Wednesday, November 7, 2007

Galloping Gertie - the Tacoma Narrows Bridge collapses [1940]

It's never too late to remember past errors (and hopefully learn from it):

1940: The newly completed Tacoma Narrows Bridge collapses during a windstorm. The collapse is filmed, helping it to become one of the more conspicuous engineering failures in American history.

The suspension bridge, spanning Puget Sound in Washington state between Tacoma and Gig Harbor, was the third longest in the United States when it opened to traffic on July 1, 1940, three years after the Golden Gate Bridge.

Following the practice of the day, the Tacoma Narrows Bridge was built to be aesthetically pleasing as well as structurally sound. Its towers were slender and graceful, and the narrow roadbed and its shallow trusses made the span very flexible. Too flexible, as it turned out. The lack of longitudinal rigidity led directly to the collapse.

The bridge's tendency to sway, even in moderate winds, earned it the nickname "Galloping Gertie" and motorists came from miles around to "get seasick" on the drive across.

Concern was expressed about the bridge's instability even during construction, resulting in the installation of hydraulic buffers to control stresses, but no one could have predicted the catastrophic failure that occurred on Nov. 7.

That morning, the center span began swaying between three and five feet in winds of only 35 to 45 mph. The bridge's construction and the effects of the winds upon it created a phenomenon known as harmonic vibration, that caused the center span to roll. It was severe enough to alarm officials, who closed the bridge to traffic at 10 a.m. The violent twisting continued -- a support cable at center-span snapped and the span was now undulating up to 28 feet -- and pieces of the roadbed began breaking off at around 10:30. The center span finally collapsed just after 11 a.m. and plunged into Puget Sound.

There were no human casualties but a dog, left behind in the only car abandoned on the bridge, perished. The remains of the bridge were left on the bottom of the Narrows, where they continue functioning as a reef.

As a result of the collapse, the methods for testing and building bridges -- including the new Tacoma Narrows Bridge that opened in 1950 -- were radically changed. Further studies also led to new theories of vibration, aerodynamics, wave action and harmonics as they apply to bridge design.

(Source: Northwest Rain)


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