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Re: SEISMIC MISSING MASS

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Juan,

I don't know of a web site but the following book has about the best explanation I have seen.

A. K. Gupta, RESPONSE SPECTRUM METHOD - IN SEISMIC ANALYSIS AND DESIGN OF STRUCTURES, Blackwell Scientific Publishers, Boston, MA, 1990.

I think the ASME B&PV code has an appendix on this topic.

As Charlie Hamilton already said in his reply, the problem is related to the number of high frequency modes included or rather excluded in the dynamic analysis. If you have a regular structure (pretty uniform stiffness and mass distribution throughout), the 90%-95% guideline for effective mass works pretty good. But when your 'structure' is more complicated, such as with a piping system having many bends, supports and lumped masses, the 'missing mass' issue becomes much more important. The basic concept is to lump all the higher modes into one equivalent static applied load at the zero period acceleration (ZPA).

I have also seen 'missing mass' referred to as the lumped masses at restrained DOF's (supports) which of course do not contribute to the dynamic response. If your mesh is very refined, these so-called 'missing masses' will be insignificant. I believe the conventional understanding of missing mass is the one associated with excluded higher frequency modes.

Walt Sawruk
Senior Consultant
ABS Consulting
Shillington, PA
Email: wsawruk(--nospam--at)absconsulting.com

At 01:54 PM 25-04-03 -0500, you wrote:
Hi,
Does anyone know what is the missing mass problem in sesimic analysis/design?

Can anyone give me a brief explanation of this method?

Do you know of a website that has more info about it?

Thank you,

Juan

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