What Makes Bridges Wobble? Your Awkward Walk

A new study asks: How many people does it takes to set a pedestrian bridge a-swaying?

Millennium Bridge
The formerly wobbly Millennium Bridge Wikimedia Commons

Walking across a wobbly bridge can make even the most stoic among us get a bit panicky. But not all bridges wobble, and scientists have long puzzled over why. Now, reports Aylin Woodward at New Scientist, they may finally have an answer. A new study details what it takes to make some pedestrian bridges get their sway on. The cause: awkward strides.​

The study, published in the journal Science Advances, suggests that the primary source of wiggling bridges is actually the stride of pedestrians trying to make it across. As people walk along the bridge, they subconsciously adjust their stride to accommodate imperceptible motions in the bridge caused by things like wind. ​As more people walk along the bridge, they also make similar adjustments, which forces them to walk in a way that matches a bridge’s frequency. ​Once a critical mass is reached, those steps exert enough force to start the bridge wobbling.

It's a vicious cycle. Once the bridge begins noticeably moving, people exert even more force to stabilize themselves, leading to even more swaying.​

As Igor Belykh, professor of applied mathematics at Georgia State University and author of the study, tells Mary Beth Griggs at Popular Science, it’s akin to boating on the water. “Suppose you are on a boat. When the boat is steady you walk with your normal gait,” he says. “If there are stormy seas you will adjust your gait to keep your balance.”

That idea, called phase locking, has been proposed as the cause for bridge-wobbling previously. But Belykh calculated a mathematical model that can be used to determine how many gait-adjusting people it takes to make structures of different sizes sway. As Belykh tells Joanna Klein at The New York Times, his formula allows users to plug in data about the bridge to figure out just how big a crowd needs to be before it sets the span a-rocking. 

It’s not just a theoretical idea. In recent years there have been several notorious bridge-wobbles. In 2000, at the opening of London’s Millennium Bridge across the Thames, the bridge swayed worryingly, causing its closure three days later and a multi-million dollar fix. In 2014, the Squibb Park Bridge in Brooklyn closed for a three-year repair after it was found to be a little too bouncy for most pedestrians.

The wobble usually isn't a gradual change. Once the crowd hits critical mass it goes from a calm bit of suspended sidewalk to earthquake mode. For the Millennium Bridge, he tells Griggs the magic number is 165 people. “At 164 pedestrians, potentially nothing, then boom, the bridge starts wobbling when you have one or two additional pedestrians,” he says. “It’s a very complex relationship.”

While the formula could be useful in helping engineers design bridges and learn to counteract the wobbles, Belykh tells Woodward it's not yet ready for primetime. “Before any of these models end up in a civil engineering code, you have to collect evidence from each of these bridge swaying incidents – detailed studies of when they happen, the bridges’ properties and the number of people – and see if these models help make a good prediction,” he says.

Until then, just hold on tight.

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