A Physics Lesson From Tragedy: The Original Tacoma Narrows Bridge Opened 81 Years Ago

On July 1st, 1940, a suspension bridge over the Tacoma Narrows, aptly named the Tacoma Narrows Bridge, opened to automobile traffic. (Location) The bridge connected Tacoma, WA the Kitsap Peninsula over Puget Sound, originally carrying Primary State Highway 14, which is present-day Washington Route 16.

Tacoma Narrows Bridge Opening, 1940. Image: Washington State Department of Transportation.

The bridge was a grand engineering marvel, as it was the third largest suspension bridge in the United States, only behind the Golden Gate Bridge in San Francisco and the George Washington Bridge between New York and New Jersey, each of which is still standing today.

However, even before the bridge was completed, workers noticed its tendency to sway in the wind. Efforts were made to correct this behavior in the bridge, but these attempts proved to be futile. One reason was the bridge itself was quite narrow - apologies to the name. It was just two lanes (one lane in each direction) wide, as traffic planners anticipated the bridge would have low traffic volumes, and at the time they were apparently not worried about building for the future. This narrow roadway made the bridge insufficiently rigid to prevent the vertical swaying that was occurring.

While some minor swaying in bridges and other large structures, such as skyscrapers, is part of the design to help withstand heavy winds, the swaying on the Tacoma Narrows Bridge was severe enough that drivers often saw the road rise several feet many times. One could even feel the bridge's sway on its opening day.

This came to a head in November. A sustained wind event on November 7th, 1940 forced the closure of the bridge, and it is thankful that authorities did so, as on that day, just over four months after opening, the bridge would collapse after swaying violently back and forth. 

The collapse of the Tacoma Narrows Bridge. (Library of Congress LC-USZ62-46682)

A reporter named Leonard Coatsworth was the last person to drive the bridge, and as his car became undriveable in the swaying, he was forced to walk by foot from the vehicle. "Around me I could hear concrete cracking. I started back to the car to get the dog, but was thrown before I could reach it. The car itself began to slide from side to side on the roadway. I decided the bridge was breaking up and my only hope was to get back to shore. 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… Towards 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". His dog, still in the car, was the only fatality in the event. The dog nor the car were ever recovered after the wreck.

One of the reasons the bridge collapse was so well studied is that it was captured on film, and was distributed shortly after the event for presentations on newsreels of the time.

You may have heard of this bridge from physics textbooks, since its collapse is often cited as a negative consequence of not factoring in mechanical resonance in its design, but the accident is more complicated than that, with aeroelastic flutter being the main cause of the crash. Basically, any sustained wind speed above 35 miles per hour would make the bridge continue to sway until it eventually tore itself apart. (A more technical explanation is available here)

The sunken remains of the 1940 bridge were added to the National Register of Historic Places in 1992 to protect it from salvagers, with the nickname "Galloping Gertie" mentioned, having been christened to the bridge during its construction, after workers noted its ability to move in the wind.

After the collapse in November, a replacement bridge was quickly proposed, but was delayed when the United States entered the Second World War, and building materials and steel became a premium commodity. Thus, the replacement bridge was delayed and wouldn't reopen until 1950.

1950 Photo of the second Tacoma Narrows Bridge. (BridgeHunter)

The 1950 Bridge still stands to this day, although it now only carries Westbound traffic, as opposed to two-way traffic. The eastbound span adjacent to it was built in 2007.

The collapse of the Tacoma Narrows Bridge, like the collapse of every bridge, are lessons that civil and structural engineers can learn from to design bridges safer. Unfortunately, the relative safety of bridges today is built on the failures of yesterday.

Thanks as always for reading!


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