Tanks designed to withstand earthquakes

In 1994, Santa Rosa, Calif., undertook a series of street and utility improvements to support the development of a new residential neighborhood. The project included the design and construction of two water tanks, which, because of the city’s proximity to two earthquake fault lines, was no mean feat.

Initially, project specifications called for the steel-welded tanks (750,000 gal. and 250,000 gal.) to be designed according to Standard D100 of Denver-based American Water Works Association (AWWA). However, the standard had last been updated in 1984, and it did not offer guidance regarding vertical seismic acceleration or the potential dangers specific to near-fault locations.

The project’s construction manager, Sverdrup Civil, Walnut Creek, Calif., hired Kleinfelder, a Pleasanton, Calif.-based consulting firm, to prepare site-specific seismic response spectra for the tank site. Also, it obtained a draft of AWWA’s Standard D100, which the association was in the process of revising. (The revised standard was published in 1996.) The draft served as the design basis for the Santa Rosa tanks, but the finished structures would incorporate additional features to accommodate the site conditions.

For example, the tank roofs were to be constructed of fillet-welded, steel plates that would be held to steel rafters by their weight and by friction. However, that design provided little vertical space between the roof and the water level, and calculations showed that seismically generated waves could strike the roof.

A taller tank would have accommodated the waves and prevented the roof from sustaining damage, but city officials rejected that option, noting that the tank’s height (60 feet) already was conspicuous. Therefore, engineers designed tank roof plates and rafters for upward loads from water waves, and the plates were welded to rafters in wave contact zones. Furthermore, shell plates were specified at a minimum of 1/4 inch thick rather than the 3/16 inch allowed by Standard D100-84.

During seismic events, water tanks sometimes fail because the anchors at the foundations tear away from the wall shell. Anchor tearing, as well as shell buckling, had occurred during the 1994 earthquake in Northridge, Calif., making them timely considerations in the Santa Rosa project.

In response to those concerns, tank designer Trusco Tank, San Luis Obispo, Calif., designed anchor assemblies in which the anchor bolts would yield before their attachments. Furthermore, it followed recommendations of the American Iron and Steel Institute, Washington, D.C., to ensure that anchor forces were distributed properly throughout the tank shell.

Finally, criteria were set to accommodate tank sliding during an earthquake. The large tank was designed with a reduction factor of 4.5, which accounts for ductile yielding of tank elements and for dissipation of kinetic energy by friction between the tank and the base. The tank base is separated from the foundation by a layer of sand, and, except for restraint by the anchors, is free to slide.

New criteria were presented to the city in 1995, and they resulted in the following additional features: * Special lateral ties were added between the roof rafters to prevent lateral buckling; * The inlet/outlet pipe was designed with adequate flexibility to allow movement between the tank and the foundation; and * The number of base anchors was increased (to 72 on the larger tank), and the anchor design was strengthened. Construction of the water tanks was completed in 1996.