PSA 9.2 — Modal Analysis: Mass Participation, Cutoff Frequency and the Mass You Left Out
A stress report for a rack header lands on your desk with one paragraph of dynamic work in it:
A modal analysis was performed. The first natural frequency is 17.5 Hz and all extracted modes lie below the 33 Hz cutoff. The system is rigid and no further dynamic analysis is required.
Three sentences, and two of them are wrong in opposite directions.
Commit to an answer before reading on: what is wrong with that paragraph?
First, 17.5 Hz is below 33 Hz, so the system is precisely not rigid in the sense that word carries in seismic work —…
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