The paper, published in the Proceedings of the Royal Society, says that the phenomenon is not related to the structural form of the bridge, but rather the behaviour of the pedestrians. The same pedestrian-structure interaction has also been identified on other bridges, including Bristol's Clifton Suspension Bridge.
The paper says that balance is achieved by changing the position of foot placement for each step, based only on the final displacement and speed of the centre of mass from the previous step. The same balance strategy as for normal walking on a stationary surface was applied to walking on a laterally swaying bridge.
Without altering their pacing frequency, averaged over a large number of cycles, the pedestrian can effectively act as a negative damper to the bridge motion, which may be at different frequency. The pedestrian can therefore inadvertently feed energy into bridge oscillations.
Senior lecturer in civil engineering Dr John Macdonald said: "It is clear that the motion of the bridge affects the force from the pedestrian, rather than the pedestrian simply applying an external force." It has generally been thought the Millennium Bridge 'wobble' was due to pedestrians synchronising their footsteps with the bridge motion. However, this is not supported by measurements of the phenomenon on other bridges, according to the research.
The researchers were surprised to find that pedestrians walking randomly, keeping balance as normal, can cause large bridges sway. This finally seems to explain the initiation of the Millennium Bridge 'wobble' and gives new insight for designing bridges to avoid vibration problems, they say.
