Stories tagged I-35W bridge
Each Saturday, learn firsthand about the I-35W bridge rebuilding project
A friend of mine has a holiday visitor from Lithuania who's a highway engineer and very interested in the I-35W bridge collapse. Looking for ways for him to learn more about last summer's bridge collapse, I just found this at the official bridge building website: View the construction progress every Saturday at 11 AM for a free Sidewalk Superintendent Talk led by project representatives who will describe the design/build process and answer questions. Everyone is welcome! Meet at the corner of Washington and 19th Avenues (in front of Grandma's Saloon).
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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.
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)
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Investigators are going to recover the pieces of the collapsed I-35W bridge and reassemble major sections (just like they do with crashed airplanes) to try to find a cause for the disaster. Seeing all the pieces in the proper order can help investigators figure out what went wrong and which bits failed first. The process could take a year or even longer.
According to this article in the New York Times ("Engineers see Dangers in Aging Infrastructure"), disasters like yesterday's bridge collapse may become more common if we don't find better ways to detect potential problems and invest in maintaining our aging roads, bridges, and other public works...
Later this afternoon, the US Army Corps of Engineers is going to lower the water level of the river by two feet in the area of the bridge collapse.
After the initial investigation is done, the Corps will open the roller gates at the Ford dam (i.e. Lock&Dam #1), dropping the level of the pool and giving emergency workers and investigators better access to the wreckage.
The silver lining of our recent drought is that the Mississippi is flowing at only 15% of normal. That means the river is shallower than it would otherwise be, and the debris in the channel isn't creating a massive dam.
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