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Lateral thinking solves puzzle of London's wobbly Millennium bridge

Most bridges could be made to sway by pedestrians in similar fashion to the wobble that forced the closure of the Millennium Bridge in London only days after it opened in 2000.

The cause of the phenomenon that gave the “wobbly bridge” its nickname has been identified by new research, offering clues that could help engineers and architects to minimise such lateral movement in future designs.

The £18 million Millennium Bridge, designed by Lord Foster of Thames Bank and the engineers Arup, was opened as the capital's first new river crossing for more than a century on June 10, 2000. The 320-metre structure was described as a “blade of light” across the Thames.

As crowds started to cross it, however, the bridge began to sway dramatically. It was closed for investigations and reopened 20 months later after dampers had been installed to prevent excessive motion, at an extra cost of £5million.

The study by John Macdonald, a civil engineer at the University of Bristol, suggests that the sway did not arise because of pedestrians synchronising their walking movements, as previous research had suggested.

It has long been understood that a large number of people walking in step can cause bridges to move: this is why soldiers are ordered to break step when marching across them. Scientists had thought that something similar happened by accident on the Millennium Bridge.

Dr Macdonald's work suggests that the natural walking movement of thousands of people was responsible. As they altered their gait to maintain balance, extra energy was transmitted to the bridge, amplifying its sway.

The effect was not related to the structure of the bridge, and could thus influence other designs. Similar if less pronounced swaying has even been observed and measured elsewhere, for example on the Clifton Suspension Bridge in Bristol.

“This could potentially affect any bridge at all,” Dr Macdonald told The Times. “A lot of people blamed the bridge for what happened in 2000, but it can happen to all sorts of designs.

“It's been seen on a large truss bridge in Canada, on a big concrete road bridge in New Zealand and on a railway bridge in Birmingham that nobody's ever heard of. The extent to which they do it varies, but the phenomenon is the same.”

His findings suggest that Arup, the bridge's engineers, got it right by installing dampers to reduce lateral movement. Dr Macdonald's calculations match the engineering firm's model and suggest that a repeat of the opening wobble is unlikely.

The study, which is published in the journal Proceedings of the Royal Society, used the latest biomechanical research into how people walk to investigate how the movement of thousands of people would have affected the bridge.

When people walk, they effectively fall from their left leg on to their right, and back again. When lateral movement of a bridge pushes a walker slightly to one side, he or she will tend to fall farther to the other. Dr Macdonald's modelling shows that this imparts a little extra energy to be bridge, adding slightly to its motion.

Normally such motion is imperceptible but with thousands of pedestrians thronging to a newly opened landmark, their combined effect was strong enough to be felt. When the bridge began to sway, pedestrians synchronised their walking movement, which added to the problem.

The movement would have been unlikely to have affected the structural integrity of the bridge, Dr Macdonald said. There could be a risk of fatigue, however, on a bridge that regularly experienced a pronounced wobble.

Reference: Mark Henderson, Science Editor. Times Online, Dec 17, 2008