Bridge designers need to realize that there can always be negative mitigation if the crowd is large enough.

Researchers at Georgia State University have come up with a surprising new explanation for why pedestrian bridges suddenly begin to shake and move when so many people are crossing at the same time.

This new interpretation contradicts the 20-year-old belief that bridges vibrate because the frequency of their vibrations corresponds to the frequency of footprints, which is scientifically known as vibration.

Large moving energy of individual steps

A New study Published in the journal Nature Communications on December 10, a team of Georgia mathematicians led by Professor Igor Belik and colleagues from the University of Bristol and the University of Cambridge showed how a high-designed bridge like the University of Leicester Golden in the UK. Gate Bridge or Brooklyn Bridge suddenly becomes unstable.

And step A statement Published in Science Alert, scientists say that when enough people pass on foot, as each one walks at their natural speed, they transfer more energy to the bridge, which begins to oscillate with it. As the bridge begins to vibrate, each person adjusts their steps to avoid falling, and this disrupts the bridge.

At the 50th anniversary of the founding of the Golden Gate Bridge in San Francisco in 1987, 300,000 people – when they came to celebrate its opening – the swing of the bridge caused massive panic. The bridge, took a sway, so caused panic and dizziness and people threw bikes and trolleys into the sea to reduce the load on the bridge.

Suspension bridges move like a boat on the sea (Getty Images)

The researchers studied data collected on 30 bridges and, using complex mathematical equations, predicted the exact transition point to the swing position of a given bridge, which determined the exact number of people moving.

And inside Press release “Our work shows that we can dramatically amplify the tiny vibrations that every person walking on the bridge creates,” Belich said on the Georgia State University website.

Negative mitigation and vibration amplification

In 2003 New York City lost power, the crowd of pedestrians returned home via the Brooklyn Bridge in the dark, and a large number of pedestrians shook the bridge so badly that some pedestrians felt the seashore and could not maintain their balance when they stopped moving.

In 2000, the Millennium Bridge in London was renamed the Rolling Bridge because its oscillating movement crossed the crowd on opening day and the bridge was closed immediately and never reopened for two years. When you watch videos of pedestrians on the Millennium Bridge you will notice that their heads and torsos move in the same direction as the wave.

The Millennium Bridge in London is stable with only 165 people walking on it (Unsplash)

A study by Belich and colleagues in the journal “Scientific advances) In 2017, it will be stable when 165 people walk over the Millennium Bridge in London, and adding another person will completely change the tables. Therefore, the researchers were interested in studying the effect of the number of intermediates at the turning point of the bridge from the state of stability, which is scientifically known as negative damping, to a state of continuous oscillation without fading or decreasing.

“Imagine a passenger walking on rough seas, adapting the boat’s left and right and forward to the boat’s vibration; they slow down the passenger’s forward movement, which Belize has scientifically called” negative mitigation or non-mitigation “, and then start motion. .

“Bridge designers need to realize that there can always be negative mitigation events if the crowd is large enough,” Belich added. “Our developed formula provides useful estimates of the number of pedestrians expected to be on a bridge.”