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Bridge Checkup

Autonomous wireless networks of sensors may give alerts before cracks appear.
June 26, 2001

When you drive over a bridge, particularly one of those really high ones over rocky rivers, you’d like to know that engineers have recently checked its safety.

Unfortunately, that’s unlikely. Most of the thousands of bridges across the country don’t get thorough tests on a regular basis, because those tests simply would cost too much.

Modern bridges, with their redundant structural supports, basically never collapse. But the effects of aging can be unpredictable, and cracks in bridge structures can put them out of commission, clobbering local traffic and economies.

Dryver Huston, an engineer at the University of Vermont in Burlington, says bridges are simply not tested often enough. He and his colleagues hope to solve this problem with a system of self-contained wireless sensors that can feed daily updates on the condition of each part of a bridge to a central computer, economically and autonomously. They will test their approach this summer on a Vermont bridge.

Crack Attacks

Engineers can do visual inspections or lug portable test equipment onto a bridge, but such methods are so time-consuming and expensive that bridges often go unchecked for years. And problems such as cracks, or deterioration to the point where cracks are likely, occur on much shorter time scales. Fractures often open and spread so far before they are detected that the bridge must be shut down for extensive repairs.

Existing bridge sensors can be left in place for long periods of time, but these typically require bulky wire networks-which are expensive and time consuming to install-and a power source. These constraints are particularly troublesome in remote areas, and such systems are only rarely employed.

Daily Data

Huston envisions a network of inexpensive sensors that deliver daily data reports via wireless for years or decades without being serviced. He outlined this vision at the Complex Adaptive Structures meeting held earlier this month in Hutchinson Island, FL.

The University of Vermont team, working with sensor builder Microstrain of Burlington, VT, and researchers at the University of Delaware in Newark, will attach over 100 penny-sized strain sensors to the test bridge. Clusters of sensors will attach to a central transmitter that collects and sends data to a main computer via wireless transmission.

The strain sensors won’t directly detect cracks. Instead, they will measure the structural load at a given point to see if this exceeds the known safe limit at that spot. “The idea would be to sense the problems in their infancy so they can be corrected before they become expensive problems,” Huston explains.

If cracks actually begin to form, Huston says, they can be detected indirectly because the readings for structural load will get strange. The researchers hope to eventually incorporate additional sensors, like tiny video monitors, that would look for cracks directly.

Renewable Energy to Keep On Truckin’

In this summer’s test, the units will run on batteries. The researchers hope to switch soon to renewable sources such as solar power, wind power or vibration electromechanical conversion (tapping into the natural vibrations of the bridge as a power source). Huston says it’s too early to say which option will win out, but he favors the vibration option.

A battery-powered version of the system probably could be available within about a year, but widespread adoption would probably take closer to a decade (barring new government regulations on bridge monitoring). Developing the necessary energy-harvesting technology for long-term autonomy will take two to three years, Huston estimates.

Sharon Wood, a civil engineer at the University of Texas in Austin, says this wireless research could prove “extremely useful.” She points to a need for better monitoring as the nation’s bridges age and as shippers push for new laws to allow increasingly heavy trucks on the highways-trucks even heavier than what the bridges were designed to handle.

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