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SF’s tallest buildings may be in seismic danger, warns report

Engineer’s assessment of SF skyline reveals shaky prospects

Photo by Dhruv Vohra/Shutterstock

On Thursday, engineering non-profit agency Applied Technology Council (ATC) released a study about the seismic safety of San Francisco’s tallest buildings, warning that standards are outdated and that the city should commit to new inspections of key structures before the next big earthquake comes.

The late Mayor Ed Lee commissioned the report last year in response to the sinking conditions at the Millennium Tower.

ATC says it spent 14 months inventorying 156 SF buildings (a few of which are not yet constructed but have received permits) that are 240 feet or taller.

In all, the report, titled Tall Buildings Safety Strategy, identifies 16 points of potential concern about the seismic stability of San Francisco’s tallest towers, including but not limited to:

  • Seismic standards may not go far enough: “The San Francisco Building Code sets minimum requirements for geotechnical site investigations and foundation design. Because they are minimum requirements, they do not fully address all of the geotechnical conditions found in San Francisco. [...] Many of the new tall building developments are challenging even these best practices due to unique soil conditions, the size and weight of the new buildings.”
  • The 1994 Northridge Earthquake in Southern California revealed potential building weaknesses that SF has not followed up on: “San Francisco’s steel buildings were not systematically inspected for weld damage, which they might or might not have sustained in the 1989 Loma Prieta earthquake. [...] The city should apply the inspection, evaluation, and repair provisions of the San Francisco Existing Building Code as applicable to pre-1989 welded steel moment frames.”
  • The city lacks a comprehensive source of data on tall buildings: “This project compiled a database with information about all buildings 240 feet or taller [...including] building location, height, occupancy, age, construction material, structural system, year, and foundation type. Prior to the creation of this database, the City had no centralized, searchable repository with this information about all tall buildings in its jurisdiction. Following the completion of this project, the City will need to develop a mechanism for maintaining or expanding the database.”
  • Plans and assessments tend to consider buildings in isolation: “[This report] does not explicitly address the likely interactions between the tall buildings, the non-tall buildings that still comprise most of downtown, and the critical infrastructure that serves the neighborhood. [...] A separate recovery plan, drawing on the present study’s findings, would bring these ideas together in a practical way to support a neighborhood and its functions, as opposed to just individual buildings with certain characteristics.”
  • One-size-fits-all standards do not, in fact, fit the biggest buildings: “SAP [Safety Assessment Program] procedures and criteria [...] are generic and not well-suited to complex or recovery-critical facilities, including most tall buildings. DBI should develop its own procedures suited to San Francisco’s tall buildings (and otherwise unique building stock) regarding such topics as limits on exterior-only inspection, limits on rapid evaluation, and damage estimates.”

The ATC report also notes that “new and existing tall buildings represent a dominant portion of the city’s business sector” and particularly of recent and upcoming construction, and that “their high concentration downtown poses an aggregate risk to neighborhood and citywide recovery not presented by other building groups.”

To see the full report and list of recommendations, go here. For a map and database of the city’s tall buildings, go here.