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Re: A new angle on "Sliding"

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AWWA D100-96 (most current adopted version I'm aware of) Section states:
"Flat-bottom tanks may be anchored or unanchored* to tesist seismic loading."

"* If an unanchored tank design is used, the maximum thickened bottom annulus ring width (radial direction) used to resist overturning shall be limited to 7 percent of the tank radius.  The maximum thickness of the bottom annulus ring shall not exceed the bottom shell course thickness.  The tank must be anchored if these criteria cannot be met."

In general, the concern with unanchored tanks from a performance standpoint is that if the shell picks up under seismic load, a dynamic impact loading increases the likelihood that shell buckling ("elephant's foot buckling") will occur when the shell slams back down.  

The East Bay Municpal Utility District in Oakland, California, in their Engineering Standard Proactice 550.1 requires: "Flat bottom steel tanks shall be anchored and provided with fixed steel roofs. The tanks shall be designed in accordance with the latest edition of AWWA D100, except as specifically modified herein.  For consistency with  AWWA D-100, tanks may be designated as either Improtant or Critical only. Use of I = 1.0 for Standard tanks will result in a nonconservative design."

EBMUD's "tank guru on seismic design" is Eric Fieberling, S.E. I'll mention your concerns to him (he is a listserver member).

I think you are on the right track in raising the questions you are raising.  Unfortunately, AWWA D100 is written as a national standard and you well know how folks in non-earthquake country don't want to hear about seismic issues. If you can convince the designer to anchor the tank, I think you will do much to help ensure a reliable water system in San Jose.

Unfortunately, as a national standard, it probably meets CBC Section 1634.4" "Tanks with Supported Bottoms", which allows the use "an approved national standard, provided that the seismic zones and occupancy categories shall be in conformance with the provisions of Sections 1629.4 and 1629.9, respectively."

Bill Cain SE
Albany  CA

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