Seismic retrofit effort in S.F. inching ahead
San Francisco is launching an effort to identify which of the city’s concrete buildings could collapse in an earthquake — and retrofit them.
On Tuesday, Mayor London Breed released an executive directive asking city officials to determine which concrete buildings are susceptible to damage in an earthquake and then develop standards to retrofit them.
The U.S. Geological Survey’s most recent estimate gives the Bay Area a 95% chance of a damaging earthquake in the next 100 years. In a major quake, buildings made out of concrete that isn’t ductile — or flexible — or that lack steel reinforcement can crack and collapse, according to Brian Strong, the city’s chief resilience officer and director of the Office of Resilience and Capital Planning.
“After the (1906) earthquake, when the city burned down, a lot of people went away from woodframe construction, and they went to concrete,” Strong said. “We ended up with a lot of these concrete buildings in these various older areas of the city, which don’t perform as well when the shaking happens. They’re less prone to burn down, but they’re more prone to collapse.”
Under the directive, the city’s Office of Resilience and Capital Planning and the Department of Building Inspection have six months to draft legislation that would create a screening process for concrete buildings. The DBI is then in charge of developing and releasing new seismic retrofit standards for the city’s building code.
San Francisco’s modern retrofitting efforts started after the 6.9 magnitude Loma Prieta earthquake shook the city in 1989. Following the quake, the city passed an ordinance requiring all unreinforced masonry buildings to be retrofitted. Then, in 2013, the city began a program to retrofit “soft-story” buildings — ones where the first floor is unreinforced, such as a garage or a retail space.
So far, the city has a preliminary list of about 3,400 buildings that could fall into the category, according to Strong. These maps — based on public data such as Google Maps, he said — give officials a general idea of the problem.
About half are commercial buildings, while industrial and residential buildings each make up about a quarter, Strong said. They are found throughout the city, but are concentrated in downtown, Chinatown, the Tenderloin and the South of Market neighborhood, according to Strong.
But without speaking to building owners and doing inspections, the city hasn’t been able to verify the list, according to Laurel Mathews, senior earthquake resilience analyst for the Office of Resilience and Capital Planning.
“We can look at a building and say there’s a lot of concrete on the outside of that building, but we don’t have an X-ray. We don’t have unlimited access to go inside these buildings and look at how much steel is in them. Some buildings look like a lot of concrete on the outside, but have some other kind of structural system on the inside,” Mathews said.
The screening process will likely include reaching out to the owners of the buildings on the list to verify their status, as well as doing public outreach to discover whether there are any additional buildings made of non-ductile or unreinforced concrete, Strong said.
Ahead of the directive being released, city officials spent a year consulting with stakeholders such as residents and retrofitting experts to develop recommendations for the program. The report identified issues such as the high cost of retrofits and the need for temporary housing for tenants displaced during construction. Strong previously told the Chronicle the cost of the concrete retrofit program would likely be higher than the soft-story retrofitting, which cost about $75,000 per building.
Any final decisions about funding and deadlines for the program will be determined later as the legislation is developed and debated, according to Strong. It is likely to be a lengthy process — retrofitting the city’s soft-story buildings took more than seven years. Strong said more than 90% of soft-story buildings — more than 4,500 total — have now been retrofitted.
“We’re going one step at a time. We’re trying to be very conscientious about giving people time and informing them of what’s happening and what this means, because we don’t want to scare people needlessly,” he said.