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Millennium Bridge
The Millennium Bridge The Millennium Bridge
Some 80,000 people crossed the bridge on its opening day and those on the southern and central spans detected vibrations.
The bridge began to sway and twist in regular oscillations. The worst of the movement occcured on the central span where the deck was moving by up to 70mm. The frequency of the oscillations increased, leaving people unnerved and unsteady.
The engineers insisted the bridge wouldn't fall down but closed it completely after an attempt to limit numbers proved unworkable.
Engineers discovered that the sideways forces of the pedestrians' footsteps created a slight horizontal wobble in the brigde.
As the structure began moving, pedestrians adjusted their gait to the same lateral rhythmn as the bridge.
The adjusted footsteps magnified the motion - just like when four people all stand up in a small boat at the same time.
As more pedestrians locked into the same rhythm, the increasing oscillations led to the dramatic swaying captured on film.
The sideways motion has been seen before, most notably in 1975 on the Auckland Harbour Bridge in New Zealand.
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