Bad vibes: The science of vibration could lead to clues in the bridge collapse
Vibration problem?: A combination of vibrations coming from various area sources could be factors in the fall of the I-35W bridge in Minneapolis on Aug. 1. (Flickr.com photo by Diversey)
I’m not a structural engineer and don’t even play one on TV, but I’m curious to see what kind of dots get connected on this scientific phenomenon and the I-35W bridge collapse.
A former chairman of the National Transportation Safety Board, who just happened to be in the Twin Cities the day of the bridge collapse, raised some interesting questions. Jim Burnett was interviewed by the Star Tribune today and said that bridge collapses usually have two main causes: vibration and fatigue cracking.
Earlier inspections of the bridge have noted that it showed signs of fatigue cracking, but not to the degree that officials felt the bridge needed immediate attention. But I haven’t heard a lot of comment in press accounts, yet, about vibration. Burnett pointed out that two primary causes of vibration were going on or near the bridge at the time of the collapse: construction work on the bridge’s road deck and also vibrations from a train passing underneath it.
“Vibration is one of things that cause cracking to propagate," he said to the newspaper. "They will be looking at that."
He was on the scene of the collapse at 5 a.m. today (Aug. 2) before sharing his preliminary thoughts on the collapse. But his analysis got me thinking to the role vibrations have played in other bridge failures.
Remember the Hyatt Regency hotel walkway failures in Kansas City in 1981? About 2,000 people were there for a dance contest. The hotel featured several walkways suspended at the second, third and fourth levels over the hotel’s atrium. When people started dancing on the walkways, the vibration of the feet moving together caused them to collapse, killing 114 people and injuring more than 200 others.UPDATE: See the comments below for clarification on the Kansas Hyatt disaster.
Digging around a little more, I found these postings by engineers at the Ask a Scientist website.
Here’s some observations from engineer Vance Calder: An army troop marching at full cadence is ordered to stop marching and walk across a bridge at each soldier’s own individual gait. The "in time" stepping produces vibration, the multitude of soldiers adding to the vibration. When trying to think of waves, think of waves in water. Opposing waves can cancel each other. But like the troops moving in the same direction, the waves can add to each other and gain extra strength.
More points from engineer James Prxewoznik include: Vibration, in general, is bad for materials. It can lead to fatigue of parts and eventual failure. There are two types of vibration: free vibration and forced vibration. Free vibration occurs through actions of forces inherent with its design. Forced vibrations come from external forces outside the design of an object. If those two forces coincide, the vibration oscillations can magnify causing an object to come apart.
There might have been a lot of vibration at play on the 35W bridge: jack-hammering and other construction work, the train passing underneath, and don’t forget, it was an extremely windy day in Minneapolis on Aug. 1. A lot more needs to be checked out, of course. But investigators will likely be looking at how many different forms of vibration combined on the bridge at the time of its collapse.
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You can appreciate what wind can do to a bridge on this You Tube video of the Tacoma Narrows bridge collapse.
The Hyatt collapse had nothing to do with dancing. It was a bad connection detail. It wasn't a dance contest, it was a Tea Dance. Most of the people on the walk ways were watching, not dancing.
To clarify people had gathered for a tea dance, but there was a dancing contest starting. People gathered on the suspended walkways to watch the contest below. Most of the reports I can find state that people were dancing on the walkways.
However, you are right that the major factor that lead to the collapse was poor design and construction decisions that resulted in the walkways collapsing not the dancing and vibration. Their construction and design could hardly support the dead weight (the load of the structure itself without people) much less the huge numbers of people who were filling the structures to watch and dance. I have corrected the post above.
Nevertheless, vibrations of this sort can be an important factor in the design and failure of structures. They just weren't the primary problem in the Hyatt disaster.
Actually the 'vibrations' weren't any part of the problem. There only 63 people total on the two walkways. 40 on the second floor and an additional 23 on the fourth floor walkway. The minimum mass per area that Kansas City code allowed would have been safe for a live loading of over 1200 people. Built to code Godzilla could have dance up there without a problem. The NSB study showed conclusively that the connection on the fourth floor was failing and had been for some time and that it was only a matter of time before it failed without any extra loading. NSB completely ruled out any vibrations or dancing as the cause. In fact, despite what witnesses claim, tv news footage being shot before and during the collapse show no dancing going on the on the walkways.
There is some more conversation about vibrations and bridge design over on another blog post about the bridge collapse.
sorry 2 everyones fam
Ok here is an idea. You get a whole bunch of Ceiling Fans and put them under the bridge. That way the force of the wind holds the bridge up.
Understanding the scienific principles of cival design is an essential part which is often disregarded by people - the accuracy and quality has to be perfect and take into consideration issues resulting in wind and tension fluctuations.
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