City of Bluffdale Triangular Tank

A 4 MG drinking water storage tank, along with a new pipeline, was constructed for the City of Bluffdale, UT, as part of a larger project to improve the drinking water service and water pressure for the lower parts of the city. The City of Bluffdale serves a population of approximately 20,000. The owner and project team evaluated several tank sites that would meet the proper elevations and fit a typical round or rectangular storage tank. However, the team found the best location to be a triangular-shaped parcel of land bounded by two roads and a canal. Despite the unique shape of the site, it proved to be the most ideal location due to its close proximity to the water treatment plant and aqueduct. Erecting the storage tank in this location would allow it to easily connect to the aqueduct and into Bluffdale’s larger drinking water system.

The City recognized that if they were to proceed with this site location and build a tank with the needed capacity, they would have to consider tank shapes beyond round and rectangular to fit the given site geometry.

Once the triangular tank shape and capacity were determined, the engineer worked with STRUCTURAL TECHNOLOGIES to explore how to maximize the structural efficiencies. It was quickly concluded that a post-tensioned AWWA D115 tank design would be 20% more cost-efficient than a conventionally reinforced design and would offer other short- and long-term advantages to the City. In addition, the team recognized that the sharp corners of a true triangle tank were neither structurally efficient nor contractor-friendly, so a rounded corner option was implemented into the design. This change allowed for horizontal post-tensioning systems to be continuous through the corner areas and eliminated dead spots in the tank where water may not cycle into the system as frequently.

A post-tensioned tank design was chosen not only for its cost effectiveness but also because of its other structural advantages. Compared to conventionally reinforced designs, all elements of the innovative triangular tank used post-tensioning. Both bonded and unbonded post-tensioning systems were used in the floor slab, walls, and roof slab. Single strand, 0.5 in. diameter unbonded tendons were used in the membrane floor slab, vertically in the walls, and in the two-way flat-plate roof slab, while bonded seven-strand systems were used horizontally in the walls.

Post-tensioning provides the structure the long-term durability benefits of prestressed concrete, including the elimination of cracks, increased watertightness, joint-free floor and roof slabs, increased corrosion protection of primary reinforcement, and improved seismic resilience. The watertightness of post-tensioned tanks is required by code to be four times more restrictive than all other tank construction types. Historically, D115 prestressed concrete tanks have been found to have no measurable water loss during the watertightness testing.

During the construction of this unique tank, the general contractor benefited from larger slab placements, flexibility in wall placement sizes to match formwork availability, and reduced congestion in formwork for ease of concrete placement.

After the final watertightness test, the tank will be backfilled and buried with 12 in. of soil on top of the roof to provide thermal insulation benefits to both the tank structure and the drinking water quality. The tank will not require routine maintenance to the structure or the post-tensioning systems, just standard internal cleaning required by AWWA. The tank will provide safe and reliable drinking water to the City and its residents for decades to come.