For over six decades, engineers have studied the collapse of the 1940 Tacoma Narrows Bridge. The experts disagree, at least on some aspects of the explanation. A definitive description that meets unanimous agreement has not been reached. The exact cause of the bridge’s failure remains a mystery.
Why is it important to know the exact cause of the 1940 bridge’s collapse? Engineers need to know how a new suspension bridge design will react to natural forces. The more complete their understanding, the better their problem solving, and thus, the stronger and safer their bridge. The fact that engineers still argue about the precise cause of the Galloping Gertie’s collapse is testimony to the extraordinary complexity of natural phenomena. Today, the 1940 Tacoma Narrows Bridge’s failure continues to advance the “scientific method.”
The primary explanation of Galloping Gertie’s failure is described as “torsional flutter.” It will help to break this complicated series of events into several stages.
Here is a summary of the key points in the explanation.
1. 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.
2. On the morning of November 7, 1940 shortly after 10 a.m., a critical event occurred. The cable band at mid-span on the north cable slipped. This allowed the cable to separate into two unequal segments. That contributed to the change from vertical (up-and-down) to torsional (twisting) movement of the bridge deck.
3. Also contributing to the torsional motion of the bridge deck was “vortex shedding.” In brief, vortex shedding occurred in the Narrows Bridge as follows:
(1) Wind separated as it struck the side of Galloping Gertie’s deck, the 8-foot solid plate girder. A small amount twisting occurred in the bridge deck, because even steel is elastic and changes form under high stress. (2) The twisting bridge deck caused the wind flow separation to increase. This formed a vortex, or swirling wind force, which further lifted and twisted the deck. (3) The deck structure resisted this lifting and twisting. It had a natural tendency to return to its previous position. As it returned, its speed and direction matched the lifting force. In other words, it moved ” in phase” with the vortex. Then, the wind reinforced that motion. This produced a “lock-on” event.
4. But, the external force of the wind alone was not sufficient to cause the severe twisting that led the Narrows Bridge to fail.
5. Now the deck movement went into “torsional flutter.”
“Torsional flutter” is a complex mechanism. “Flutter” is a self-induced harmonic vibration pattern. This instability can grow to very large vibrations.
Tacoma Narrows Failure Mechanism – original sketch contributed by Allan Larsen
When the bridge movement changed from vertical to torsional oscillation, the structure absorbed more wind energy. The bridge deck’s twisting motion began to control the wind vortex so the two were synchronized. The structure’s twisting movements became self-generating. In other words, the forces acting on the bridge were no longer caused by wind. The bridge deck’s own motion produced the forces. Engineers call this “self-excited” motion. It was critical that the two types of instability, vortex shedding and torsional flutter, both occurred at relatively low wind speeds. Usually, vortex shedding occurs at relatively low wind speeds, like 25 to 35 mph, and torsional flutter at high wind speeds, like 100 mph. Because of Gertie’s design, and relatively weak resistance to torsional forces, from the vortex shedding instability the bridge went right into “torsional flutter.”
Now the bridge was beyond its natural ability to “damp out” the motion. Once the twisting movements began, they controlled the vortex forces. The torsional motion began small and built upon its own self-induced energy.
In other words, Galloping Gertie’s twisting induced more twisting, then greater and greater twisting.
This increased beyond the bridge structure’s strength to resist. Failure resulted.
1. What were the constraints on this project? In what ways was the project’s planning and scope management appropriate and when did they begin taking unknowing or unnecessary risks?
2. Identify the risk factors that you consider most important for the suspension bridge construction. How would you assess the riskiness of this project? Why?
3. What forms of risk mitigation would you consider appropriate for this project?
4. What if Clark Eldridge’s original design for the 1940 Tacoma Narrows Bridge had been built, instead of Leon Moisseiff’s? Would it have blown down on November 7, 1940 like Galloping Gertie?
5. What recommendations would you make to keep the current (replacement) bridge from suffering the same fate?