SUSPENDED ANIMATION
THE TACOMA NARROWS BRIDGE
by Jason Zasky

It's an ominous sign when a bridge opens and the public equates the new structure with an amusement park ride. That's exactly what happened when the Tacoma Narrows opened in July 1940. Attracting passengers from far and wide, people would drive across simply to experience the excessive undulations of the roadway, soon earning it the nickname of 'Galloping Gertie.' When its spectacular collapse was captured on film in November of that year its place as the most remarkable engineering failure in history was assured.

The Tacoma Narrows Bridge collapsing on November 7, 1940

Connecting the Olympic peninsula with Washington State's mainland, the Tacoma Narrows' main span measured 2,800 feet, making it the third longest suspension bridge in the U.S. Most notable about the design, however, was its surprisingly slender frame. Measuring only 39 feet across, the two-lane bridge was extremely narrow relative to its length, making the roadway prone to moving up and down in waves. At times, travelers reported that cars driving ahead of them or following behind were temporarily obscured from view.

"There's a long history of bridges that have failed. We're about due...and cable-stayed bridges bear careful watching."

The Tacoma Narrows wasn't the only new suspension bridge of the period that suffered from excessive motion. The Golden Gate bridge [1937], New York's Bronx-Whitestone [1939], and Maine's Deer Isle bridge [1939] also demonstrated an alarming tendency to undulate in the wind and all had to be retrofitted with extra cables and/or stiffening devices. While it's unclear whether the Narrows' principal engineer, Leon Moisseiff, was aware of the problems plaguing these other new bridges (he almost certainly was), he argued for cost-cutting adjustments to the original design and against initiatives that would detract from the bridge's appearance. Meanwhile, an advisory engineer named Theodore Condron strongly and repeatedly urged that the Tacoma Narrows be built at least 25% wider. Condron, a lone dissenter, was ignored, though his fears quickly proved justified as the bridge began behaving abnormally even during the construction process.

On November 7, 1940, with the wind blowing steadily at 40-42 mph, one of the extra stiffening cables that had been added to the Tacoma Narrows suddenly came loose and the roadway began to twist back and forth in increasingly violent fashion. The bridge was quickly closed to traffic leaving newspaper editor Leonard Coatsworth, his cocker spaniel Tubby, and an investigating University of Washington engineering professor named F.B. Farquharson on the bridge in its final heaving moments. After Coatsworth lost control of his car he was forced to abandon it with Tubby still inside. At this point, the story becomes the stuff of legend. "I heard two versions back when I was in steel design class," says Brian McDonald, principal engineer at Exponent Failure Analysis Associates in Menlo Park, CA. "In one the professor saved the dog. In the other, when he opened the car door the dog bit him and he came back [without the pooch]."

Moments after the two men crawled off the bridge deck to the relative safety of the toll plaza, a 600-foot section of the center span gave way, plunging upside down into Puget Sound where it lies today as a sort of artificial reef. The side spans (the sections of roadway on the outside of the two towers) held but sagged dramatically in the aftermath, while the steel towers were so disfigured that they had to be removed before a replacement bridge could be built. In 1992, the sunken remains were placed on the National Register of Historic Places to protect them from souvenir seekers.

"...We've done a very bad job of convincing the public that there's a need for risk management."

The collapse of the Tacoma Narrows provided the impetus for civil and structural engineers to begin incorporating aerodynamics into bridge design. "There are now design equations that engineers can use to predict the wind loads on structures," says McDonald. "If it's a major project or a unique or exotic structure, they'll build a scale model that includes the surrounding terrain, then blow wind across it and measure."

The replacement version of the Tacoma Narrows was unveiled in 1950 and utilizes a potpourri of different design techniques that help control wind behavior. "There's very little daring about it and you honestly can't blame them," says Mark Ketchum, vice president of San Francisco-based bridge engineering firm OPAC. "They were pretty conservative the second time around." And while its 50-year lifespan has been relatively uneventful, a major incident occurred during its construction when a moderate-sized earthquake flung one of the newly mounted (but as yet unbolted saddles) off the top of one tower, punching a hole in and sinking a barge in the water below. Currently, an architecturally similar bridge is being designed to run parallel to the existing Tacoma Narrows and is scheduled for completion in 2005.

In the past 15 years, the futuristic-looking cable-stayed bridge has replaced the suspension bridge as the favorite of engineers and public officials, owing to its aesthetically pleasing appearance and cost-efficiency. A cable-stayed bridge (such as the Sunshine Skyway in Tampa, FL), is supported by cables that run down to the deck from one or more towers. Ironically, the post-construction behavior of these bridges has been eerily reminiscent of suspension bridges in the 1930s, except that the issue is cable oscillation rather than movement of the roadway. In short, the cables are demonstrating an alarming tendency to vibrate, behaving much like a garden hose does when you lay it on the ground and "whip it."

