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RE: Circular Concrete tank

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Based on a PCA publication, "Circular Concrete Tanks Without Prestressing,"
the hoop stress behaves pretty much as if the base can slide (no cantilever
action) until you get down to below about .40 of the depth, then the hoop
stress gradually drops off as the effects of base fixity kick in. The
vertical moments in the wall are negligible until you start to get below
about .70 of the depth, and can be positive or negative and depend on fixity
assumptions at the base. The safe thing to do is check it using both hinged
and fixed support at the bottom. 

The above numbers are for illustration and are different depending on the
tank's aspect ratio and wall thickness, since the relative sharing of load
in membrane vs flexural action depends on the wall stiffness. I would not
rely on a "rule of thumb" to design something like this. 

You could design something for a fat tank based on pure cantilever action,
but it will still have wall tension and the tank will be full of cracks if
you ignore it. Similarly, you can design something that won't fall over
based purely on hoop tension all the way down the shell, but you may get
horizontal flexural cracks and the tank will have way more hoop steel than
it needs toward the bottom.

Get a copy of this PCA pub. I think you will find it very helpful.

-----Original Message-----
From: richard lewis [mailto:rlewistx(--nospam--at)juno.com]
Sent: Tuesday, February 25, 2003 7:25 AM
To: seaint(--nospam--at)seaint.org
Subject: Circular Concrete tank


I am about to design a circular concrete tank.  It is a temporary
retention/detention pond for stormwater runoff.  It has about 120 feet
diameter.  The walls are about 10 feet high.  My question has to do about
using hoop stresses for design.  Is there some rule of thumb for a height
to diameter ratio for a circular tank where hoop stresses are no longer
of any benefit and the wall behaves more like a cantilever retaining
wall?

Rich

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