Assessing earthquake damage using laser sensors

When installed in a multi-story building, the new sensors directly measure interstory drift, helping to determine if the structure is safe after an earthquake.
When installed in a multi-story building, the new sensors directly measure interstory drift, helping to determine if the structure is safe after an earthquake. (Pixabay)

Scientists have developed a low-cost, laser-based sensor that can monitor and rapidly assess earthquake damage to a building, according to an online article on Physics World.

According to the article, the sensor is designed to be installed in multilevel buildings in earthquake-prone regions and can determine whether floors in a building have shifted relative to each other. The sensor’s inventors say that the device could speed assessment of the conditions of critical buildings, like hospitals, in the wake of a disaster.

The earthquake sensor system developed at Lawrence Berkeley National Laboratory shines lasers from the ceiling to detectors on the floor to measure interstory drift.
The earthquake sensor system developed at Lawrence Berkeley National Laboratory
shines lasers from the ceiling to detectors on the floor to measure interstory drift.

During earthquakes, horizontal ground motions can cause various floors of multilevel buildings to be displaced sideways relative to each other. This is a phenomenon that engineers call “interstory drift”. Assessing this drift is key to ensuring that a building is safe to use after an earthquake and to identify needed structural repair work.

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But up to now, methods to assess building conditions have been expensive and time consuming. A traditional approach uses accelerometers, installed in key points around a building that determine the forces exerted on the structure during an earthquake and subsequently calculate the extent of drift. This method is impeded by the frequency limitations of the sensors and are sometimes unsuccessful in the event of permanent structural damage. Also, accelerometers are expensive to implement on a wide-spread basis.

“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, of the Lawrence Berkeley National Laboratory and the University of Nevada, who led the research.  

In response, McCallen and his colleagues have been developing sensors that—when placed within a building—can directly and rapidly measure interstory drift and potentially relay their findings to a disaster response center.

Their device, called a discrete diode position sensor (DDPS), shines a laser from the ceiling of a room to a sensor pad on the floor. This autonomous detector contains an array of inexpensive, photo-sensitive diodes that can determine, by measuring the displacement of the laser beam, if, by how much and in what direction the ceiling has drifted relative to the floor after an earthquake. From this, engineers can determine the quake’s effect on the building’s integrity.

“Previous generations of DDPS were quite a bit larger than the system we are now able to deploy,” said 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, the researchers have only tested the drift sensor in the laboratory, putting the device through its paces in three rounds of trials on a shake table. But in coming months, the team will install sensors in a multi-story building at the Lawrence Berkeley National Laboratory. The structure is located adjacent to California’s Hayward Fault Zone, one of the most potentially dangerous earthquake regions in the US. In the future, the devices could be installed in buildings throughout regions that are particularly earthquake-prone.

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