Because you are a the most reliable person who doesn’t text and drive, when you roll over a bridge your cell phone is stuck, while it’s probably giving you directions while you’re streaming the WIRED podcast. But in the background, your device is also collecting accelerometer data. Someday, this may help you identify problems with the bridge you are crossing.
Each bridge has its own “frequency,” or how much vibration travels through it, then into the car and your phone. (Tall buildings, which are shaken by wind or earthquakes, also have frequencies.) “Stiffness, weight, length—all these factors will affect frequency,” said Thomas Matarazzo, an architect and engineer. at MIT and the United States Military Academy. “If we see a significant change in the shape of the bridge, then the modal frequency will change.” Think of it as a bridge temperature—changes can be a symptom of other diseases.
In the US, most of the bridge infrastructure was built to support the traffic culture after World War II, and it is getting old and unstable. Funny Among the Weird: Earlier this year, a bridge in Pittsburgh collapsed hours before President Joe Biden was scheduled to visit the city for a construction summit. The 2007 collapse in Minneapolis killed 13 and injured 145, and the 1993 failure of a railroad bridge near Mobile, Alabama, killed 47 and injured more than 100.
To monitor for cracks, corrosion, and other defects, some bridges are equipped with expensive sensors that can detect modal changes. But most bridges around the world — there are 600,000 major bridges in the US alone — don’t have these sensors. (They didn’t set-it-and-forget-this: It takes hundreds of sensors to cover a very long bridge, and you have to replace their batteries and download data every few months.) Instead, bridge operators rely on slow, labor-intensive monitoring. .
So engineers need a better way to analyze modal frequencies, cheaply and in real time. In a new paper in the journal Nature Communications Engineering, Matarazzo and colleagues describe the use of ordinary cell phones in transit vehicles to accurately calculate the modal frequencies of the Golden Gate Bridge. This could pave the way (sorry) for a future where thousands of phone calls back and forth across the bridge can measure the health of the bridge, alerting inspectors to problems before they are visible to the naked eye.
The researchers began with a controlled experiment, in which they collected data by driving on the Golden Gate Bridge with cell phones on their backs. They know all the variables: What kind of car, their speed, where they are at any given time, and where the phones were in the car. As they drove, the phones collected data from their accelerometers, which measured the movement – in this case the vibration of the car. This allowed the researchers to accurately measure the bridge’s modal density, which matched data from traditional sensors that had already been installed remotely.