Re: Midterm Question - Sort of

[paul o johnson <pojhome@FLASH.NET>]

Leigh Palmer wrote:

> It always bothers me when the Tacoma Narrows Bridge collapse is cited as an
> example of resonance. It's not. The wind was steady, more or less, and there
> was no significant component at the bridge's resonant frequency. It is a case
> of nearly pure DC excitation.
>
> It's a great film, but it's an example of aeolian excitation, like blowing on
> a taut piece of grass between your thumbs. I have many good demonstrations of
> resonance. Using the Tacoma Narrows Bridge as an "example" can only obscure
> the concept in the mind of a student.

Now just a doggone minute, Leigh. I've been quite happy with my understanding of
this bridge collapse, viz, a resonance condition caused by the frequency of
vortex flipping. From Goodstein's explanation in Mechanical Universe, the
vortices of the wind around the cables flips forth and back at a rate which is
proportional to the DC wind speed. As that rate grows nearer to the bridge's
natural frequency, the vibration amplitude increases until ... kaboom.

Is this hogwash? Is not aeolian excitation caused by this very effect?

poj
Collin County College