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RE: A new angle on "Sliding" - unanchored tanks

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Thanks to all who have responded and please keep it coming. I have received
great input to my question so far. Just a point of clarification though
(which I should have put in the original posting). The Tank does not have a
base, per se. It is comprised of a wall (shell) and a roof that is attached
at the perimeter to the wall and is supported by intermediate columns within
the tank. However, for the base, the water just rests on a thin layer of
concrete that has no attachment to the rest of the structure. 

Ben Yousefi, SE
San Jose, CA

-----Original Message-----
From: Richard L. Hess [mailto:rlhess(--nospam--at)hesseng.com]
Sent: Friday, November 15, 2002 2:46 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: A new angle on "Sliding" - unanchored tanks


I think I would differ with you on this. On a tank with this radius to
height ratio only a very small part, if any, of the bottom will ever be in
the uplift condition and the anchorage actually results in increased  demand
on the tank because of  loss of damping and energy absorption that is
present when the tank can move a small amount.  This has been shown in some
of the recent papers on the subject and I was told by Warren Mitchell who
wrote the paper with Wozniak in 1978 for API which was the basis for 650
App. E, that the intent was not to promote more anchorage, but rather, to
promote designing tanks with  larger r/h ratios wherever possible to prevent
rupture during major seismic events.  You have to be very careful in
designing and constructing the subbase and base, but then it is better to
let it move a little.  As long as it does not rupture, it is a minor thing
to jack up a portion of the tank and repair the base  if and when the big
one occurs.
Richard Hess S.E.
----- Original Message -----
From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Thursday, November 14, 2002 4:13 PM
Subject: RE: A new angle on "Sliding" - unanchored tanks


> AWWA D100 does permit "unanchored" tanks in seismic regions, although it
> states that "properly designed anchored tanks retain greater reserve
> strength to seismic overload than unanchored tanks". A study of seismic
tank
> failures by the American Lifelines Alliance (associated with FEMA and
ASCE)
> indicated that "anchored tanks should perform better than unanchored
tanks."
> Consequently, I specify that tanks in seismic regions "shall be anchored
> tanks". Tank designers often request an option to provide an unanchored
tank
> but I stick with my specified requirements based on the improved seismic
> performance.
>
>
> William C. Sherman, PE
> CDM, Denver, CO
> Phone: 303-298-1311
> Fax: 303-293-8236
> email: shermanwc(--nospam--at)cdm.com
>
>

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