A soft story building is a multi-story building in which one or more floors have windows, wide doors, large unobstructed commercial spaces, or other openings in places where a shear wall would normally be required for stability as a matter of earthquake engineering design. A typical soft story building is an apartment building of three or more stories located over a ground level with large openings, such as a parking garage or series of retail businesses with large windows.
Buildings are classified as having a “soft story” if that level is less than 70% as stiff as the floor immediately above it, or less than 80% as stiff as the average stiffness of the three floors above it. Soft story buildings are vulnerable to collapse in a moderate to severe earthquake in a phenomenon known as soft story collapse.The inadequately-braced level is relatively less resistant than surrounding floors to lateral earthquake motion, so a disproportionate amount of the building’s overall side-to-side drift is focused on that floor. Subject to disproportionate lateral stress, and less able to withstand the stress, the floor becomes a weak point that may suffer structural damage or complete failure, which in turn results in the collapse of the entire building.
Soft story failure was responsible for nearly half of all homes that became uninhabitable in California’s Loma Prieta earthquake of 1989, and was projected to cause severe damage and possible destruction of 160,000 homes in the event of a more significant earthquake in the San Francisco Bay Area, California. As of 2009 few such buildings in the area had undergone the relatively inexpensive seismic retrofit to correct the condition. In 2013, San Francisco mandated screening of soft story buildings to determine if retrofitting is necessary. source
Please read more on San Francisco’s Mandatory Soft Story Retrofit Ordinance.