Researchers in Texas are cranking up an elaborate array of acoustic sensors on a cable-stayed bridge to "listen" for the sounds of hidden damage as it occurs.
The Fred Hartman Bridge, a 2,214-ft-long, twin-deck, eight-lane structure crossing the Houston Ship Channel in Bayport, has been fitted with 600 acoustic sensors to study the cables. The installation and monitoring of the system, called SoundPrint, is by Pure Technologies US Inc., Columbia, Md.
|(Photo courtesy of Pure Technologies/ Oliver Tozser)|
The project is part of a Texas Dept. of Transportation funded study of the consequences of weather-induced oscillations on stays. "The system can actually hear wire breakages in the cables and pinpoint them within a couple of feet," says Brian Merrill, manager of the bridge construction and maintenance branch at TexDOT.
John Breen, a civil engineering professor at the University of Texas, Austin, says many cable-stayed bridges experience oscillations. "It's a problem world-wide. A lot of time it's due to wind-driven rain."
Scientists at the university's Ferguson Structural Engineering Laboratory have been oscillating big cables in the lab and dissecting them for examination. They found SoundPrint accurately reports fatigue breaks. Testing the sensors on a real bridge is the next step and the Hartman Bridge, which has oscillation problems, was chosen.
"We have a video that shows the cables with a real excited mode of oscillation," says Merrill. "It was light rain with about an 8-mph wind and we had 6-ft oscillations in a stay. About every third cable was vibrating, and at various speeds, different cables would vibrate." Engineers tried dampening the stays with a cross connection. "It cracked like a whip," Merrill says.
Tom Lovett of URS Greiner, Tampa, Fla., the bridge's engineer, notes that vibrations are not unusual in cable-stayed bridges. He says URS has not been involved with the bridge since it opened in 1995, but says designing dampening systems has been a trial-and-error process. "The state of the art has advanced quite a bit," he adds.
The oscillations appear only in specific light wind and rain conditions. University researcher Carl Frank says tests suggest the movement can cause significant fatigue cracking near the cable ends, but the damage progresses slowly. "It is not a crisis situation," he says.
The monitoring array costs $375,000.