Tacoma Narrows resonator followup

[SSHS KPHOX <kphox@MAIL.CCSD.K12.CO.US>]

PHYS-L@lists.nau.edu writes:
> I don't understand the question. If this relates to the bridge,
> then the frequencies of vibrations of the cables supporting it
> are all much higher than the torsional modes of the bridge.
> Vibrations of the cables, per se, had nothing to do with the
> aeolian oscillation of the bridge deck, though of course their
> elasticity played some role in determining those frequencies.
> (I recall that at least the two lowest modes of the bridge
> were seen in the dramatic amplitude motion.)

I was operating under a misconcepton, that has always bothered me, that it
was the cables that were acting as the aeolian harp which then stimulated
the deck. If the cables had been all the same size and tension it might
have worked for me. Thanks
> > 4. What mechanism started and supported  the oscillations of that bridge?
>
> The bridge deck, initially more or less having the wind velocity
> in-plane, got a little bit out of plane and the wind reinforced
> this distortion. A vortex was shed, allowing the distortion to
> relax and, it turns out, overshoot its equilibrium position, at
> which time the opposite polarity distortion is driven to
> increase by the wind until another vortex is shed, the deck
> relaxes past equilibrium, usw. When the wind was gentler, this
> oscillation did not grow in amplitude to bridge failure, which
> gave the bridge its nickname of "Galloping Gertie".

This also makes great sense to me that the entire bridge deck is acting as
an aeolian harp. These vortex pushes strike me as giving pulses that match
the natural frequency of the bridge deck. As I rad my H&R, my prime
source, this is resonance. Why is it not?


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