Informing a Retrofit Ordinance: A Soft-Story Case Study

The San Francisco Community Action Plan for Seismic Safety (CAPSS), among other tasks, addressed the threat that regional earthquakes pose to 4,400 older, soft-story, high-occupancy woodframe residential buildings, like the apartment buildings that collapsed in the 1989 Loma Prieta earthquake. They house 8% of the city's population, in a city with a rental vacancy rate near 3%. A risk analysis was performed to estimate their post-earthquake safety and repair costs under 4 scenarios and 4 what-if conditions. We used a method related to HAZUS-MH, but using detailed characteristics of 4 index buildings that served as proxies for the broader population. Post-earthquake safety was characterized in terms of ATC-20 (Applied Technology Council 1989 et seq.) tag color, plus a collapsed/not collapsed metric. Results were peer reviewed by respected engineers who added their own judgment. Results were presented to approximately 80 self-selected stakeholders, including tenants, owners, and other parties. The stakeholders identified, discussed, and selected policy recommendations that included a City ordinance for mandatory retrofit. It seems likely that the City will enact such an ordinance. If that outcome is deemed "success," we attribute it to four factors that might be emulated elsewhere. First, these buildings probably do represent a leading threat to the City's viability. The memory of the 1989 Loma Prieta earthquake and photographs of similar damage in 1906 are evidence of a real, ongoing threat. Second, the risk estimate focused on an important community value: safety tag color, rather than repair cost (though we presented both). Third, while acknowledging the many uncertainties involved, we limited our discussion of probabilities, focusing instead on realistic outcomes of a few realistic earthquakes that could occur any day, and certainly will occur eventually. Fourth, we made no policy recommendations, allowing the public (through the stakeholder committee) to determine what they thought was best for them and for the City.