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Archive for Thursday, July 31, 2008

Strict building codes limit quake damage, suburb’s residents find

July 31, 2008

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— The latest in plywood-reinforced walls, tied-down foundations, strengthened concrete and stronger welds - if an earthquake had to hit somewhere in densely populated Southern California, this relatively new suburb of 80,000 people was about the best place possible.

Chino Hills was just a few miles from the epicenter of Tuesday's magnitude-5.4 quake, yet it withstood the shaking with almost no damage at all, even while other communities farther away saw fallen bricks, cracked walls and windows, warped door frames and broken water mains and gas lines.

One big reason: Chino Hills went up mostly in the 1990s and was built to the stringent earthquake standards that the state wants to see adopted everywhere across California before the Big One strikes.

"I was wandering around out there after the quake and it struck me that there's no building there that's more than 10 years old. They're all built to the most recent codes, and I think that's true of the whole Chino area," said Lucy Jones, a seismologist with the U.S. Geological Survey. "The type of earthquake that used to be a major event just isn't anymore."

Since the 1930s, California has gradually boosted its building standards. Each severe quake has prompted new rules, with the most recent major overhaul coming after the 1994 quake in the Northridge section of Los Angeles that killed 72 people.

After that disaster, the building code was amended to require, among other things, the use of plywood to reinforce Sheetrock walls in homes. The new rules also prescribed a different way of welding steel that makes the welds less likely to become brittle and crack.

In Chino Hills, 30 miles southeast of Los Angeles, about 70 percent of the homes date to the mid- to late 1990s, and the city doesn't have a single unreinforced mortar building, said Winston Ward, the city's chief building official. Most commercial buildings are no more than four stories tall.

Experts said the quake could have produced more damage if it had been centered elsewhere.

Only about 20 percent of buildings statewide are constructed to the standards used in Chino Hills. Of the remaining 80 percent, only about half could withstand a moderate or large earthquake, said Chris Poland, chief executive at Degenkolb Engineers in San Francisco. Tuesday's quake was considered moderate.

A state law passed in 1986 mandated that cities catalog all unreinforced mason structures - mostly old brick or stone buildings - and then take steps to retrofit them, a process that includes inserting steel reinforcing bars in exterior walls, and bracing the interior walls with steel, too.

But the law left the retrofitting plans to individual jurisdictions. As a result, about 30 percent of the state's nearly 26,000 brick-and-mortar structures could collapse in a large quake, according to a 2006 report by the California Seismic Safety Commission.

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