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Re: Fundamental Frequency of Tall Steel Tank

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David-
You can probably get an estimate accurate enough for preliminary design by assuming the tanks are fixed at the base and treating them as a cantilever beam.  f=SQRT(K/M) in hertz where K = stiffness of the beam and M = mass.  You can use a lumped parameter or distributed parameter model depending on the level of precision you are looking for.  You should probably look at both the bounding conditions of tank full and tank empty in determining your mass.  For such a tall, small diameter tank, I would expect the convective frequency to be quite close to the impulsive frequency making a single degree of freedom (rather than 2 dOF model usually used in simple tank design) model adequate for preliminary purposes.  Also, with the high aspect ratio of the tanks, sloshing effects of the liquid will be small.
 
With such a height to diameter ratio, you may want to look at vortex shedding to get a sense of how close the wind driving frequency will be to the tank frequency.
 
Bill Cain, S.E.
2510 Prince Street
Berkeley CA 94705
(510) 305-6520
 
 
-----Original Message-----
From: davidfinley(--nospam--at)bizsea.rr.com
To: seaint(--nospam--at)seaint.org
Sent: Fri, 6 Oct 2006 7:59 AM
Subject: Fundamental Frequency of Tall Steel Tank

I'm doing some preliminary sizing for the foundations for some tall cylindrical steel tanks - 8'-0" diameter x 66'-0" high.  They will be in a low seismic area, so wind loading is my concern right now.
 
I need to estimate the fundamental frequency of the tank in order to determine the Gust Factor per ASCE 7.  Can someone point me to a good reference?  Is there a reasonably conservative value I can assume for this preliminary work?
 
Since I am just considering wind at this point, the empty tank will control and therefore I don't need to consider the added effects of the liquid contents in determining the fundamental frequency, right?
Thanks,
David Finley
M. David Finley, P.E., P.A.
2086 SW Main Boulevard - Suite 111
Lake City, FL  32025
386-752-6400
 

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