Pounding of San Francisco-Type Soft-Story Corner Buildings

We investigated 1920s-era four-story wood-frame corner buildings common to San Francisco, many of which were damaged in the city's Marina District during the 1989 Loma Prieta earthquake. Such buildings can have relatively weak and flexible first stories and are referred to as soft-story buildings. We calibrated two building computer models to simulate actual earthquake response. We then performed computer analysis to assess collapse performance under 22 hypothetical pounding situations of both as-built and retrofitted soft-story buildings. For this building type, we found that a typical pounding situation increased the collapse rate by 14%, and for retrofitted versions, pounding slightly decreased the collapse rate. However, there were factors that appear to significantly increase the rate, especially in combination: negligible building separations and multiple adjacent buildings having low effective damping and large mass. Based on the results, we outline an approximate way to account for pounding within the context of current design procedures. An upper bound on the pounding effect was found to be equivalent to about a 30% increase in demand.