residential area. 104 pounds in the business center and 120 pounds in the low district along the Nemasket River, reduced by about 20 pounds by frictional losses during an ordinary fire.