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RE: Signs for Response Spectrum Analysis

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Daniel –


Actually, in RISA the definition of a Dominant mode is totally left up to the user.  The program never makes an attempt to automate this decision.  That’s actually the reason why we never ask for a cut-off.  We expect the user to review their modes first and then decide if any of the modes are dominant.  Also, the program default is to NOT consider dominant modes for signage.  Therefore, the only time when this comes into play is when the user specifically requests it. 


If the user decides that there is a dominant mode, then they just check a box in the RSA analysis options that says “use the “dominant” mode for signage”. When this box is checked then the RISA will always assume that the dominant mode will be the one with the highest mass participation in that direction.  If not checked, then all RSA results will have a positive sign like you would otherwise expect for RSA results. 


We could have asked the user to specify which mode they wanted to call the “dominant” model.  But, I don’t know that would improve upon that feature all that much. 


Also, interface issues are very important to us.  Therefore, if you have ideas about how this option could be (or should be) more visible to the user then I wouldn’t mind hearing them…. Though it may make more sense for you to respond privately rather than to bore the list with mundane details of our program interface.  Of course, this may just be an situation where you had not explored the RSA options in our program very much before you posted that comment.  If so, then no need to follow up with me. 




Josh Plummer, SE


RISA Technologies


(949) 951-5815 (voice)

(949) 951-5848 (fax)


From: Daniel Popp [mailto:drp181(--nospam--at)]
Sent: Thursday, August 06, 2009 9:03 AM
To: seaint(--nospam--at)
Cc: Josh Plummer
Subject: Re: Signs for Response Spectrum Analysis



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