A Cheap Wireless Way to Monitor America's Crumbling Bridges

America’s infrastructure is crumbling. Our bridges, one of the most potentially dangerous parts of our roads and highways, are absolutely falling apart. According to the U.S. Department of Transportation, as of December 2008 161,892 bridges were classified as structurally deficient or functionally obsolete. That’s more than a quarter of the bridges nationwide, a fraction that rises to a third in urban areas.

According to the American Society of Civil Engineers, transportation infrastructure in the U.S. gets a D rating. The organization says the U.S. needs to be spending $17 billion a year to “significantly improve” our nationwide system of bridges. We currently spend about $10.5 billion a year. The ASCE also says bridges are usually designed to last 50 years; our bridges average 43 years old nationally. The problem’s bad, and only getting worse.

At this point, it’s less a question of if bridges start collapsing than when, and with inspections rarely more than annual affairs, remotely monitoring their condition is key. Newer bridges are built with sophisticated wired sensor arrays that measure a huge number of structural parameters over the life of the bridge to help monitor the bridge’s health and maintenance needs while providing warning before a catastrophic failure. But for older spans, installing wired arrays is prohibitively expensive.

Thankfully, that’s about to change. Mehdi Kalantari, an electrical engineering researcher with the University of Maryland, has developed a new bridge-monitoring system that’s completely wireless and is about one-one-hundreth the cost of comparable wired systems.

Kalantari said over the phone that, when trying to think of applications for his past wireless communications research, his interest was piqued by reports showing the poor condition of the country’s bridges. Then, after the 2007 collapse of a Minneapolis bridge that killed 13 and injured 145, he delved fully into the subject.

"The main motivation was that, again, since this was before the tragic events in Minneapolis, the motivation for the work that I was doing on bridges was the public information available on the overall state of infrastructure," he said. "The information showed that something like 12 percent of the operational bridges in the U.S. are structurally deficient. To us, that information clearly showed the need."

Kalantari’s system consists of tiny sensors, most powered passively and others by battery, that constantly transmit specialized data on a bridge’s stress levels. The sensors are personalized to each bridge’s design, weak points, ambient weather conditions and known issues, like cracks that aren’t yet structurally threatening.

"The sensors are designed to be extremely low powered and very small and of course wireless," Kalantari said. "The sensors are specifically designed for main span attachments, a component of the bridge that could be critical."

The sensors include strain gauges and accelerometers, which are normal tools for structural assessment, along with sensors for vibration and thermometers, which both provide data that can help estimate material fatigue. The data are sent to a central computer that gets a constantly-updating picture of the bridge from a structural level.

"[The sensors] start measuring structural quantities that affect the structural integrity," Kalantari said. "At this point they include strain, which is the technical term for the amount of load that is being carried through a member of the structure; other things like vibration and acceleration; and some measure temperature; or, in the presence of a crack in the structure, whether it’s growing. The sensors are well tailored, not only for the components of the bridges but for the structural quantities that they are intended to measure and track so the information can be used to [assess] structural issues of the bridge."

Kalantari said the sensors are designed to last twenty years with next to no maintenance. Because they are wireless, they not only eliminated the need for routing and weatherproofing huge looms of wires on bridges not designed for them, but also have are easily and flexibly installed. Right now each sensor costs about $20, with an average-sized highway bridge requiring about 500 of them. Kalantari founded Resensys LLC to commercialize the system, which should start increased production this fall.

“Potentially hundreds of lives could be saved,” Kalantari said in a press release. “One of every four U.S. highway bridges has known structural problems or exceeded its intended life-span. Most only get inspected once every one or two years. That’s a bad mix. … This new approach makes preventive maintenance affordable – even at a time when budgets are tight. Officials will be able to catch problems early and will have weeks or month to fix a problem.”

Photo via University of Maryland