When will the millennium bridge work

A.� The Millennium Bridge, the pedestrian bridge linking the north and south side of the Thames, was opened last year. However it had to be closed almost instantly because when people walked on it, it wobbled rather dramatically.

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A definite re-opening date has yet to be announced but engineers hope it will be by the end of this year.

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Q.� What caused the wobble

A.� Rather problematically for a pedestrian bridge, the wobble was caused by the pattern or step that people fell into when they walked on the bridge.

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Q.� Why does everyone walk in step

A.� When in confined spaces, people tend to fall into step mainly because there isn't enough room for everyone to maintain their own individual step.

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On the millennium bridge, if the group of people is large enough then their collective rhythmic step is sufficient to create a lateral force strong enough to cause the bridge to sway slightly.

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To exacerbate the situation everyone naturally tries to steady themselves in respondse to the rhythm of this sway by walking in time to it so that eventually everyone on the bridge is walking in time causing it to wobble significantly.

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Q.� Why does walking, putting one foot down after the other, create a lateral force

A.� When we walk we create a downward force as we bring each foot down onto the surface, but at the same time we also create a side to side force as our weight is transferred from one side of our body to the other.

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If just one person produced this force, no problem, but when everyone on the bridge produced this force at the same time, due to synchronised steps, it proved to much for the bridge.

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Q.� But isn't the bridge a rigid structure

A.� No, it's designed to move a little, to allow it to be flexible. If it was completely rigid it would be unduly susceptible to stress.

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Q.� Can it be fixed

A.� The engineers certainly hope so. Damping solutions are being installed underneath the deck of the bridge that will absorb the lateral and downward forces of the people walking above.

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Do you want to know when something will work Clik here to ask.

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by Lisa Cardy