WHAT IS A SOFT STORY?
A soft-story residential building is one that has open parking or commercial space on the ground floor with residential upper floors. They are usually 2-4 stories and have been built prior to more recent earthquake codes.
The issue with having a first floor that is structurally weaker than the more rigid floors above is that these buildings become subject to large side-sway and are susceptible to collapse or damage beyond repair in large earthquakes (see picture below).
Recent studies done by Bay Area governments show that in a large earthquake on the Hayward or San Andreas fault, a majority of the uninhabitable housing will likely be in soft-story buildings.
The risk of soft-story buildings is property damage, loss of housing, and potential loss of life in the case of collapse.
WHAT TO DO ABOUT IT…
For smaller wood framed soft-story buildings, the easiest retrofit is sometimes adding plywood shear walls or pre-manufactured shear panels. Since adding walls may disrupt the open space and challenge the existing parking on the ground floor, this solution is not always an option.
Another common solution in retrofitting soft-story conditions is adding a steel moment frame or steel moment columns. This is most commonly used to preserve the existing parking spaces of the building (see picture above). These structural elements are designed to take the lateral forces of the earthquake on the open/weak side of the building.
There are a couple other possibilities, depending on the building geometry and retrofit criteria. But regardless of the design, these seismic retrofits would need to be prepared by a licensed civil or structural engineer.
CITY SOFT-STORY PROGRAMS
Soft stories pose a great risk to cities because if many of these buildings collapse, there will be a large loss of housing and a greater strain on post-earthquake recovery.
Bay Area cities have taken measures to determine how much inventory they have of these soft-story buildings. The ordinances are to alert the general public of these buildings’ vulnerability during large earthquakes and provide incentive for retrofits that make the buildings more safe.
Below you will find some links to some of the Bay Area Cities’ soft-story programs. For further information or discussing the options of what to do with your building, give me a call. I’d be happy to consult with you and discuss the possibilities.