On the day of the collapse -- known as the Pearl Harbor of Bridge Engineering -- Gertie was galloping fast and hard. Leonard Coatsworth, a Tacoma reporter, was driving across the bridge with his dog Tubby in the car. Here is his account of what happened:. Professor Farquarson was there doing his measurements and ran out and tried to save Tubby, but the dog bit him and he gave up the effort.
Tubby was the only fatality. The cause of the failure was solid girders, which took wind and acted like sails girders with perforations would have let the wind pass through.
Also, the bridge was not stiff enough or heavy enough to withstand the wind of the Tacoma Narrows. The collapse terminated Moisseiff's career and he died less than three years later. Clark Eldridge, who accepted some of the blame, took work with a San Francisco contractor working for the U. Navy on Guam. At the outbreak of World War II, he was taken captive by Japan and became a prisoner of war for three and a half years.
After the war Eldridge returned to Washington state and resumed work as a consulting engineer and contractor. The collapse reverberated as a personal tragedy in the lives of both men. See also Richard S.
Note: This essay was expanded on November 15, , and corrected on November 12, A Dream Come True On July 1, , a clear day with blue skies, some 10, people turned out for the dedication and opening of the bridge. The cause, it was later determined, was flutter. Flutter on airplane wings is significantly different than flutter on bridges, however, aerodynamically speaking. Wings experience much greater air speeds than bridges do, and their flutter behavior arises from the response of the wing to the aerodynamic forces affecting it.
In the case of bridges like Gertie, which experience relatively slower wind speeds and slower vortex wakes, aerodynamic forces are not a driving factor for bridge flutter.
While these forces can nudge the bridge—making it even harder for it to dampen its vibrations—these aerodynamic forces pale in comparison with the forces of the bridge as it bends and twists. The flutter effect on Gertie that led to catastrophe lasted for about 45 minutes; a wing can shake itself apart in seconds.
But just as feedback can be useful in music, flutter isn't always destructive. Inventor Shawn Frayne has attempted to exploit it for energy. His Windbelt , which debuted in , uses the principles of flutter and negative damping to generate electricity in high wind.
The key component is a taut membrane designed to flutter. Whereas a quantitative definition of resonance involves an external energy source causing the vibration, flutter, by contrast, is considered a kind of instability in the structure itself, a flaw resulting from the free response of the structure when exposed to air flow.
Nevertheless, resonance is no small concern for people who build bridges and buildings and airplanes and anything that shakes. When an opera singer's voice meets a pane of glass at its natural frequency, we not only hear the effects of this resonant vibration, but ultimately see them: the glass shatters. The glass is exhibiting a tendency common to any moving system: it absorbs the energy of the singer's oscillating voice.
This is known as forced harmonic oscillation. But when the frequency of those oscillations match the glass' natural frequency of vibration—its resonant frequency—it absorbs dramatically more energy than usual. When it can't absorb any more, it collapses.
On certain bridges, oscillations can be caused by the periodic force of people moving across it. In April , a battalion of French soldiers was crossing the Angers Bridge when it collapsed , killing over of them. The culprit, it was theorized, was the lockstep march of the soldiers, creating enough of a periodic force to match that of the bridge.
Since then, it has been standard practice for soldiers to break step when crossing bridges. On opening day in , London's Millennium Bridge felt the resonant effects of pedestrians swaying in time to its own sway. Something similar happened on June 12, , the opening day of London's Millennium Bridge. As a mass of pedestrians crossed the short steel suspension span, they created a different kind of oscillation: not up and down but sideways.
According to a study by Steven Strogatz, a mathematics professor at Cornell University, the bridge was moving in lockstep with the slight lateral motion of all the people crossing the bridge who were themselves inadvertently swaying as they walked. As the bridge began to sway ever so slightly in one direction due to winds, the pedestrians naturally leaned slightly in the other direction to keep their balance, and, not unlike soldiers moving in lockstep, they did this at exactly the same time.
Eventually their swaying reinforced the movement of the bridge as it swayed. Still, the nickname "The Wobbly Bridge" stuck. The authorities closed the bridge and later that year installed tuned mass dampers to prevent future oscillations. Shortly afterward, officials speculated that strong river currents caused by melting snow upstream had loosened one of the bridge's vertical supports.
The most likely culprit, physicists suspect, were the sort of aeroelastic forces that had caused Gertie to gallop—but not collapse—in a gale. To avoid so-called "resonance disasters," engineers of machines with engines must ensure that the frequencies of the component parts do not match each other. Like bridges, buildings and trains and other large spans use shock mounts to absorb resonant frequencies and dissipate the absorbed energy.
