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.
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