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This Day in History, 1940

TACOMA BRIDGE COLLAPSES

A favorite of Physics teachers everywhere! Have you ever seen the film?

Only four months after its completion, the Tacoma Narrows Bridge in Washington State suffers a spectacular collapse.


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When it opened in 1940, the Tacoma Narrows Bridge was the third-longest suspension bridge in the world. Built to replace the ferry system that took commuters from Tacoma across the Tacoma Narrows to the Gig Harbor Peninsula, the bridge spanned 2,800 feet and took three years to build. To save cost, the principle engineer, Leon Moisseiff, designed the bridge with an unusually slender frame that measured 39 feet and accommodated just two vehicular lanes.

The Tacoma Narrows Bridge opened with great fanfare on July 1, 1940. Human traffic across the waters of the Tacoma Narrows increased dramatically, but many drivers were drawn to the toll bridge not by convenience but by an unusual characteristic of the structure. When moderate to high winds blew, as they invariably do in the Tacoma Narrows, the bridge roadway would sway from side to side and sometimes suffer excessive vertical undulations. Some drivers reported that vehicles ahead of them would disappear and reappear several times as they crossed the bridge. On a windy day, tourists treated the bridge toll as the fee paid to ride a roller-coaster ride, and the Tacoma Narrows Bridge earned the nickname "Galloping Gertie."

Attempts were made to stabilize the structure, but they were in vain. On November 7, with a steady wind blowing at 42 mph, the roadway began to twist back and forth in an increasingly violent fashion. Before closing the span, the toll keeper on the bridge's west side let one last motorist pass, Tacoma News Tribune copy editor Leonard Coatsworth. Halfway across the bridge, Coatsworth lost control of his car. When the roadway tipped so sharply that it seemed his car would topple off, he decided to flee on foot. He tried to retrieve his daughter's black cocker spaniel from the back seat of the car, but the
dog snapped at him and refused to budge. Coatsworth ran to safety and called the Tribune, who dispatched a reporter and photographer to the scene.

Tribune photographer Howard Clifford was the last man on the bridge before the center span broke off at 11 a.m. and plunged 190 feet into the turbulent Tacoma Narrows. Trapped on the suddenly destabilized side spans, he narrowly avoided being thrown off and ran to safety. The sole casualty of the disaster was the cocker spaniel in Coatsworth's car, which fell into the Narrows and disappeared beneath the foam.

At the time, the engineering community was perplexed about how a bridge designed to withstand winds of up to 120 mph could collapse in a wind of 42 mph. Experts still disagree on the exact cause of the bridge's destruction, but most agree the collapse was related to resonance, a phenomenon that also comes into play when a soprano shatters a glass with her voice. In the case of the Tacoma Narrows, the wind resonated with the natural frequency of the structure, causing a steady increase in amplitude until the bridge was destroyed.

After the Tacoma Narrows disaster, bridge builders took care to incorporate aerodynamics into their designs and build structures with complex frequencies. Wind-tunnel testing of bridge designs eventually became mandatory. A new Tacoma Narrows Bridge was finally erected in 1950, complete with a wider roadway, deep stiffening trusses under the roadway, and other features designed to dampen the effect of wind. In 1992, the remains of Galloping Gertie in the Tacoma Narrows were listed on the National Register of Historic Places.
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Personal experience: In the late 1990s, I went searching on the Internet for information about the bridge, and was surprised to find a page giving real time photographs of the current bridge. I was able to show my students Washington residents on their morning commute...

Comments

I saw the film in physics class in high school. This page has links to some of the films, but I haven't seen the entire film anywhere since 12th grade.
They're building another bridge across the Narrows; too much traffic for the existing one. This will also make it a toll crossing. (Each bridge will carry traffic one way.)

The old bridge's remains are also known as a habitat for octopi.

Washington State also has a number of floating bridges, which have had their own set of issues. The Hood Canal bridge sank in a storm when a hatch on one of the pontoons was blown open and the pontoon filled with water, and one of the Lake Washington bridges killed a driver when the drawbridge mechanism had a short circuit and moved while traffic was flowing.

Incidentally, the Hood Canal bridge situation led to an interesting court fight. The bridge had been a toll bridge, but with the proviso that when the bridge was paid for, the tolls would be discontinued. (Sound familiar to any Mass Pike users?) Since the replacement bridge was paid for by the insurance, it was argued that the new bridge should not charge a toll. During the court battle, you could ask for a receipt in order to get a refund if the tolls were declared invalid, as they eventually were.
I babysit for a nine-year-old whose father is an architectural engineer. One afternoon we got to talking about bridges: the different kinds of suspension bridges and how they approach the porblem of distributing the road's weight. I found photos online of the Brooklyn Bridge, the Golden Gate Bridge, and the new bridge in Boston. Then I remembered Galloping Gertie (I must've seen the short movie in school, but I can't remember when), and found a copy of the movie online. We also read a contemporary news article about its collapse. I was proud to be able to teach the well-informed fourth-grader something that he hadn't already learned from his father or at school.
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