Re: Midterm Question - Sort of

[Samuel Held <Held@RHIP.PHYS.UTK.EDU>]

Paul,
        Good catch.  The problem with the Tacoma Narrows was that it was
a suspension bridge and your example is sorta of what I was trying to
explain.  I neglected the fluid dynamics though.  NB: this incident lead
to mandatory wind tunnel testing of models of all new bridges.  All over
the loss of one dog.

Sam Held


-----Original Message-----
From: paul o johnson [mailto:pojhome@FLASH.NET]
Sent: Wednesday, July 07, 1999 12:43 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: Midterm Question - Sort of


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