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Re: Resonance reply to ChrisW

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>Follow up question: Do you disregard the frequencies with low 
>participation factors?
Depends on the excitation. If you have strong excitation even low 
participation factors contribute to response.

>I have 80% participation factor in the vertical direction for mode 1 and 
>86% vertical & 16% lateral for mode 2. 
Modes 1 and 2 will likely beat, since the main response is vertical. 
Ideally the two frequencies are isolated because they differ by more than 
20%  but you can't count on the accuracy of the calculated frequencies 
because FEA isn't the real world. Your first two may be on top of each 
other or close enough for cross-talk. 

>How does low mass participation affects resonance?
The participation factor is an indication of the strength and direction 
of the response. Participation factors aren't percentages, no matter what 
STAAD tells you. I suspect the program is reporting the modal or 
effective mass as a percentage of the total mass. The effective mass is 
the square of the participation factor, and reflects the amount of mass 
actually in motion due to elastic deformation.
High participation factors indicate that a lot of the structure is 
involved in a particular mode, but thephysical response in that mode will 
depend on the excitation direction. 

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw


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