On the day it opened in 2000, London's Millennium Bridge rocked and swayed so much that it had to be temporarily closed.
At the time scientists thought they had pinpointed the cause, but a new paper in the journal Proceedings of the Royal Society A casts doubts on their findings and offers up a new explanation.
The wobble, said scientists at the time, was due to a phenomenon called 'pedestrian lock-in', whereby the pedestrians altered the way they walked to compensate for slight movement in the bridge. When they did, they synchronised their steps with the vibrations of the bridge, causing the wobble movement to increase.
However John Macdonald from the University of Bristol has found flaws in this reasoning. Instead, he believes that it is all down to the way that pedestrians balance when they walk regardless of whether they synchronise their steps.
Macdonald looked into studies of similar movement recorded on bridges such as the Clifton Suspension Bridge and the Changi Mezzanine Bridge, which lacked evidence to show that pedestrians had synchronised their footsteps. He created models of the way people balance when they walk, and used them to show the forces that would be applied to the bridge with each step.
These models revealed that even when pedestrians stepped at random, not in sync with the bridge's movements, the way they placed their feet in order to balance provided a force which acted as a 'negative damper'. That means they increased the movement of the bridge rather than acting as shock absorbers.
This research provides useful insight for tackling problems of movement in future bridge designs.