Bridges are designed for people walking on them. That's the whole job. So it's worth knowing about the one that had to close because people walked on it.
Two days
The Millennium Bridge, a footbridge over the Thames between St Paul's and Tate Modern, opened on Saturday 10 June 2000. According to the engineers at Arup, who designed it, up to 100,000 people crossed it that day, with up to 2,000 on the bridge at any one time.
With that many people on it, the deck started to sway from side to side. When the crowd thinned out, the movement stopped. On Monday 12 June the bridge was closed.
Nobody did anything wrong
There was no marching band and no crowd jumping in time. People were just walking. So what happened?
Arup's explanation was a loop. A walking person pushes very slightly sideways with every step. When the deck starts to move sideways, people adjust how they walk to keep their balance, and those adjustments push the deck in time with the sway. More sway, more adjusting, more push. The engineers called it synchronous lateral excitation, and blamed people falling into step with the bridge.
Later tests put the tipping point at around 165 people. Below that number, the bridge copes. Above it, the loop takes over.
Maybe not even in step
The "falling into step" part has since been questioned. A study published in Nature Communications in 2021, by a team including Igor Belykh of Georgia State University, found that the wobble can start before walkers fall into step at all. In their model, each person's small corrections for balance are enough on their own to feed the sway.
So who broke it? In a sense, everyone, and politely.
The fix
The bridge stayed closed for almost two years. Engineers added dozens of dampers under the deck to soak up the movement: 37 viscous dampers plus tuned mass dampers, according to the people who designed and supplied them. In November 2000 the full remedy was priced at £5 million. The bridge reopened in February 2002 and hasn't wobbled like that since.
Londoners still call it the Wobbly Bridge.
What's illustration
Our video tells this story with an illustrated crowd. Otto wearing number 165 is a joke: nobody was given a number, and nobody was the one who tipped it over. The people, the light and the colours are drawings, not footage.
Sources
- Fitzpatrick and Ridsdill Smith (Arup), "Stabilising the London Millennium Bridge", Ingenia, August 2001
- Science News, 2017, on how a crowd's walk can make a bridge wobble
- Georgia State University news release on Belykh et al., Nature Communications, December 2021
- Belykh et al., "Emergence of the London Millennium Bridge instability without synchronisation", Nature Communications, 2021
- Building, "Arup puts reputation on the line over bridge", 24 November 2000
- Taylor Devices, case study of the Millennium Bridge dampers
- Wikipedia, Millennium Bridge, London
