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Re: Friction factor for plastic flat bottom tank

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That provision (for friction due to gravity loads) is for the anchorage of
elements of structures, non-structural components and equipment (Section
1632.1, 1997 UBC).  For large (100,000 to 12,000,000 gallon) bottom
supported water tanks we allow friction to be used to resist sliding (UBC
1629.1) of the tank foundation system but use positive anchorage from the
tank to the foundation.  The load factors provide for a reduction of dead
load (UBC Sect. 1612.2.1 or 1612.3.1) which reduces the net vertical load
available to mobilize friction of the overall system. AWWA D-100, under
some conditions, does allow resistance to sliding via friction (i.e.,
"unanchored tanks") but it personally makes me uncomfortable as close as we
are to the Hayward Fault. 

For a small plastic tank that Sasha describes, the anchorage of the tank to
the foundation is difficult since the bottom and side walls may deform
substantially allowing the tank to "walk" and uplift, producing the
potential for pipe breakage at connections.  

One anchorage system I've seen for cross-linked polyethylene tanks
(Poly-Cal tanks out of Stockton, CA) is a series of angle posts with cables
encircling the tank. The system does not directly connect to the tank.  It
acts sort of like a net to catch the tank when it moves.  Although I'm
uncomfortable with this design for a high seismic area, I can't suggest
anything better since plastic is hard to fasten to.

Bill Cain, SE
Oakland, CA

At 05:35 PM 6/2/98 -0400, Lew Midlam wrote:
>I think there's a provision in the code that prohibits using friction to
resist lateral loads, but I couldn't find it in a quick flip thru the pages.
>Lew Midlam, PE
>> Hi,
>> 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?
>> Thanks,
>> Sasha Itsekson, PE