If you've been on a bridge with a lot of people on it at the same time, you may have experienced some swaying and wobbling. There is a new explanation for the movement in the structure.
The thought was that people fall into step with each other while they're walking, causing a synchronized pendulum effect as everyone moves from left foot to right foot and back again. The model is called the Kuramoto.
The new hypothesis shows that bridge oscillations can begin with many pedestrians walking with their own rhythm. Each person tries to stay upright once the swaying starts, with these adjustments destabilizing the bridge even further.
"Imagine passengers walking on a boat in a storm," says Belykh.
They will adapt their motion in response to the shaking of the boat. They will slow down their forward motion.
The transfer of energy from footsteps to the bridge is an example of negative damping, which can cause much larger end results. If enough people apply force to it, it will eventually be made to move.
The team crunched the numbers using observations of different bridge swaying events, experiments and modelling to reach their conclusions, although some events were recorded in more detail than others. The records did not mention pedestrians walking in sync.
The Millennium Bridge in London was used as the primary case study for putting forward the Kuramoto model. Video analysis showed heads and torsos of pedestrians moving together.
The Millennium Bridge is in London. There is a picture of (Johan Mouchet/Unsplash).
"This explanation has been part of the scientific zeitgeist, it was so popular," says Belykh.
The Brooklyn Bridge in New York started to sway in 2003 when an East Coast power failure caused so many people to walk over it. Pedestrians were unable to keep their balance if they stood still.
The researchers found that bridges in general are more vulnerable to swaying than previously thought.
The researchers want to do more work on the movement of people in crowds in order to back up the explanation that naturally varying footsteps rather than synchronized walking causes this effect.
The threshold for each bridge is outlined in earlier work by the team, and it's about 165 people for the Millennium Bridge. It's not easy to calculate, but in the future, engineers could figure this out in advance.
"Bridge designers should be aware that there could be dangerous instances of negative damping," says Belykh.
Our formula gives useful estimates for the number of pedestrians using the bridge.
The research has been published.
