Karman vortices and natural frequencies

erik wrote in news:0914ed01-213c-45d2-9750- snipped-for-privacy@c30g2000hsa.googlegroups.com:

There is nowhere enough information to make a judgement here. It depends on, among other things, the amount of badness that happens if it hits resonance and the with of the peak in a curve resonance response vs. frequency. Some structures have a narrow peak and some a broad peak. An easy, and therefore almost useless answer, is 5 standard deviations from the resonance peak.

Off the top: the Q factor of a resonant element indicates by how much the input disturbance amplitude can grow in the resonant region. The higher the Q the narrower the resonant band of frequencies. You would expect a civil design to be rather low Q and lossy. In this case, the resonant region is wider. Even so inputs in the band outside 0.7 to 1.3 times Fr should excite only small oscillations.

Brian Whatcott Altus OK

the weird thing is, there is a european standard about lighting columns, that prescribes a loadfactor to be applied, which is dependent upon the nat. freq. of the column. But which nat. frequency ??? every construction has a lot of those.... I can compute the frequencies, but have to decide which one to apply to the factor-calculation.... So I try to find out which nat. frequency is near the karman frequency... But even then, I could find that the corresponding oscillation mode is such, that there is no danger... true ?

I am stepping too close to the edge of my experience but still, Tacoma means that not exciting a resonance is a 'very good thing'.

You would prefer excitations to

1) stay away from any mode or 2) stay away from hi-Q modes but better: 3) render excitations harmless by counter measures like helical coiled fins around a light standard exposed to the breeze.

Brian W

It is suggested, that Tacoma was not a resonance issue, but a case of self-exiting vibration. I suppose these phenomenae are somehow related...