New Sensor Improves Earthquake Response Efforts

Optics Makes a Difference in Monitoring Seismic Structural Health
easuring building interstory drift has been a factor in assessing buildings for post-earthquake damage for some time, yet finding a reliable method to do so has been fraught with challenges. Traditionally, engineers mounted strong motion earthquake accelerometers at select elevations to secure data on the back-and-forth and side-to-side force imposed on a shaking building. But processing the acceleration data from these instruments to obtain building drift displacements is very challenging due to the frequency limitations of the sensors, especially when buildings have sustained permanent displacements associated with damage. Even more difficult is receiving data quickly enough to inform decision-making on continuity of operations and occupant safety. In addition, because typical building accelerometer-based instrumentation can be quite costly, systems tend to be very sparse with accelerometers on relatively few buildings.

DDPS leverages a promising new alternative for directly measuring building interstory drift that combines laser beams with optical sensors. This technique centers around projecting laser light across a story height to sense the position at which the light strikes a detector located on the adjacent building floor to directly measure structural drift. The tool developed at Berkeley Lab relies on utilizing a laser source and position sensitive detector. Making use of a geometric array of small, inexpensive light-sensitive photodiodes, the sensor is able to instantly track the position of an impinging laser beam.

“Previous generations of DDPS were quite a bit larger than the system we are now able to deploy,” says McCallen. “Based on design advancements and lessons learned, the sensor is a quarter of the size of our original sensor design, but features 92 diodes staggered in a rectangular array so that the laser beam is always on one or more diodes.”

So far, DDPS has held up to three rounds of rigorous experimental shake table testing.

“The rigorous testing the DDPS has undergone indicates how the drift displacements measured on the three testbeds compared to representative drifts that could be achieved on an actual full-scale building undergoing strong shaking from an earthquake,” McCallen said.

Why DDPS Is Smart for Cities
The most populous town affected by the earthquakes in southern California earlier this month was Ridgecrest itself, a city of 29,000 which sits at the epicenter of a magnitude 7.1 earthquake which took place on July 5. Even though this is a small population center, the building damage estimates are still in the $100-million range.

If an earthquake of that magnitude were to hit Los Angeles 150 miles to the south of tiny Ridgecrest, or San Francisco, nearly 400 miles north, literally hundreds to thousands of buildings would be at stake for damage. In that scenario, the ability to measure and display key interstory drift information immediately after an earthquake would provide critical new data for making informed decisions on building occupancy – giving first responders information to help guide their efforts to evacuate a building, and municipalities the potential to maintain functional use of important facilities such as hospitals.

In addition, understanding a building’s drift profile would allow a quick determination of building damage potential, letting building inspectors know where to look for potential damage. This will be an important capability in moving beyond time-consuming and challenging manual inspections of hundreds of buildings after the next major urban earthquake.

McCallen noted, “The major earthquakes that struck in southern California this past week serve as a reminder of the risks associated with seismic activity across many regions of the United States. These events put an exclamation point on the need for continued societal focus on earthquake readiness and resilience, including an ability to provide the sensors and data analysis that can rapidly measure infrastructure health and inform the most effective response after the next major quake.”

This research was funded by the U.S. Department of Energy’s (DOE) Nuclear Safety Research and Development (NSR&D) Program managed by the Office of Nuclear Safety within the DOE Office of Environment, Health, Safety and Security.  An objective of the NSR&D program is to establish an enduring Departmental commitment and capability to utilize NSR&D in preventing and/or reducing high consequence-low probability hazards and risks posed by DOE and NNSA nuclear facilities, operations, nuclear explosives, and environmental restoration activities.