On November 7, 1940, the Tacoma Narrows Bridge failed during a windstorm. During construction, workers had reported* a noticeable amount of “galloping” (some give is desirable), but nothing like what occurred on November 7. As 42 mph gusts pummeled the four-month-old suspension bridge, the road started to oscillate visibly. At one point, one side of the road was 28 feet higher than the other (at 05:03 you can see two folks stumbling across the bridge).
After an hour, a number of the bridge’s cables snapped, forcing the center span to collapse. The cause: aeroelastic flutter. Essentially, the amplitude of the oscillations increased exponentially as the wind continued to hit the bridge. When the energy grew greater than what the bridge was naturally built to absorb (called “damping”)… well, boom went the dynamite. This is why today’s bridge designers put their models through wind tunnels.
Today, the old Tacoma Bridge’s steel girders and half-mile of roadway comprise one of the world’s largest human-made reefs in the waters of Puget Sound.
In 2004, Shawn Frayne used the concept of aeroelastic flutter to develop the “Windbelt“, a small device that generates energy by placing magnets at the ends of a small fluttering belt. His company, Humdinger, is working to commercialize the devices. The Windbelt is not yet available for sale.
Humdinger’s FAQ explains the difference between “resonance” and aeroelastic flutter: “While the effect of aeroelastic flutter is not necessarily a resonant effect, but rather a positive feedback loop that reaches a limit cycle, some variations of the Windbelt technology do use resonance between two or more components on the generators to increase efficiency.”
*It’s said they chewed lemons to combat nausea on the job.
Henry Petroksi wrote a book about bridge failures (To Engineer is Human) that features a chapter on the Tacoma Narrows Bridge.
Reader’s Digest version: look at how thin (to to bottom) the road deck is. Compare it in your mind to the decks on other suspension bridges you’ve seen. The deck wasn’t stiff enough to resist the wind.
So now we know. You need to make sure the road deck is thick enough.
The wind belt is such a simple concept. I’m sure there are tons of engineers out there smacking themselves for not coming up with it sooner.
As you may expect, aircraft suffer from this problem too:
http://www.youtube.com/watch?v=ca4PgyBJAzM
http://www.youtube.com/watch?v=iTFZNrTYp3k
http://www.youtube.com/watch?v=kQI3AWpTWhM
http://www.youtube.com/watch?v=8D7YCCLGu5Y
http://fr.myspace.com/index.cfm?fuseaction=vids.individual&VideoID=3452299
I have also seen flight-test footage showing flutter in C-5 wings and 737 control surfaces, but I think the respective manufacturers don’t want those videos floating around the internet…
Luckily, while flutter is not simply a resonant phenomenon (it increases non-linearly since the more a part deflects, the greater the aerodynamic force that pushes it to deflect, as long as it doesn’t stall), it can often be solved relatively easily by changing the resonant frequency of the structure to put that frequency outside the range that could be reached by aerodynamic forces pushing the structure up and down. Unfortunately for aircraft designers, this usually means adding weight, since it involves either increasing the damping (making the structure less springy by using materials that are less elastic, so it doesn’t “bounce back” so quickly from the extremes of the deflection cycle and is slowed down as it moves between one extreme and the other), the stiffness (making the structure stronger so it flutters more quickly), or just the weight and moment of inertia (making the structure heavier so it flutters more slowly).
Of course the power generating magnets will tend to dampen any resonance. A great design would have a resonant requency tightly coupled to 60hz to generate useful AC. That would probably be pretty hard to do.
The wiki page on “Artificial Reefs” has at least two bigger ones, so I suggest you remove ‘today’ and/or insert ‘unintentional’
#5: Good catch. Text amended.
The video of that bridge undulating has been re-used so many times, I’m pretty sure I knew all about the thing and the reason for the collapse by the time I was 10 years old.
Every time my parents took me over the bridge that replaced it, I secretly hoped to catch it in the act. It wasn’t until quite some time later that I realized they would have fixed the flaws during reconstruction.
Actually, all I can find on the Wikipedia page on artificial reefs regarding size starts with:
“The world’s largest artificial reef was created by the purposeful sinking of the USS Oriskany.”
The Oriskany is nowhere near a half mile long. So I’m not sure which are the two reefs that you (anon5) were saying are larger?
This footage has always had a rather Winsor McCay quality for me.
The ‘Windbelt’ system looks interesting, but you can the commercial problems in their description.
Their ‘micro’ system seems intended for unattended dataloggers etc .. devices that would require AA batteries.
According to their FAQ, their ‘micro’ generator generates the energy equivalent of dozens to hundreds of AA batteries over the life of the generator. But a AA battery has about 2.5kJ of energy in it. (They would use carbon-zinc as a guideline, because it gives their most favourable figures)
So 100x 2.5kJ = 250kJ
But you can buy a specialised ‘lithium-thionyl chloride’ battery with a 20 year life that has about 250kJ of energy in the size of a D-cell.
They cost under $20 USD in one off quantities.
That’s a hard spec to compete with.
Mac
Note to the car– dude, DRIVE!!!! GO!! GET OFF OF THE F**KING BRIDGE !!!! Pedal to the metal!!! jeez….