Because you’re a very responsible person who doesn’t text and drive, when you roll over a bridge your smartphone is stuck to the dash, where it is perhaps giving you directions while streaming a WIRED podcast. But in the background, your device is also gathering reams of accelerometer data. One day, this could help diagnose problems with the very bridge you’re speeding across.
Every bridge has its own “modal frequency,” or the way that vibrations propagate through it—then subsequently into your car and phone. (Tall buildings, which sway in the wind or during an earthquake, have modal frequencies too.) “Stiffness, mass, length—all these pieces of information are going to influence the modal frequency,” says Thomas Matarazzo, a structural and civil engineer at MIT and the United States Military Academy. “If we see a significant change in the physical properties of the bridge, then the modal frequencies will change.” Think of it like taking a bridge’s temperature—a change could be a symptom of some underlying disease.
In the US, much of the bridge infrastructure was built to support car culture after World War II, and it’s getting old and unsound. Irony among ironies: Earlier this year, a bridge in Pittsburgh collapsed hours before President Joe Biden was scheduled to visit the city to talk about infrastructure. A 2007 collapse in Minneapolis killed 13 and injured 145, and the 1993 failure of a railroad bridge near Mobile, Alabama, killed 47 and injured over 100.
To monitor for cracks, corrosion, and other defects, some bridges have expensive sensors that detect how their modal frequency changes. But the vast majority of spans around the world—there are some 600,000 highway bridges in the US alone—lack these sensors. (They’re not set-it-and-forget-it: It takes hundreds of sensors to cover a particularly long bridge, and you’ve got to swap out their batteries and download data every few months.) Instead, bridge operators rely on slow, labor-intensive visual inspections.
Engineers, then, need a better way of monitoring modal frequencies, ideally cheaply and in real time. In a new paper in the journal Nature Communications Engineering, Matarazzo and his colleagues describe how they used ordinary smartphones in passing cars to accurately estimate the modal frequency of the Golden Gate Bridge. That could pave the way (sorry) for a future in which thousands of phones going back and forth across a bridge could collectively measure the span’s health, alerting inspectors to problems before they’re visible to the human eye.
The researchers began with a controlled experiment, in which they collected data by driving across the Golden Gate Bridge with smartphones on their dash. They knew all the variables: What kind of car they were in, their speed, their location at any given time, and where exactly the phones were in the car. As they drove, the phones collected data from their accelerometers, which measure movement—in this case the car’s vibrations. This allowed the researchers to accurately measure the modal frequency of the bridge, which matched data from traditional sensors that had already been deployed along the span.