Brian Whatcott wrote:

On Tue, 29 Jun 2004 16:48:11 -0400, Bryan Martin
wrote:
//
The designers of the Tacoma-Narrows bridge didn't give enough consideration
to its aerodynamics or stiffness. It shouldn't have been flexing to that
degree at all. It was a solid road deck built from I-beams in an area known
for frequent high winds. Modern suspension bridges are usually built from
steel trusses for stiffness and have open steel grid decking down the middle
of the span to kill any lift generated by the roadway.

It's not a kind site. The current bridge there gallops too a little,
in high winds.

I have never noticed any motion. My mother cried when the old one went down. It
was back to waiting in line for the ferry. The original design was an upside
down U in cross section with solid sides. It didn't exactly generate lift. It
would flex a bit under a wind load (35 mph made it gallop like crazy) and twist
and dump, twist back with the U slightly facing the wind and do it again. The
real problem was harmonic oscilation. The darn thing had a harmonic mode that
could get pumped up over time with very little wind.

The replacement is all open gridwork with four wind grates in the roadway the
whole length. My brother, who is a ranger in the Tetons (was on Lou Dobbs the
other night about rescue operations), has his arrest ticket framed from the
time he walked the cable from end to end when he was a teenager.

-- Charlie Springer