Prestressed Tank Provides Secure Storage
The City of Signal Hill, 30 miles south of downtown Los Angeles and boasting less than 10,000 residents, was confronted with the task of increasing its water storage and living up to the town motto, "A small unique community that is economically independent, prides itself in personalized service to the residents and business community that it serves and a community which has created and works to maintain a high degree of livability for its residents."
In order to achieve its goals, the city hired RBF Consultants, a consulting engineering firm based in Irvine, CA. RBF has extensive knowledge in water storage facilities and decided to use a buried, circular, prestressed concrete tank to satisfy the city's water storage needs.
RBF analyzed the site the city had selected and recognized that a circular tank would more efficiently use the site than a rectangular conventionally reinforced tank and would also offer the city the highest quality, longest lasting and lowest maintenance water storage structure. Burying the tank also would allow for development of a park on top of the structure.
A prestressed concrete tank also provides a connection at the wall base and wall top that can withstand the high seismic forces commonly generated by Southern California earthquakes. This connection, typically referred to as a "free connection," allows each component of the tank (i.e. wall, wall footing and roof) to act independently of one another, which further enhances the already beneficial ductile property inherent of concrete. To date, prestressed concrete tanks have endured several devastating earthquakes in the western United States without any significant damage.
Schuler Engineering Corp. of Corona, CA, was awarded the general construction contract for the project and completed the tank floor, footing, wall, columns and roof concrete pours. The tank design consisted of a six-inch-thick membrane floor, perimeter ring foundation, a 12 inch vertically and circumferentially prestressed wall and round columns supporting a 15-inch-thick flat roof slab capable of sustaining the weight of park service equipment, the park and its visitors.
DYK Inc., based in El Cajon, CA, was awarded the tank prestressing contract, which consisted of vertically post-tensioning the tank wall with 84 high-strength Dywidag threadbars and circumferentially prestressing the wall with more than 5 1/2 miles of galvanized seven-wire strand. The circumferential prestressing provided adequate force to resist the water load of the tank and account for sloshing of the tank contents should the structure undergo an earthquake. The circumferential prestressing was then covered with several layers of shotcrete.
The tank now provides 1.2 million gallons of both fire and potable water storage for the City of Signal Hill. With a diameter of 100 feet, a 21.7-foot-tall wall and a column supported flat roof, the tank at the corner of Skyline Street and Dawson Avenue is artistically decorated with icons associated with Signal Hill's history. From this serene park setting the residents can learn about the "small unique community" that they live in as well as enjoy breathtaking views to Catalina Island more than 20 miles offshore, without ever knowing that they are standing on top of their drinking water supply.