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RE: Problem with Bouyancy for a Reinforced Concrete Tank

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Thanks to all of your responses.  They were very much appreciated.
 
Tom Bodkin, E.I.T.
Carroll Engineering
-----Original Message-----
From: Tom Bodkin [mailto:TBODKIN(--nospam--at)carrollengineering.com]
Sent: Friday, June 28, 2002 8:17 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: Problem with Bouyancy for a Reinforced Concrete Tank

 
-----Original Message-----
From: Tom Bodkin
Sent: Friday, June 28, 2002 8:14 AM
To: '%22seaint-ad(--nospam--at)seaint.org%22'
Subject: Bouyancy Factor for a Reinforced Concrete Clarifier

I am designing a 14' high, 55' dia. concrete tank to be almost fully buried.
 
When checking for bouyancy, I find that a cross section that looks reasonable (and consistant w/the existing clarifier tank on site) weighs about as much as the water it displaces. This gives me a safety factor of 1.0, not 1.5 like I am aiming for.
 
I am using the PCA Manual for Circular tanks w/o presressing.
 
I am using 120pcf for dry soil and 70 pcf for bouyant soil as recommended in the geotechnical report, and ground water is pretty high (on site wells indicate that g.w. is 2' below grade)
 
The PCA manual seem to make some conservative assumptions like neglecting the weight of the soil "wedge" around the tank which would give me a lot more resistance.
 
Can anyone offer any suggestions so I don't have an unreasonable design?  The existing tank was designed/built in 1988 is just a little smaller and has a F.S. of 1.10.  It survived Hurricane Floyd.  They did use ground water relief valves about midheight which I plan on using to help some. 
 
Thanks in advance!!! (first time user)
 
Thomas W. Bodkin
Carroll Engineering