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Re: Signs for Response Spectrum Analysis

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You rightly say that defining a "dominant" mode requires engineering judgment.  How does RISA-3D define "dominant"?  There is only a checkbox for "Use Dominant Mode for Signage"; the user is not allowed to input a cutoff, meaning that the program must have one internally.

A second, broader question:  If this requires engineering judgment, why then is it programmed into RISA and not even visible to the user?  This is not an attempt to ding RISA;  all structural software seems to have this tendency.

Best regards,
Daniel Popp, S.E.

From: Josh Plummer <joshp(--nospam--at)>
To: seaint(--nospam--at)
Sent: Thursday, August 6, 2009 10:47:04 AM
Subject: RE: Signs for Response Spectrum Analysis

Parag -

If you have a mode that "dominates" over the other modes then you should be
able to use the relative signs of that mode to determine which members are
in tension or compression and such.  You'll probably still want to use two
LC's for each of the EQ loads. At least when you're designing a combined
footing for a moment frame then your analysis won't try to tell you that
both columns are in uplift at the same time.

For what it's worth RISA-3D does give the user the option of using the
dominant mode to provide signage to the RSA results..... The real judgment
call then is what constitutes a "dominant" mode.  Certainly a model with 90%
mass participation would.  Probably a single mode with 75% mass
participation would as well.  But, once you start getting below that it
becomes questionable.


Josh Plummer, SE
RISA Technologies

From: Parag Bande <pabande(--nospam--at)>
To:  seaint(--nospam--at)
Subject: Signs for Response Spectrum Analysis

Dear All,
In Response Spectrum Analysis (RSA), due to modal combinations (CQC or
SRSS) signs for the member reactions/forces gets lost. Is there any
accepted methodology to apply signs to these forces during load
combination ? When EQ load is subtracted from other loads in load
combination it means that direction for concern EQ is reversed. However 
structural response for any kind of horizontal load one cannot get all
members (forces) behaving similar manner, means all bending cannot be
hogging/sagging or all members cannot be in compression/tension.
Thanks in advance

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