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Re: Seismic Loads on Large Tanks - API 620

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The graph for "K" and other factors in API was derived from equation.  The
equation was presented in a paper - "Basis of Seismic design provisions for
welded steel oil storage tanks" by Wozniak and Mitchell, in the 1978
proceding of API.  The paper developed the sesmic chapter in API 650 and API
620.  It is also the basis for seismic design in the AWWA.

Here is the derivation for "k":

k = 0.578/ (sqrt(tanh(3.67/(D/H))))

I have it on mathcad for our tank design.  It is much easier than going
through the graphs.  I am sure that is what the supplier is doing - or has
one of the commercial programs which calculates the API design loads.

- Aswin

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From: Paul Meyer <PMeyer(--nospam--at)HASimons.com>
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: Seismic Loads on Large Tanks  - API 620

I am checking the seismic design of a large tank using API 620.

In API-620 Appendix L, there is a factor "k" that is used to calculate the
period of the first mode of sloshing.  "k" is derived as a function of the
ratio of diameter to height of the tank.  A graph called figure L-4 is given
that plots "k" as a function of D/H.  The value of "k" is slightly less than
0.6 for all values of D/H less than about 1.8.

The tank suppklier has indicated that the value of "k" used in his design is
0.5783933.  Either he has much sharper eyes than I do (the graph is about 1
inch high) or he is deriving the value of 'k' numerically.  I cannot find an
equation for "k", and the value of 0.578...seems about right, but I am
curious about the derivation of the value of "k."

Any help would be appreciated.