The Norumbega Covered Water Storage Tank is 1,300 feet long, 625 feet wide and 25 feet deep, with a surface area of more than 17 acres. It holds 115 million gallons of drinking water and is one of the largest tanks in the country. The 12-inch roof slab with 12-inch-deep drop panels is supported by 2-foot-diameter columns at 25 feet on center each way and 24/18-inch-thick exterior / interior walls. A 12-inch base slab and 24-inch-thick localized footings bear the weight of the structure and the contained water. The design includes three exterior valve vault structures (the largest is 34 by 34 by 24 feet deep) along with a roof-slab-supported 25 by 50-foot control building.
Thornton Tomasetti’s contributions to the pre-bid phase of the project helped lead to a successful low bid of $89 million for the first ever design-build contract awarded by the MWRA. Our recommendations at this early project stage allowed the team to streamline construction and optimize cost. Our engineers provided structural sizing and reinforcing quantities for the major tank components (wall, columns, base slab, roof slab and footings) to compare pricing of various structural schemes.
Our engineers studied feasibility and made recommendations to impose construction equipment loading on the tank roof and base slabs. Our design subdivided the tank into three cells by incorporating a double cell divider wall along the cell interface. In concert with the interior/exterior walls, they act as shear walls to resist the hydrostatic, soil and other lateral loadings. Our design maximized the spacing of expansion joints, increasing continuity of the roof and base slabs to resist vertical and lateral loadings. Our engineers also created detailed drawings for the design-build bid submission to the MWRA.
Innovation in Design
Thornton Tomasetti engineers attended numerous meetings with the Norumbega Constructors and their subcontractors to fine-tune the structural design and construction. Some of our value engineering contributions included vertical extension of the internal baffle walls to support the roof slab while, in turn, eliminating rows of supporting concrete columns; relocation of the external precast concrete piping to an internal channel within the tank footprint; and eliminating concrete capital forming by increasing the drop panel thickness
Among many innovations, our engineers initiated a concrete mix test program that resulted in a concrete with less than half the allowable shrinkage specified by MWRA, decreasing future cracks and subsequent repairs for the 100,000 cubic yards of concrete used on the project. The tank’s three cells can each be filled or emptied using the internal valve vault structures. Two feet of soil with surface planting and roadways covers the completed tank.
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