Wireless sensor networks may be on the verge of providing a simple solution to one of the nation's greatest infrastructure challenges -- the inspection of hundreds of thousands of aging bridges.
Engineers predicted at the NIWeek conference in Texas this week that the new sensor technologies will be key as government agencies and other bridge owners begin dealing with pending regulations that could call for structures to be inspected twice as often as they are today.
"This would be a revolution for the bridge inspection process," Jeremiah Fasl, a structural engineering Ph.D. candidate at the University of Texas, who is studying the application of the sensor networks, told Design News in an interview.
Conventional visual bridge inspections aren't going away. But with about a quarter of the country's 600,000 bridges nearing functional obsolescence, and with calls for more frequent inspections on the rise, structural engineers are looking for better ways to get the job done.
Sensor networks could provide an answer. In Austin, Texas, a steel bridge on Interstate 35 has been outfitted with a wireless sensor network to study the usefulness of the technology. University of Texas researchers glued resistive-type strain gauges to bridge surfaces and then hard-wired them to data acquisition nodes built by National Instruments. The nodes gather data from the sensors, then send it wirelessly to an NI 9792 wireless sensor network gateway. The gateway, which is essentially a Kleenex box-sized headless computer assigned to the task of "talking" to the nodes, relays the sensor data to the cloud, where structural engineers can analyze it.
Most of the information gleaned from the sensors is strain data caused by impact loads from 18-wheelers and garbage trucks.
"If we can count the fatigue cycles resulting from those strains, then we can estimate when the bridge is nearing the end of its fatigue life," Fasl said. "Then we can go out to the bridge and do a visual inspection and look for cracks in the steel."
Fasl explained that the technology is a tool, not a solution unto itself. But by knowing the status of a bridge's fatigue life, engineers have found they can prioritize visual inspections, thereby assigning more importance to bridges nearing the end of their useful lives. An older, heavily used bridge, for example, might need inspections every three months, whereas a newer one not undergoing high strains might need inspections every one to two years.
For bridge inspectors, the technology could be a godsend. Bridge inspectors -- typically equipped with a truck, ladder, tape measure, and steel center punch -- often face a difficult physical task when they inspect a bridge. The new technology wouldn't eliminate that task, but it would enable them to focus on the bridges that most need inspection, and not waste effort by climbing on to bridges that are in good structural health.
"They get really tired of having to climb up on bridges to get data," Mike Neal, a LabVIEW product manager for National Instruments, told me. "Our goal is to take the burden off of them, so they can look at what really matters to them, which is the data."