At buildings like Taipei , which is located on an active seismic zone, a ton pendulum—a tuned mass damper—is designed to absorb the effects of resonant frequencies.
In , a story building in Seoul had to be evacuated after vertical tremors began shaking it violently for about ten minutes. At first this puzzled engineers. Their Tae Bo workout was apparently twice as intense that day, making their fancy footwork synch up with the building's structural resonance. In describing other examples of resonance, the Motherboard article regurgitated the fallacy that Galloping Gertie was wrecked by resonant frequencies.
A correction has been issued. But there are possibly thousands of other un-corrected articles and books, including in some from very large and seemingly reputable sources , including Encylopedia Britannica and the Harvard math department.
But even in the years following the collapse, the Federal Works Agency Commission report of the ensuing investigation found that it is. It was found that there is no sharp correlation between wind velocity and oscillation frequency such as is required in case of resonance with vortices whose frequency depends on the wind velocity. At a glance of the edited footage, it's tempting to think that Gertie was brought down by resonance, given the vivid visual evidence of a bridge that undulates before it collapses, not unlike the wine glass that shatters under the vibrations of a singer's voice.
In fact, resonance is a similar phenomenon to flutter, to the extent that it involves the "reinforcement" of existing oscillations and can lead to a dramatic and possibly destructive amplification of energy. Bilah and Scanlan write that the phenomenon acting on the bridge "would appear not to contradict the qualitative definition of resonance… if we now identify the source of the periodic impulses as self-induced [rather than external], the wind supplying the power, and the motion supplying the power-tapping mechanism.
That is not how forced resonance is typically described the matching of an external force's oscillations with that of another object , or what is implied when people blame only the force of the wind for the bridge's collapse. But the resonance explanation has persisted, thanks to repeated mistakes by physics teachers and textbook writers and science journalists, and buttressed by the convenience of the video evidence.
The bridge's engineers had forgotten many lessons from the early days of suspension bridges. Somehow, the media, teachers, and scientists misremembered the new lessons. How did the incorrect explanation persist for so long? In their paper about the event, Bilah and Scanlan cite 30 sources that mention resonance as a cause of the bridge's failure.
Ultimately, they point their fingers at a mix of rough, semi-empirical guess work and the "telephone" effect. It's easy to see why: the math and physics involved can seem complicated. And the unforgettable image—a bridge undergoing large periodic motion as an external force applies energy to it until it collapses—is, to physics teachers and textbook writers, an irresistible scientific example, an eye-popping way to wake up the kids at the back of the classroom. An early assertion that resonance was to blame appeared in that New York Times story two days after the collapse.
Resonance has been named by the Times as a culprit in the Tacoma Narrows collapse three times since that story. But strangely, a story published on page 1 the previous day included a more accurate account of the collapse that had nothing to do with resonance.
Andrews, one of the bridge's engineers, pointed to the closed stiffening trusses along the sides of the bridge's deck. The wind hitting them "caused the bridge to flutter, more or less as a leaf does, in the wind. That set up a vibration that built up until the failure occurred. In mainstream and science media, however, that idea got drowned out by resonance. The textbooks written by David Halliday and Robert Resnick in the early s included photographs of the Tacoma Narrows Bridge in its section on resonance, and concluded that the "wind produced a fluctuating resultant force in resonance with a natural frequency of the structure.
The professors speculate that Edson mistook the vortices that induced the earlier rolling motion with the ones that began to form as the bridge twisted higher and higher: "We see the flutter vortex trail as a consequence, not a primary cause. In this version of the bridge film originally published by Franklin Miller, the first caption erroneously describes its "resonance vibration". Meanwhile, in a report to accompany an educational videodisc, the American Association of Physics Teachers points a finger at Franklin Miller , who published and distributed the first and most famous footage of the collapse for use in classrooms around the country.
The term "resonance vibration," notes the AAPT, was "indeed erroneously used in one of the captions in the film first edited in There is a crucial irony in the AAPT's paper. Its authors failed to note that the footage contained in the accompanying DVD is itself erroneous. While Monroe shot at 24 frames per second, Elliot had switched his camera to run at 16 frames per second, possibly to preserve film.
