Soft story structures are structures built on first stories with more than 80% open area on one first-story wall or more than 50% open area on two adjacent first-story walls.
Because of population density and high land values, mixed-use construction became prevalent, integrating soft stories so that the first story of buildings can be used as commercial real estate or parking and have a dramatically different layout than the upper stories.
These soft story structures carry a major flaw. In earthquakes, they pose a serious risk to occupants. Not only do these soft stories make the structures weaker, but they were also built to obsolete seismic standards that incorrectly predicted earthquake effect.
Many cities, such as the City of San Francisco, have decided that soft story structures must be retrofitted to meet current guidelines for occupant safety.
Approximately 5,000 soft story structures in San Francisco require seismic strengthening. In 2013, the City of San Francisco passed the Mandatory Soft Story Retrofit Ordinance and received over a 99% response. At this late point in the program, remaining structures have limited time to comply with this important ordinance to protect their structures and their lives.
Soft Story retrofits consist of a number of components including pre-constructed bracing, such as moment frames or shear walls, and new concrete footings. These bracings are added on the first floor to protect occupants. New footings serve to hold down the building during the shaking of an earthquake. Together, these various components complement each other to provide comprehensive earthquake resilience.