It is, perhaps, the most widely-know bridge collapse this side of London. On November 7, 1940, the nearly-6,000-foot Tacoma Narrows Bridge in Washington state underwent a series of spectacular oscillations during a wind storm before dramatically collapsing into Puget Sound.
The spectacular event was captured by four different people with 16mm film cameras. The resulting footage gained widespread fame and was viewed the world over.
Now, for the 75th anniversary of the Tacoma Narrows Bridge collapse, a team of researchers from Texas State University has uncovered a historical error related to that film footage that has skewed perceptions of the disaster for decades.
Texas State physics faculty members Donald Olson and Russell Doescher, Texas State physics undergraduate Joey Hook, along with East Carolina University physicist Steven Wolf, publish their findings in the November 2015 edition of Physics Today, with an expanded article in the November 2015 issue of The Physics Teacher.
Bridge to Oblivion
The original Tacoma Narrows suspension bridge opened with much fanfare on July 1, 1940. On November 7 of that year, however, a low pressure system approached the Washington coast and blasted the bridge with 40-mile-per-hour winds. That violent weather system continued on to the Midwest, where it became known as the Armistice Day Storm, killing 145 people in the region and sinking two freighters on Lake Michigan.
The bridge had earned the nickname "Galloping Gertie" the months prior because of a tendency to oscillate up and down even during mild winds. Shortly after 10 a.m. on the day of the collapse, the oscillations suddenly turned into a torsional, twisting motion. At 11:02 a.m., the stresses became too much for the structure and a 600-foot section of the roadway broke loose and fell into Puget Sound. By 11:10 a.m., most of the long center span had followed.
In 1961-1963, Franklin Miller converted original 16mm footage of the collapse into an 8mm film loop that was shown in physics classrooms across the country into the 1980s. By 1982, video had largely supplanted film, and that 8mm film loop was converted--along with additional archival footage--to video.
Because video formats--videodiscs, VHS tapes and DVDs--operate at a rate of nearly 30 frames per second (fps), and 16mm film could operate at 12, 16, 24 or even 36 fps, the conversion was not straightforward. Throw in the fact that the 8mm film loop operated at 18 fps, and the transfer grew even more complex. Assuming the existing 16mm film footage had a running speed of 24 fps, the technicians converted every four film frames into five interlaced video frames to make up the fps shortfall.
Conversion of Errors
By timing the torsional oscillations, the Texas State researchers determined the bridge goes through 18 twisting cycles per minute on the existing video. Stopwatch measurements taken on November 7, 1941, however, timed the bridge cycles at 12 per minute--a significant discrepancy. The Texas State researchers were able to prove that the original 16mm camera that filmed the oscillations was running at the slower 16 fps, not the 24 fps assumed when the conversion to video was done. When the film frames are viewed at the slower speed, the torsional cycles match the eyewitness stopwatch measurement of 12 cycles per minute.
Because of mistaken assumptions made during conversion of the film in 1982, viewers of the modern video have been viewing a sped-up version of the scene, with the bridge twisting and bucking much more wildly than the oscillations seen by eyewitnesses on November 7, 1940.