| The suburban city of Rancho Cucamonga is one of the fastest growing metropolitan areas of the Inland Empire, having experienced a population growth of nearly 30 percent since 2000. |
With an inside diameter of 161'6", service water depth of 41'7", and wall height of 44'6", the tank incorporated a 12.5-inch-thick bi-axially compressed wall and a flat slab roof. Preceding the application of the external prestressing strand and shotcrete encapsulation, the cast-in-place corewalls were constructed using over 830 cubic yards (CY) of concrete, and each wall section was poured full height, which eliminated the need for horizontal jointing. The 6-inch-thick tank floor utilized 600 CY of concrete, and the cast-in-place flat slab roof required 810 CY of concrete poured in two sections.
Critical to the operational enhancements of the treatment plant upgrade, the new 6.0-MG tank included baffle walls to meet requirements for desired chlorine contact time. By restricting and redirecting the flow of the contained water, the baffles promote a circuitous flow-through path for the treated water prior to delivery.
Seismic Considerations
Given that the tank is located in the state of California and due to the high probability that it may be subjected to high seismic forces sometime over its service life, seismic design was of particular importance. Specially anchored yet flexible seismic connections, a form of base isolation, were incorporated at the wall base and top in order to maximize the ductility of the structure under a seismic event. Additionally, integrally-cast roof tie-down connections were included to restrict upward movement of the tank roof under seismic excitation of the tank's contained water.
Accounting for hydrodynamic and hydrostatic loading, as well as differential temperature and moisture conditions, the reservoir wall was prestressed in two directions: vertically and circumferentially. The latest prestressing technology was utilized by DN Tanks for the project. The vertical prestressing design included one hundred and thirty-two (132) 1.25-inch-diameter threadbars embedded in the tank's corewall. After the corewall reached adequate strength, a hydraulic ram was threaded onto the vertical threadbars at the top of the wall and was subsequently tensioned to 137 kips of force.
External circumferential prestressing was completed by DN Tanks' specialized equipment, which continuously stressed the prestress material to a force of 14,950 pounds with a high degree of precision (± 1.5% tolerance). Further, approximately 44.7 miles of galvanized 7-wire strand was applied to the tank corewall while being continuously recorded and electronically monitored to ensure the highest quality possible. A total of 461 wraps of stranded cable were applied to provide the load resistance.
Prestressing not only ensured an economical project but also gave the structure the strength and durability to endure horizontal and vertical ground accelerations. The state-of-the-art prestressing machine allowed the tank to be constructed faster and stronger with a high degree of precision. Upon completion of prestressing, the stranded cable was encased and protected with shotcrete.
Looking Forward
With California facing its fourth year of one of the most severe droughts on record, the District has taken a proactive approach to providing a sustainable water supply. With drought conditions likely to persist, improvements to the District's LWMWTP will enhance its ability to provide reliable, high-quality water to meet current and future customer needs.
About the Author: Judy Horning is a marketing associate with over 21 years of experience researching and writing articles about water and wastewater storage tank projects for DN Tanks. DN Tanks is a specialist in the design and construction of AWWA D110 prestressed concrete tanks used for potable water, wastewater, chilled water, and other liquids.
