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Rigid Structure Critera

Rigid Structure Critera

(OP)
Section 15.4.2 of ASCE 7-05 sets forth some criteria for determining whether or not a structure is rigid.  Specifically it states that a structure is rigid if it has a fundamental period less than 0.06.  My question is this - What if the mass participation for the fundamental period (first mode of vibration) is extremely low (say less than 1% of the total mass).  In other words there is only one small section that is really oscillating under this frequency (say a short section of platforming)?  It would seem to me that the majority of the structure is actually rigid but the ASCE criteria forces you to classify it as a non-rigid structure.  Are there any guidelines as far as judging fundamental frequency and mass participation when determine if a structure is rigid or not? Thanks in advance.

RE: Rigid Structure Critera

No.  If you want to investigate whether a structure meets the rigid criteria, you'd have to do a modal analysis of your structure to determine your periods and see where you fit on the code defined response spectrum.

The only time mass participation comes into play is if you have to do a modal response spectrum analysis to determine your seismic forces.  There you would use only the modes that give you at least 90% mass participation

As an aside, if you have a very low mass participation then wouldn't that particular mode not contribute to the overall seismic response of the structure anyway? There would be very low mass.

RE: Rigid Structure Critera

Generally speaking I would say it is up to the engineer to decide what the "fundamental" period of the structure is. Yes, you can use a computer dynamic analysis to help you.  But, you make the decision.

If the mode that dominates the behavior of the structure has a period of less than 0.06 then I would be perfectly comfortable saying that this is a rigid structure.  

And, I would say that mass participation is a good indicator of how important that mode is.  If the mass participation for that mode is over 50% then I would call that the dominant mode.  If the mass participation is very low, then I would not call it a dominant mode.  

Where it would get tricky is if you had one mode with 40% mass particpation at 0.05 seconds and another mode with 20 or 30% mass participation at 0.1 seconds.  I would not normally think of that as a rigid structure.  Though I might design it that way if I thought it would be more conservative.  


 

RE: Rigid Structure Critera

(OP)
Thank you for the replies.  I was personally leaning towards the same conclusion you both reached that low mass participation would indicate the frequency was not a dominate mode.   

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