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Re: Seismic Design of Nitrogen Storage Tank

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Rich,
 
Look in the ASCE  Guidelines for Seismic Evaluation and Design of Petrochemical Facilities, page 4.A-4: T=7.78/10^6*(H/D)^2*(12*W*D/T)^(1/2).  For your tank, safely assume 1/2" shell thickness.
 
This formula is recommended for tanks on legs, but actually is for a cylindrical column.  I discovered that legs significantly affect the tank rigidity, and may require special consideration during the T analysis.  
 
Yours appears to be a 6000-gallon LIN tank of some earlier construction.  Calculated more or less accurately, the period of such tank is about 0.16 sec.  
 
It should be noted that after all research I've done, the formulas of ASCE7 13-15 appear to adequately reflect the force - consistently with AWWA D100 and the above ASCE book.  At the end of the day, it all comes down to a good old 0.3*W (ASD, CA away from faults), so it may not be worth your time to go really deep into that. 
 
Regarding sloshing - back in 2001, I had a long talk on the subject with Bob Wozniak who apparently is one of (if not the) main structural gurus behind the AWWA D-100.  He informed me that for smaller D/H, sloshing becomes a force-reducing factor and can be safely ignored (as is routinely done).  This can be also derived from the AWWA formulas.
 
V. Steve Gordin, SE
Irvine CA
 
 
  
----- Original Message -----
Sent: Tuesday, November 04, 2008 14:01
Subject: Re: Seismic Design of Nitrogen Storage Tank

Steve,

Thanks for the response.

I think I found that I can use ASCE 7-05 15.7.8 for my design.  I believe
the exception of 15.7.13.1 will let me do this for a liquid nitrogen tank.
What I'm wondering is just how to get the Impulsive force (tank) and the
convective force (sloshing) for my system.  I reviewed the example problems
of FEMA 450 and they seem to refer to API 652, 650 and AWWA D100 at just
the critical time of explaining something.  Unfortunately I do not have
these publications.

I seem to have one hangup that I was hoping some Standard would solve.
That is to calculate the fundamental period of the tank structure.  To
calculate the impulsive force I need to know the fundamental period of the
tank.  The only information I have of the tank are the dimensions.  I'm
researching how I can calculate the fundamental period of the tank vessel.
I have a copy of '2006 IBC Structural/Seismic Design Manual'.  Example 53
gives an equation of T=7.65x10^-6*(L/D)^2*(wd/t)^0.5.  It doesn't say where
this equation came from some I'm not sure it is valid in my case.

To calculate the convective force I again need to know the fundamental
period of the tank.  Any insight on this would be appreciated.

Rich


On Tue, 4 Nov 2008 10:23:33 -0800, "Steve Gordin"
<sgordin(--nospam--at)sgeconsulting.com> wrote:
> Rich,
>
> There is no magic here, the "industry standard" is to apply ASCE7 Chapter
> 13 and 15.  It should be noted that, in spite of the presumed engineering
> simplicity of the problem, there are several issues in this design that
are
> usually missed, including - but not limited to: 1) "diagonal" direction
of
> the lateral force; 2) punching shear on thinner pads; 3) asymmetric
> location of three legs on the pad (while the tank may be centered on the
> pad, the moment arm for concrete design is longer in one direction). 
>
> Also, the pads for this type of installations tend to accommodate more
> equipment than just the tanks (for example, vaporizers that may become
> quite heavy).  In these instances, a beam-on-elastic foundation approach
> may be suggested for more accurate evaluation of moments (watch the pad
> uplift, though - soil does not provide resistance in this direction).  In
> some complex cases I use the FEA for the pad as supported by the one-way
> springs.
>
> HTH,
>
> V. Steve Gordin, SE
> Irvine CA
>
>
>

>
>    
>  
>
>
>   ----- Original Message --


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