Scientists conduct test quakes and study the results on a one-storey stone masonry structure
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 to withstand killer earthquakes.
On Monday,researchers at the Englekirk Structural Engineering Center,run by the University of California,San Diegos 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 centre has test-shaken other structures made from other kinds of materialssuch as last years test of a three-story parking garageto 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. The test was modelled after the magnitude 6.7 Northridge quake that killed 57 people and damaged 40,000 structures in 1994. Sensors attached to the masonry and wood framing fed data into computers.
Initial observations by the assembled experts yielded some surprises. At 80 per cent of the Northridge intensity,about half the bricks on one outside wall came tumbling to the ground. At 120 per cent 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 per cent shake appeared to do little further damage to the exterior. And the 200 per cent 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 analysing 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 Association.
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,its a failure of the connection system, said Klingner. Earlier tests at 25 per cent and 50 per cent of the Northridge quakes intensity had shown no damage.