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The Structural Sensors on a Fracture Critical Bridge

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The latest in a series of reports about concerns over construction defects in the new San Francisco-Oakland Bay Bridge suspension span involves cracked welds in the girders that form the span’s roadway. Others that have emerged include suspect foundation concrete, rusted tendons in the skyway that connects the suspension span to Oakland, broken anchor rods, and corrosion on the main cable. Many of the issues were first reported by The Sacramento Bee.

As a “fracture critical” structure, in which some vital structural elements lack redundancy or backup, the bridge would seem to be a perfect candidate for an installed system for structural health monitoring technology. We asked the engineers what sensors are in place and what are they there for.

ENR: Does the Bay Bridge have any embedded structural sensors and if so, what technology are they and could you describe how they function?

Marwan Nader, vice president, T.Y. Lin International: The Bay Bridge has two kinds of sensors: accelerometers and humidistat and temperature/relative humidity data logger (T/RH)

ENR: What are they designed to pick up?

Nader: The accelerometers are designed to measure the response of the bridge during a seismic event and wind vibrations. The humidistat and T/RH  are designed to control and measure the relative humidity in the dehumidified zones.

ENR: Have they been effective?

Nader: Yes. The information gathered from the humidistats and T/RH provide the owner with the knowledge that the dehumidified zones are kept below the specified relative humidity to avoid corrosion. The accelerometers are used to monitor wind vibrations.

ENR: What have they found thus far?

Nader: The humidistats and T/RH have helped owner to calibrate the system. The accelerometers will help the engineer assess the performance of the bridge after an earthquake.

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