Wednesday, November 7, 2012

The Fascinating Story of the Tacoma Narrows Bridge



It was on this date in 1940 that the Tacoma Narrows bridge collapsed.


It is not often that I write about physics on this blog, but I feel confident enough in my understanding of the Tacoma Narrows bridge collapse of 1940 that I can create a post about it. Furthermore, all the knowledge you are about to enjoy comes straight from what I learned in my remedial physics class back in 1995.  This is going to be a research-free post!  

Here goes:

Lookit that bridge!  It's collapsing right in front of your eyes.  What caused such a calamitous end to something made of steel, concrete, and cable?  Most people would jump to the obvious conclusion that it failed because of an earthquake, but there was no earthquake in Washington state on November 7, 1940.  The design of this bridge was so faulty that it was destroyed by wind.

The Tacoma Narrows bridge was not long for this world.  It was built in 1938 and lasted only two years before collapse.  During its short life, it was nicknamed "Galloping Gertie" because it buckled so violently in the high winds of the Tacoma narrows.  The roadway would warp front-to-back in the wind so much that drivers would sometimes lose sight of the cars ahead of them.

On November 7, 1940, a sideways wind struck the bridge in such a direction and at such a speed as to cause a deadly feedback loop.  The frequency of the wind twisted the poorly-designed bridge's roadway into a wave, which was just the right frequency to build upon itself.



Any bridge has a natural frequency that might liead to this type of positive feedback loop; one only hopes that it would not coincide with naturally-occurring rhythms.  It is for just this reason that troops, when marching over bridges, are commanded to "break step," or stop marching in unison.  The fear is that an entire army marching across a bridge in lock-step might be enough to enact this sort of positive feedback effect and bring down the structure.  (I seem to remember Mythbusters proving this theory to be true, but, in the spirit of a research-free post, I'm not going to check into it.)

If you've ever leaned a guitar against an amplifier that is turned up too loud, you will hear the effects of a positive feedback frequency.  The sound feeds from the amplifier through the pickups and back out of the amplifier, increasing exponentially as it goes.  It just keeps adding to itself until something fails.

So, what happened on November 7, 1940?  The high winds in the Tacoma Narrows began rocking the bridge at just the right (or wrong, I suppose) frequency, causing a feedback loop whose effects you can see on the old movie reels from the day.  Soon, the strain was enough to bring the entire structure down.

The bridge pitched for over an hour, and there was fortunately enough time for everyone to get off of it before it failed.  Only one car was on the span when it collapsed.  The driver survived by crawling on his hands and knees to safety, but his pet dog was so scared that he could not be coaxed out of the backseat.  He was the only casualty of this spectacular destruction.

Here's a more in-depth look at it (in color.)

(Note: If you know anything about physics and would like to refute anything I just wrote, please feel free to do so in the comments.)

2 comments:

  1. i came over to leave a comment on your guernica post but can't find it. i laughed when i read it. it brought back fond memories of my own genius children at that age ;) i always loved their artistic endeavors. they made me look at things in a different light.

    i'm a regular reader of your blog, but i read it from google reader and don't usually click through. since i'm here, i'll also say i remember seeing a video of this bridge's collapse... it was used in a school classroom in a lesson on different types of bridges. stunning!

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    1. Hello! Thanks for reading. (And, yes, that Guernica post was up for just a moment before I had to take it down again. It's up permanently now.)

      Now that my kid has accomplished his Guernica, I'm just waiting to see what his blue period has in store.

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