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RE: Cable Tie Down for FRP Tanks

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FRP and other plastic tanks are ideal for lots of process applications, but
you almost have to look at each one case by case. A lot of these get
purchased and show up on the job site without any thought having been given
to how they will be anchored.

They need some sort of clip restraint at the base to keep them from sliding,
and if there are no hold-down lugs incorporated in the tank shell near the
base to attach to, I have seem arrangements where straps are attached to
clips at the top of the shell, or sometimes a cable frame is built around
the tank and independently supported. People get pretty creative about
solving this problem. The ones I have seen do not run loose across the top
to the other side. 

I would design the straps to take all the uplift except for the dead weight
of the shell and top. You can't count on the weight of the contents to help
with the resistance unless the bottom is reinforced sufficiently to mobilize
the weight without tearing loose from the wall at the bottom.

The usual approximate formula for anchor tension would be T = 4M/nD - P/pi*D
where m is the overturning moment, n is the number of equally spaced
anchors, D is the diameter, n is the number of anchors, and P is the dead
weight of the shell and roof.

You also need to consider whether the tank shell can buckle. It will be in
compression on the toe side and also on the heel side from the anchor. For
thin plastic walls, and a low modulus of elasticity, buckling resistance
isn't the greatest.

If you're lucky, the manufacturer has installed attachment clips and tested
them to vouch for their tearout capacity.

-----Original Message-----
From: Acie Chance [mailto:achance(--nospam--at)lacsd.org]
Sent: Monday, February 07, 2005 1:48 PM
To: seaint(--nospam--at)seaint.org
Subject: Cable Tie Down for FRP Tanks



    I am checking the hold down of a Fiber Reinforced Plastic tank due to
wind and seismic.  Using free body diagrams I get twice the tension in the
cables as the original engineer did.  I know this method is used on a
regular basis but I have not been able to find the method explained.  If any
one has a book or other document with the  method explained and/or a worked
out example they could e-mail or fax to me it would be helpful.  The
calculation I am checking is mostly a spreadsheet with results and few
formulas.
   I am not sure everyone on the list is interested in this problem so my
fax number is (562) 908-4270 and my e-mail address is achance(--nospam--at)lacsd.org.

Thanks
Acie Chance

PS  I do not want to hear about bad spelling and grammar.


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