Workers rappel down the side of the Brooklyn Bridge to install highly sensitive accelerometers to measure vibrations as part of a seismic vulnerability study. (Photo: Aliya Turner)
Dozens of bridges in the world are now studded with advanced sensors that monitor the bridges’ structure and could avert collapses. From IEEE Spectrum’s “Responding to Disasters: From Prediction to Recovery” special, Prachi Patel reports on what it will take to rig more bridges with this technology.
GELLERMAN: In August 2007, the I - 35 West bridge in Minneapolis collapsed into the Mississippi river. Rush hour traffic was bumper to bumper, as eight lanes of the span came crashing down. Thirteen people died - more than 140 were injured.
Sensors could have warned engineers the bridge was about to buckle but this one didn't have them, and even today, most bridges in the United States don't have them.
But that may be changing, as Prachi Patel reports. Her story is part of the IEEE Spectrum program “Responding to Disasters, from Prediction to Recovery.”
NEWSCASTER: The bridge over Interstate 35 West has absolutely collapsed. It collapsed onto….
PATEL: The 40-year old bridge in Minneapolis came down because of a faulty gusset plate. Not something a bridge inspector would have noticed, but a strain sensor at that gusset might have picked it up. Dozens of the largest and most complex bridges in the world are already studded with hundreds of sensors, like strain gauges and tilt meters.
MOON: We measure things like acceleration, and rotations or tilts, and also this kind of internal stress that the members are feeling, this kind of internal stretch that they undergo.
PATEL: Franklin Moon is a civil engineering professor at Drexel University.
Workers install strain gauges and accelerometers, which measure deformation and vibrations respectively, to the Burlington-Bristol Bridge north of Philadelphia as part of a permanent structural monitoring system. (Photo: Jeffrey Weidner)
MOON: What the technology does is capture the way a bridge responds, you know, whether something has changed. If something fundamentally changes in the structure, which typically is not a good thing, you can pick it up by monitoring that. I think there are a lot of parallels, it gets use and maybe over-use, but there are a lot of parallels between, you know, human health care and what we should be doing for our bridge population. Just like when you turn 40, or 50, or 60, you're kind of expected to undergo different types of tests for health reasons, etc.
PATEL: Well, the average age of an American bridge is 42. Most were designed to have a 50-year lifespan and many have exceeded that. So why aren't we installing sensing systems on more of our aging bridges? Here's Andrew Foden, an engineer at an infrastructure consulting firm.
FODEN: There's about 530,000 bridges in the United States. For standard highway bridges, which are the bridges you drive across every day and don't even notice you're going over them, sensor technology is very rarely deployed, except for in experimental conditions. Sensor technology is more widely used on signature bridges, such as, you know, the George Washington Bridge, or Sunshine Skyway Bridge in Florida, those types of structures.
PATEL: A signature bridge like those Foden mentions would typically cost hundreds of millions, or even billions, of dollars to replace, he says.
A worker attaches equipment to monitor the integrity of the Burlington-Bristol Bridge that connects New Jersey to Pennsylvania. (Photo: Jeffrey Weidner)
FODEN: For a typical highway bridge that would cost somewhere between 500,000 and two million dollars, it just doesn't make sense to apply 200,000 dollars worth of sensors. The sensors themselves are generally relatively low in cost. You're talking about from tens of dollars to a couple hundred dollars to even the most expensive sensors are typically just a thousand to two thousand dollars. The real cost is the labor involved in installing the sensors, and then there's another component of the cost which is analyzing the data and making some sense of it.
PATEL: Drexel's Franklin Moon agrees that cost is holding back the use of bridge monitoring technology. But there are other reasons, too.
MOON: The infrastructure is government-owned. This isn't like Boeing, where, you know, they may decide we're gonna make an investment into creating a better airplane and then eventually we'll make a profit at the end of that because of that investment. You don't have that same driver for innovation when it comes to public infrastructure.
PATEL: But Moon says that's changing with the possibility of public-private partnerships and private companies operating leased infrastructure.
MOON: So it's kind of an exciting time for infrastructure in that way.
PATEL: Not only that, sensors, and methods for analyzing the massive amounts of data they generate, are also getting better and more economical every day. Which means we should soon see many more U.S. bridges getting smarter and safer. For Living on Earth, I’m Prachi Patel.
GELLERMAN: That story is part of the IEEE Spectrum, National Science Foundation program “Responding to Disasters, from Prediction to Recovery.” For more information, go to our website - LOE dot org.
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