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RE: Dynamic analysis

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Several individuals have suggested that the model should be modified if
there is not enough mass in the lower modes.  While models with a large
percentage of the mass in higher modes, or lower frequency modes with
little mass may be indicitive of modeling errors, they may also be correct.
 You need to study the model and verify that it reflects the real
structure.  Then if the model gives results you do not like, consider
changing the real structure.

If you are operating at a level where you believe that 90% of the mass
should occur in the first 5 modes or that the first two modes should be
translational, then I would suggest that you review your dynamics with
emphasis on mode shapes, participation factors, and how the response
spectrum values scale the modal results.

Mark Gilligan 

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