[TheForge] WAS: BLACKSMITHING CHAT WITH OLD FRIENDS - Bridge

Fri Jun 28 17:37:00 2002

Only on the last day were they trying to escape and not until the
oscillations became so severe the roadbed began breaking up under their
feet.

Father likes to talk about how the word would go out in the Seattle/Tacoma
area when Gertie was dancing and people would take turns driving across it.
The contest being who could make the shortest time run and keep the car in
it's lane. On REALLY good days the contest was walking across and staying on
the sidewalks.

Galloping Gertie didn't get it's nickname due to a single catastrophic
failure. The Tacoma Narrows bridge did that every time the wind blew above a
certain speed, around 17 mph if I recall correctly. It also wasn't the first
bridge to collapse due to wind harmonics and if it's designers had checked
it wouldn't have.

The Tacoma Narrows bridge was state of the art, built with better steel than
ever before available in quantity. This allowed them to design it with the
shallowest main support girders ever seen and still have a huge safety
margin over and above material yield margins. It was a beautiful bridge,
more graceful than any suspension bridge ever. It was truly a work of art.

Unfortunately there is only so many hours in a day and so much an engineer
or team of engineers can be expected to know or take into account. An
aircraft designer could've pointed out a couple basic flaws in the design. A
Roman drill instructor could've explained a basic principle of bridge
construction and harmonics they didn't realize applied. And so on but hind
sight is 20-20 eh? <grin>

Fact is wind harmonics hadn't been a serious factor up till then in america.
A number of years earlier in england there was a catastrophic suspension
bridge failure due to wind harmonics but communications being what they were
it's small surprise the designers of the Tacoma Narrows bridge hadn't heard
of it, or perhaps didn't understand the cause, or perhaps didn't believe it
could happen to good old american knowhow, or . . . . ? Who knows.

Anyway, the main roadbed girders were shallower (narrower webs in the "I"
beams) than ever before and due to innovative structural design much
stronger
than previous bridge girders. The shallow girders were the first of three
main interrelated problems. The other two being equal length sections and
round suspension cables.

What happened to Galloping Gertie was basically pretty simple. As the wind
driven vibration hit the suspension cables resonant frequency, the resultant
sine waves induced in them, shortened the cables minutely. When the cables
shortened they lifted the bridge deck. This changed the angle of attack of
the bridge deck relating to the wind, creating lift and slackening the
cables.

The cables only lost tension once of course (when it collapsed) but they did
slacken enough to allow the wind greater effect and the side that tightened
(low side) got a significant frequency increase that acted like a spring to
give the bridge deck a little more oomph on the rebound. Where the shallow
bridge deck girders come in is also several fold but the two biggest factors
were: one they didn't break the wind like deeper girders do, (they were
actually aerodynamic) and secondly they weren't rigid enough even though
they were more than strong enough to carry the load. The flexibility was
designed in as it's earthquake country.

Okay so far, all bridges flex in the wind, some alarmingly but here is where
the final fatal flaw comes in. Every section of the bridge was the same
length. The entire bridge had the same resonant frequency. This means that
when the bridge started to resonate in the wind it developed positive
feedback loop oscillation. The perfect, ever increasing sine wave so obvious
in the film.

It's like kicking on a swing, every swing you add just a little and before
long you're at the top of the stroke, push any farther and you crash. This
is exactly what happened to the Tacoma Narrows bridge. It wasn't that the
wind was stronger that day, it was how long it blew at exactly the wrong
speed that killed her.

Dad not only tells about how Gertie danced in the wind but how beautiful the
sound of the wind in the cables was. He said you couldn't really hear the
music unless you were out on the bridge. As the deck oscillated the
suspension cables tightened and loosened with the same sine wave and the
tune oscillated with it. Dad says it was beautiful if frightening to walk
the deck while Gertie was galloping.
Galloping Gertie was not only visually beautiful, she was architecturally
and structurally beautiful but she was also aerodynamically and harmonically
tuned and that song and dance killed her.

I saw a special on TV a few years ago and then picked up a book. I also only
used some of Dad's first hand reminiscences. So if I have any of this wrong
it's my CRS acting up again. <grin>

Also, if you take a look under any bridge built in the past 50 years or so
you'll notice the abutments and piers are skewed in relation to the deck,
even if the road makes a straight shot across the . . . whatever. This is to
keep the bridge from having a resonant frequency. Even so if you're on a
bridge while a dog trots across it'll frighten the stuffins out of you. The
darn deck bounces more than if a raft of semis were hurtling across at
ridiculous speed.

Frosty
------------------------
If it ain't forged
it ain't real.
Wrought iron is.
The FrostWorks

Meadow Lakes, AK.

----- Original Message -----
From: "Darrell" <[email protected]>
To: <[email protected]>
Sent: Sunday, June 23, 2002 3:58 PM
Subject: Re: [TheForge] WAS: BLACKSMITHING CHAT WITH OLD FRIENDS - Bridge

> They are not "walking across it", they were trying to get off it.
>
> Darrell

> ----- Original Message -----
> From: "Ray Miller" <[email protected]>
> To: <[email protected]>
> Sent: Sunday, June 23, 2002 3:29 PM
> Subject: [TheForge] WAS: BLACKSMITHING CHAT WITH OLD FRIENDS - Bridge
>
>
> > I believe it to be "Galloping Girtie" the Seattle Tacoma Bridge. There
> > is film of it oscilatting prior to collapse. There are actually some
> > fools walking across it!!!
> >

> >
> > Ray Miller
> > Cincinnati
> >
> > Jeff Harding wrote:
> >
> > >Ralph;
> > >
> > >   I recall the same bridge, they have a photo of it just as it
> > >collapsed and the road bed was in the shape of a perfect sine wave.
> > >Can't remember where it was either.
> > >
> > >   Jeff   ><>
> > >