Even before it opened, the men building the bridge across the Tacoma Narrows had given it a name. The deck heaved so freely in the wind that some workers chewed lemon peel to settle their stomachs, and once cars were allowed on, drivers reported watching the vehicle ahead sink out of sight and rise again, as though riding a long ocean swell. They called it Galloping Gertie. Far from frightening the public, the motion became an attraction: people drove out from Tacoma on breezy days just to feel the roadway ripple. The bridge lasted four months and six days. On the morning of 7 November 1940, in a wind of roughly 40 miles per hour — brisk, but nothing like a storm — the familiar gallop changed into something new, and within hours the third-longest suspension bridge in the world lay at the bottom of Puget Sound.
The bridge was the extreme expression of a fashionable idea. Its designer, Leon Moisseiff, was one of America's most admired bridge engineers, a consultant on the Golden Gate and a champion of "deflection theory", which held that long suspension spans could be far lighter and more flexible than earlier engineers had dared. At Tacoma the theory was pushed to its limit. The main span stretched 2,800 feet, yet the deck was only 39 feet wide and was stiffened not by deep open trusses but by solid steel plate girders a mere eight feet deep. The result was slender, graceful and cheap. It was also, in cross-section, close to the worst aerodynamic shape possible: the solid girders caught the wind like a sail, and the ribbon-thin deck had little resistance to twisting.
On the final morning the bridge stopped galloping and began to twist. The two halves of the roadway rotated in opposite directions, the deck tilting through something like 45 degrees while lamp posts whipped back and forth. Leonard Coatsworth, a local newspaper editor, abandoned his car mid-span and crawled to the towers on hands and knees; his daughter's cocker spaniel, Tubby, refused to leave the car and became the disaster's only fatality, biting the man who tried to rescue him. Barney Elliott and Harbine Monroe, owners of a Tacoma camera shop, filmed the death throes on 16mm colour film. Their footage of the writhing deck, which tore loose and plunged into the Narrows at about eleven o'clock, became some of the most watched engineering film ever shot, later selected for America's National Film Registry.
The myth of resonance
For decades, physics textbooks told a tidy story: the wind's push happened to match the bridge's natural frequency, so the oscillations grew — resonance, the same reason soldiers are told to break step on footbridges, a rule usually traced to the collapse of Britain's Broughton Suspension Bridge under marching troops in 1831. The story is neat, memorable and wrong. Forced resonance needs a periodic push whose rhythm matches the structure's own, and the wind that day was essentially steady; nor did the eddies it shed match the frequency at which the deck was twisting.
What killed Gertie is now called aeroelastic flutter, and it is more sinister than resonance because it feeds itself. Once the deck began to twist, its changing angle to the wind altered the aerodynamic forces acting on it, and those altered forces pushed the twist further. Instead of damping the motion, the airflow pumped energy into it on every cycle — engineers call this negative damping. No rhythmic gusts were required; a steady wind above a critical speed was enough. Flutter was already the nightmare of aircraft designers, who had seen it tear the wings off aeroplanes. Tacoma proved that a bridge could, in effect, fly itself to pieces.
The aftermath had its own dark comedy. An insurance agent who had written a large slice of the bridge's cover had quietly pocketed the premium, gambling that a brand-new bridge could not possibly fall down; he went to prison. The official investigation drew in Theodore von Kármán, the great Caltech aerodynamicist, whose wind-tunnel tests on a model of the deck showed that the shape itself was the culprit. The wreckage was never raised: the old deck still lies on the bed of the Narrows, one of the world's largest man-made reefs, and was added to America's National Register of Historic Places in 1992.
A replacement opened in 1950 — wider, stiffened with deep open trusses and pierced with gaps to let the wind pass through. Locals inevitably christened it Sturdy Gertie, and it still stands. The deeper legacy is procedural: since 1940, no major suspension bridge has been designed anywhere without its deck being tested in a wind tunnel. Galloping Gertie performed for only nineteen weeks, but her ghost sits in on every bridge design meeting in the world.
Quiz nuggets
- The Tacoma Narrows Bridge opened on 1 July 1940 and collapsed on 7 November 1940 — a life of four months and six days.
- Its 2,800-foot main span was the third longest in the world at the time, and its designer was Leon Moisseiff.
- The only fatality was Tubby, a cocker spaniel left in Leonard Coatsworth's abandoned car.
- The cause was aeroelastic flutter, a self-feeding twisting instability — not simple resonance, despite generations of textbooks.
- The 1950 replacement was nicknamed Sturdy Gertie; the original deck remains underwater as an artificial reef, listed as a historic place in 1992.