The Tacoma Narrows Bridge opened to traffic on July 1, 1940. This suspension bridge connected Tacoma with the Kitsap Peninsula across the Puget Sound in Washington State. Just four months later, on November 7, 1940, the bridge dramatically collapsed in a windstorm. This unexpected failure prompted widespread inquiry into its causes.
A Bridge Unlike Any Other
The Tacoma Narrows Bridge departed from earlier designs with its slender and lightweight construction. Designed by Leon Moisseiff, it featured an exceptionally narrow roadway, measuring just 39 feet wide for its nearly 2,800-foot main span. This resulted in an unprecedented span-to-width ratio of 1 to 72. Instead of deep, open trusses that allowed wind to pass through, the bridge used solid, eight-foot-deep plate girders to stiffen its deck.
This design was influenced by “deflection theory,” which suggested a bridge’s inherent flexibility and self-weight could provide stability. Engineers believed the main cables could absorb lateral wind pressure, allowing for a lighter, more economical structure. Using these shallow plate girders contributed to its sleek aesthetic and reduced construction costs. However, this choice also meant the deck presented a solid obstacle to the wind, forcing airflow to move above and below it.
“Galloping Gertie”
From its opening, the Tacoma Narrows Bridge gained an unusual reputation for its noticeable movements, earning it the nickname “Galloping Gertie.” Motorists sometimes experienced the roadway rising and falling by several feet.
The bridge’s response to winds included visible vertical oscillations. Engineers and the public were aware of these phenomena, but the potential for catastrophic failure was not fully understood. Attempts were made to mitigate these movements, including the installation of tie-down cables, but these measures proved ineffective.
The Day of Collapse and Aeroelastic Flutter
On the morning of November 7, 1940, strong winds swept through the Tacoma Narrows. Initially, the bridge exhibited its familiar vertical undulations. Around 10:00 a.m., the bridge’s movement shifted from an up-and-down motion to a dramatic twisting or torsional oscillation.
This change was caused by aeroelastic flutter, a complex aerodynamic instability. Unlike simple resonance, aeroelastic flutter involves a self-exciting and self-amplifying oscillation. The wind’s energy interacted with the bridge’s flexible structure, causing it to twist, and this twisting motion generated additional aerodynamic forces that further amplified the twisting. The bridge’s solid plate girders, unlike open trusses, obstructed wind flow, creating vortices that reinforced this twisting. The torsional vibration grew continuously, reaching up to 45 degrees of tilt and a vertical displacement of 28 feet. This severe motion led to the failure of structural components, and the main span collapsed into the Puget Sound.
Revolutionizing Bridge Engineering
The collapse of the Tacoma Narrows Bridge had a lasting impact on civil engineering and bridge design. This event highlighted the need to consider aerodynamic forces on structures, which were not fully understood in previous designs. It prompted a significant shift in how engineers approached the design of long-span bridges, moving beyond deflection theory alone.
Extensive research into aeroelasticity began, including wind tunnel tests. These tests, using scale models, became an established part of the design process to predict and prevent failures. Modern bridge design now incorporates features to enhance aerodynamic stability, such as open truss structures instead of solid girders, and often includes longitudinal slots in the deck. The lessons from “Galloping Gertie” led to more comprehensive design codes and practices, ensuring contemporary bridges are built with increased resilience against wind-induced oscillations.