One of the main purposes of the Tacoma Narrows bridge being built was for quicker transportation of military equipment & personnel between Tacoma & Bremerton's military bases. The 1940 Narrows Bridge was built "primarily as a military necessity" to link McChord Air Field south of Tacoma and the Puget Sound Navy Shipyard in Bremerton. This important fact is often is often overlooked today. But it was well known to area residents and local newspapers in 1940. The Narrows Bridge proponents had strong support from the United States Navy, because of its shipyard in Bremerton, and the Army, because of its installations at McChord Field and Fort Lewis.
The official "start date" according to the construction contract was November 25, 1938 and proceeded at breakneck speed; something however, was amiss. Once the deck had been built the construction workers saw that it would move up and down whenever there was a strong wind. Henceforth the construction workers and eventually the wider public, came to know the bridge as Galloping Gertie.
Vertical oscillations of the roadbed occurred even during the construction phase and raised questions about the structure's stability. Some breezes as low as four miles per hour caused oscillations, while stronger breezes often had no effect. Prior to the bridge's opening on July 1, 1940, hydraulic buffers were installed at the towers to control the stresses. The undulations continued, however, and further studies were undertaken at the University of Washington. Their recommendation of the installation of tie-down cables in the side spans were implemented, but to little effect.
Local folks lost no time in nicknaming the bridge "Galloping Gertie." Fascinated by Gertie, thousands of people drove hundreds of miles to experience the sensation of crossing the rolling center span. The disappearance and then reappearance of cars often highlighted the experience. For four months, the Washington Toll Bridge Authority thrived as traffic had trebled from what had been expected. Although concerns about the bridge's stability had been voiced, bridge officials were so confident of the structure, they considered canceling the insurance policies in order to obtain reduced rates on a new one.
So Galloping Gertie went on her way, bucking and jerking during any windy spell. It became a spectacle and a tourist attraction, bringing people in from miles around to observe the solid bridge become almost as malleable as rubber. This effect was caused by the wind vibrating the concrete at exactly the speed concrete responds too, leading to the concrete adding more to the vibration which in turn causes the wind to vibrate the bridge more and so on. This amazing phenomenon is known as Aeroelastic flutter. Even in a light breeze, Gertie's undulations were known to produce waves up to ten feet tall. Sometimes these occurrences were brief, and other times they lasted for hours at a time. Numerous travelers shunned the route altogether to avoid becoming seasick, whereas many thrill-seeking souls paid the 75-cent toll to traverse Gertie during her more spirited episodes.
Throughout the early morning hours of Thursday, November 7, 1940, the center span had been undulating three to five feet in winds of 35 to 46 miles per hour. Alarmed by this constant motion, highway officials and state police closed the bridge at 10:00 A.M. Shortly thereafter the character of the motion dramatically changed from a rhythmic rising and falling to a two-wave twisting motion. The twisting motion grew stronger with each twist; span movement had gone from 5 foot to 28 foot undulations. This twisting motion caused the roadbed to tilt 45 degrees from horizontal one way and then 45 degrees from horizontal the other way.
For about 30 minutes, the center span endured the twisting. At about 10:30 A.M., a center span floor panel dropped into the water 195 feet below. The roadbed was breaking up, and chunks of concrete were raining into the Sound. At 11:02 A.M., 600 feet of the western end of the span twisted free, flipped over, and plunged down into the water. Engineers on the scene hoped that once this had happened, the remainder of the span would settle down. The twisting continued, and at 11:09 A.M., the remaining bridge sections ripped free and thundered down into the Sound. When this happened, the 1,100 foot side spans dropped 60 feet, only to bounce up and then settle into a sag of 30 feet. As for the center span, it rested on the dark and tide-swept bottom of the Narrows.
I do not hold copyrights to either the photos or the music used in this slide show. I am aware that many photos are fuzzy but please remember that the people who took the photos were excited amateurs recording exciting events on the spur of the moment.
The Tacoma Narrows Bridge disaster of 1940 resulted in sweeping changes in bridge design.
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Be well!
Nina