In 2006, a concrete panel weighing several thousand pounds fell onto traffic in Boston's Big Dig tunnel, crushing a car and killing a motorist. The alleged cause -- and subject of a multi-million dollar settlement -- was faulty epoxy that allowed bolts in the ceiling to wiggle loose.
Mechanical engineer Joe Guarino of Boise State University in Idaho is developing an early warning system to prevent such catastrophic joint failures in the future. His team listens to the sounds made by vibrating bolts. Their analysis of how these sounds change as the bolts unscrew has revealed certain frequencies in these noises that could be monitored to check the health of bolts in buildings, bridges, and tunnels.
Guarino and his team work on a full-scale structural model made of steel beams and girders connected by bolts. They tap the structure with a hammer, causing it to vibrate. The sounds made by the vibrating bolts are recorded by an electronic stethoscope, similar in design to the stethoscopes used by doctors. Then the engineers unscrew the bolts a bit, tap the structure again, and listen for changes in the sounds. "Any slight relaxation in a joint can change the way it vibrates," says Guarino.
Using a pattern detection technique called the continuous wavelet transform, the team can pick out which ranges of frequencies change the most. Their results suggest that the signatures of unscrewing may be found in certain mid-to-high frequencies. During the talk, Guarino will be explaining and playing these sounds, available for use by the press.
The research is still in the preliminary stages of lab testing. But Guarino hopes to eventually take it into the field to check for bolts that are vibrating loose or degrading through exposure to the elements. "If we're successful, this could lead to implanting permanent, inexpensive accelerometers that could monitor joints continuously," says Guarino.
The talk "Acoustic detection of bolt detorquing in structures" by Joe Guarino will be presented at the 157th Acoustical Society of America Meeting to be held May 18-22 in Portland, Ore.
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