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Re: Resultant Seismic Force

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There are two procedures for calculating the seismic load on this type of
structure :

1. The API 650 Method  -  The American Petroleum Industry has a procedure in
calculating the dynamic effect of the fluid inside the tank. Although this
procedure is used for ground supported tanks, this could be adopted for your
case in question. This procedure calculates for the seismic force for the moving
fluid, location of line of action of the seismic force on the moving fluid,
calculates for the seismic force on the structure / vessel alone, and the
location of line of action of this second seismic force. Please refer to the API
650 - Specifications for Welded Oil Storage Tanks for details.

2. You can assume the fluid to be static and compute for the seismic force using
the equivalent static force method for non-building structures. The dead load is
equal to the sum of the fluid weight and the dead weight of the structure /
vessel. The seismic force can be assumed to act on the structure's center of
gravity, giving due consideration to the weight of the fluid.

We use the first procedure when the dynamic effect of the fluid is substantial
or when the client requires us to use that method. Otherwise, we use the second

Kind regards,

Roy Sharif M. Sison, RCE

Vyacheslav Gordin wrote:

> Problem:
> Vertical cryogenic vessel (tank) on three legs in seismic Zone 4.
> Diameter:      4-to-10 feet
> Shell height:  10-to-50 feet
> Leg height:     about 2 feet
> Weight:          15-to-150 kips.
> Should the resultant of the seismic force be located at:
> a) tank center of gravity; or
> b) at 2/3 of the height?
> The opinion of fellow professionals is crucial for me making the decision to
> either accept the method ?a? (customary for the industry), or try to push
> through method ?b? (as ? in my opinion - more corresponding to the physical
> nature of the problem).
> So, what do you think?
> Steve Gordin, S.E.,
> Irvine, CA
> P.S. Anyway, if we have something as simple as a CMU property wall ? where
> would you apply the resultant of the seismic force?  SG
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