On This Date In History: The Tacoma Narrows Bridge opened in July 1940 to great fanfare. With a span of 2800 feet, it was the third longest bridge in the world. The builder decided to save money by making the bridge only 39 feet wide to support just two lanes of traffic which seems pretty short sighted given that it was probably a pretty good bet that traffic in the years ahead would continue to increase. Nevertheless, the bridge only had two lanes but was designed to withstand winds of 120 mph. People flocked to the bridge after it’s opening, not just because it was faster than the ferry, but also because word had traveled that the moderate winds that are common in the area would make the bridge sway and even undulate up and down. It was pretty cool to see. Drivers reported losing sight of the autos in front of them when the bridge waved. The bridge gained the moniker “Galloping Gertie.”
All the fun and games came to a crashing end on this date in 1940 when steady winds of just 42 mph made the bridge sway…and then leap. It continued throughout the day until the entire structure failed. The bridge had been closed but the last man let across lost control of his car and, in fear of being tossed 190 feet into the Tacoma Narrows below, he fled on foot. He tried to retrieve the family cocker spaniel from the back seat but the pup would have none of it. As he reached the end of the bridge, the bridge collapsed spectacularly with the dog as the only casualty.
The result of the collapse was analyzed and is a common subject in college physics classes. Basically, the wind was in tune with the natural harmonics of the bridge. The more the wind blew, the greater it increased the amplitude of the harmonics creating a wave. Its a similar phenomena as when a singer breaks a glass. To avoid a similar fate, bridges today, including the one that replaced “Galloping Gertie” have damping and aerodynamic features built in and wind tunnels are used to test the ability of the structure to resist wind and prevent compromise of the integrity of the structure.
Here is a different, color film on video of the collapse
Whatever man builds, can come down. Gravity and other forces are constantly at play. There are some things in life in which compromise can be a good thing. But engineers cannot afford to not get it right the first time. It kinda reminds me of what US Grant often said, “man proposes; God disposes.” These days we often teach our kids that close is good enough. If that is your methodology, mommas don’t let your babies be engineers.