The owners of thousands of concrete buildings in San Francisco will be required to undertake seismic screenings to determine whether their structures are at risk of collapsing during a major earthquake.
The San Francisco Chronicle reported that a preliminary survey using public records, old insurance maps, and street-level images estimates the city may have up to 4,000 concrete buildings.
The concrete structures are mostly industrial, commercial and multifamily buildings, which are concentrated in the Tenderloin, South of Market and downtown areas, the report said.
The Board of Supervisors unanimously adopted an ordinance last week that gives the city six months to notify building owners who will be required to commission an engineer to complete a seismic screening of their property.
The owners will then have 18 months to submit a completed seismic screening form to the city, at an estimated cost to landlords of $300 to $3,200 per building. The seismic screening forms will include engineer-validated information about each building’s age, structure type and history of past retrofits.
Laurel Mathews, a senior earthquake analyst for the city, told the supervisors that it’s impossible to know how many buildings in the preliminary survey are truly vulnerable without an engineer’s assessment.
The concrete buildings include a wide variety of designs and construction methods, she said, and the city needs to determine whether buildings with concrete facades have steel-reinforced interiors.
“We don’t really have a clear understanding yet of how many of these buildings there are in the city,” Mathews said. “It’s really difficult to know the true risk and impact without first understanding the scale of the challenge.”
According to a city-commissioned study in 2010, concrete properties could account for 50% of the fatalities should San Francisco get hit again by an earthquake as powerful as the 1906 disaster, which measured 8.3 on the Richter scale.
The city is aiming to determine how many concrete structures fall into two at-risk building types.
The first type is non-ductile concrete buildings that lack steel reinforcement to withstand side-to-side shaking and were built before 1990, when the city building code was amended with stricter standards after a 6.9 magnitude earthquake hit the Bay Area in 1989.
The second category of at-risk buildings is “tilt-up” or rigid-wall, flexible-diaphragm buildings, which are often warehouses, grocery stores or manufacturing facilities. In powerful earthquakes, the walls of tilt-up buildings can detach from their roofs, causing structural damage.
Retrofitting a concrete building can be a lengthy and expensive process involving the displacement of residents and costs of $50 to $200 a square foot. Thus far, a handful of Southern California cities, including Los Angeles, Santa Monica and West Hollywood, have mandated retrofits of concrete buildings.
The ordinance passed last week in San Francisco sets voluntary retrofit standards for “early adopters” who elect to make the upgrades. The new law says that if the city passes a mandatory retrofit program in the next 20 years, building owners who retrofit now will be exempt from it.
Supervisor Myrna Melgar, who introduced the seismic screening ordinance, said the engineering assessments will not automatically be followed by a mandatory retrofit program.
“At this point, people should not expect that it will be mandatory,” Melgar told the Chronicle. “But if we find that it is a problem, that people are not solving it and that it will have bad consequences, then somebody at some point will make it mandatory.”
The ordinance requires the city to create a searchable online database so the public can see which concrete buildings have been screened.
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