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Soft-Story Buildings: An Overview

What is a soft-story building?

The term “soft-story building” describes a building with a level or story that is significantly more flexible than the stories above it, which can be hazardous in the event of an earthquake. In multi-unit wood-frame buildings, weakness at the ground level stems from large openings in perimeter walls or insufficient interior partition walls. Examples of perimeter openings include garage doors, tucked-under parking or large windows. During a strong earthquake, the ground level walls of a soft-story building are unable to support the stiff and heavy mass of the stories above as they move back and forth. These ground level walls can shift sideways until the building collapses, crushing the ground floor.

Past earthquakes have resulted in loss of human life and serious injuries due to the damage caused by soft-story buildings. For example, in the 1994 Northridge earthquake, 16 people were killed and 34,000 housing units were left uninhabitable by soft-story collapses, while in 1989 Loma Prieta earthquake 7 lives were lost and 7,000 housing units were rendered uninhabitable due to soft-story building issues.

How can I tell if my building is a soft-story building?

It is not possible to know for certain that a building is a soft-story building simply by looking at it from the street. Although large openings at the ground level are a strong indicator that a building may have a soft-story, many characteristics affect a building’s response to an earthquake, including the contribution of interior partitions, strength of walls and previous retrofits completed. Only a building-specific analysis conducted by an engineer can determine if a particular building is a soft-story building.

Using the figure below, an initial visual inspection can estimate the degree of openness of the particular exterior wall.  If at any exterior wall,  solid section of the wall at the ground floor are less than 80% of the the solid walls above, then it is likely that a soft story condition exists.  However, the degree of openness criteria is not the only indicator of whether a building is “safe” or “unsafe” – rather, the criteria reflects a decision by the city of Alameda to put those buildings with the largest openings on a suspect list of buildings required to complete an Engineering Report.

Soft-Story Building Diagram

What is being done to prevent soft-story building damage?

Bay Area cities such as Berkeley and Fremont have already implemented their own soft-story ordinances to prevent damage in event of an earthquake.  Oakland and Alameda have passed soft story ordinances but have not yet began notifying the owners, while San Francisco, San Leandro and others are currently developing soft-story ordinances to be implemented in the next few years.

In the next few posts, we’ll talk about what Alameda is specifically doing to mitigate soft-story building damage.

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