"The largest cable-stayed bridge is Tartara in Japan," says Ketchum. "If you stand at the top of its towers you can see these traveling waves coming up and down and you feel this thump as the energy pulse hits the tower." Ketchum has also visited the cable-stayed Rama IX bridge in Bangkok and reports: "It shakes constantly at 10 to 15g vertical acceleration, which is kind of like what the 1989 [San Francisco] earthquake was except it's doing this all the time." Recently, when Ketchum drove out onto Rama IX he found that his car was bottoming and topping its suspension. "Bang! Bang! Bang! It was shaking that hard," he recalls.

"To build [the Gibraltar Bridge] would require the world's deepest water structures, the world's tallest towers and longest span structures—all at the same time."

Henry Petroski, professor of civil engineering at Duke University and author of "Engineers of Dreams" and "To Engineer is Human" concurs that cable-stayed bridges are demonstrating behavior not anticipated and being built longer and more slender than is wise—the same ingredients that contributed to the Tacoma Narrows disaster. "There are these warning signs," he says. "There's a long history of bridges that have failed, and they generally fall in 30-year cycles. We're about due . . . and cable-stayed bridges bear careful watching."

Ketchum relates the cycle to alcoholism skipping generations. "My mom's old saying is that if you see your parent as an alcoholic you won't become one yourself because you see how nasty it is. It skips that generation but then the next generation doesn't see the danger so they fall into that trap. There hasn't been a horrible collapse for a while and maybe its skipping generations of bridge design," he concludes.

Ketchum attributes many of the problems exhibited by today's bridges to a lack of adequate pre-construction engineering—a combination of naivete among owners regarding design challenges and an unwillingness to provide the necessary financial resources. "We're designing bridges with less and less engineering effort and less and less attention to what I consider the necessary aspects of whether or not it's going to perform right. A suspension bridge with 500-foot tall towers, 10 million pounds of wire, 25 million pounds of steel plate, and 300,000 yards of concrete that has to be dangled from those towers is a little bit different than a ramp and an overpass," he notes. The Rama IX bridge sounds like the perfect example: "Here we are 12 years after it was built doing engineering studies to make sure it isn't tearing itself apart," says Ketchum. "Maybe that's our fault. As a professional group we've done a very bad job of convincing the public that there's a need for risk management."

Risk management may become a more significant issue as public officials call for ever-larger and more expensive bridges. To handle the increasing volume of automobile traffic, future structures will be bigger and longer than ever before. Currently, the Messina Straits bridge in Italy is being designed with two towers that will be a remarkable 3 km apart. But the mega-project of the future could be the three-tower Gibraltar Bridge, which would connect Spain and Morocco. If the project comes to fruition it most likely would require two 5 km spans flanked by two 2 km spans for a total of 14 km. "To build those it would require the world's deepest water structures, the world's tallest towers and longest span structures—all at the same time," says Ketchum.

While it's been sixty years since the Tacoma Narrows disaster, the fundamental issues remain the same. If public officials continue to push to envelope, squeeze pre-construction engineering budgets, and fail to anticipate the behavioral idiosyncrasies of cutting-edge design, it seems only a matter of time before there's a high profile collapse. An old adage comes to mind: "You can pay me now or pay me later."

SWAY IT AIN'T SO
LONDON BRIDGE IS FALLING DOWN?
by Jason Zasky

When the Millennium Bridge opened this past June it was expected to be a source of pride for the city of London, offering a pedestrian link across the Thames River and spectacular views of the city. After all, the picturesque single-arch footbridge was the first new river crossing in London in over a century. Instead, the structure had to be closed within days of opening due to excessive swaying, behavior its engineers curiously attributed to "synchronized walking."

While its European designers claim that this pedestrian effect was unpredictable, several U.S. engineers have indicated that the problem is well documented. In an interview with Failure, Mark Ketchum, of San Francisco-based design firm OPAC, who engineered the similarly-designed (but non-pedestrian) Arroyo Cangrejillo bridge in Argentina, noted, "the problems exhibited on the Millennium Bridge are source-similar to the problems that plagued a footbridge connecting the BART [Bay Area Rapid Transit] station to the football stadium in Oakland, CA, about 25 years ago."

In a nutshell, the Millennium bridge has the same vibration frequency laterally as it does vertically, so when the structure moves up and down that energy bleeds over, leading to a side-to-side motion. "The Millennium has got problems," says Ketchum. "They are going to have a hard time retrofitting their way out of that one."

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RECOMMENDED LINKS
http://www.tacomanarrowsbridge.com (Tacoma Narrows Bridge Project)

http://www.opac.org (OPAC Web site)

MULTIMEDIA
http://www.britannica.com/bcom/eb/article/6/0,5716,72716+1+70874,00.html (Color film of the Tacoma Narrows collapse)