Oct 17 2016
The Rainbow Bridge in southern Utah is a bridge of solid rock carved by water during the last ice age. At 290 feet tall it’s one of the highest natural bridges in the world and held sacred by Native Americans in the region. But it doesn’t stand still. Like all structures it moves in response to vibration.
Last year the University of Utah obtained permission to measure resonance at the Rainbow Bridge. Their report, published in Geophysical Research Letters, found that the bridge is affected by both natural and human activity. Wind can make it hum. Waves from man-made Lake Powell on the Colorado River, only a mile away, make it sway a little. During two days of measurements the bridge felt three earthquakes, one of which was a man-made earthquake in Oklahoma.
The report includes this video of the Bridge’s eight modes of resonance. The animation is exaggerated so you can see the movements. “Mode 7” at the 0:23 mark looks positively scary!
Read about how the Rainbow Bridge moves at Resonance in Rainbow Bridge: University of Utah study listens to the natural bridge vibrate and sing.
And speaking of scary bridge movements …
Man-made bridges are engineered to move just a little in response to wind and other forces, but they mustn’t move too much or they break. In a spectacular case of poor design the Tacoma Narrows Bridge, built to withstand 120 mph winds, collapsed on 7 Nov 1940 in a 40 mph wind only four months after it opened. The cause was attributed to resonance, though more recently to aeroelastic flutter. Watch the bridge collapse below. Read the whole story here.
Rainbow Bridge from Wikimedia Commons, click on the image to see the original.
video of Rainbow Bridge from University of Utah
Video of Tacoma Narrows Bridge collapse by Barney Elliot, 7 Nov 1940 via Wikimedia Commons)