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Re: Friction factor for plastic flat bottom tank

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Anchoring a plastic tank may just trade one type failure for another. 
First, is the tank actually designed for seismic loads, including
sloshing pressures?  Second, is the attachment to the tank strong enough
to prevent uplift without tank rupture?  To just prevent sliding, a safer
method is to anchor a curved steel angle as a curb around the base, with
soft filler material (optionally) in between.   In any case, tank
friction should be discounted, since tanks are well known to "walk" and
tip as they respond to seismic motions.

After the 1994 Northridge Earthquake, I investigated one such failure of
a 5,000 gallon plastic water tank.  The base was left intact and about
two feet from its original position.  The tank shell and head were about
twenty feet away, having smashed into and damaged a heavy steel building
(at an exit location).  Apparently, the tank rupture had been quite
violent, and the water had BLOWN COMPLETELY THROUGH an adjacent Type V
building, crashing food service equipment out through the oposite wall. 
Potentially life threatening?  You bet.

This tank was later replaced with a proper seismic design and
installation.  The lower four feet of the new tank was reinforced with
another wall of FRP to prevent similar rupture.  The anchors were tied
from the top to the foundation with cables.  

It may be best to consult with a tank manufacturer if you are dealing
with a new installation, or if the original vendor is available.  If not,
proceed cautiously.

Russ Nester

On 02 Jun 98 10:36:31  itsekson(--nospam--at) (ITSEKSON SASHA) writes:
>I am designing the anchorage of the flat bottom plastic tank 18ft high 
>by 12 ft. diameter.
>It's supposed to be supported on the existing foundation and in 
>designing the anchorage (Hilti HVA's) I would like to include the 
>frictional resistance at least for part of the total shear.
>I know that friction between concrete and plastic is considerably 
>smaller that between steel and concrete (about 0.3).  Does anybody 
>have any references in mind or have any opinions on these values?
>Sasha Itsekson, PE

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