Re: Midterm Question - Sort of

[Jim Green <JMGreen@SISNA.COM>]

> Well, that's a bit like saying that the steady bowing of a violin
> string is an example of resonance. There are certainly *normal modes
> of oscillation* for the bridge (and the violin string), and these
> modes can be excited by an external source of energy in a nonresonant
> interaction. Resonance I take to refer to the excitation of a normal
> mode by an *applied* periodic forcing function, clearly not the case
> encountered here. The applied forcing function is, more or less, DC.



I dunno Leigh, _I_ use "resonance" to mean the vibration in a normal mode
no matter how the "vibration" is caused.   I don't know that there is a
divine definition for the word "resonance"  but surely the bridge reached a
normal mode vibration (I would say "resonance" but the mileage of others
may vary)


| It was not the cables of the bridge that shed vortices at the bridge
> normal mode oscillation frequency, but rather the bridge deck itself.
> That is, indeed, the mechanism responsible for aeolian oscillation.
> I believe the bridge actually flexed in more than one of its normal
> modes.

Well yes the bridge oscillated in a vertical plane AND torsionally about
the horizontal axis.

BUT the cause of the break up was that the normal mode frequencies changed
so that the bridge was more nearly in "resonance" -- This because one of
the cable collars slipped.

Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen