Point of Collapse
Jomo Prince considers himself a veteran of the East River bridges. As a high school student at Manhattan Center for Science and Mathematics, he would often walk home over the Brooklyn Bridge. On 9-11, he used the Queensboro to flee the city. And today, he drives across the Manhattan Bridge "all the time."
But on August 14, the night of the blackout, the 28-year-old computer technician for the architectural firm Kohn Pedersen Fox experienced something he'd never felt before. Packed shoulder to shoulder with pedestrians, he could feel the Brooklyn Bridge sway so much that if he stood still he couldn't keep his balance. Then there was the groaning.
"I looked over to the right and thought, What in the world is that noise?" says Prince. "It was coming from the cables, and I didn't want to find out. I picked up my pace four-times and I was gone."
Prince's experience has been echoed by many who crossed the Brooklyn Bridge during the massive blackout trek. In fact, the New York City Department of Transportation acknowledged that it received numerous reports from concerned residents. On the bridge, pedestrians reported feeling seasick, having to weave as they walked, and hearing noises ranging from creaking to grinding.
In the last two years borough dwellers working in Manhattan have twice been forced to evacuate, and both times the bridges have played a pivotal role in the exodus. After the chaos of 9-11, walking home during the recent blackout seemed almost trite. But with the rising number of incidents that force the city to empty by whatever means necessary, we are placing a burden on the Brooklyn Bridge that has some engineers concerned. They argue over whether the 120-year-old bridge was designed to handle hundreds of thousands of pedestrians at one time and what damage age has wrought, despite periodic repairs.
But John A. Ochsendorf, a structural engineer who is also an assistant professor of building technology at M.I.T, raises the key point: "There haven't been many times in the life of this bridge when it's been packed with pedestrians."
One issue on which engineers can agree may shock most people: Contrary to logic, pedestrian traffic is actually heavier than vehicular traffic. Pack people into the same square footage of a car or SUV, and the humans weigh more than the vehicle.
What's more, pedestrian movement interacts with a bridgevibrating it, in the words of several engineersin a much more chaotic and little understood way. While cars and trucks move their weight in a smooth, uniform manner, pedestrians constantly shift weight from side to side and strike the bridge in an up-and-down motion. Witnesses observed the bridge reacting to both types of pedestrian stress on the day of the blackout.
For this reason, armies traditionally "break step" when they march over a bridge. In 1850, the Angers suspension bridge collapsed over the Maine River in Angers, France, as 478 French soldiers marched in step across it, killing 226. Though a storm raged at the time of the accident, experts blame the soldiers' marching for causing one of the anchoring cables, embedded in concrete, to tear loose.
Allan McRobie, a lecturer in engineering at Cambridge, says flatly that bridges are not designed for the dynamics of crowd interaction. He called pedestrian traffic and bridge engineering "a gray area" where a complex web of action and reaction is at work.
"The basic point is that engineers don't know what happens when a crowd walks over a bridge," he says. And whenever you get an unknown in engineering, an associated risk exists because of the lack of control, McRobie says. "If someone tells you there's no danger and it's all completely understood, they're not really telling the truth."
Like many contacted for this article, Ochsendorf of M.I.T. says he wondered about the bridge's load and reaction when he watched the blackout coverage on television. For a decade, engineers have learned how foot traffic vibrates a bridge. "The problem of lateral vibration has been noted for more than a century, but no one has become concerned about it as a design problem until the last couple of years," Ochsendorf says.
Citing the weight and motion concern, he said large numbers of pedestrians create some of the heaviest loading that a bridge would ever carry. "It is definitely something that should be worried about," he says. Imagine thousands of people on the Brooklyn Bridge, stepping randomly yet at the same time. Now imagine both the pedestrian promenade and the inbound vehicle lane packed with people, as they were on August 14.
DEFLECTION BY DESIGN
Other engineers, however, argue that August 14 provided a perfect test case for the Brooklyn Bridgewhich it obviously passed. Masanobu Shinozuka, chair of UC-Irvine's civil and environmental engineering department, calls suspension bridges "very flexible" by their structural nature. "Movement is not a cause for concern," says Shinozuka, who used to commute between his New Jersey home and a teaching job at Columbia via the George Washington Bridge. "I felt the GWB vibrating, but it's all within the design consideration."
These engineers also point to the New York City marathon route, which pours across the Verrazano and then leaks over the Queensboro, and to San Francisco's Golden Gate, which deflects (that is, flattens out) as much as one foot when runners race over it.
