As an engineer I've always been intrigued by the Tacoma Narrows bridge collapse. Just the thought of how wind can excite the bridge's structural resonance to a point where it collapses is pretty incredible.
So I was excited when vibration expert (and author of our PDF handbook An Introduction to Shock & Vibration Response Spectra) Tom Irvine used an enDAQ sensor (formerly Slam Stick) to perform a test on the Huntsville University Drive pedestrian bridge... and then blogged about it. He mounted the vibration data logger to the center of the bridge with double sided tape and jumped up-and-down to excite the bridge's vibration modes. With the vibration data he was able to draw some interesting conclusions. Read on for Tom's results and some other examples (with some videos) of bridge excitation:
Tom Irvine's Bridge Resonance Results
Tom prepared a report on the pedestrian bridge's natural frequency and posted it to his vibration blog. Using the vibration data captured and the free VibrationData Toolbox he developed he was able to determine the bridge has a fundamental frequency of about 2.2 Hz with 0.16% damping.
The London Millennium Bridge Swaying
One of the reasons I find Tom such a great vibration analysis resource (check out his vibration blog that is chock full of incredible information), is that he is able to tell a story with his results. He doesn't just stop with the numeric vibration analysis, he connects the results to a real world experience.
In Tom's report he discusses how a pedestrian's natural gait typically corresponds to a frequency of about 2 Hz. This is obviously very close to the 2.2 Hz resonance in the bridge that he measured. He goes on to note that there is an instinctual tendency for pedestrians to synchronize their steps with the bridge motion and with one another. And this has lead to issues on pedestrian bridges when the bridge's natural frequency is excited.
The London Millennium Bridge had this happen back in 2000, (check out the video below); after this incident dampers were added to the bridge. As this University of Cambridge article on the design states, "as a result [of the added 37 linear viscous dampers and over 50 tuned mass vibration absorbers] this bridge is now probably the most complex passively-damped structure in the world. This same group published a detailed report on how the London Millennium bridge swayed in the first place which included some linear feedback modeling.
Tacoma Narrows Bridge Collapse
Seeing that video of pedestrians exciting a bridge's resonance probably reminds you of that famous Tacoma Narrows bridge collapse -- the one I discussed earlier. This bridge's resonance was rather easily excited by wind conditions which ultimately led to the bridge collapsing.
2020 Humen Bridge in China
Recently the Humen Bridge in China experienced a similar phenomenon on May 5th 2020. Popular mechanics has an article that explains more why this started on the bridge, it seems that a recently installed retaining wall on a pedestrian bridge acted as a "wind-capturing sail."
Will the Huntsville University Drive Bridge Collapse?
Most likely not. Although the bridge has a resonance that is close to a frequency pedestrians typically walk at and it has a low damping ratio, the pedestrian traffic is far too low. As Tom notes though, it may be interesting to have the local university students explore adding dampers to the bridge.
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