As Suspension Bridges Age, A Search For Failures
A picture of a weather simulation chamber containing a suspension bridge cable. - Engineers created this weather simulation chamber, which has heat lamps, artificial rain and salt, to test the accuracy of their sensors embedded in the cable. (Joe Palca/NPR)
How do you know when a suspension-bridge cable is about to fail? That's the question engineers at Columbia University in New York are trying to address in a new experiment.
Most suspension bridges share the same basic design: Giant cables strung between two towers are anchored to the shore on either side. The roadway hangs suspended below, with all its weight pulling on the main cables.
And these bridges are built to last; some of them are more than 100 years old, including New York City's Brooklyn Bridge. San Francisco's Golden Gate Bridge is more than 70 years old. But none of these bridges will last forever.
(Joe Palca/NPR)
Cables — bundles of thousands of thick steel strands spun together — are typically visually inspected once or twice a year. But Columbia engineers are looking for a more systematic way to inspect cables — and, in the process, to predict a cable's life span.
So they are testing a system where tiny sensors are embedded inside the cable bundles to track the cable's condition round-the-clock.
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STEVE INSKEEP, host:
Some engineers are working on another mystery. These engineers are at Columbia University in New York, and they want to know how ell the cables on suspension bridges are holding up. They're working on what they hope will be a more accurate way of telling when a failure is imminent. Their study is relevant for bridges anywhere in the world, but this project has particular importance to people in New York.
NPR's Joe Palca has our report.
JOE PALCA: There are 10 suspension bridges in the greater New York City area. There's the Bear Mountain Bridge, the Mid-Hudson Bridge, the Triborough Bridge, the Manhattan Bridge, the Williamsburg Bridge, the George Washington Bridge, the Throgs Neck Bridge, the Bronx-Whitestone Bridge, the Verrazano Bridge, and, of course, the granddaddy of them all, the one I'm standing on now: the Brooklyn Bridge.
These bridges were all built to last, but none of them will last forever. How long they'll last is a question they're trying to answer about eight miles north of the Brooklyn Bridge in a cavernous room on the campus of Columbia University.
Professor RAIMONDO BETTI (Civil Engineering, Columbia University): The uniqueness of these bridges is that some of them are over 100 years old.
PALCA: That's Raimondo Betti. He's a professor of civil engineering and engineering mechanics at Columbia University. Betti is in charge of the program to build and test a new sensor system that will measure the health of bridge cables.
Prof. BETTI: One of the problems that these bridges have is that there is corrosion inside these cables. And so the question arise: What is the strength of the cables after 100 years of service?
PALCA: Most suspension bridges share the same basic design: giant cables are strung between two towers and are anchored to the shore on either side. A roadway is suspended below the cables. The cables themselves are bundles of thousands of thick steel strands. Right now, cables are inspected visually once or twice a year. That's okay, but obviously, a rapidly developing problem can be missed.
Raimondo Betti's idea is to embed tiny sensors inside the cable bundles to track the cables' condition round the clock. Now, it's one thing to put a sensor inside a cable. It's another to know whether it's giving you reliable information. So Betti and his colleagues have built themselves a large glass-walled chamber, and inside they've put a 20-foot length of bridge cable with their sensors embedded in it.
Prof. BETTI: It's an exact replica of a 20-inch diameter cable. Now the Williamsburg Bridge, the Manhattan Bridge, those are bridges that have similar size.
PALCA: Over the next six months, they'll record data from the sensors as they bake the cable with heat lamps and douse it with water, the kinds of things a New York bridge cable might be exposed to.
Prof. BETTI: We are trying to create conditions that are more aggressive than the one out there, because otherwise the system will not be tested. And six months from now, we'd open the cable.
PALCA: Open the cable and see how well the data they recorded matched the actual damage to the cable. Of course, even if Betti's sensor system works as he hopes, it doesn't solve the problem of what to do if a cable on a bridge connecting, say, Brooklyn and Manhattan is showing dangerous signs of weakness.
Prof. BETTI: It's impossible to replace a bridge like that in a densely-populated area like New York City. You don't have the luxury of saying, I'm closing the bridge for five years, the traffic will go somewhere else.
PALCA: It will take some clever engineering and a lot of money to replace a cable. The new sensor system could let them know when that's absolutely necessary.
Joe Palca, NPR News. Transcript provided by NPR, Copyright National Public Radio.
