Suspension Bridges: The Rise and fall of The Tacoma Narrows Bridge
By: Alexander Heredia
November 7, 1940, was the day that one of the most advanced and ambitious engineering projects failed. The Tacoma Narrows Bridge, once celebrated as a feat of modern engineering, twisted and collapsed under winds of only 42 miles per hour, plunging into Puget Sound, Washington four short months after its opening. This shocking event not only surprised the nation but also revolutionized civil engineering, challenging existing beliefs about bridge design and wind resistance. What went wrong with "Galloping Gertie," and how has this notorious failure influenced the future of bridge engineering?
The construction of the Tacoma Narrows Bridge took place on July 1st, 1940, in Pierce County, Washington. The bridge was made of mainly steel and concrete; features included: Carbon steel girders (a large iron or steel beam or compound structure used for building bridges and the framework of large buildings), pairs of deep I-beams that supported the bridge, suspension wires around 20 ½ inches in diameter that were made out of 5,500 tonnes of steel wiring, bases that needed more than 30,000 cubic yards of concrete.
Leon Moisseiff designed the bridge to be the most flexible ever constructed. Engineers of the time believed that the design, even though it exceeded ratios of length, depth and width that had previously been standard, was completely safe [1].
A short four months after its construction, on November 7th, 1940, the bridge tragically collapsed. At this time, engineers made bridges to withstand speeds of up to 120 miles per hour. Even with hundreds of tonnes of concrete and steel, the bridge collapsed under winds of 42 miles an hour—far below the limit of the bridge.
Why did the bridge collapse?
The collapse of the bridge was an event that stunned the masses; how could one of the most modern bridges in the US collapse with light winds?
In March 1941, the Carmody Board announced its findings. "Random action of turbulent wind" in general, said the report, caused the bridge to fail. This ambiguous explanation was the beginning of attempts to understand the complex phenomenon of wind-induced motion in suspension bridges. Three key points stood out:
(1) The principal cause of the 1940 Narrows Bridge's failure was its "excessive flexibility;"
(2) the solid plate girder and deck acted like an aerofoil, creating "drag" and "lift;"
(3) aerodynamic forces were little understood, and engineers needed to test suspension bridge designs using models in a wind tunnel [2].
This incident caused bridge engineers to rethink their knowledge of the vertical motion of suspension bridge decks under wind loads. Suspension bridges move vertically (up and down) to account for earthquakes, strong winds, and heavy traffic. This is vertical motion; the bridge vibrates or oscillates in a specific pattern and time frame. That time frame and pattern is known as the vibrational mode. An example of this is The Golden Gate Bridge. The Golden Gate Bridge exhibits vibrational modes; these modes cause the bridge to oscillate (move up and down) around twice per second [3].
The flexibility of the Tacoma Narrows bridge was excessive, leading to its vulnerability to twisting forces. In general, the 1940 Narrows Bridge had relatively little resistance to torsional (twisting) forces. That was because it had such a large depth-to-width ratio, 1 to 72. Gertie's long, narrow, and shallow stiffening girder made the structure extremely flexible [4].
The Tacoma Narrows Bridge collapse exemplifies aeroelastic flutter, which is an unstable, self-excited structural oscillation at a specific frequency where the motion of the structure extracts energy from the airstream [5]. When the wind blew across the bridge, it created vortices that matched the bridge's natural frequency. This frequency caused the bridge to begin vibrating; these vibrations intensified and led to the collapse of the bridge. Unlike simple harmonic motion, aeroelastic flutter is a complex interaction where the structure's movements feed back into the aerodynamic forces.
How do other bridges and complex structures benefit from this?
The collapse of the Tacoma Narrows Bridge served as a lesson for other modern suspension bridges. An example of this would be the Akashi Kaikyō Bridge located in Kobe, Awaji Island. Engineers completed the construction of the Akashi Kaikyō Bridge in 1988 and equipped it with aerodynamic deck designs to reduce wind resistance. Engineers also used extensive wind tunnel testing and computer simulations to make sure that the bridge would stay stable even under the harshest winds.
Conclusion
The collapse of the Tacoma Narrows Bridge was a pivotal moment in civil engineering history. It highlighted the importance of understanding aerodynamic forces and the dynamic behavior of structures. The disaster accented the necessity for rigorous testing and the need for innovations in design and construction practices.
—---------------------------------------------------------------------------------------------------
Alexander Heredia is a 9th grader in Georgia primarily focusing on biological related sciences, engineering and biomedical engineering.
Sources
“Tacoma Narrows Bridge collapses” (https://www.history.com/this-day-in-history/tacoma-narrows-bridge-collapses)
“Tacoma Narrows Bridge history - Bridge - Lessons from failure” (https://wsdot.wa.gov/tnbhistory/bridges-failure.htm)
“How the Bridge Vibrates” (https://www.goldengate.org/exhibits/how-the-bridge-vibrates/)
“November 7, 1940: Collapse of the Tacoma Narrows Bridge” (https://www.aps.org/archives/publications/apsnews/201611/physicshistory.cfm)
“Aeroelastic Flutter” (https://engineering.jhu.edu/fsag/research/aeroelastic-flutter/)