Officials have known as far back as the 1971 Sylmar quake that such buildings can collapse, but efforts to address those weaknesses have stalled because of the high cost of retrofitting the buildings.
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The devastating earthquakes in Japan and New Zealand have focused more attention on the vulnerability of such buildings.
For California, the quake in Christchurch was particularly relevant because it served as a fresh reminder of the major weaknesses in so-called non-ductile concrete buildings. Numerous concrete structures failed in New Zealand during the Feb. 22, magnitude-6.3 earthquake, including the six-story Canterbury Television building, where 100 people were believed to have been buried, including many TV station employees and 60 students.
The collapse of the CTV building, which housed a local TV station, clinic and English-language school, was particularly shocking. It was built in 1986, 15 years after the Sylmar earthquake, and after codes had been changed in New Zealand and California to improve buildings' ability to withstand shaking. Yet the CTV building collapsed in a manner consistent with a brittle concrete building, said Thomas Heaton, professor of engineering seismology at Caltech.
Experts both in Christchurch and Los Angeles said such buildings are worrisome because it's so difficult for people inside to survive if large slabs of concrete fall on top of them.
"They're killers. In my opinion, they could take many thousands of lives in a Southern California earthquake, especially one inside the Los Angeles Basin," Heaton said. "When they fail, the failures are just unsurvivable. You just end up with a pile of floor slabs, one on top of another."
Identifying potentially unsafe buildings is crucial, Heaton said, because being in a building designed to withstand major shaking is a "big part of your survivability in an earthquake," Heaton said. "If you choose poorly, you're definitely in harm's way."
In the last decade, the Los Angeles City Council has considered a plan to identify brittle concrete buildings. But the plan's author, Councilman Greig Smith, was forced to shelve the plan as support evaporated and the recession left little money to address the risk.
The California Seismic Safety Commission has also recommended that the state identify brittle buildings, both public and private, and figure out a way to reduce the risk. But lawmakers have taken little action.
"Given the economic situation now, no one has got any money to do it," said Richard McCarthy, the commission's executive director.
Many of the larger buildings in question were constructed in the 1950s, '60s, and '70s, during the post-World War II building boom in which bigger was better, and companies throughout a growing California needed larger, multistory buildings that could house many more employees and residents, and larger parking structures that could hold more cars.
But in creating taller, slender buildings, engineers unknowingly took greater risks, and some buildings carried a fatal flaw. Made out of a concrete frame or walls but lacking sufficient reinforcing steel, these buildings were found to be brittle during an earthquake, as if the frame was built out of saltine crackers. When shaken during an earthquake, the floors and columns can snap, collapsing the entire building.
And they did.
During the 1971 Sylmar earthquake, the first floor of the just-built six-story Olive View Hospital was pulverized, killing three people. The nearby Veterans Administration hospital all but disintegrated into a pile of rubble. Forty-seven people died there.
California has made efforts to improve the safety of some structures since the Northridge earthquake in 1994, focusing on freeways and hospitals. But some hospitals have struggled to comply with state law that they be quake-resistant. In 2013, 258 buildings at 84 hospitals will remain at significant risk of collapsing in an earthquake, according to data supplied by hospitals to the state. California has 426 hospitals.
Identifying problem buildings requires a review of design records and on-site inspections.