introduction
oscillation
test programme
results
solution
modifications

video

The bridge opened to the public on 10 June 2000 when an estimated 80,000 to 100,000 people crossed it. As with all bridge structures, the Millennium Bridge is subject to a degree of movement. However, when large groups of people were crossing, greater than expected sideways movements occurred. The maximum sway of the deck was approximately 70mm. In order to fully investigate and resolve this phenomenon the decision was taken to close the bridge on 12 June. The movement and its effect on the crowds can be seen from the video footage.

A programme of research was undertaken during the summer of 2000. A solution was then developed using the results of these tests. Arup has warned other bridge designers of their findings and the British Standard code of bridge loading is being updated to cover the phenomenon, now becoming referred to as Synchronous Lateral Excitation.

The research indicated that the movement was caused by the sideways loads we generate when walking. Chance correlation of our footsteps when we walk in a crowd generated slight sideways movements of the bridge. It then became more comfortable for people to walk in synchronization with the bridge movement.

This instinctive behavior ensures that the sideways forces we exert match the resonant frequency of the bridge, and are timed such as to increase the motion of the bridge. As the magnitude of the motion increases, the total sideways force increases and we becomes more correlated.

The sway movement is not specific to the Millennium Bridge. The same excessive sway movement could occur on other bridges, future or existing, with a lateral frequency under ~1.3 Hz and with a sufficient number of pedestrians.

During the investigations Arup discovered that other bridges with completely different structures to the Millennium Bridges have swayed sideways when crowded, for example the Auckland Harbour Road Bridge during a demonstration in 1975, which can be seen in the video footage. These cases have not been widely published and as a result the phenomenon has not become known to practicing bridge engineers.