Tom Cocola, the spokesman for the New York City Department of Transportation, says his department was first alerted to the movement of the Brooklyn Bridge by an engineer from the Department of Citywide Administrative Services, but that the DOT had engineers on the scene all afternoon. He stresses that in situations like the blackout, the DOT sends staff to highly traveled infrastructure such as bridges and ferries. If they had felt there was a danger at the bridge, Cocola says, they would have alerted the police. (Department engineers also observed the Queensboro, Williamsburg, and Manhattan bridges.)
The department's director of bridge inspection, Bojidar Yanev, who arrived at the bridge at 8 p.m., experienced what he called "minor swaying by general suspension bridge standards." In an informal DOT report written August 15, he noted that the movement alarmed pedestrians, most of whom do not know that suspension bridges can "[sway] by several feet once every two to three minutes."
People noticed the movement more than they normally would have because many walked on the inbound traffic lane along the outer edge of the bridge, where movement is more perceptible, he wrote.
Yet there are two problems with Yanev's analysis, according to witnesses. First, many people on the center promenade reported feeling the movement so much that it affected their ability to walk and made them feel ill. Several witnesses said the bridge not only swayed from left to right, but bounced, jerking the cables and vertical suspenders.
Second, much of the heavier foot traffic had passed over the bridge between about 5:30 and 7 p.m., many witnesses reported. At 8, some argue, the movement was not as pronounced as during the peak of the exodus.
The DOT's Cocola doesn't refute that pedestrians in the center lane could feel the movement, but says it fell within a normal range. He also reiterates that department engineers were at the scene well before Yanev.
One of the best examples of the unknown interaction between people and bridges took place on June 10, 2000, on London's Millennium Bridge. When the structure opened, to great fanfare, pedestrians eagerly hopped on.
But the bridge immediately started to sway, and the oscillations increased as more people piled on. Eventually pedestrians had to adjust their movement to that of the bridge, exacerbating it even more.
In the book Sync: The Emerging Science of Spontaneous Order, published earlier this year, Cornell University mathematician Steven Strogatz writes that within minutes, the bridge "began to wobble, 690 tons of steel and aluminum swaying in a lateral S-shaped vibration like a snake slithering on the ground." Though the pedestrians originally walked at random, the bridge's movements forced them into synchronized stepprecisely what soldiers try to avoid when crossing a bridge.
Every engineer interviewed for this article said no one was in danger on the Millennium Bridge, yet its builders closed it, and it remained shut for a year. "There was danger in the fact they didn't know what was going to happen," McRobie says.
In an interview, Strogatz says he hopes engineers considered synchronized walking in their calculation when they designed the Brooklyn Bridge. Though he says he didn't think the Brooklyn Bridge was in any danger on August 14, he says the phenomenon "can take off in an explosive way. If it does move significantly, enough to change people's pattern of walking, their gait can cause a problem."
In his report, the DOT's Yanev wrote that the Brooklyn Bridge "is probably the best secured Bridge against such movements going out of control." John Roebling designed it with three separate systems "for that specific purpose," he wrote, noting that it has a suspension system, a diagonal stay system, and a stiffening truss. "Roebling himself famously said if anything happens to one of [his] systems, 'The bridge may sag, but it will not fall.' "
The Brooklyn Bridge opened on May 24, 1883. It has had its share of tragedies and scandals, from the dozens of men who died building itincluding designer Roeblingto a scam that almost resulted in dodgy wire being strewn throughout its cables. Yet experts and laypeople alike consider it to be the most beautiful and best-designed bridge ever built. Spiro Pollalis, an engineer and professor of design technology at Harvard, suggested that New Yorkers "turn the lemon to lemonade." He says that the events of August 14 should be investigated, but that no one should panic. Instead, people should use the blackout as a reminder of the "jewel" in Lower Manhattan and walk across it more often. "It's my favorite bridge," he says.
M.I.T.'s Ochsendorf says he doesn't believe the bridge would ever collapse, but that his biggest concern would be cables or vertical suspenders snapping on the bridge. That occurred in 1981, killing a pedestrian; the cables have since been replaced.
But Cambridge's McRobie offers a philosophical perspective: "When you have a crowd on the bridge, you're at the very limits of human knowledge. We understand the bridge very well, but we don't understand the crowds."
And should another catastrophe force him to again walk from Manhattan to his Bedford-Stuyvesant home, Jomo Prince will try out a new plan to get over the East River. "I'm going to take the Manhattan."
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