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Tacoma Narrows Bridge

The Tacoma Narrows Bridge was a mile-long suspension bridge across the Tacoma Narrows of Puget Sound near Tacoma, Washington in the United States. Its spectacular self-destruction provided an object lesson in the dangers of resonance effects in engineering.

The bridge itself was solidly built, with girders of carbon steel anchored in huge blocks of concrete. However, shortly after its construction in July 1940, it was discovered that winds through the Tacoma Narrows were able to set up a resonance in the bridge, causing it to sway and buckle dangerously in windy conditions. From this behavior the bridge gained the nickname, "Galloping Gertie."

It has been suggested more recently, as a product of careful physical research, that though resonance may have played a part in the collapse, the main cause of the collapse was actually the simple aerodynamic forces of wind. No matter what the frequency or position of the wind pressure gradient, the bridge would still have collapsed.

In November 1940, during one such period of high winds, the waves built up in the bridge were amplified to the extent that the bridge's structure collapsed under the stresses. From the account of a driver stranded on the bridge during this event:

"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... 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 final destruction of the bridge was recorded on film. The Tacoma Narrows Bridge Collapse (1940) is preserved in the US National Film Registry, and is still shown to engineering students as a cautionary tale.

Several oscillation modes were in the bridge's vibration. The mode it vibrated in when it collapsed was the second torsional mode, which allowed the cable to stay the same length, one half going up while the other went down. The collapse started with the cable loosening.

The bridge has since been redesigned and rebuilt with stiffening struts and openings in the roadway to let wind through.

External link: The Complete Tacoma Narrows Bridge Info Center (http://www.firebirdz.net/tnb/)



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