Even as investigators continue to work to recover victims and pinpoint the cause of Wednesday's catastrophic collapse of the Interstate 35W bridge over the Mississippi River in Minneapolis, one topic getting more attention immediately is the availability of systems that can help highway officials better judge the health of the nation's bridges.
Sensor technology to monitor the steel and concrete health of highway bridges already exists and is in use in many areas of the country, while new and improved monitoring systems continue to evolve in myriad research labs and universities.
Vendors, including Physical Acoustics Corp. in Princeton, N.J., and Pure Technologies Ltd. in Calgary, Alberta, already offer intricate portable systems using sensors that can "hear" cracking in bridge cables or steel components so engineers can better assess the conditions of structures.
Meanwhile, engineering research projects, including one at the University of Missouri-Columbia, were already under way long before this week's bridge collapse to advance the science of bridge monitoring. At the school, work is being done on a large-scale sensor system that would be fastened to several concrete bridge piers below a span to alert officials about even the slightest tilting or swaying of critical piers supporting a bridge.
"There's lots of research projects looking at health monitoring for bridges," said Glenn Washer, an assistant professor of civil engineering at the school and an expert in the field. "There are some technologies right now that are in use monitoring bridges."
Minnesota Department of Transportation officials could not be reached this morning to determine if any such systems were in use on the collapsed bridge or on any other spans in the state.
Washer said that existing monitoring systems from Physical Acoustics and Pure Technologies collect "acoustic emissions" given off by fracturing metal. "If a piece of steel is under force and it fractures, it gives off a tremendous amount of energy" in the form of sound waves that can be detected and interpreted by a sensor system. It's not uncommon for some of the individual steel strands that make up bridge cables to break over time, he said. Officials can use the sound-sensing systems to track how many of the thousands of strands in a cable are damaged before maintenance is required.
What makes the research done by Washer's team different is the focus on monitoring bridge supports for problems. A prototype is expected to be ready for testing and in the field in about six months, he said. "I think there will be more of a focus on these kinds of technologies and more investment" in light of the Minneapolis bridge collapse.
The system is designed to use 10 to 20 tilt meters that would be attached to specifically located bridge piers to monitor their vertical positions. The tilt meters use inexpensive, readily available sensors that work on the same principle as liquid-filled bubble levels. When the sensor tilts, the liquid moves across electrodes, creating an electronic signal. That signal can be collected by 10 to 20 electronic channels on the sensor and analyzed by special algorithms and software being developed by the team.
Data from the monitors can then be transmitted to highway officials for evaluation. The system will run on typical Microsoft Windows-based PCs.
Bridges are normally inspected visually every two years, but such inspections can't delve deeply into the condition of a structure. So while inspectors can see external signs of damage, they can't see deeper issues in the concrete, steel supports or even in the footings and pilings below the water under a bridge. Bridge inspections can take several days to two weeks to perform, depending on the size of the structure.
"We're trying to develop technologies that can improve that process," Washer said, noting that researchers around the nation are looking at these problems.
Fatigue cracking, which is already seen as one potential factor in the Minneapolis bridge collapse, could be found using X-rays and ultrasonic detection, but those methods are costly and slow for a large span and are not typically used for regularly scheduled inspections, he said. "There's so much bridge [to cover] for a sensor that's an inch in diameter that you'd never do all of it," Washer said.
In addition, he said, the simple presence of fatigue cracks won't automatically set off alarm bells because the cracks take time to develop, are usually fixable and don't lead to immediate structural failure. Even the ultrasonic detection can't be used everywhere because of the time needed to check a single bridge, he said. "It is used where necessary."
Washer's research team is also looking at ways of embedding acoustical sensors as bridges are built, right into the wet cement surrounding the steel reinforcement rods that strengthen the roadway. That would allow officials to track internal corrosion of the rods as the bridge ages.
Terry Tamutus, sales director at Physical Acoustics, which makes the Sensor Highway II acoustic emissions detection system, said the system can evaluate sounds emanating from any materials, including steel cables, trusses and concrete in a bridge. "[It] can tell that there are cracks many months before they're visible" on the surface, he said.
Sound waves move more efficiently through solid objects than through air, making any sounds easier to listen out for, Tamutus said. "It's not amazing. It's simple. Doctors use stethoscopes all the time. If you put your ear on a train track, you can hear a train approaching from far away."
The Sensor Highway II systems, which are portable and can be moved from bridge to bridge as needed, usually cost between $20,000 to several hundred thousand dollars each. Typically, evaluations take between one day and a week.
In the long run, however, technology, inspections and maintenance can only do so much, said Washer. "You [can] never eliminate the risk completely of a bridge collapse," he said. "There's always going to be a possibility of failure."