Shaking out clues to quake damage
It took two years of planning, a month of construction and then just four 30-second bursts of shaking.
And from that shaking, academics and building-industry specialists hope to add to their knowledge about how to prepare California, and the nation, to withstand the killer earthquakes thought to be slouching toward us.
On Monday, researchers at the Englekirk Structural Engineering Center, run by the UC San Diego’s Jacobs School of Engineering, used a device to violently vibrate a one-story masonry veneer structure that was built to resemble many homes and businesses in Southern California.
The center has test-shaken other structures made from other kinds of materials -- like last year’s test of a three-story parking garage -- to see how much damage is wrought by the pushing and pulling of a simulated earthquake.
The structures are constructed on a shake table, a concrete platform connected to hydraulic pistons.
Monday’s test was modeled after the magnitude 6.7 Northridge quake that killed 57 people and damaged 40,000 structures in 1994.
The test structure was subjected to intensities that were 80%, 120%, 150% and then 200% of the Northridge quake. Sensors attached to the masonry and wood framing fed data into computers.
Initial observations by the assembled experts yielded some surprises. At 80% of the Northridge intensity, about half the bricks on one outside wall came tumbling to the ground. At 120% intensity, the other half came crashing down.
“I was not expecting to see this at this level,” said Richard E. Klingner, professor of civil engineering at the University of Texas at Austin.
But the 150% shake appeared to do little further damage to the exterior. And the 200% test only knocked loose a few bricks on a different wall.
Finding out what the test results mean will fall to professors and graduate students who will spend weeks analyzing the numbers and devising computer models so that the information can be extrapolated to provide insight into how other structures might react to quakes. Films of the shaking will also be examined.
The research is part of a $950,000 project funded by the National Science Foundation and the National Concrete Masonry Assn.
The wall whose bricks fell first had been constructed with a nail-like device that fastened the masonry to the wooden structure.
The walls where bricks did not fall as easily were made with a screw-in device.
“This is not a failure of the masonry, it’s a failure of the connection system,” said Klingner. Earlier tests at 25% and 50% of the Northridge quake’s intensity had shown no damage.
At Monday’s tests, the masonry on the outside was not the only focus. The experts were also interested in what happened to the drywall, whether it cracked or crumbled, and whether it provided structural support.
Gary Hart, an emeritus professor at UCLA now working for a consulting firm in Marina Del Rey, said questions about drywall are often key to insurance claims involving earthquakes.
Among the specialists watching the tests and examining the building was Junqing Lei, a civil engineering professor at Beijing Jiaotong University, now visiting at UCSD. She said that while her country has earthquake research facilities, it does not have an outdoor facility akin to UCSD’s shake table.
Benson Shing, a structural engineering professor at UCSD and the lead investigator on the project, said one goal is to help upgrade building codes to improve earthquake resistance.
“This shows us there’s a lot of room for improvement,” Shing said as he stood near the fallen bricks.