On Mon, 12 Feb 1996, Barry Kutner wrote:
> Big Don <bigdon@eskimo.com> writes:
>
> > A classic case of this is the first Tacoma Narrows bridge which was most
> > certainly designed to withstand a bumper-to-bumper traffic jam of
> > fully-loaded 18-wheelers in both directions in combination with *static
> > loads* from some peak windstorm. Yet it failed when lightly loaded in a
> > rather moderate garden-variety windstorm. For those who have never seen
> > the video of what came to be known as "Galloping Gertie," the wind set up
> > a swaying/twisting action in the main span that increased in amplitude
> > until the whole works deposited itself in Puget Sound.
> >
> As I recall from engineering classes in college, the problem with the
> bridge was it became a resonant structure, and therefore required very
> little energy to get it swinging back and forth. It had to do with the
> type of suspension and supports used. [...]
That's exactly the point. Go watch, for example, a long boom flopping
with the gusts, or the tower twisting and untwisting when hammered by the
rotor slop stops being hit. Whatever the tower/antenna system's natural
frequencies in torsion and bending are, they will be excited by this action.
And that is what these proposed computer programs cannot begin to deal with
adequately. For example, some natural resonances will depend on the guy
tension which is a spring of sorts. This is a big unknown and rarely
stays constant. How often does anyone check that with a calibrated
measuring device, assuming they ever did during the initial
installation of the tower.
Steady-state wind load analyses don't address any of this.
Big Don
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