The Tacoma Narrows Bridge

Or why engineers must be detail oriented!

The Puget Sound region of the Pacific Northwest encompases some 20,000 square miles of land. Nowhere in that region do the mainland and the Olympic Peninsula come as close as they do at the Tacoma Narrows. State officials realized that in order to make the spectacular but sparsely populated peninsula region more accessible, they would need to span the Narrows. In realization of that, the state created a toll bridge commission in 1937, with the authority to build, construct, and operate toll bridges in the State of Washington.

In 1938 or so, Washington State Department of Highways bridge engineer Clark Eldridge came up with a plan to span the Tacoma Narrows with a 5000 foot two-lane suspension bridge, which when completed would be the third longest suspension bridge in the world. Between 1938 and 1939, numerous bridge experts reviewed and modified Eldridge’s design. At that time, the appearance of being streamlined was considered important, and the increasing popularity of automobile transportation and the decline of the railroads lessened the load requirements for the bridge. One of the engineers, Leon Moisseiff, made the significant substitution of a open stiffening truss with a shallow plate girder, resulting in a much lighter and streamlined looking bridge.

On July 1, 1940, the Tacoma Narrows Bridge opened to traffic, linking the Washington mainland with the Olympic Peninsula. But even during the construction phase unusual things happened to the bridge: gentle breezes caused vertical oscillation of the roadbed while stronger breezes often had no effect. A variety of band-aids were installed to combat this prior to opening and the University of Washington commited to further study the problem.
In no time at all, the locals nicknamed the bridge, “Galloping Gertie.” Thousands drove hundreds of miles to experience crossing the undulating center span. For four months, the Toll Bridge Authority enjoyed a boom time as traffic was about three times what they had expected. Concerns about the bridge’s stability had been voiced, but the bridge officials were steadfast in their confidence in the structure.
Scarcely four months later, in the early morning of Thursday, November 7, 1940, the center span experienced three to five foot oscillations in 35 to 46 mph winds. Bridge officials were properly alarmed by this and with the assistance of the state police, the bridge was closed to traffic at 10AM. A short time later, the motion changed from the usual rhythmic undulation to a two-wave twisting motion. With each twist the motion grew stronger; the span movement changed from 5 foot to 28 foot undulations to twisting severe enough to cause the roadbed to alternately tilt 45 degrees to the horizontal in each direction.
For about 30 minutes, the center span endured, but at approximately 10:30AM, one of the center span floor panels dropped into the water 195 feet below, followed quickly by more chunks of concrete. At about 11AM, a good part of the western end of the span twisted free and plunged down into the water. Bridge officials were hopeful that once this happened, the remainder of the span would settle down. Alas, the twisting continued, and nine minutes later, the remaining bridge sections became part of Puget Sound.
Professor Burt Farquharson from the University of Washington managed to capture the bridge’s final minutes on film. Watching the film, you see a stalled car on the bridge…on the wrong side of the road. It belonged to a reporter who had abandoned it after the engine stalled. The violent twisting motion threw the car to the other side of the road. A dog was stranded in the car, and Professor Farquharson risked his life by walking the centerline of the span (which was the center of the twisting motion and thus motionless) to attempt a rescue. Sadly he was unsuccessful and the dog was the only casualty of the bridge’s demise.

This short piece of film has become a classic example of engineering failure, shown to  mechanical engineering students as a vivid example of what happens when you don’t pay attention to the details. Of course, the Government commissioned a study of the bridge's failure. Ultimately the original designers were exonerated, and the resonance theory of failure advanced. In other respects, the report was vague and inconclusive (in true government fashion), leaving the exact cause of failure as a subject of debate and study for more than forty years.

Many years ago, I saw this piece of film on TV as part of the You Asked For It show. I was completely enthralled by this brief encounter and always wanted more. I’ve traveled Gertie’s replacement countless times, and I’m happy to report that the new bridge exhibits none of the unusual behavior of its more famous but short-lived predecessor.

Now, thanks the the Internet, I have found a number of web sites that explain (in painful detail) what happened that fateful day. Explore the links below and enjoy. Many have animation or movie clips where you can see Gertie doing her thing.

