Q: After the 2005 disaster in New Orleans we decided we should go ahead with the seismic retrofit of our home in El Cerrito, Calif. Can you recommend any research that I should do before doing the project? I am confused as to what methods are recommended for homes in my area and whether what’s recommended to me by experts who do this work is right for my home.
A: The Gulf Coast has hurricanes; the Midwest has tornados; and the San Francisco Bay Area has the ever-present threat of an earthquake.
As the baseball playoffs and World Series approach we often reflect on the Loma Prieta quake of Oct. 17, 1989. The Marina District was in flames; a section of the Bay Bridge collapsed; and an elevated section of Interstate 880 fell.
We have often wondered why the 1879 Victorian house that Kevin lived in and the 1920s Craftsman bungalow Bill was renovating came through unscathed.
We believe the bolted concrete foundation we had recently installed on Bill’s house, like the upgraded foundation on Kevin’s Victorian, along with some of the construction techniques of yesteryear, contributed to the survival of these buildings.
Educating yourself is a great idea. Here are a few resources to get you started.
Six principal factors influence how and why buildings are damaged in an earthquake. They are:
1. The depth of the quake and the strength of seismic waves reaching the surface.
2. The duration of the quake.
3. The proximity of the building to the quake’s center.
4. Geological and soil conditions.
5. Construction details, including materials, structural systems and plan configuration.
6. Building condition, including maintenance level.
The first four factors are beyond our control. The last two factors are what the retrofitting process is all about.
Building type and construction methods determine the behavior of a house in an earthquake. Balloon framing, where the studs run from foundation to roofline, such as on Kevin’s wood-frame Victorian, allowed the building to absorb the shock.
Diagonal 1-by-6-inch subflooring acted as a diaphragm in both homes to lessen the effects of the shock waves.
On the other hand, many Bay Area buildings have been built with several large openings that put the building at risk for significant damage during a quake. An example of this is a garage that is built under the living space of a house.
These wide, unreinforced openings create what is known as a “soft story” that is prone to damage during a quake. Structural reinforcement is critical for these buildings.
Since the Loma Prieta quake, retrofitting buildings to protect against quake damage has taken on a heightened priority for Bay Area cities. We did a brief Web search using the keywords “seismic retrofit” and came up with a large number of hits.
These three Web sites are our favorites:
1. The Association of Bay Area Governments publishes a slide presentation developed by the Federal Emergency Management Agency (FEMA) titled “Training Materials for Seismic Retrofit of Wood-frame Homes.”
It contains 242 slides showing damage to wood-frame homes and both proper and improper retrofit techniques. We recommend it as a good overview. The presentation can be found at www.abag.ca.gov/bayarea/eqmaps/fixit/training.html.
2. For a more in-depth review of retrofitting older buildings we recommend an article titled “The Seismic Retrofit of Historic Buildings” by David W. Look, AIA, Terry Wong, PE, and Sylvia Rose Augustus.
This is a detailed overview and contains sections on planning, assessing costs, questions to ask and a helpful glossary of terms. It’s published by the National Park Service and can be found at www.cr.nps.gov/hps/tps/briefs/brief41.htm.
3. Finally, the city of San Leandro, Calif., publishes an earthquake handbook touted as “a high-impact, full-color, 16-page booklet that provides residents with a plain-English explanation about earthquake risks in the community.”
According to the Web site, the city also offers quarterly earthquake strengthening workshops given by the city’s building department. The workshops are directed at the do-it-yourselfer but also are helpful in learning how to get the best service if one chooses to hire a contractor.
The city also maintains a “Homeowner’s List of Earthquake Contractors.” Go to http://www.ci.san-leandro.ca.us/city-contractlist.html. The Association of Bay Area Governments maintains a similar list.
