Nicknamed “Galloping Gertie” for its tendency to bend and undulate, the Tacoma Narrows Bridge had just opened to traffic on July 1, 1940. In a now infamous failure, in the face of moderate winds the morning of Nov. 7, 1940, the bridge started to repeatedly twist. After an hour of twisting, the bridge collapsed. One fatal engineering assumption led the bridge to shake itself apart.
At the time, many designers believed that wind could not cause bridges to move up and down. That it actually can may seem like an obvious fact now, but that incorrect assumption cost about US$65 million in today’s dollars and a dog’s life.
Small vertical movements allowed the bridge to twist. Near the end, the bridge twisted in ways the designers had never anticipated. This twisting stressed the bridge until the Tacoma Narrows Bridge collapsed.
By assuming no vertical movement from wind, the engineers didn’t study how parts of the bridge would flutter in the wind before they built the bridge. This oversight ultimately doomed the bridge.
The Tacoma Narrows Bridge collapsed in 1940 because its designers assumed it wouldn’t flutter up and down in the wind, but it ended up being slender enough that the wind caused it to move up and down.
University of Washington Libraries Digital Collections
This failure illustrates an idea that many engineering students learn during their coursework: All engineering calculations are based on models. Safe design requires engineers to recognize the assumptions in their models and to ensure the design’s safety despite any limitations.
I am an expert in computational modeling, which I teach at Olin College. In my classes, I talk about models and teach engineers to use them safely.
Learning to use models carefully is important: As the famous statistician George Box said, “All models are wrong – some are useful.”
Models and their engineering use
Models are interpretive frameworks that help scientists and engineers connect data to the real world. For instance, you likely have an everyday sense for the strength of objects: If you bend a piece of wood with enough force, it will break. A stronger board can take more force.
Engineers have models that make this everyday sense more precise.
Engineering strength depends on an interpretive framework that relates forces, the size of an object and their ratio − which represents mechanical stress. What engineers call “strength” relates to this computed stress.
Considering strength helps engineers select a material that is strong enough to build a bridge.
An interpretive framework − a model − for strength, used in engineering. Force, F, and size or area, A, are used to compute stress, sigma. Sigma is then used to determine strength.
Jorge Stolfi/Wikimedia Commons, CC BY-SA
But all models leave out details from the real world. To compute stress, an engineer needs to describe…
