Everybody has experienced that strange sensation when a bridge starts to sway from side-to-side.
Scientists had thought it was due to people falling into step with each other while walking, causing a synchronised pendulum effect as everyone moves from left foot to right foot and back again.
But a new study has dismissed this so-called Kuramoto model, suggesting instead that bridges begin swaying when people walk with their own individual rhythm rather than copying each other.
Theory: According to new research, bridges tend to move more smoothly when people follow their individual rhythms than following each other’s.
Study shows how people can create bridges that are as wide as the Golden Gate (pictured).
Once these oscillations start, each person then tries to adjust their steps to stop themselves falling, which in turn destabilises the bridge even further.
The theory was put forward by researchers from Georgia State University.
‘Think of passengers walking on a boat rocking side-to-side in a stormy sea,’ said mathematician Igor Belykh, from Georgia State University.
‘They will adapt their motion both laterally and in a forward direction in response to the shaking of the boat.
‘In particular, they will slow down their forward motion.’
The researchers said this transfer of energy from footsteps to the bridge, and the swaying it causes, is an example of negative damping, where small vibrations cause much bigger end results.
They used the example of a rusty swing in a playground and how this can eventually be made to move if it has enough people applying force to it.
The scientists used observations of various bridge swaying events, as well as modelling and other experiments to come to their conclusion.
From the data they looked at, there was limited evidence of pedestrians walking in sync with one another.
The Millennium Bridge in London, which was closed for two years because of excessive wobbling, had been used as the main example supporting the Kuramoto model.
This is because video analysis showed the heads and torsos of pedestrians moving together as one.
‘This explanation was so popular, it has been part of the scientific zeitgeist,’ said Belykh.
However, Nobel-prize winner Brian Josephson was among the first to challenge the synchronization explanation of London Millennium Bridge instability.
In 2003, another bridge-swaying accident occurred.
The US East Coast blackout caused thousands of people to walk across the Brooklyn Bridge, New York.
Pedestrians claimed they felt sick and had trouble standing still.
After these oscillations begin, everyone tries to adjust their steps so they don’t fall, which further destabilizes bridge.
The Millennium Bridge in London (pictured), which was closed for two years because of excessive wobbling, had been used as the main example supporting the Kuramoto model, which suggested that synchronised walking was to blame for the swaying of a bridge
This study revealed that bridges were more susceptible to oscillations than originally thought.
They now want to analyze the movements of crowds more to support their hypothesis that natural variations in footsteps causes bridges to move, not synchronised walking.
A number of pedestrians crossing the bridge may also be used to determine the threshold that a bridge will oscillate before engineers can calculate it.
It is approximately 165 for Millennium Bridge, however.
Belykh states that bridge designers need to be alert for potential instances of adverse damping.
“Our formula allows us to estimate the number of people who will be using bridges,”
Nature Communications has published the results of this study.