To Care for Aging Bridges, Minnesota Taps the Power of Drones
How do you inspect a bridge for signs of dangerous deterioration? It’s not easy.
You’d either need to lower yourself on rope over the guard rails, or use specialized equipment like a “snooper”—a heavy-duty truck with a long, extending arm. Those can set back transportation departments as much as $1 million each.
And you need a lot of them: More than 55,000 bridges are in need of repair or replacement in the U.S., according to the American Road and Transportation Builders Association. Then there’s the labor cost, public inconvenience, and safety hazard of lane closures, which create especially vulnerable environments for crashes.
That’s why Minnesota, land of many lakes and some 20,000 bridges, began experimenting with drones in 2015. Unmanned aerial vehicles can scan entire bridge lengths and nose into hard-to-reach cracks for a fraction of the cost of traditional inspection methods. A new short documentary by Van Alen Institute, in collaboration with CityLab, shows how.
Drones can hover close to aging trusses, piers, and bearings, snapping thousands of high-resolution images. Special software then stitches these images into 3D models, which engineers can examine on computers; a single click on a small detail can call up a library of photos stretching back in time—a useful reference for timing a structure’s deterioration rate.
Some drones, like the senseFly Albris model, have cameras that peer straight up and down, rather than merely ahead. That’s useful for bridge inspections, explains Barrett Lovelace, a regional manager with Collins Engineers, Inc.:
“When we get underneath bridges, we can see underneath the deck,” he says.
Access is a major challenge for rigorous inspections. So other drone models, like the decahedral Elios, are designed to crawl, spider-like, along high abutments or narrow holes.
It’s no accident Minnesota is pioneering this use of a new technology. On an August evening in 2007, a span of I-35 over the Mississippi River in Minneapolis buckled, snapped, and plunged into the water and rocky shore below. The bridge—one of the busiest in the state—had been carrying bumper-to-bumper cars, trucks, and a school bus. Thirteen people died, and 145 more suffered injuries.
That tragic incident became the face of a national infrastructure crisis, and a wake-up call for Minnesota. In 2008, the state launched a $2.5 billion program aimed at overhauling hundreds of structurally deficient or fracture-critical bridges. A decade later, most of them have been replaced or repaired, or are soon to be. And protocols for planning bridges are now much more stringent, since the cause of the 2007 collapse turned out to be a design flaw—not, in fact, a maintenance issue.
According to Sarah Sondag*, a metro bridge inspection engineer with the Minnesota Department of Transportation, drones will never entirely replace in-depth human inspection. “They’re one tool,” she says in the film. Soon, MDOT will begin to identify which types of drones are most effective than others, on which types of structures, in order to develop a statewide plan.
Testing these whirring, uncanny devices on Minnesota’s state bridges tends to draw an audience, but that’s a good thing, Sondag says. “I think if we use [drones] effectively, we can show that we’re using the public money effectively, and efficiently performing these inspections.”
The one drawback to using these unmanned crafts? Lovelace says inspectors are getting less exercise.