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Re: steel - vibration analysis

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Josh - when I read Allen's post, I generally agreed with it, though I may have misread what I wanted to read. Where framing changes length within a bay, it is correct that the variation in frequency tends to reduce or eliminate footfall vibrations because of the large number of different vibration modes at play - there is no large area in which the vibrations may be sustained.

The example in 7.2 is also a very regular structure, with the diagonal areas both on the perimeter and around the atrium apparently not affected by the vibration problems - the primary areas of stiffening were in back-to-back regular orthogonal bays. (I only skimmed the article to see what you were referring to, let me know if I missed some of it) You can still have issues in non-orthogonal bays if the primary frequency is related to the girder, but the joists/beams should tend not to promote resonance unless they are all of similar stiffness, or all fall in the lousy range. Vibrations of plates and shells is greatly complicated by non-regular geometry, and can effectively only be performed numerically. If you have an issue with an unusual structure, the first thing to do (barring obvious deduction) is to determine your mode shapes, frequencies, and modal mass participation - then you can start to isolate where the issues are. Your comparison of extending the vibration theory from wide flange to tube is not analogous- extending from a wide flange to a non-linear non-prismatic curved member would be more appropriate. And, of course, the theory doesn't extend to such members.

(now where did I put that set of nomex khakis...)

Jordan


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