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Re: Signs For RSA

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Parag - 

If your maximum mass participation is only 15% then you clearly do NOT have
a dominant mode for that structure.  Some thoughts: 

1) Have you been strategic with how you modeled your mass?  Too often I see
folks using 500 modes for an RSA analysis when they could have gotten away
with 20 modes if they just modeled their mass more efficiently.  By more
efficiently I mean that you lump the mass at locations that to not
contribute significantly to LOCAL modes.  Often times this means lumping the
mass at the major beam column intersections rather than as distributed loads
or point loads along the mid-span of the beams.  In your case, that will
probably not result in a dominant mode.  But, it should still help you
reduce the number of modes that you're dealing with.  

2) If you don't have a dominant mode, then what you have to do (as others
have suggested) is to use two load combinations for each earthquake load
combination.  One with a positive factor and one with a negative factor.  

3) Then when you are reviewing your results you need to remember that the
MAGNITUDES of your results are accurate, but that their signs might not be.
This usually only comes into play when you're comparing or combining
multiple results together.  Examples might be combined footings or mat
foundations, story shears, story moments, et cetera.  

4) This loss of signs for your analysis results is really the main reason
why so many engineers avoid Response Spectra Analysis. There really is no
substitute for experience in dealing with this issue.  Once you've done it a
few times, it's much less intimidating.    


Josh Plummer, SE
RISA Technologies
(949) 951-5815 (voice)
(949) 951-5848 (fax)

From: Parag Bande <pabande(--nospam--at)>
To:  seaint(--nospam--at)
Subject: Re: Signs For RSA


I am not using RISA-3D but using NISA. How to define contributing/ 
dominant mode if I have a very complicated structure in which I am going 
to excite it up to  around 500 modes and cut off at 33 Hz. The mass 
participation in each mode varies from 5 to 15%. It is quite difficult 
to have judgment for a particular mode to be selected as contributing 
mode for signs. Softwares can not decide the contributing mode. Is there 
any other parameters (other than mass participation) by which we can 
find contributing mode in the direction of earthquake.

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