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RE: Friction factor for plastic flat bottom tank

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I thank everybody who responded to my post.

As Bill noted in his response it is a common practice in the design of flat bottom tanks to resist the sliding by friction at the bottom, and to provide appropriate anchorage for the uplift forces ( as per API 950 for example).  If you look at standard the anchorage design with chairs welded to the tank etc. it is hard to imagine to engage the anchors in shear action as their mode of failure would be flexure rather than shear.  The reason for the chair design is to distribute the stresses to some height of the tank wall as the bottom and wall plates are usually is not very thick, and to avoid the "elephant foot" type of failure that was typical for unanchored or poorly anchored tanks.
	|         |     / a.b. chair
             tank wall  |         |   /
	|         | /
	|         |/
 ------------------------------/ |
tank bottom	          | anchor bolt

That's the design of the steel tank.  With plastic it is a little different story.  The particular tank that I am working on has a thickened edge (with slightly oversized holes at the a.b. locations) at the intersection of wall and bottom plate sticking outward, and the uplift anchorage in achieved by providing a sort of L-shaped clips that are attached "upside - down" at the thickened edge and tied down by the anchor bolts.  Something like this:

             tank wall 	|
	|___ |___ anchor clip
                                |__   |      |
	|      \|      |
thickened edge	|       |\     |
tank bottom	         | anchor bolt

Anyway, here is what I did for this particular application.  I assumed 0.15 friction factor for plastic to concrete so to design a.b. for half of the total base shear.

Thanks again,

Sasha Itsekson, PE
From: 	Bill Cain, S.E.[:bcain(--nospam--at)]
Sent: 	Tuesday, June 02, 1998 8:07 AM
To: 	seaoc
Subject: 	Re: Friction factor for plastic flat bottom tank


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