A 40-mile-an-hour wind twisted the Tacoma Narrows Bridge into Puget Sound on November 7, 1940, four months after it opened; the federal board that studied the wreck blamed the wind, and engineers still argue about how it worked

On the morning of November 7, 1940, a new suspension bridge across the Narrows of Puget Sound twisted itself into the water. It was not a hurricane that did it, and it was not a sudden snap.

Why did the bridge move in the first place?

The Tacoma Narrows Bridge opened to traffic on July 1, 1940, and it had a habit that workers noticed before a single car crossed. Its deck rippled up and down in light winds, a motion the crew nicknamed "Galloping Gertie."

Nobody had seen a deck do that before. The motion looked alarming without being dangerous, until it was.

The rippling did not stop when the bridge opened to the public. Traffic kept crossing a structure that moved under its own weight and under the wind, and the people who drove it had no way to know which movements mattered.

What happened on November 7?

The wind readings that morning began at 38 miles an hour at 7:30 a.m., and two hours later engineers clocked 42 miles an hour near the east end of the span, according to the Washington State Department of Transportation's history of the bridge. Some accounts round the whole event to 40.

At 10:03 a.m. the roadway began a lateral twisting motion, and it never settled back. The transportation department ties the switch from up-and-down motion to twisting to the slip of a cable band at midspan on the north cable. Each swing carried the deck further than the one before, tilting one way and then the other with growing amplitude until the structure could no longer hold together.

At 11:02 a.m. a 600-foot section of the eastern half of the center span broke free. By 11:10 a.m. the bridge was gone. The collapse took about an hour from the first twist to the last, which is long enough for a person on the span to notice that something was wrong and short enough that most people on it had no time to act.

Who was on the bridge that morning?

Leonard Coatsworth, a news editor for the Tacoma News Tribune, drove onto the span with a black cocker spaniel named Tubby in the back seat. Accounts disagree about whose dog Tubby was: the state transportation department says it belonged to Coatsworth's daughter, while one popular retelling reports that his wife said it was hers.

Coatsworth got out of the car, and he was not the only person who tried to save the dog. Howard Clifford, a photographer, ventured onto the center span toward the car and had to turn back.

F.B. Farquharson, a University of Washington engineering professor who had come to watch the bridge move, reached the car at about 10:55 a.m. and decided to try. When he reached inside, the terrified dog bit his finger, and he retreated to safety moments before the span fell (per the transportation department's account). The three men lived. Tubby did not, and the dog is the only death on record.

Did resonance bring it down?

Physics textbooks have long taught the collapse as resonance, a story in which wind sets the deck swinging at its natural frequency until something gives. Billah and Scanlan argued in 1991 that this version oversimplified both the physics and what the engineers understood at the time. The example most people reach for is a column of soldiers marching in step across a bridge, a story whose details change with each telling, and it is easy to see why the simpler picture stuck.

The engineers who studied the wreck said something narrower. A federal board whose members included Othmar Ammann, Theodore von Kármán and Glenn B. Woodruff reported in 1941 that the vertical oscillations were probably induced by the turbulent character of the wind, and that the torsional motion, once it started, tended to increase in amplitude. The report, The Failure of the Tacoma Narrows Bridge, was submitted to the Federal Works Agency after the collapse, and it set the terms for the debate that followed.

What is still argued?

Billah and Scanlan's case is that the failure was self-excited torsional flutter, an aerodynamic instability in which the deck feeds energy into its own twisting. On their reading, vortex shedding could not have driven the twisting mode, because the twist did not match the rhythm of the shed vortices. The wind, in other words, did not need to keep time with the bridge for the bridge to keep moving.

Later work on nonlinear structural models has put more weight on the structure itself, and no single explanation has won unanimous acceptance among the researchers who study it. The tidy version is wrong about the physics and thin about the record, and the field has not agreed on a replacement.

My own judgment is that flutter is the better account, because it explains why the motion grew rather than merely continued. A resonance story can describe a bridge that shakes; only a feedback story describes one that tears itself apart in an hour.

Why does it still matter?

Every long span built on a windy coast still faces the question the Narrows left open in 1940: how far a deck can move before the motion starts feeding itself. The engineers who design those spans now work from the wreck, not from the textbook picture, and the difference shapes how they test a deck before it carries a single car.

The bridge's failure remains the case students meet first, which is why the dispute over it matters beyond Puget Sound, and why a bridge that lasted four months still shapes how a span is drawn on a drafting table today. Anyone who signs off on a span over water is asking the same question the Narrows asked that November morning: how much motion before the wind takes over?

Sources

• Washington State Department of Transportation, Tacoma Narrows Bridge History: Collapse (undated)
• Washington State Department of Transportation, Tacoma Narrows Bridge History: Tale of Three Bridges (undated)
• Billah, K. Yusuf and Scanlan, Robert H., Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks (1991)
• Structure magazine, Revisiting the Galloping Gertie (undated)
• Today I Found Out, The Only Victim of the Tacoma Narrows Bridge Collapse Was a Three Legged Dog (2012)
• Wikipedia contributors, Tacoma Narrows Bridge (1940) (undated)