November 3, 2011

Column | Korky Koroluk

How sensors on New Orleans bridge give early warnings of structural problems

It may well be the smartest bridge anyone has built yet.

When they turned on the power on a new bridge over Lake Pontchartrain in New Orleans, it became a sort of poster child for the use of information technology in bridge design and construction.

A total of 340 devices, ranging from simple strain gauges through inclinometers, accelerometers to weigh-in-motion sensors, are embedded in or attached to the nine-kilometre, twin-span structure, providing more information than bridge designers, builders or operators have ever had before.

The original bridge, built in the 1960s, was damaged beyond repair by the storm surge generated by Hurricane Katrina in 2005. The replacement bridge is higher and sturdier than the old one, and incorporates a sophisticated network of devices that monitored the structure during construction, and will also monitor it through its service life.

The short-term monitoring reached a climax during a lateral load test in February, 2009. Engineers lashed one pier of the eastbound span to one pier of the westbound span using post-tensioning cables, then used two 600-ton hydraulic jacks to apply 1.8 million pounds of horizontal pressure. They wanted to confirm that the foundations would perform as planned if the bridge was struck by a passing ship or barge. They did. Engineers had predicted three-quarters of an inch of movement. That’s what they got. Instruments were placed in the precast pile foundations by placing them in the rebar cages for the piles at the casting yard. Then the concrete was poured, enclosing corrosion monitors, as well as PCB conduits which were included to accommodate possible new instrumentation.

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By having things like corrosion monitors, strain gauges, heat sensors and inclinometers in place from the outset, the firm that supplied the electronic system, Geocomp Corp, was able to monitor every aspect of the piles, the concrete pour, curing, pre-stressing, and transportation to storage. Then it monitored loading onto barges to take them down the Mississippi River from Memphis to New Orleans, off-loading, and positioning for installation. Then it monitored every blow as the piles were driven into the lakebed below. Engineers were thus able to confirm that the piles were properly cast, handled and installed. The 340 devices included in the finished bridge will ensure that engineers will know more about this bridge than any other. They will know more about its day-to-day performance, its maintenance needs, and will have earlier warnings of possible problems should they arise

The whole system cost $650,000, which isn’t much for an $800-million project. It’s a big bridge, with the potential for coming under some extreme loads. Not every bridge needs all that sophisticated equipment.

American infrastructure is in no better shape that Canada’s, which is to say, it’s pretty bad. And bridges are on the minds of a lot of people just now because a new report from an organization called Transportation for America, tells us that the U.S. has more than 70,000 bridges that are “structurally deficient,” which the federal government defines as needing substantial repair or outright replacement.

More than 18,000 of those bridges are concentrated in large metropolitan areas. Included is a major bridge over the Ohio River at Louisville, Ky., which carried 55,000 cars a day until it was abruptly closed in September after cracks were found in two of its supporting steel beams.

The U.S. report notes that the high number of deficient urban bridges means about 210 million cars are crossing deficient bridges every day in the country’s biggest cities. Also, the cost of fixing or replacing deficient bridges is about $70.9 billion. Federal spending on bridges is currently just over $5 billion annually.

All this adds up to a disaster waiting to happen. But no one knows where or when.

Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@dailycommercialnews.com

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