The 6.0 magnitude earthquake that struck Napa, CA, in August caused more than 200 injuries and structural damage to many homes and businesses throughout the area. The quake was the largest to hit the San Francisco Bay Area since the Loma Prieta earthquake (6.9 magnitude) in 1989, prompting the governor to declare a state of emergency.
I have done several posts about San Francisco’s Soft-Story Retrofit Ordinance and some of NEES-Soft testing related to soft-story retrofits. The soft-story ordinance only addresses multi-unit residential units and does not require retrofit of single-family homes. Cities are reluctant to mandate seismic evaluation and retrofit of single-family homes for a number of reasons that I won’t discuss here. The draft Earthquake Safety Implementation Program (ESIP) for San Francisco will not recommend mandatory retrofit of single-family homes until 2030.
The good news is homeowners can retrofit their homes without waiting for the government. A couple years ago in this post, I discussed some of the tools available to retrofit existing buildings.
One of these tools is the 2012 International Existing Building Code (IEBC). The IEBC has provisions for repair, alteration, addition or change of occupancy in existing buildings and for strengthening existing buildings. For alterations, these provisions may not comply with current IBC requirements, but they are intended to maintain basic levels of fire and structural life safety. The IEBC also provides prescriptive provisions for strengthening existing buildings against earthquake damage, which include strengthening residential houses on raised or cripple wall foundations.
Cripple wall failures are a common type of damage observed in older homes, caused by inadequate shear strength in the cripple wall. An additional failure point is the attachment of the wood sill plate to the foundation. Having a strong connection between the wood structure and the concrete foundation is critical in an earthquake. Since the work required to strengthen these connections is typically performed in a crawlspace or unfinished basement, it is a relatively low-cost upgrade that is extremely beneficial to structural performance.
Our website has information for retrofitting your home. The Seismic Retrofit Guide has information about how earthquakes affect a home and the steps to take to reinforce the structural frame of a house. The Seismic Retrofit Detail Sheet is intended to help building departments, contractors and homeowners with seismic retrofitting. It includes common retrofit solutions for reinforcing cripple walls and foundation connections.
One business owner in Napa chose to retrofit her building when she purchased it. You can see her video narrative here.