Jul

13

Tacoma narrows bridgeAs an engineer, it is indeed fascinating that even after all of this time, and with enormously sophisticated computer tools, there is still contention as to what caused the Tacoma Bridge to fail.

I don’t think this is a shortcoming of modern engineering; rather it underscores just how darned complicated these systems can be, how many ways in which they can interact with themselves, other structures, and the environment, and how difficult it can be to plan for every possible occurrence, while still keeping cost, schedule, and original design intent within reason.

Perhaps one of most interesting observations of all, however, about the Tacoma disaster, is one that never was: there has not subsequently been a bridge failure that spectacular since. Thus, even without computers, Wiki, massively online resources, or even a pocket calculator, the engineers of the day managed to figure out what not to do in subsequent bridge designs to avoid such failure in the future. That, to me, is incredibly impressive.

It would have been nice to corner one or two of those folks and get their thoughts on what they think happened.

-Jon

Jon Longtin, Ph.D.

Associate Professor and Undergraduate Program Director

Department of Mechanical Engineering

SUNY Stony Brook

Stefan Jovanovich adds:

One of the rent-a-faculty members who was allowed to teach structures for a semester at the school of Architecture at Harvard in the early/mid-60s had been an ironworker in his youth. He showed us the footage of Galloping Gertie's spectacular collapse and then told us a story. He said that he had known two men who worked on the Tacoma Narrows, and they had both told him that the thing was going to be a lemon because the bridge deck "had too little steel in it". The point of the lecture, from our teacher's point of view, was that we future architects might want to solicit the opinions of the people who will actually build our perfectly imagined castles in air.

The Bronx-Whitestone Bridge in NY was cast from the same mold as the Tacoma Narrows. Its web site has a picture of the original bridge deck and the retrofit made at the Tacoma Narrows collapse when Warren Trusses were added. The Wikipedia web site for Othmar Ammann says that the trusses have been removed and replaced with fairings to solve "the wind problem". (Can the New Yorkers confirm whether or not this is true?)


Comments

Name

Email

Website

Speak your mind

5 Comments so far

  1. JK on July 13, 2010 11:02 am

    According to Wikipedia, it`s “flutter” that caused it.

  2. BridgEngr on July 13, 2010 4:27 pm

    The Tacoma Narrows bridge collapse was due to the fact that the natural frequency of the structure matched the wind frequency thus causing resonance. The effect of resonance in structures tend to amplify loading on the structure due to dynamic amplification.

    Suspension bridges tend to be inherently “less stiff” than other types of bridges mainly because of the need to span long distances while minimizing dead weight. The original Tacoma Narrows did not have a stiffening truss which today is an important feature in suspension bridges used to stiffen the overall structure. Compare the old with the new Tacoma Narrows and you will see the lack of a stiffening truss below deck in the old bridge.

    Today, we design bridges by performing not only static analysis but also dynamic evaluation due to seismic and wind loading. In addition, long span bridges tend to undergo wind tunnel testing on scaled models.

    Bridge design is a fascinating and exciting career that I have been doing for the past 15 years. In addition, I have had the privilege of being on the design team that designed the new Tacoma Narrows bridge.

  3. Tim on July 13, 2010 10:57 pm

    The bridge failed because no rigorous structural analysis was performed prior to construction. Had such an analysis been performed, it would have revealed a strong resonance peak at approx 0.2 Hz, and would have resulted in additional stiffening being applied along the longitudinal axis of the design.

    Today, with fast processors and cheap memory, we are able to construct digital 3D models where things like frequency and impulse response can be easily characterized, and design liabilties can be quickly identified. By comparison, only pencil, paper, and slide rule were available in 1938.

    There is no mystery here. The bridge failed because a strong resonance was unintentionally introduced into the design. The wind provided sufficent impulse energy to initiate motion. The bridge resonance amplified the motion to the point of structural failure.

  4. Gary Rogan on July 13, 2010 11:20 pm

    This just goes to show that it’s pretty rare to have a major structure that is all of sudden “decides” to vibrate self-destructively for no apparent reason. It’s all the more amazing that markets have a tendency to self-organize themselves every so often into cascading stampedes.

  5. Gregory Rehmke on July 14, 2010 5:58 pm

    It is interesting to contrast “Type A” problems and “Type B” bridge problems. “Type A” would be when under-design causes the bridge to fail. But what about over-design that drives up the cost of the bridge? We want super-safe bridges that are sure not to fail, but by “over-design” I mean bridges that have way more design and construction than needed to be absolutely safe.

    That may sound like a good thing, but it raises costs and ten over-designed redundantly safe bridges could be more costly than having a bridge fail from time to time and be rebuilt. All bridge design and construction expenses have opportunity costs. The money, engineering, and construction expertise could have been used elsewhere, so an over-designed bridge means the rest of the world is slightly under-designed and more prone to failures.

    I would expect government agencies and bridge regulator/managers to regularly over-invest and mal-invest in bridge design and construction, just as the FDA insists on expensive and unnecessary testing of new drugs.

    The consequence of over-regulation of bridges is fewer bridges with less capacity for cars, thus more congestion. (Not to mention building bridges in the wrong place anyway, like Nowhere, Alaska.)

    The rebuilt Tacoma Narrow Bridge had only two lanes, I think, each way for decades, leading to severe traffic congestion each work day for decades. Finally a second bridge was constructed a few years ago, and a toll added to pay for it.

    But for decades the too-narrow Narrows bridge was “free” during peak hours, causing millions of lost hours stuck in socialism-caused traffic jams.

Archives

Resources & Links

Search