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Resources: Newsletter Articles: Earthquake Retrofit

October, 2000

The recent earthquake in the Napa Valley has been a reminder that we should all prepare our homes for the next big quake. There are things you can do now to help protect your home and its contents when an earthquake hits. The last few earthquakes along the West Coast have caused catastrophic structural damage. The Association of Bay Area Governments (ABAG) has performed studies to help predict the number of homes that would be "red-tagged" by local building departments as "unsafe for occupancy" after a major earthquake. It was determined that of the homes located near the fault, where the shaking intensity will be at its peak, it is expected that at least 12% of the homes built prior to 1940 that have not been retrofitted will be red tagged. Only 1.8% of the newer and retrofitted homes are expected to be uninhabitable, making retrofitting a worthwhile investment.

In 1972, building codes were introduced in order to ensure that some basic strengthening methods were used in new construction such as bolted sill plates to the foundation and plywood sheer walls. Most homes constructed prior to the adoption of this code have not been built to survive a major earthquake.

Checking your home's earthquake fitness is easy and requires a flashlight and a willingness to get a bit dirty. Start in the crawl space underneath your home. Is the house properly bolted down to its foundation? The mud sill should be bolted at six-foot intervals (or as specified on your plans), and a bolt should be located within one foot of every joint or step in the mud sill, but no closer than nine inches to the end of the board.

Next, examine the cripple wall. Make sure your cripple walls are braced with plywood to resist motion. Even if your cripple walls have cross bracing, they are not strong enough for earthquakes unless plywood has been added. Finally, check for faulty materials in the concrete and the wood framing to make sure it's in good condition. Sometimes the concrete used in foundation is too porous and crumbly to provide adequate strength.

Do you see any obvious evidence in the wood of dry rot or insect damage? If so, the damaged wood will need to be removed and replaced. It's a good idea to hire a structural pest control inspector to look for damage not easily seen except by a trained eye.

If you decide you want to have your home retrofitted, you need to determine if you want to do the work yourself or hire a contractor. Whichever route you chose be sure that you have gathered as much information as possible regarding current codes and requirements. There is a basic criterion that needs to be followed to perform a proper retrofit. To have this work performed you should call a qualified expert in the retrofit field. However, if you want to do the work yourself, be sure you seek expert advice.



Sherry Benninger

sherrybenninger@grubbco.com

The GRUBB Co., 1960 Mountain Blvd., Oakland, CA 94611

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