ARTICLE: Soft-story Retrofits to Become Mandatory in the City of San Francisco
A renewed effort to urge property owners to seismically retrofit thousands of buildings in The City will now move from "voluntary to mandatory," according to Mayor Gavin Newsom, as buildings failed to get upgraded...
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What is seismic retrofitting?
Seismic retrofitting is the process of reinforcing existing structures, which would provide more resistance against forces due to earthquake.
What is the process of seismic retrofitting?
A building which is vulnerable to earthquake must be evaluated by a licensed structural engineer, and a retrofitting plan be engineered.
The engineered designs usually involve the construction of a primary resisting element such as moment frames and sheer walls. Often, additional reinforcing connections between the existing building elements such as foundation to frame and between beams and posts are recommended as well.
Why is it necessary?
Prior to the development of modern seismic codes in late 1960's, most structures were built without adequate reinforcements for seismic protection.
What types of buildings require seismic retrofitting?
While the seismic codes have since been getting progressively more stringent, most structures built prior to the 1997 UBC code may benefit from additional reinforcement.
Among residential structures, most single or two story wood-frame houses without adequate foundation bolting or plywood sheer walls in the crawl space can benefit from retrofitting. Also, most older hillside houses, which are supported by posts and piers, perform poorly in an earthquake.
Among the commercial structures, the most vulnerable are buildings with soft story conditions as well as un-reinforced masonry buildings.
Soft story condition refers to multi story buildings with a ground level design, mostly comprised of columns and short span walls to accommodate retail windows and garage openings. The upper levels are typically used as offices or apartments.
Due to this particular vulnerability, and because of health and safety concerns, many Bay Area municipalities are in the process of providing incentives to the owners of these buildings to retrofit their buildings.
