Tacoma Narrows Bridge Is Falling Down
When engineers make mistakes, the results can be both spectacular and expensive. And if someone happens to be at the right place at the right time with a movie camera, immortality of the sort no engineer wants is inevitable. The collapse of the Tacoma Narrows Bridge on the morning of November 7, 1940, only four months after it opened (and 67 years ago today), is the perfect example. The film of itshot by the owner of a local camera store, Barney Elliott, made newsreels all over the world and has appeared endlessly on television and in physics classes ever since. It ranks right up there with the destruction of the Hindenburg, Pearl Harbor, and, more recently, the eruption of Mount St. Helens on the list of all-time great film clips.
The need for a bridge across the waterway separating the city of Tacoma from the Kitsap Peninsula, at the bottom of Puget Sound, had been clear since the late nineteenth century, but only in the 1920s did serious planning begin. The Great Depression dealt the plans a setback, but in 1937 the state legislature created the Washington State Toll Bridge Authority and appropriated $25,000 to study the Tacoma Narrows project.
The resulting design called for a bridge 6,000 feet long, with a center span of 2,800 feet. Since the bridge was intended to accommodate only two lanes, its width was just 39 feet, making for an elegantly narrow design of the kind that was much in fashion in the 1930s. But it also made for a very light bridge. For comparison, New York’s George Washington Bridge, completed in 1931 and the longest in the world at the time, was eight lanes wide, and its roadway weighed 31,590 pounds per linear foot. The Tacoma Narrows Bridge weighed only 5,700 pounds per foot. Its lightness made it much more vulnerable to strong winds.
In addition, suspension bridges need to be stiffened to distribute evenly the weight of vehicles passing over them. The stiffening is provided by girders running along each side of the roadway. For aesthetic reasons, the engineers of the Tacoma Narrows Bridge wanted the stiffening to be as shallow as possible, to keep the bridge light and graceful. Unfortunately, it was far from clear in the 1930s how much stiffening was the minimum consistent with safety.
For many years, the rule of thumb was a length-to-depth ratio of 150 or less. The Williamsburg Bridge in New York City, for example, is perhaps the only truly ugly suspension bridge in the world, but it’s safe; the Williamsburg’s massive stiffening provides a length-to-depth ratio of just 40. Originally, the depth of the Tacoma Narrows Bridge’s stiffening girders was designed at 25 feet, which would have given it a ratio of 112 (the length in this case is that of the center span, i.e., the part between the towers). But in order to save money and maximize the bridge’s slender profile, a consulting engineer, Leon Moisseiff, argued that stiffening only 8 feet deep would be enough. The state’s chief engineer objected, but the federal Public Works Administration, which was largely funding the project, sided with Moisseiff. The eight-foot depth gave the bridge a length-to-depth ratio of 350, way below the standard.
In addition, instead of using trusses, a series of open steel triangles pointing up and down, as stiffening, it was decided to use plate girders, solid sheets of metal. Plate girders are cheaper than trusses, but they have much more surface area—they are “bluff” in engineering terms—and thus catch the wind much more.
In hindsight, it is clear that the Tacoma Narrows Bridge was a disaster waiting to happen. It didn’t take long.
Early on in its short career, the bridge acquired the nickname Galloping Gertie because it tended to undulate along its length in any appreciable wind. Drivers reported seeing oncoming cars appear and disappear because of the wave-like motion of the bridge. While means were discussed for lessening this effect, almost no one, either engineers or the general public, thought a collapse was likely. Indeed, the insurance agent who had written an $800,000 policy on the bridge for the state was so confident of the bridge’s integrity that he pocketed the $70,000 premium instead of forwarding it to the insurance company. He went to jail for two years. Other insurance policies covered most of the bridge’s cost.
On the morning of November 7, winds across the strait reached near gale force, about 40 miles an hour. The bridge undulated, but no more than usual, and cars crossed it uneventfully. At 10:00 A.M., Leonard Coatsworth, a news editor for the Tacoma News-Tribune, drove onto the bridge. He was headed for his family’s vacation home and had their dog, Tubby, a black spaniel, in the back seat. Suddenly the bridge began to exhibit an entirely new kind of motion. In addition to undulating along its length, it began to twist from side to side. This was far more dangerous, for as the wind continues to blow, the amplitude of the twisting increases until the wind decreases or something gives under the strain.
Coatsworth was the last person to drive onto the bridge. His car became uncontrollable, and when he got out, he was immediately flung to the pavement. “Around me I could hear concrete cracking,” he reported. “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." He crawled on his hands and knees, which were soon bleeding and raw, back to the toll plaza.
Two workers who had been stationed inside the bridge’s east tower hurried to safety when the twisting started, while at the west end of the bridge a pair of delivery men jumped from their van just before the tilting roadway knocked it onto its side. They clung to the pavement, afraid to move. After a nerve-wracking delay, workmen backed a truck onto the bridge and carried them to safety.
Word began to spread that the bridge was in trouble. The Tacoma News-Tribunedispatched a photographer, Howard Clifford, and a reporter, Bert Brintnall. Barney Elliott left his camera shop with a 16mm movie camera and hurried to the site. Also among the growing crowd of onlookers taking motion pictures and still photographs was Professor F. Bert Farquharson of the University of Washington. He had been hired before the bridge officially opened to investigate ways of taming its oscillations, and he happened to have chosen this day to shoot footage for his engineering studies.
A little after 10:30, the wind died down briefly. Clifford, who had been shooting from the toll plaza, tried to go to Tubby’s rescue, but he couldn’t get to the car. He had to tuck his camera under his arm like a football and run back, often getting flung to the ground. His legs were black and blue from the hips down. Barney Elliott had been on the span earlier taking movies, but he also returned to the toll plaza.
Around 10:55 Farquharson, who of all people should have known better, made another attempt to rescue Tubby. He retreated when the presumably terrified and disoriented dog bit his finger. Farquharson reached safety just in time, for shortly after 11:00 a.m. the structure reached the limits of its strength. The suspender cables, which hung from the main cables and supported the roadway, began to snap one by one. When the weight of the roadway exceeded what the remaining supporting cables could bear, 600 feet of it tumbled into the water below. It took with it Leonard Coatsworth’s car and poor Tubby, the only fatality in what, thanks to Barney Elliott, is one of the most famous, if least lethal, disasters in American history.
As always with engineering failures, lessons were learned. Engineers became a lot more cautious about pushing the limits of suspension-bridge design. Many recently completed bridges, such as New York’s Bronx-Whitestone Bridge (another Moisseiff design), finished in 1939, had their stiffening reinforced to lower the ratio of length to depth. No major suspension bridge has suffered a serious structural failure since, and with modern computers able to test new designs with a high degree of certainty, another Galloping Gertie is very unlikely.