Some of the most susceptible structures to shaking damage are soft-story apartments and condominiums. A soft-story residential building is one that has open parking or commercial space on the first floor and housing on higher floors built prior to recent codes. In an earthquake, ground shaking causes such structures to sway and sometimes collapse. A soft-story collapse can have particularly disastrous consequences considering that they crush cars and kill people occupying the open areas.
ABAG modeling has shown that, in both a large earthquake on the Hayward or San Andreas faults, two-thirds of the uninhabitable housing units will likely be in soft-story residential buildings.
Local Governments: Soft-story mitigation and policy opportunities
Estimates of Number of Soft-Story Buildings in the Region
The following table summarizes the number of buildings identified as having a potential soft-story by various building inventories. The year built, units,and stories indicates the criteria used to estimate the number of buildings. More information about the inventories and soft-story mitigation programs in these cities (and others) can be found below the table.
|City/County||Total Buildings||Total Units||Year Built||Units||Stories|
|San Francisco||2,800||29,000||< 1973||≥ 5||≥ 3|
|San Francisco (estimated)||4,600||18,400||< 1973||4||≥ 3|
|San Francisco (estimated)||3,400||10,200||< 1973||3||≥ 3|
|Oakland||1,479||24,273||< 1990||≥ 5||≥ 2|
|Oakland (estimated)||1,060||4,240||< 1990||4||≥ 2|
|Oakland (estimated)||370||1,110||< 1990||3||≥ 2|
|Entire Santa Clara County||2,630||33,119||any||any||≥ 4|
|San Jose (incl. in county total)||1,093||10,923||any||any||≥ 4|
|Berkeley||400 (320 wood-frame)||5,000||< 1995||≥ 5||≥ 2|
|Alameda (estimated)||275-300||3,500||< 1985||≥ 5||≥ 2|
|Sebastopol||55||unknown||< 1990||≥ 2||≥ 2|
In 2013 San Francisco passed legislation that requires the evaluation and retrofit of “multi-unit soft-story buildings,” defined as: Wood-frame structures, containing five or more residential units, having two or more stories over a “soft” or “weak” story, and permitted for construction prior to January 1, 1978.
The Resilient City – The Dilemma of Existing Buildings, San Francisco Urban Research Center (SPUR)
The City of Oakland is taking steps to identify soft-story multi-unit buildings vulnerable to collapse in earthquakes. Past earthquakes have demonstrated that these buildings pose a safety risk to tenants and occupants, a financial risk to owners and risk the recovery of the City and region. In 2008 Oakland surveyed its multi-family buildings with five or more units and in 2009 Oakland passed an ordinance that required the owners of these buildings to complete a simple evaluation of the ground floor. The 2013 report documents the data collected thus far as a result of that ordinance and recommends next steps the City and residents can take to reduce damage to multi-unit wood-frame soft-story buildings in an earthquake.
City of Oakland Ordinance for Mandatory Seismic Screening of Multiple Story Buildngs Constructed Prior to 1991, July 2009
City of Berkeley Inventory Documentation
Santa Clara County (Including San Jose)
Inventory Documentation: Inventory of Soft-First Story Multi-Family Dwellings in Santa Clara County, Collaborative for Disaster Mitigation San Jose State University – Includes numbers of soft-story buildings in each city.
City of Alameda award winning soft-story program
City of Richmond potential soft-story inventory
City of Fremont ordinance 10-2007 Requiring Mandatory Retrofit of Apartments
Cities on the Peninsula between San Francisco and San Jose also tend to have a large number of soft-story buildings. However, no specific numbers are available for any of these communities. These buildings are also common in the denser portions of Marin County than in the other North Bay counties.