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Steady on

发布时间:2019-03-07 05:18:01来源:未知点击:

By Ian Sample Engineers for London’s £18m Millennium Bridge have finally worked out how crowds of pedestrians made it wobble so alarmingly on its opening day. The answer is a simple relationship. As the bridge sways, people’s feet exert sideways forces on it to keep their balance. These forces increase directly with increases in the extent of swaying, say the bridge’s consulting engineers, Arup. “This is absolute magic,” says Pat Dallard, Arup’s structural advisor for the bridge. The relationship underlies the positive feedback loop in which the more the bridge sways, the more people’s unsteady walking synchronises to make it sway. The insight comes eight months after safety fears closed the bridge. It follows a series of unsuccessful experiments carried out by the engineers and academics around the UK. The engineers got to the root of the problem by first measuring how much the bridge wobbled when crowds were walking on it. From this, they worked out how much sideways force the crowd must be exerting on the bridge to achieve the swaying. When they looked at the results, they found that the force people pushed into the bridge as they walked increased directly with the extent of swaying. The swaying begins with slight vibrations in the bridge structure caused by wind and people joining the bridge. This prompts people to start adjusting the way they walk. “You place your feet further apart to steady yourself, just like when you’re on a train,” says Dallard. This increases the sideways force each person exerts on the bridge as they walk, he says. But everybody in the crowd steadies themselves in the same way – because they all feel the same swaying motion – so they end up walking in step. The co-ordinated crowd is then pushing pulses of sideways forces into the bridge, which makes the bridge sway even more. This closes the positive feedback loop, because the more the bridge sways, the more force people exert to keeping standing. To fix the problem, Arup plan to fit the bridge out with vibration-absorbing dampers, at a cost of £5m. The dampers will be strapped beneath the walkway of the bridge, and will prevent both sideways and vertical vibrations becoming too large. According to Arup, once a decision has been made on who will fund the remedial measures, the work could be completed within months. A full-length feature on the troubles of the Millennium Bridge will appear in New Scientist magazine in March. More at: