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:
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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