Did Resonance caused by the vibration of a jack hammer being used cause the bridge to collapse?

NNN0LHI

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Wed Aug-01-07 07:10 PM
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Did Resonance caused by the vibration of a jack hammer being used cause the bridge to collapse?

Edited on Wed Aug-01-07 07:18 PM by NNN0LHI

http://www.lightandmatter.com/html_books/3vw/ch02/ch02.html

Chapter 2. Resonance

Soon after the mile-long Tacoma Narrows Bridge opened in July 1940, motorists began to notice its tendency to vibrate frighteningly in even a moderate wind. Nicknamed �Galloping Gertie,� the bridge collapsed in a steady 42-mile-per-hour wind on November 7 of the same year. The following is an eyewitness report from a newspaper editor who found himself on the bridge as the vibrations approached the breaking point.

�Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car... I jammed on the brakes and got out, only to be thrown onto my face against the curb.

�Around me I could hear concrete cracking. I started to get my dog Tubby, but was thrown again before I could reach the car. The car itself began to slide from side to side of the roadway.

�On hands and knees most of the time, I crawled 500 yards or more to the towers... My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb... Toward the last, I risked rising to my feet and running a few yards at a time... Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows.�

The ruins of the bridge formed an artificial reef, one of the world's largest. It was not replaced for ten years. The reason for its collapse was not substandard materials or construction, nor was the bridge under-designed: the piers were hundred-foot blocks of concrete, the girders massive and made of carbon steel. The bridge was destroyed because of the physical phenomenon of resonance, the same effect that allows an opera singer to break a wine glass with her voice and that lets you tune in the radio station you want. The replacement bridge, which has lasted half a century so far, was built smarter, not stronger. The engineers learned their lesson and simply included some slight modifications to avoid the resonance phenomenon that spelled the doom of the first one.

2.1 Energy in Vibrations

http://www.madsci.org/posts/archives/may98/892678504.Eg.r.html

Resonance is the property, which most objects have, of vibrating more strongly when exposed to an external force which is itself vibrating at the object's natural frequency. This is most easily illustrated by using a 440-hertz (A above middle C) fork to tune your guitar. If you were to strike the fork, then hold it close to the strings, a properly-tuned A string (as well as the D string; its first harmonic is an A) would vibrate, driven by the vibrations of the tuning fork. If you did this, then quickly damped the tuning fork, you would -- if your guitar is in tune -- hear the A and D strings sounding a very faint 440-hertz A.

If you were to put enough oomph into the 440-hertz tuning fork, you could cause your A and D strings to snap! This is what is happening when, in the old Memorex commercial, Ella Fitzgerald breaks a wine glass by singing a particular high note.

The old Tacoma Narrows bridge was named Galloping Gertie after its completion in July 1940 because it vibrated rather strongly whenever there was a little wind. Crossing it was like a roller-coaster ride, and Gertie was quite popular. However, on November 7, 1940, a day of rather high winds, Gertie took on a 30-hertz transverse vibration (like sending waves down a rope by moving the end up and down) with an amplitude of 1� feet! It later took on a twisting motion of about 14 hertz and quickly tore itself in two.