Sensing Danger in Bridge Cables

The Carleton Laboratory on the first floor of the Mudd Building is home to what Prof. Raimondo Betti calls “the beginning of a new era in infrastructure testing.”  After more than two years of preparation, a simulated bridge cable 20 feet long, 20 inches thick and made up of nearly 10,000 galvanized bridge wires, enclosed in an environmental chamber to accelerate deterioration, is being subjected to a stress test. As the weathering chamber subjects the cable to accelerated aging, it is providing the means for Betti to measure the success of the method he has developed to monitor deterioration of bridge cables.

Nearly 10,000 steel wires form the dummy cable created to test the sensors developed to monitor the deterioration of bridge cables that hold up many of New York’s major bridges.
Betti’s goal is to develop a state-of-the-art corrosion monitoring system to be used in main cables of suspension bridges. Buried in the dummy cable are 76 sensors, several microphones, and two pre-corroded strands for blind tests. For six months, the cable will be subjected to a regimen that will simulate decades of wear for a suspension bridge, such as that endured by New York’s Williamsburg, Manhattan, and Brooklyn Bridges.

“New York City has among the oldest suspension bridges in the world,” says Betti, a professor in the Department of Civil Engineering and Engineering Mechanics. “Their cables have a life span of about 150 years and they will continue to deteriorate if nothing is done. If a cable goes, the entire bridge goes.”

The city’s bridges are approaching an age that causes concern for their well-being: the Brooklyn Bridge is 126 years old, the Williamsburg Bridge is 106, and the Manhattan Bridge is 100. Betti estimates that the loss of a New York City suspension bridge would cost $10 billion. While replacing a cable is costly, about $100 million he says, it is much less than replacing the entire bridge.

His research is aimed at finding ways to safely extend the life of existing suspension bridges and is a collaboration between SEAS, Parsons Transportation and Physical Acoustics, and the Federal Highway Administration.The aging chamber in the Carleton Lab is the means to provide that answer. It subjects the cable to a schedule of water, heat from heat lamps, and cooling by an air-conditioner. At the same time, the cable is subject to the same tension as if it were stretched on a bridge: 1.2 million pounds, provided by hydraulic jacks.

The sensors Betti has developed are measuring corrosion rates, temperature, humidity, acidity, and chlorine content. The microphones will detect cracking in the steel strands. Readings from both the sensors and microphones are fed into a computer system for evaluation. Once the six-month test period is over, Betti will evaluate the results of what will be the equivalent of decades of outdoor exposure. The answers he finds should help provide a prescription to insure a longer life for all suspension bridges.

Join Professor Betti for a demonstration on Friday, June 5, at 2 p.m. To make a reservation, click the Alumni Reunion Weekend icon at www.engineering.columbia.edu.