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Vibration vs. Wave Propagation Analysis

Vibration vs. Wave Propagation Analysis

(OP)
The other day I was having lunch with a engineer I know. A subject we talked about began with me expressing frustration with displacement/modal based dynamic analysis (in programs like STAAD) that sometimes didn't predict vibrations (that occurred in real life) at other levels /locations of buildings. He suggested to me a wave propagation model could possibly solve those issues. I personally have never used such a model (I'm not even aware of any software that does). So I thought I'd ask here: has anyone here used such a technique? If so, were you satisfied with the results? Does it indeed "spread out" the vibration "better"?

RE: Vibration vs. Wave Propagation Analysis

Do you recall some details of project with "missed" vibration ?

RE: Vibration vs. Wave Propagation Analysis

(OP)

Quote:

(Tmoose)

Do you recall some details of project with "missed" vibration ?

Yes (in fact, I was asked to look at it to see what went wrong). It basically was a compressor on bolted down to a steel frame (up on the 4th floor of a building). It caused some noticeable vibrations at other levels that the model failed to predict. Looking at it, I think the main culprit was likely the estimate for the foundation stiffness at the column bases. Modifying that, I got a bit closer to what was observed in the field.

But that comes back to my original question: does a "wave propagation" model transfer these vibrations better? I would assume that it has similar boundary constraint issues (i.e. to a normal displacement/modal based dynamic analysis).......but, in the situation i describe above (i.e. the original model), I would assume (famous last words) that if it is propagating waves, it would have propagated them to the other levels (at least to some degree) regardless of the foundation stiffness assumption.

RE: Vibration vs. Wave Propagation Analysis

We used to use wave propagation models to model the interior of cars, for use with noise cancelling. Essentially that means the phase angle of the mode is no longer +/- 90 degrees, so you get travelling energy, whereas your standard modal assumes quadrature, hence no energy transfer. The problem as I see it is that we were working from real results, not dodgy computer models.

Cheers

Greg Locock


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RE: Vibration vs. Wave Propagation Analysis

(OP)
Thanks Greg......good info.

RE: Vibration vs. Wave Propagation Analysis

Is this not related to the fact that what you get is related to forced vibrations, rather than resonance modes?

RE: Vibration vs. Wave Propagation Analysis

(OP)

Quote:

(rob768)

Is this not related to the fact that what you get is related to forced vibrations, rather than resonance modes?

Is this directed at me or Greg?

RE: Vibration vs. Wave Propagation Analysis

(OP)
Question (to anyone who may be interested).....reading more on this (wave propagation methods of vibration analysis), it appears that this sort of analysis is appropriate once forcing frequencies get above about 100-200 Hz or so (especially in the kilohertz range). One explanation I've seen is because such higher modes are involved. But using a typical FEA formulation, you only have as many modes as you have degrees of freedom. How does the fact its a wave propagation analysis change that? (Some of the material I've read shows frequency modulated stiffness matrices for elements. Not sure if that plays a role or not.)

RE: Vibration vs. Wave Propagation Analysis


you have to look at the accoustic velocity in the individual components to assess whether wave propagation is significant relative to the natural frequencies of your structure. This establishes the cut-off frequency of the accoustic propagation.

many times you can rely on the quasi-static analysis for frequencies below cut-off.

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