Some experts in Turkey fear the Bosphorus Bridge may share the fate of the Tacoma Narrows Bridge, which collapsed due to wind-induced vibrations in 1940. AA photo
Vibrations created by the footsteps of the tens of thousands of people who crossed the Bosphorus Bridge on Sunday caused the span to swing dangerously, experts have said, a claim the Istanbul Highways Directorate disputed.
According to experts, the phenomenon behind the swinging of the bridge, called “resonance,” occurs when an external vibration – such as that caused by the wind or heavy pedestrian traffic – matches the natural sway of the span, creating a feedback loop that amplifies the motion, potentially to the point of collapsing the bridge.
The provincial highways directorate said in an official statement Monday, however, that the swinging of the bridge was due to resonance and that the motion observed in the bridge’s electricity pillars was within safe limits.
“In order to create resonance, all the pedestrians would have to have been taking their steps at the same time, as in military order, creating only one frequency level,” the statement said. It added that the bridge’s systems issue a warning if safe swinging limits are exceeded and no such warning was given during Sunday’s Intercontinental Istanbul Eurasia Marathon.
Professor Ahmet Vefik Alp, an engineer, disagreed, saying the situation on the bridge was very dangerous. “A great threat was avoided,” he told daily Milliyet on Sunday. Asked why the same thing did not happen during last year’s marathon, the only time each year that the bridge is open to pedestrians, Alp told the Hürriyet Daily News & Economic Review that the incident was the result of coincidence. “This time, the resonances overlapped,” he said.
This year’s marathon, which included a 15-kilometer race and an 8-kilometer “fun run,” also saw a large increase in the number of people participating, in part due to the sunny weather Sunday. Officials with the Istanbul Governor’s Office and the highways directorate, as well as police officials and bridge engineers, will meet with event organizers Tuesday for a routine evaluation of the marathon. They are also expected to discuss the swinging on the bridge and the massive amount of public participation in the event.
“Of course the [resonance] issue will be on the table. And not only the physical security of the bridge, but also the long time it took to get the crowd off the bridge and the problems caused by this will also be discussed,” Ozan Çakır, the coordinator of the Eurasia Marathon told the Daily News. According to Çakır, more than 200,000 people crossed the bridge as part of the marathon-related events. Due to the higher-than-expected participation, it took 100 minutes longer than the estimated time to reopen the bridge to traffic.
If bridge engineers and police officials conclude that the masses of people on the bridge put the structure and human life at risk, the organizers will reconsider how they structure the public race on the bridge, Çakır said.
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Construction experts said the fact that the resonance effect stopped after a time showed that the bridge had overcome the risk of collapse. A similar effect famously caused the collapse in 1940 of the original Tacoma Narrows Bridge in the United States.
Engineer Alp said pedestrians should no longer be allowed to cross the Bosphorus Bridge and advised officials to examine whether the motion created Sunday had caused any damage to the bridge.
Architect Orhan Erdil also said the resonance of the bridge created a dangerous situation that could have led to the bridge’s collapse, daily Milliyet reported. According to Erdil, the motion was caused by having so many people run over the bridge at the same time. “If a division of soldiers marched over a bridge, the same thing could happen again,” he said.
The number of participants in the Eurasia Marathon was limited in 2000 for bridge security after the massive attendance in a prior event. Some 150,000 people participated in 1999, less than the estimates for this year.
source: here