MadSci Network: Engineering |
What caused the bridge Galloping Gertie to collapse?
One "hertz" is one cycle per second.
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 1½ feet! It later took on a twisting motion of about 14 hertz and quickly tore itself in two.
Wind-tunnel testing of bridge designs is now used to insure against similar disasters.
Information in this answer was taken from this excellent site, which includes both detailed explanations and video clips. Here's another, smaller site.
Dan Berger | |
Bluffton College | |
http://cs.bluffton.edu/~berger |
Try the links in the MadSci Library for more information on Engineering.