When would a structural engineer ever need to know about the effects of aeroelastic flutter?
When the 40mph winds cause the Galloping Gertie suspension bridge to collapse in 1940.
The amount of force and energy air exerts can create an instability in an otherwise stable, fixed structure. Among structural engineers, this phenomenon is known as aeroelastic flutter. Although typically associated with airplanes, aeroelastic flutter also has applications to ground transportation, like it did for the Tacoma Narrows Bridge (aka "Galloping Gertie"). "Galloping Gertie" was a suspension bridge that spanned the Tacoma Narrows Strait of Puget Sound in Pierce County, Washington. When it was completed in July of 1940, it was considered "a triumph of man's ingenuity and perseverance". Only four short months later, it would be called "the Pearl Harbor of engineering". But what caused that stigma?
On the morning of its collapse, Tacoma Narrows Strait had wind speeds of 38-42+ mph. These winds were blasting the new bridge on its broadside, making it undulate, or "gallop". As the wind continued, what was a small ripple of 2-5 feet turned into a massive lateral twisting motion of 28 feet at an angle of up to 45 degrees. This alternating twist is aeroelastic flutter. When the bridge twisted up, gravity and the tension of the bridge caused it to twist back down. It twisted down too far, so the wind hit the same side in the opposite direction, making it twist back up even more violently. This cycle of violent lateral twisting eventually tore the bridge to pieces. Because of aeroelastic flutter, the central portion of the bridge was resting on the bottom of Puget Sound in less than two hours.
Galloping Gertie's collapse was ultimately a result of the flexibility of the bridge and its inability to absorb dynamic forces caused by the wind in the Narrows. Structural engineers ran a series of tests throughout WWII to better understand the aerodynamics of a fixed object and find a solution. Their new bridge design could withstand winds of up to 125 mph. To combat its flexibility and increase its strength, structural engineers:
- Used lightweight concrete for the bridge's floor
- Created open steel gratings between driving lanes
- Enlarged anchorages
- Utilized the increased foundation pressures
With its new design, Galloping Gertie could once again take its place as a triumph of engineering. It became "a combination of men's dreams, fortitude, and inventive ingenuity...a masterpiece of engineering skill."
Tacoma Narrows Bridge (1940). (n.d.). Retrieved from https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)
Tacoma Narrows Bridge: Lessons From the Failure of a Great Machine. (n.d.). Retrieved from http://www.wsdot.wa.gov/TNBhistory/Machine/machine3.htm
The Bridge and Aeroelastic Flutter
Good explanation of aeroelastic flutter. (2012). Retrieved from https://www.physicsforums.com/threads/good-explanation-of-aeroelastic-flutter.624992/
Footage of the Bridge Collapsing
Tacoma Narrows Bridge Collapse (Sound Version) (Standard 4:3) (1940). (2012, December 30). Retrieved from https://www.youtube.com/watch?v=nFzu6CNtqec
Cheta, M. (2011, December 15). Tacoma Narrows Bridge was brought down by aeroelastic flutter, not resonance. Retrieved from http://tehgeektive.com/2011/12/15/tacoma-narrows-bridge-brought-down-aeroelastic-flutter-resonance/
Tacoma Narrows Bridge. (n.d.). Retrieved from http://www.lib.washington.edu/specialcollections/collections/exhibits/tnb
Tacoma Narrows Bridge: "Galloping Gertie" Collapses November 7, 1940. (n.d.). Retrieved from http://www.wsdot.wa.gov/tnbhistory/connections/connections3.htm