The city of Ridgecrest, California, was at the center of a magnitude 7.1 earthquake on July 5. Even though the city is a small population center, with roughly 30,000 residents, building damage estimates were still in the $100-million range.

Now, an optical sensor could speed up the time it takes to evaluate whether buildings like those in Ridgecrest are safe to occupy shortly after a big quake.

An optical sensor could speed up the time it takes to evaluate whether buildings are safe to occupy after an earthquake. Source: LBNLThe technology captures and transmits data depicting the relative displacement between two adjacent stories of a shaking building. Researchers said the tool is able to provide reliable information about building damage immediately following an earthquake, and could speed up efforts to safely assess, repair and reoccupy buildings.

Scientists and engineers at Berkeley Lab, Lawrence Livermore National Laboratory and the University of Nevada-Reno began working in 2015 to design an optical method of measuring what is known as interstory drift within buildings. After four years of research and testing at the University of Nevada’s Earthquake Engineering Laboratory, the Discrete Diode Position Sensor (DDPS) will be deployed later in 2019 in a multi-story building at Berkeley Lab. The Energy Department's laboratory sits adjacent to the Hayward Fault, considered one of the most dangerous faults in the United States.

The sensor combines laser beams with a position sensitive detector to directly measure drift between building stories. Source: Diana Swantek/Berkeley Lab“Until now, there’s been no way to accurately and directly measure drift between building stories, which is a key parameter for assessing earthquake demand in a building,” said David McCallen, a senior scientist in the Energy Geosciences Division at Berkeley Lab and faculty member at the University of Nevada. He led the research effort.

The trouble with accelerometers

Measuring building interstory drift has been a factor in assessing buildings for post-earthquake damage for some time. Traditionally, engineers mounted accelerometers at select elevations to secure data on the back-and-forth and side-to-side forces imposed on a shaking building. But processing the acceleration data to obtain building drift displacements can be 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 costly, systems tend to be sparse with accelerometers on relatively few buildings.

Lasers and optical sensors

DDPS 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. Making use of a geometric array of small, relatively inexpensive light-sensitive photodiodes, the sensor is able to track the position of an impinging laser beam.

DDPS would be positioned between building stories to detect interstory drift and transmit data about building damages to response planners. Its debut comes as governments make post-earthquake building inspection and reoccupation a central focus of response planning, and as 5G remote connectivity becomes a reality. Source: Diana Swantek/Berkeley LabThe current sensor is around a quarter of the size of the researchers' original sensor design, and features 92 diodes staggered in a rectangular array so that the laser beam is always on one or more diodes.

The ability to measure and display key interstory drift information immediately after an earthquake could provide critical new data for making informed decisions on building occupancy. First responders could have additional 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. The researchers said this would be an important capability in moving beyond time-consuming manual inspections of hundreds of buildings after the next major urban earthquake.

This research was funded by the U.S. Department of Energy’s Nuclear Safety Research and Development Program managed by the Office of Nuclear Safety within the DOE Office of Environment, Health, Safety and Security.