As Suspension Bridges Age, A Search For Failures

  • Playlist
  • Download
  • Embed
    Embed <iframe src="http://www.npr.org/player/embed/100947959/101024294" width="100%" height="290" frameborder="0" scrolling="no">
  • Transcript
Two men stand on the Brooklyn Bridge, inspecting it in 1922. i

In July 1922, the Brooklyn Bridge was closed to automobile traffic after inspectors identified a slipped cable. Bettmann/Corbis hide caption

itoggle caption Bettmann/Corbis
Two men stand on the Brooklyn Bridge, inspecting it in 1922.

In July 1922, the Brooklyn Bridge was closed to automobile traffic after inspectors identified a slipped cable.

Bettmann/Corbis
A picture of a weather simulation chamber containing a suspension bridge cable. i

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 hide caption

itoggle caption Joe Palca/NPR
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.

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.

It sounds simple, but it's not. It's one thing to put a sensor inside a cable; it's another to know whether it's giving you reliable information. To test the sensors, civil and mechanical engineer Raimondo Betti and his colleagues have built themselves a large, glass-walled chamber, complete with the technology to simulate the weather conditions that barrage real-life bridge cables.

  

Inside the chamber, they've put a 20-foot length of bridge cable with their sensors embedded in it.

  

"It's an exact replica of a 20-inch diameter cable," says Betti. "The Williamsburg Bridge, the Manhattan Bridge — those are bridges that have [cables of] similar size."

  

During the next six months, Betti and his team will record data from the sensors as they bake the cable with heat lamps and then douse it with water and salt.

  

"We are trying to create conditions that are more aggressive than the ones out there," says Betti, "because otherwise the system will not be tested."

  

After six months, they'll open the cable and see how well the data they've recorded matches 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 Brooklyn and Manhattan is showing dangerous signs of weakness.

  

"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, and for five years the traffic will go somewhere else.' "

  

It would take some clever engineering — and a lot of money — to replace a cable. The new sensor system could let engineers know when that's absolutely necessary.

Comments

 

Please keep your community civil. All comments must follow the NPR.org Community rules and terms of use, and will be moderated prior to posting. NPR reserves the right to use the comments we receive, in whole or in part, and to use the commenter's name and location, in any medium. See also the Terms of Use, Privacy Policy and Community FAQ.