According to a study published last month in Physics Today by Donald Olson and colleagues at Texas State University and East Carolina University , the films were converted to early film reels for classrooms as if they both ran at 24 frames per second. This led to a pair of innocent but crucial conversion errors that have since been immortalized on the AAPT's DVD and on other film reels, videotapes and websites. The film shot from the shore is roughly accurate, showing the bridge's slow swaying and twisting and eventual collapse.
But in many versions, the footage Elliot shot on the bridge makes the movement look about 40 percent faster than it really was. In the footage, the oscillations of the bridge appear to be about 18 cycles per minute, but Farquharson's own stopwatch that day measured a torsional frequency of 12 cycles per minute.
The first conversion mistake happened in the early s, when the film was converted for use by Franklin Miller in a series of physics classroom film loops that played at 18 fps in 8mm projectors. The second error happened in , when three scientists used Miller's loops to produce the AAPT's videodisc, The Puzzle of the Tacoma Narrows Bridge Collapse , which contains Miller's film, additional archival film footage, and interactive material.
Due to the conventions of US and other TV signals at the time, the format operated at 30 fps. By examining every frame of the videodisc sequences, he and his colleagues observed that the technicians made the leap from 24 to 30 fps by "stretching" every 4 frames into 5 frames through a technique known as telecine, which aims to make the converted video appear natural and at normal speed. They later confirmed this theory with one of the video's producers.
As a result, they write, "viewers of the modern video formats see the torsional oscillations significantly sped up 18 cycles per minute , compared to… the more majestic and lower frequency oscillations 12 cycles per minute measured by eyewitnesses on Nov. The powerful video evidence coincides with a particular tendency of ours, I think: to interpret what we see according to what we expect to see. This phenomenon, sometimes described as "the observer effect," is so feared by scientists that various methods have been developed to prevent it.
And yet it lingers, influencing experiments and everything else. After the collapse, authorities pledged to rebuild the bridge immediately. But the war effort changed all that: the bridge's side spans were melted down for steel, a valuable commodity for the country's military machine.
The remains of its once-undulating deck were left under the waves of Puget Sound, where they now form a giant artificial reef. In , "Galloping Gertie" was placed on the National Register of Historic Places, in part to deter scavengers, and in part to part to memorialize an event that has been misremembered if not forgotten. In general, for scientists and engineers, Gertie's failure represents progress.
And understanding the other failures—how for so long so many failed to accurately describe or depict why she collapsed—yields yet more lessons. When you watch the footage, it turns out that what's really going on is hard to see. And it's made even harder when you're told--perhaps against your own intuition--that resonance was to blame. Vibrations helped break apart the bridge, that's clear. But being vague about how those vibrations originated—or being sloppy about how they are represented on film—can easily lead to inaccuracies and misrepresentations and the loss of important details, however tiny they may be.
But the engineers examining the collapse weren't swayed by how the collapse looked or by what they heard. The Tacoma Narrows' replacement, completed in , was a major on advance on its predecessor, with open foot stiffening trusses compared with Gertie's 8-foot trusses , wind grates and hydraulic shock absorbers nickname: "Sturdy Gertie.
Today the two bridges are now the 38th longest bridges in the world. Speaking of engineering concerns, in general, Washington is now emblematic of the U. A year after the collapse, David L.
Glenn, the PWA's field engineer, revealed that he had not signed off on the bridge when it was completed in July He had submitted a report warning of faults in design and refused to recommend acceptance of the structure. The situation that gave rise to the original bridge's risky design echoes patterns that existed prior to the explosion of the Space Shuttle Challenger and the GM ignition switch debacle.
Cultures of groupthink discourage dissent, stepping up, speaking out, admitting fault, and making redesigns even when they are essential. Those institutional problems are all the more scary because the big new things institutions produce often come with flaws that are unexpected and can be hard to fix. You can design for what you know, but you might not design for what you don't.
Not just the unknown knowns, to borrow a burdened phrase but the unknown unknowns. Motorists crossing the 2,foot center span sometimes felt as though they were traveling on a giant roller coaster, watching the cars ahead disappear completely for a few moments as if they had been dropped into the trough of a large wave.
The original bridge was a suspended plate girder type that caught the wind, rather than allowing it to pass through. As the wind's intensity increased, so did Gertie's rolling, cork-screwing motion -- until it finally tore the bridge apart. Then after 29 months of construction, a new, much safer Tacoma Narrows Bridge opened on Oct. The new bridge spans 5, feet -- 40 feet longer than "Galloping Gertie" -- and is part of Hwy.
0コメント