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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.