Seismic Design: Considerations in Water Storage Construction

Sponsored by
Aerial view of 2.5-million-gallon prestressed concrete tank and pump station project in the suburb of Alpine, Calif DN Tanks.
Aerial view of 2.5-million-gallon prestressed concrete tank and pump station project in the suburb of Alpine, Calif. Photo courtesy of DN Tanks.

By Judy Horning

Located in Southern California, the Padre Dam Municipal Water District delivers water to a population of over 103,000 residents in the eastern suburbs of San Diego. The District takes pride in providing a safe and reliable water distribution system to the communities it serves. Driven by this commitment and the need for additional fire flow capacity following the devastation of many area homes during the tragic Cedar Fire, the District replaced an existing 1.5-million-gallon (MG) water storage tank with a 2.5-MG prestressed concrete tank in the suburb of Alpine, Calif.

The existing "East Victoria" tank was a partially-buried, hopper-bottom, non-prestressed, cast-in-place concrete reservoir constructed over fifty years ago. It required continual maintenance to keep it in safe operating condition and to comply with California Department of Public Health regulations. Even though structural evaluations established that it could be retrofitted, hydraulic modeling indicated that the site required an increase of one million gallons of capacity to improve operational reliability, provide emergency storage and increase fire flow availability.

As such, Padre Dam Municipal Water District hired a project team consisting of civil and structural engineer AECOM; geotechnical consultant Ninyo & Moore; general contractor Spiess Construction Company; tank subcontractor Ward Henshaw; and tank prestressor DN Tanks.

Tank Design

After careful consideration, the District, with AECOM, determined that a 2.5-MG prestressed concrete tank offered the optimal solution for water storage at the site. The East Victoria tank floor, footings, columns, walls, and roof were all designed and constructed with concrete. With an inside wall height of 28' and a diameter of 125' the tank incorporated a 10" thick, poured-in-place corewall and a flat slab roof. The walls were constructed using cast-in-place concrete, and each section was poured full height, eliminating horizontal jointing.

Partially-buried prestressed concrete tank Padre Dam MWD.
Partially-buried prestressed concrete tank offered the optimal solution for water storage at the site. Photo courtesy of Padre Dam MWD.

Seismic Considerations

As the tank was located in California and likely to be subjected to significant seismic forces sometime over its service life, seismic design was important. Specialized seismic connections were incorporated at the wall base and top - similar to a base isolation system in essential buildings. They were designed to allow for maximum ductility under a seismic event to ensure the structure would continue to perform should the tanks undergo horizontal and vertical ground accelerations.

The tank corewalls were both circumferentially and vertically prestressed utilizing the latest in prestressing technology. Circumferential prestressing was completed by DN Tanks' strandwrapping machine, which applied the desired force to the hot-dipped galvanized, 7-wire strand with a high degree of precision to 14,950 pounds. A recording of the strand application was continuously electronically monitored and documented, ensuring that a tight tolerance of the applied force was maintained. Vertical prestressing utilized 96 1¼" diameter high-strength threadbars embedded in the wall and spaced approximately 48"on center. This elongated each of the high-strength steel threadbars with a hydraulic ram threaded onto the vertical threadbar at the top of the wall and stressed to 137.3 kips of force.

Prestressing the corewall both circumferentially and vertically yielded an efficient structure to contain the large loads produced by the stored water and the subsequent backfill and design live loads. Prestressing not only ensured an economical project but also gave the structure the strength and durability to undergo horizontal and vertical ground accelerations.

Summary

Water storage tanks are an integral part of the District's long-term program to create a safe and reliable water distribution system able to meet current regulatory compliance and improve water quality, reliability and capacity.

About the Author: Judy Horning is a marketing associate with DN Tanks.

DN Tanks is exhibiting at AWWA's ACE14 expo in Booth 1021.

Sponsored by

TODAY'S HEADLINES

Innovative carbon-neutral wastewater treatment plant unveiled in California

The EPA recently joined the Bureau of Reclamation, California Energy Commission and Congressman Col. Paul Cook at the ceremonial start of Victor Valley Wastewater Reclamation Authority's carbon-neutral energy project located at ITS wastewater treatment facility in Victorville, Calif.

Scottish Water to spend £3.5 billion on infrastructure improvements

Utility Scottish Water will invest £3.5 billion investment over six years to improve drinking water supplies, wastewater discharge and water mains...

Conquering Everest’s water contamination problem

A Ball State University team from Indiana is working to bring clean drinking water to a small, isolated community in mountainous Nepal that lies in the middle of the heavily congested pathway to Mount Everest...

MD counties to undergo major six-year water, wastewater design project

Louis Berger has been awarded a $4-million, three-year contract with the Washington Sanitary Suburban Commission for a water and sewer design project focusing on financing water and sewer reconstruction programs and inspection and repair of critical water and sewer infrastructure.

FOLLOW US ON SOCIAL MEDIA