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
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
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