Soft-story buildings have one or more floors that are structurally weak and can collapse during an earthquake. They often have open spaces like parking lots or retail spaces, making it difficult to install shear walls. Building codes prohibit their construction, and older buildings need upgrades to make them safer. In the San Francisco bay area, one in six structures is a soft-story building.
A soft-story building is a multi-story building with one or more floors that are “soft” due to structural design. These floors can be particularly dangerous in earthquakes because they cannot cope with the lateral forces caused by the building swaying during an earthquake. As a result, the soft story may fail, causing the so-called soft story collapse. If you’ve ever seen images of massive damage after a major earthquake, you’ve probably seen a number of examples of soft story collapses, because it’s a leading cause of damage to private residences.
Soft-story buildings are characterized by having a story that has a lot of open space. Parking lots, for example, are often soft stories, as are large retail spaces or floors with lots of windows. While the unobstructed soft story space might be aesthetically or commercially desirable, it also means there are fewer opportunities to install shear walls, specialized walls designed to distribute lateral forces so that a building can cope with the rocking characteristics of an earthquake. .
If a building has a floor that is 70% less stiff than the floor above it, it is considered a soft-story building. This soft story creates a major weak point in an earthquake, and as soft stories are classically associated with retail spaces and car parks, they are often on the lower floors of a building, meaning that when they collapse, they can take the whole building down, causing serious structural damage that can make the structure totally unusable.
Many earthquake-prone regions have building codes that specifically define a soft-story building and prohibit the construction of such buildings. When builders apply for a permit to build a new structure, engineers can review proposed plans to ensure no soft stories are incorporated, and recommendations for design improvements can be included in the rejection letter, if the building department decides the building will not be structurally sound in the event of an earthquake.
Older buildings are of great concern to building departments and emergency services in areas where earthquakes are common. These buildings are often in need of upgrades to make them safer in the event of an earthquake, and such upgrades can be relatively inexpensive, especially when compared to the cost of replacing a soft-story building after it collapses. In the San Francisco bay area, a region notorious for its earthquakes, one in six structures are estimated to be soft-story buildings, meaning many neighborhoods could suffer catastrophic building collapses in a major earthquake if the owners refuse to comply. Some insurance companies that offer earthquake insurance will refuse to insure if a building is classified as a soft-story building, due to increased liability.
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