Wednesday, November 7, 2012

Tragedies in Science: The Collapse of the Tacoma Narrows Bridge

Since it's collapse in 1940, the Tacoma Narrows
Bridge has been rebuilt, this time as a
twin suspension bridge. Image courtesy: WSDOT (Flickr CC)
When you think of iconic bridges of the world, the Tacoma Narrows Bridge, which spans a narrow strait in the Puget Sound south of Seattle, may not come to mind immediately.  But that bridge (today it's actually a set of twin bridges--one for each direction of traffic) has an incredible history that serves as a serious cautionary tale for engineers.

The original Tacoma Narrows bridge, completed in 1940, was a long time in the making but short-lived once it was finally built. The idea of building a bridge across the sound to connect the city of Tacoma to the Kitsap Peninsula dates back to at least 1889.  During the 1920s and 30s a number of citizen campaigns and leaders supported various designs and approaches to building a bridge

One of the major sticking points for all of these efforts was the cost. In 1938, the state of Washington applied for federal funding to build the bridge. The state bridge engineer, Clark Eldridge, drew up a fairly standard suspension bridge design with an estimated price tag of $11 million, but officials were skeptical that a project of that magnitude could pay for itself in bridge tolls.  In the end, they granted the state $6.4 million and required them to hire Leon Moisseiff, a bridge engineer from New York who said the bridge could be built for much less than $11 million.

Moisseff's plan was indeed much less expensive because his design used far less steel and called for the Tacoma Narrows Bridge to be very light and narrow compared to other suspension bridges of the time. When they saw the plan, engineers at the Washington State Highway Department protested, calling the bridge design "fundamentally unsound."  But that did not stop the project.  Construction began on November 23, 1938.

During construction, workers noticed that the bridge bounced.  Soon after opening in July 1940, the bridge became known by engineers and motorists alike for "the bounce" or "the ripple."  It was rated to withstand winds up to 120 miles per hour, but even in a light breeze the bridge moved.  Motorists reported waves, sometimes up to 5 feet tall, undulating from one end of the bridge to the other.  Sometimes the bouncing was short-lived, but on occasion it was reported to last for up to six hours. The bridge earned the nickname Galloping Gertie.  Some locals steered clear of Gertie out of fear or to avoid sea sickness; others sought it out as a thrill ride.

In the spring of 1940, the Toll Bridge Authority contacted Moisseiff who reported that two of his other recent bridges were having similar movement issues, but at a smaller scale. The Authority then hired engineering professor F. Bert Farquharson from the University of Washington to find a solution to the problem.  Farquharson and his students built scale models of the bridge and conducted wind tunnel experiments.  In October, engineers added temporary tie-down cables to anchor the bridge to the seafloor and diagonal cables between the bridge's deck and the main cables to brace it.  This helped limit the bridge's movement, but not enough.

In early November, as workers repaired one of the tie-down cables that had broken loose from Gertie's galloping, Farquharson completed his wind tunnel studies.  He noted a torsional (twisting) motion in the bridge model under high wind conditions.  "We watched it," Farquharson later told reporters, "and we said that if that sort of motion ever occurred on the real bridge, it would be the end of the bridge."

Guided by Farquharson's findings, the Toll Bridge Authority began drawing up a contract to have wind deflectors installed on the bridge. The morning of November 7, the state bridge engineer, Clark Eldridge was sketching plans for the deflectors, pricing materials, and preparing a rapid response that would have had the entire bridge covered in wind deflectors within 45 days. But they were too late.

By 7:30 am, 42-mile-per-hour winds were rushing up the Narrows from the Southwest and hitting Gertie broadside.  She began to gallop. Farquharson, Eldridge, and a crowd of spectators began to gather, photographing and even taking video of the bridge.  At 10:03 am, the bridge suddenly began the lateral twisting motion that Farquharson had feared.  Pieces of concrete from the roadway began to break free, and at 11:02 am, a 600-foot span collapsed into the sound.  Galloping Gertie had failed.

Footage from the Nov 7, 1940 collapse of the Tacoma Narrows Bridge. Courtesy: Internet Archives

Fortunately, no humans and only one dog died during the collapse, but the outcome could have easily been much worse.  In a situation like this who is to blame?  Did the collapse of the bridge result from an ethical breach or an honest miscalculation?  Does the designer of the bridge bear responsibility?  What about the federal officials who put construction costs above other considerations--including perhaps safety--when selecting a design and funding the project?

Answers to ethical questions like these are not always straightforward.  Fortunately, though, there are standards of conduct and general principles that members of the scientific community abide by.  You're probably familiar with the Hippocratic Oath that doctors take, declaring that (among other things) they vow to do no harm to their patients.  In the U.S., the National Society of Professional Engineers' code of ethics declares that engineers shall "hold paramount the safety, health and welfare of the public." General standards relating to scientific methods and to topics of study (especially human and animal subjects) also guide researchers.

So what if Clark Eldridge's original (more expensive) design had been built instead of Leon Moisseiff's lighter, cheaper version?  Structural engineers who reviewed the original plans for the State of Washington concluded that the bridge would likely still be standing.

For more about ethical standards and dilemmas in science and engineering, read our Scientific Ethics module.

For more about how models of all types are used in research, visit our module Research Methods: Modeling.

For more about the history and design of the Tacoma Narrows Bridge (and other suspension bridges), check out the Washington State Department of Transportation's More Than a Bridge site.

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