Why the Millennium bridge experienced unexpected swaying?

Due to the crowd- 👫 loading and synchronous lateral excitation caused on an opening day, the London Millennium bridge was swaying too much in an uncomfortable manner for pedestrians use.

Synchronised walking of pedestrians caused an occurrence of the natural frequency of sway/lateral movement less than 3 Hz.

Crowd loading caused outer span oscillations of about 50 mm at 0.8 Hz and at the centre span, a sway of about 75 mm at 1 Hz was recorded.

The solution was to install Viscous Dampers to alternate lateral supports and increase vertical damping, therefore, the bridge will not oscillate again.