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Chart: L.A. mayor's proposals for concrete, wood quake retrofits

Chart: L.A. mayor's proposals for concrete, wood quake retrofits
Left, a building collapsed at both ends in the Northridge earthquake (Ricardo DeArantha/Los Angeles Times). Sixteen people died at the Northridge Meadows apartments, right. (Los Angeles Times)

The mayor's proposals for mandatory retrofits target two of the riskiest types of structures built in Los Angeles before 1980: brittle concrete buildings and multi-story wooden buildings supported by weak columns on the ground floor.

Concrete buildings

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Retrofit deadline: 30 years

Cost of retrofit: Varies. Could be more than $1 million for a 10-story building

Affected buildings: About 1,500 need further study to determine if retrofit is needed

Vulnerabilities: Many don't have enough steel reinforcement to hold columns in place during shaking. The collapse of two concrete office towers killed 133 people in a 2011 quake in New Zealand.

Wooden buildings

Retrofit deadline: 5 years

Cost of retrofit: About $60,000 to $130,000 for a modest-sized apartment building

Affected buildings: About 17,000 need further study to determine if retrofit is needed

Vulnerabilities: These types of buildings have collapsed during both the 1989 Loma Prieta and 1994 Northridge earthquakes. Sixteen people died when the Northridge Meadows apartment complex pancaked.

Source: Mayor's office, L.A. Department of Building and Safety, University of California

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