Effect Measure

The American Scientist is the magazine of the scientific society, Sigma Xi. You can get it on the news stand, and that’s where I bought the latest issue (March-April, 2009). One of the articles is by historian of engineering, Henry Petroski, and it’s about the Tacoma Narrows Bridge. The bridge was a product of Depression infrastructure funding but instead of being an investment in the future it only lasted four months. It is famous because its spectacular collapse was caught on film (below the jump) and for many years was shown in thousands of undergraduate calculus and elementary physics classes as an example of forced resonance.

What’s forced resonance? Lots of physical structures have a “natural frequency.” A pendulum of a certain length, for example, swings at a characteristic rate (provided you don’t make its maximum angle from the vertical too large). That’s how a pendulum clock keeps time. Children on swings are like pendulums and also have a characteristic swing rate. It is common for parents to get behind the swing, wait for it to reach its greatest height, and then give it a push. The push is therefore synchronized to the swing’s natural frequency, and if you push it hard enough the swing will go really high. On the other hand If you push at some other time, say at the lowest point coming back, the swing will be reduced or even stopped altogether. You get the picture. Synchronizing the push with the natural frequency is what is called forced resonance. What does that have to do with the Tacoma Narrows Bridge?

Actually, nothing. It turns out that the collapse of the bridge probably had to do with a different and still poorly understood phenomenon, aeroelastic flutter. The original theory was that a gusty wind set up periodic forcing at just the right frequency (the natural frequency of the bridge structure) resulting in a forced resonance that tore the bridge apart. Since most wind gusts are not at all regular but very chaotic (literally and figuratively), no one now thinks this is a proper explanation, but the whole episode resulted in new standards for suspension bridge design.

The bridge was one of the longest of its time (exceeded only by the contemporary projects of the George Washington Bridge and the Golden Gate Bridge), and connected mainland Washington with the Olympic Peninsula. The two are separated by the inlets and coves of Puget Sound, which extends some 60 miles southward from Seattle to Olympia. The narrowest part of the Sound is around Tacoma and that’s where the bridge was built. Construction started in 1937, and half the cost was met by FDR’s Public Works Administration, the other half a load from another FDR New Deal agency, the Reconstruction Finance Corporation. Both required Washington State to hire a consultant for such a huge bridge and they engaged Leon Moisseiff, then the world’s leading suspension bridge engineer. For aesthetic reasons Moisseiff produced a level bridge that was extremely narrow — just two lanes with almost no sidewalks. Instead of conventional steel trusses it had 8 foot steel girders in the deck. The bridge was thus much narrower and lighter than any other bridge its length, which produced an unusual flexibility.

The bridge soon earned the nickname of Galloping Gertie because it would wave up and down to the point where cars crossing it could see vehicles in front of them appear and disappear as the bridge undulated. Instead of a disincentive, this became an attraction and toll revenues exceeded expectations for the first four months of the bridge’s life, which began on July 1, 1940. But on November 7, four months later, during a windstorm with gusts over 40 miles per hour the movement suddenly changed character. Instead of only moving up and down like a wave, the deck also began to twist about its center line. Within hours it was the most spectacular bridge failure every captured on film.

There are a number of longish versions on YouTube, but I chose this newsreel version for its breathless entertainment value:

Comments

  1. #1 BlowMeDown
    March 26, 2009

    I noticed that under the hed of this post (in category:) there appears ‘engineering’, ‘infrastructure’, ‘movies’, then ‘terrorism’. I looked for allusions to “terrorism” in the piece and was puzzled until I realized that this was your comment on the quality of the structural engineering evident in the project. Good Call..!! ..and totally appropriate.

  2. #2 revere
    March 26, 2009

    LOL. Thanks. It was supposed to be Technology (just above it on the list) and I clicked the wrong thing. I just changed it.

  3. #3 MJ
    March 28, 2009

    This was the first home movie I ever saw. Scared me to death! I was six years old and a neighbor bought a home projector. His first movie was of the Tacoma Bridge going down. We watched it over and over. I think the kids were waiting for the man to go down with the bridge, but he never did. LOL

  4. #4 Pierce R. Butler
    March 29, 2009

    Ah, if only our modern journalism were so professional and objective…

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