Not to be Outdone

The Tacoma Narrows bridge may have been the first bridge to sink in Washington State, but certainly wasn't the last. In 1940, the state opened the Lacey V. Murrow Bridge, a floating or pontoon bridge that carried traffic across Lake Washington in Seattle across US Highway 10, later known as Interstate 90. In 1961, the state opened the Hood Canal Bridge, another pontoon bridge spanning the Hood Canal, a saltwater canal in Puget Sound. The HCB is significant because of the water depth and tidal activity (80-340 feet, with 16.5 foot tidal swings). In 1963, the state opened the Evergreen Point Bridge, spanning Lake Washington again, but this time to the north. Sadly, this bridge is the victim of short-sightedness, and it now carries traffic several times greater than anything the designers had envisioned. It is now a floating traffic jam. Affluent neighborhoods surround it at both ends, and the likelihood of increased bridge capacity along this route is unlikely because of pressure from the NIMBYs. Finally in 1989, the state built their latest floating bridge, a parallel structure to the Lacey V. Murrow Bridge.

In 1979, a severe windstorm and worker oversight (access hatches into the pontoons were left open, allowing water to enter. Mark this; it's a continuing theme) caused the west-end of the bridge to sink into Hood Canal. Read a bit more about this failure here. The Hood Canal Bridge does have an interesting website, which I recommend, especially if you're planning to cross it. You would think that the state's Department of Transportation would learn from its mistakes, but...I guess that with a bureauacracy, anything is possible. Read on.

In 1990 the Lacey V. Murrow Bridge sank while undergoing renovation as part of the completion of the western terminus of I90. An additional floating bridge was contructed adjacent to it and carrying traffic. The plan was to renovate the original bridge, and ultimately it would carry traffic eastbound while the new bridge would handle westbound and HOV traffic. On Thanksgiving Day 1990, a severe storm with high rainfall, high winds, heaped insult onto injury and the bridge sank. The engineering failure analysis can be found here. You'll see the continuing theme re-emerge here, as pontoon A5 submerges below the waves of Lake Washington.

The Department of Transportation has lost 3 major bridges in 50 years. Their expertise doesn't end with bridges; they also manage the State Ferry System, which is considered an extension of the highway department. Neither does their expertise with failure, as the new ferries purchased in the last 30 years have been plagued with a variety of propulsion system problems. But that's for another page somewhere else because this page is about bridges. And then there's the small matter of freeway design. I could go on...

Oh yes, at present, all of the floating bridges are still floating, and the Tacoma Narrows Bridge is still suspending.

Links of Interest

Follow this link for a more detailed account of the early days of the Narrows Bridge, the story of its creation, demise, and replacement. There is a table giving numerous bridge specifications and underwater maps showing the placement of the wreckage of the original span, now a marine hostel.

The Narrows Bridge community has errected this very complete site about their favorite bridge. There's information about the proposed third (second?) bridge here. (There's a Java-enabled version that you can access by going back to the root site). Lots of pictures and links. Very nicely done.

One of the theories given for the bridge's failure was resonance; that the wind flowing through the deck of the bridge excited the structure to its natural resonance, causing the failure. This link explores that theory.

Doug Smith, a fourth year student at Carleton University in Ottawa studied the bridge's failure for a class project. Some photographs and text from his report were lifted and presented here. You'll also find a link to a camera shop in Tacoma selling a videotape (not Prof. Farquharson's) of the bridge's last moments.

Mark Ketchum's bridge failure site. He explores not only the TNB failure, but also other bridge failures. The Billah and Scanlon paper from the American Journal of Physics can be downloaded from his site.

Here is a Freshman Feature from the University of Iowa. It cites references refuting the resonance theory and introduces a new one.

This paper, from the Undergraduate Engineering Review, (University of Texas @ Austin) explores and evaluates the theories of the failure. There is a good glossary of terms as well as many references.

Want to build a bridge? Explore different bridge types and learn how they work to solve the problem of spanning two points in space.

The Context for World Heritage Bridges explores bridge building throughout the world, from ancient times to present. This is a site worthy of your time.

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Last modified 8/31/2001