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RE: A new angle on "Sliding"

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There certainly is some friction between the floor plates and the foundation
that will resist movement (and dissipate energy) but I wouldn't count on the
tank not moving. I am aware of several steel tanks that have shifted
slightly in far less than the design earthquake. Also, depending on how
severe of an earthquake you experience, unanchored tanks can lift up quite a
bit while they are in motion. 

I recently did a seismic evaluation for a couple dozen unanchored ground
storage tanks at a facility near San Francisco, and based on the minimal
predictive methodology I was able to locate in the literature, calculated
potential translations and uplifts as high as 12 to 18 inches for some
tanks. If this isn't enough to break the bottom connection near the shell,
it is certainly going to cause havoc with the pipe connections if they don't
have a lot of flexibility. It's the primary disadvantage of unanchored
tanks, many of which have otherwise survived earthquakes unscathed.

I have also seen some papers suggesting, based on observed damage and
analysis, that current AWWA/API methodology for the design of unanchored
tanks is not sufficiently conservative and that the tanks are more
susceptible to "elephants foot" buckling on the compression side of the
shell than the current design methodology suggests. Unfortunately, the
consensus nature of these standards makes it take a very long time for
changes to get adopted.



-----Original Message-----
From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)ci.sj.ca.us]
Sent: Thursday, November 14, 2002 3:37 PM
To: seaint(--nospam--at)seaint.org
Subject: A new angle on "Sliding"


We are currently reviewing a steel water tank that is approximately 300 feet
in diameter and approximately 40 feet high. The designer is proposing to not
have any anchorage from the tank to the perimeter ring foundation that the
tank wall is resting on. His point is that, there is sufficient friction
between the tank wall and the foundation to resist the seismic forces from
the tank itself and any sloshing forces from the water!

I have not in, my entire professional life, seen a case where a structure of
this magnitude would not use any positive anchorage from the superstructure
to the foundation. There is some code language in the UBC, such as sections
1605.2.3 and 1809.3, that deal with anchorage of walls and buildings. And to
me it is clear that what is being referred to in those sections is positive
anchorage. However, the designer is insisting that using friction is an
acceptable method of resisting seismic forces in tank design. Any input from
those experienced with this type structure is appreciated.

Ben Yousefi, SE
San Jose, CA

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