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RE: Tacoma Narrows bridge collapse

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Colleagues:
	Prior to the article in SEI, I would have said that all that could
have been said had been said by the late Dr. Scanlan at Johns Hopkins.  A
very good paper by Billah and Scanlan, at least in regards to debunking
the myth that it was purely a vortex shedding phenomena, can be found on
Mark Ketchum's website www.ketchum.org (which you should visit anyway if
you're a bridge person!).  From what I recall of the SEI article, I would
guess that there's still a dispute as to the role of the vorticies in the
instability (whether they were a cause of, or a result of, the motions).
If you're interested in the phenomena, I strongly urge you to read the
Scanlan paper.
	As to whether we can reconstruct the failure, at the time the
bridge collapsed Dr. Farquharson at U. Washington had already constructed
a scale model in a wind tunnel and was in the process of testing it.  As
it turned out, he was able to recreate almost exactly the pathological
behavior of the prototype structure.
	There were several bridges constructed prior to and concurrent
with the Tacoma bridge which also exhibited "lively" performance in the
wind.  The Thousand Islands, the Deer Isle, and the Bronx-Whitestone come
to mind.  I'm not sure whether they would have shown the same dramatic
instability as the Tacoma, but in the post-disaster climate it was
certainly not desireable to have your suspension bridge bounce in the
breeze!

HTH
Thomas P. Murphy, Ph.D., P.E.
Harrsiburg, PA



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