London's wobbly bridge reopens

The bridge was forced to close within days of opening in June 2000

LONDON, England -- London's Millennium Bridge has re-opened to the public after a £5 million ($7 million) repair programme to correct a wobble which forced its closure after just three days.

The £18.2 million ($26 million) bridge -- designed to look like a shaft of light over the water -- swayed alarmingly when it opened in June 2000.

Queen Elizabeth II officially inaugurated the completed bridge at a dedication ceremony which was preceded by a service in St Paul's Cathedral.

But after just three days the bridge's embarrassed operators had to close the 320-metre long structure, which runs from St Paul's Cathedral north of the River Thames to the Tate Modern art gallery on the south bank.

Pedestrians gave different verdicts on the bridge.

Gill Gregorowski, of Richmond, south west London, who was with her husband

Paul, said: "It's pretty solid and there doesn't seem to be much movement at all."

But Mary Kidd, from Dulwich, south London, who was on the bridge on its opening

day two years ago, said: "I enjoyed swaying around then. It's a shame it's not more wobbly."

Engineers wrestled for months with the problem before submitting a recommended-solution report in September 2000.

Not only did they come up with a solution -- the installation of 37 viscous dampers, which act like shock absorbers, and 54 tuned-mass passive stabilizers, essentially weights on springs, which dampen the vertical motion -- they also gave a name to the problem.

A team at the Cambridge University Structure Laboratory discovered "synchronous lateral excitation," which basically is a science for how people walk.

The bridge, seen here with St Paul's Cathedral in the background, is London's only dedicated pedestrian bridge

The team found that each human step not only exerts a downward force, but there is a slight push toward the outside of each foot as it falls.

People walking on a swaying surface tend to compensate by spreading their legs further apart.

"What's more important is that you change the phase of your footfalls to match what the bridge is doing," said laboratory spokesman Allan McRobie, who describes this tendency to walk in time with the sway as an unconscious "biofeedback" effect.

"It wasn't well known. There had been some observations, but it's certainly not well known," he said.

If enough people are on the bridge, their lateral push eventually exceeds the structure's damping force, or its resistance to movement.

Engineers found that the bridge was stable with 156 walkers, but add 10 more and the movement increased suddenly.

Most of the dampers are underneath the bridge deck, and none compromises the overall design.

The work was completed in January this year, after which a series of tests was carried out to ensure that the problem was solved.

The engineers are confident that the bridge is solid, and that the public will forget the criticism that followed the first opening when it re-opens on Friday.

"I think when people are on it, they will just be glad it's there," McRobie said.

The Millennium Bridge is the capital's first dedicated pedestrian bridge.

It is also the first new river crossing in central London for more than 100 years, since Tower Bridge was opened in 1894.