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November 7, 1940 – The Collapse of “Galloping Gertie”

Posted November 05, 2018 12:00 AM by RSBenner

If you have ever taken a physics course, whether in high school or college, there is a very good chance that you heard about the collapse of the Tacoma Narrow Bridge. In fact, if you are an engineering student, you probably have learned more about this bridge than the average student. Why? Because not only did this bridge collapse due to flaws in the design and construction, it collapsed in an amazingly dramatic fashion and was captured on film. This film, commonly shown as part of many physics or engineering curricula, leaves a lasting impression in the student’s memory.

Construction of the Tacoma Narrows Bridge began in September of 1938. It was a suspension bridge built over Puget Sound between the city of Tacoma and the Kitsap Peninsula in Washington State. At the time of construction, it was the third-longest suspension bridge in the world, by main span length. However, as soon as the deck of the bridge was built, the workers realized that something was not right. The deck of the bridge would move vertically during windy conditions – so much so, it was given the nickname “Galloping Gertie.” Some of the workman were even forced to chew on lemons to lessen the feeling of seasickness they experienced while working on the bridge.

Although the state’s engineers assured the local papers that the ‘bounce’ was normal, they began taking steps to eliminate the movement. They contracted for a wind tunnel study of the bridge to be performed to find the cause and a permanent solution. In addition, four hydraulic jacks were installed at the towers to act as shock absorbers. Although this modification was not very effective, the bridge was opened to traffic on July 1, 1940.

In October of 1940, as a preliminary suggestion from the wind tunnel study, tie down cables (1-9/16” diameter anchored restraining wires) were installed along the bridge’s side and mid-spans. This seemed to reduce the ‘bounce’ of the side spans, but was not effective at reducing the movement of the center span. The increased winds of autumn were now taking their toll on Gertie.

During the morning of November 7, 1940, a strong, icy wind was lashing at the bridge, causing two- to five-foot-high movements of the mid-span. This was followed later by a “lateral twisting motion” which was tilting the roadside up 28 feet one side, then the other, at an angle of up to 45 degrees. Approximately one hour after this twisting motion began, the Tacoma Narrows Bridge failed, and the center span of Galloping Gertie fell into Puget Sound.

The Tacoma Narrow Bridge was a suspension bridge and, as such, was designed to move. But why did this bridge move so much and, therefore, ultimately fail? As described in many physics and engineering books, the cause of the collapse is attributed to forced resonance: aeroelastic flutter produced by the wind that matched the natural frequency of the bridge. Several factors contributed to this. The wind tunnel study, completed just days before the collapse, determined that the solid stiffening girders, used underneath the roadbed, were partly to blame. Typically, bridges employed an open lattice beam truss, allowing the wind to pass through. However, this solid girder design caused the wind to be diverted above and below the structure, therefore, attributing to the flutter. Other factors identified throughout the investigation were the narrow deck design (only two lanes) and the overall lightness of the bridge partly due to the cost saving measures of limiting the amount of steel used during construction.

Although a tragedy financially, there was no loss of life from the collapse (except for a cocker spaniel, Tubby). And, as a direct result, research into aerodynamics-aeroelastics expanded, which helped influence future bridge designs preventing this from occurring again.

However, Galloping Gertie still lives on in physics and engineering classrooms and students’ memories.

References:

http://www.wsdot.wa.gov/tnbhistory/connections/connections3.htm

https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)

Video showing the collapse:

https://commons.wikimedia.org/w/index.php?title=File%3ATacoma_Narrows_Bridge_destruction.ogv

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#1

Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/05/2018 9:38 AM

I was surprised to see how long the center section stayed together for so long. The concrete roadway looked to me like a sheet of cloth billowing in gentle wind, and the whole structure moving as one piece. Why didn't the roadway crack and crumble faster?

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/05/2018 10:58 PM

As a physics teacher, I have viewed the video (most of the times as a film loop) well over a hundred times (while showing it to my students), yet never tired of watching it! I always offered to show it a second time to each class, and they nearly always voted to see it again.

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/05/2018 11:15 PM

I first watched this during a physics class back in 1980 - damped versus driven harmonic oscillations, and resonance frequencies was the topic, if I remember correctly.

Never did hear of any casualties.... that is until I just read about poor old Tubby.

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#4

Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/06/2018 5:06 AM

Was born and raised nearby and have crossed the "new replacement" bridge 100's of times...but not once without a lump in my throat.

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/06/2018 11:44 AM

There are a number of bridges around the world where soldiers are ordered to break step when crossing. It's slightly alarming that the structure is that close to instability.

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/06/2018 4:32 PM

Nicola Tesla experimented with devices that would supply a periodic impulse to structures to build up resonant energy.

https://en.wikipedia.org/wiki/Tesla%27s_oscillator

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/07/2018 10:57 AM

Thanks, Rixter, I'd never heard of that one. Tesla seems to straddle the groud between bonkers and genius.

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/06/2018 8:58 PM

Sometimes you're the windshield, sometimes you're the bug.

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#9

Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/07/2018 10:00 PM

Works just like the reed in a woodwind instrument, only on a larger and much more expensive scale!

Airflow separation causes Von Karman vortices which generate alternating (up and down) lift forces on the bridge...

https://www.youtube.com/watch?v=f3LmjJ1N7YE

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/07/2018 10:45 PM

Thanks! I hadn't seen that... Really interesting!

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/12/2018 10:35 PM

I think that this type of vortex-shedding mechanism explains things better than "aeroelastic flutter" or simple resonance.

Resonance with what? From ~1968 explanations as seen in my physics class, one would think that the wind was gusting in periods that matched the natural frequency of the bridge. Not very likely, especially for any length of time.

However, with a near-constant wind, vortices will shed alternately top/bottom from the bridge structure, and hence create possible resonance.

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/12/2018 11:05 PM

I think you have a very valid point!

I've always assumed the resonance was due to a specific velocity of the wind interacting with the road surface and alternately with the beams below, but I like your idea better...

And of course the two halves of the center span clearly had essentially identical frequencies, and were in resonance with each other. I presume that making one of the two support towers higher than the other would diminish that resonance.

My current bridge worry is the 'new' Oakland-SF Bay bridge, built with Chinese steel, several pieces of which have already failed. ...but that's worth another thread.

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#13
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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/13/2018 10:00 AM

I also prefer the term "vortex shedding." My experience is with poles and wires; we have probably all witnessed "galloping" of wires (conductors) between poles. Many higher voltage conductors have dampers (Stockbridge is one brand) on them to prevent galloping at resonance by adsorbing a bit of energy. Resonance happens at low wind velocities for those who don't already know.

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

11/13/2018 12:21 PM

Now that you mention it, I do remember seeing "galloping" of wires/cables, but the last time I specifically remember was well over 50 years ago, so a lot of younger folk may never have seen it.

The specific case I most remember was one of those major telephone cables with a lead sheath around 2 inches in diameter. The center of that span was going up and down a total of around 4 feet; it had to be exerting some major forces on the wooden poles. I always assumed that in this case the resonance was between one or more of the poles and the cable.

The dampers I'm most familiar with are simple sheet-metal "flags" placed well off the center of the span to effectively create two shorter and non-equal spans. I frequently see these on phone cables. I used to be able to see one from my living room window, but the trees have grown to hide all those cables.

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Re: November 7, 1940 – The Collapse of “Galloping Gertie”

12/29/2018 2:27 AM

Beautiful!

If you're not standing on it! ;-( ;-)

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