Soft Story – Many properties, residential, commercial and apartment buildings have what is called a “soft story” condition.
This term is used to describe any building that has a habitable room or rooms above a garage, carport or porch area that was not specifically designed to transmit shear or lateral forces to the story above. Failures of these types of building or structure with soft story conditions can lead to loss of lives in an earthquake. Many counties in California are currently drafting ordinances to require retrofitting of all soft story buildings.
On the 107th anniversary of the 1906 San Francisco Earthquake (April 16, 2013), San Francisco Mayor Ed Lee signed into law the Mandatory Soft Story Retrofit Ordinance. This legislation requires the evaluation and retrofit for “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. These residential buildings with a soft story condition are particularly vulnerable to severe damage and collapse during an earthquake. During the 1989 Loma Prieta earthquake, 7,700 soft story residential buildings were rendered uninhabitable and, during the 1994 Northridge earthquake, over 34,000 housing units were rendered uninhabitable. Currently, over 58,000 San Francisco residents and close to 2,000 businesses are located in approximate 2800 soft story buildings that have not been retrofitted. The compliance time line for meeting the Wood-Frame Soft Story Seismic Retrofit Program includes:
- Tier I Buildings must be retrofitted by September 15, 2017 – These buildings contain educational, assembly, or residential care facility uses.
- Tier II Buildings must be retrofitted by September 15, 2018 – These buildings contain 15 or more dwelling units.
- Tier III Buildings must be retrofitted by September 15, 2019 – These are the remaining soft story buildings not falling within another tier.
- Tier IV Buildings must be retrofitted by September 15, 2020 – These buildings contain ground floor commercial uses or are located in a mapped liquefaction zone.
In California, there is an earthquake hazard disclosure law requiring the seller to disclose to the buyer at the time of sale the existence of certain known earthquake hazards, such as lack of bolting, existence of cripple walls with no shear paneling, hot water heaters that are not properly strapped, etc.. One of the conditions required to be disclosed as a hazard is the existence of a “soft story” condition. Visualize two dominoes standing on their small ends. Load four or five bricks on the dominoes with no difficulty because the dominoes are strong enough to carry the load, or downward force. But, if you go to the side of the dominoes and apply a tiny shear force: a shake or with even a breath; the whole thing will come crashing down… more
Potentially Earthquake-Unsafe Residential Buildings — a (Very Rough) List
San Francisco’s Department of Building Inspection has kept a preliminary list of potentially dangerous “soft-story” buildings since 2009, but inspectors say it has not been verified by actual building inspections, and was never intended for public consumption. Some of the addresses the city generated might be wrong.
See more at: SFPP
For more information, please visit the City and County of San Francisco Earthquake Safety Implementation Program.