The Millenium Bridge over the Thames was opened on 10 June 2000, but was closed three days later after complaints of people feeling “sea sick”. The problem was caused by the people walking synchronously. When people walked across the lightweight bridge it vibrated at its resonance frequency and people subconsciously adjusted their walking pace so that it matched the resonance frequency of the bridge, as this causes the least resistance to a person walking. However when several hundred people are all exciting the bridge at the resonance frequency, it causes the bridge to sway. Engineers at ARUP tested the bridge and analysed the data. The remedy was to install dampers to reduce the vibration.
Read more about this story on the ARUP web site.
Read the article on the BBC web site.