Structural Health Monitoring is Sensitive Subject
 OSMOS |
 OSMOS OSMOS used four strategically placed fiber-optic strands to monitor stress on a massive truss barged by ocean transit from Alabama to New York. |
A question begged by the Minneapolis bridge collapse is whether current inspection practices are adequate or whether unused or underutilized monitoring technology could improve them. Predictably, vendors of such tools think so, although researchers and engineers are more reserved.
Except during load tests, monitoring devices were not installed on the I-35W bridge because “monitoring would have been very difficult given all of the critical locations,” says Daniel L. Dorgan, Minnesota’s chief bridge engineer. He also says engineers were concerned about drilling into truss members and possibly weakening them. Besides, when failure is sudden, monitors are no help, Dorgan says.
But experts in the field of structural health monitoring say the purpose of instrumenting structures is less about being able to yell fire when disaster is imminent than about helping engineers evaluate structural response to strain. The benefits aren’t limited to warnings of impending failure. They may also lead to the confident extension of service life.
There are several tracks moving forward in sensor technology. There are the marketed products that either focus on localized changes in areas of known concern or others that shoot for a more global understanding of the strain/response behavior of an entire structure. Then there are the research scientists trying to figure out where to go from here. The applied products are beginning to be widely used. but not much in the U.S., vendors say.
Los Angeles-based Material Technologies Inc. is monitoring seven bridges in Pennsylvania and has plans for 17 in Massachusetts in the next year. Its Electrochemical Fatigue Sensor is a nondestructive crack inspection tool that measures microscopic fractures in metal structures and monitors fatigue in real time.
Bill Berks, vice president, says the system uses reports from previous inspections to identify locations for test patches. The patches contain a liquid electrolyte. Berks says the material under test responds to it by producing a very small current, which is measured and pro-cessed to look for patterns that can indicate whether a crack is present and growing. “We can detect cracks down to 0.01 inches in length,” says Berks.
“On a very large bridge, with many joints, we would spend three or four days and do a dozen sensor locations a day,” Berks says. A more typical small bridge test usually costs $8,000 to $12,000. He claims the system can track the growth of a crack with 99% accuracy.
Gary Hoffman, civil engineer with Applied Research Associates, Mechanicsburg, Pa., is impressed. “All the testing I’ve seen shows that we have a new tool here that gives consistent reports about what’s happening with fatigue cracks on bridges,” he says. Hoffman, who retired last year as a chief engineer with Pennsylvania Dept. of Transportation, says the company tested three of its bridges last year and found unknown cracks that were expanding on one of them.
 Lifespan Technologies Lifespan Technologies� piezoelectric gauges capture peak strain data. |
A different approach is taken by Peter J. Vanderzee, president of Lifespan Technologies, Alpharetta, Ga. His firm installs self-powered piezoelectric gauges to capture peak strain readings on structural members. The systems can be simple arrays that store data for periodic download, or they can be elaborate ones sending live signals via the Internet, coupled with concurrent environmental data. “We have limit alarms we can put on any of the sensors,” he says.
He says he believes “the best of all possible worlds” for large structure would be to use the data to calibrate a finite element model of the entire structure. “It tells you exactly where the stresses are,” he says. But he adds that it would be important to collect data over a long period to capture the full range of temperature cycles. “If you don’t go through a temperature cycle you are going to miss a lot of important data,” he says.
That’s also the observation of Fruma Narov, president of New York City-based UA-OSMOS, a branch of a French company that has been using fiber-optic strain gauges to monitor structures for a decade. It has 500 installations worldwide, but only 20 of them are in the U.S., she says.
Narov is an engineer and she takes an engineer’s approach to analyzing a structure to determine the most fruitful points for monitoring. Her firm monitored stress in a 360-ft-long, 4.8-million-lb steel truss barged from Alabama to New York City in the fall of 2004 with just four monitoring strands. “A truss may be easier than a stringer bridge, where every girder works independently,” she says. “It’s ridiculously not expensive.”
Her firm’s technology uses fiber-optic strands strung between terminator blocks epoxied to the face of members being monitored. Light signals passed through the strands are cabled to a processor that can take 100 readings per second. Changes in the strand length change the wavelength of the light transmitted. It can be coupled with other sensor data, such as wind, temperature or accelerometers, and preset to report average readings over time, or to jump to a real-time recording if any sensor registers an anomaly.
The owner of one bridge under scrutiny is concerned about wind. Strain gauges on beams bridging upper chords tracking stretching and compression of the members reveal they are far more responsive to temperature changes than wind gusts, and they lengthen when temperatures drop and compress when the temperature rise, suggesting a far more complex picture of strain building elsewhere.
“You can’t get that data visually,” Narov says, adding that the reliance on visual inspection puts engineers in a terribly stressful position of having to make judgments without empirical data.
 Alamos National Laboratory Miniature chopper from Los Alamos Lab is set to fly up to installed sensors tucked into high-interest but difficult-to-reach locations, power them up with a blast of microwave and ping them for data. Its test flight was July 31. |
Other researchers, such as Mike Watson, manager of the applied physics group at Pacific Northwest National Laboratory, say that vendors’ technology can make valuable contributions. But he and others hope that the adoption of such equipment will not drain research funding. He says there is more work to be done, particularly in the area of understanding the message in the data.
Researchers like Chuck Farrar, at Los Alamos National Laboratory, are investigating other promising technologies as well as working to solve data acquisition issues that make the monitoring of complex structures tough. He currently is working on an “active” sensing project using a piezoelectric patch to “pulse" structures with a signal tailored to the suspected damage. Applied in difficult-to-reach locations, it can be charged by a microwave burst and interrogated by a receiver on a miniature helicopter. The 12-lb chopper is expected to make its debut during tests on a bridge 130 miles south of Albuquerque, N.M., by the end of this month.
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