Stories tagged vibration

Aug
02
2007

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)
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)
tangled roadwayI’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.

Mar
20
2007

Vibrations create cool patterns

Vibrating surfaces create sound waves. In the video below a plate of metal is vibrated with higher and higher frequencies. Salt spinkled on top of the plate can only stay where the surface is not vibrating. The nonvibrating areas are called nodes. Distance between nodes is shorter for higher frequencies.Standing waves
Standing waves
The patterns you see are known as standing waves. We have an apparatus like this at the Science Museum of Minnesota on the third floor. You might also try to make standing waves in a stretched slinky.

Try doing this yourself

I once stretched rubber from a large balloon over the front of a large car speaker. Using an amplifier and frequency generator, I was able to make similar patterns in salt sprinkled on top of the sheet of rubber. You could try an electric keyboard to produce the different frequencies of sound.

Want to see a longer video?

If you have time here is a link to part 1 of some extraordinary film clips of Hans Jenny experiments from the 1960's and early 70's(28 minutes).