Building Age and Earthquake Safety

Earthquake safety in the design of wood frame houses has increased over the last 50 years, so the age of your building has much to do with how well it is designed to resist earthquakes. In general, the newer the building, the stronger it is likely to be in an earthquake. Building age can be determined from the following sources:

1) tax assessor's file
2) Sanborn map
3) building permit file
4) utility records

The local building department is a good place to start looking for your building's age because it will usually have a copy of the Sanborn map and other records. If none of those sources indicates the age of the building, the architectural style of the house can give an indication. Older buildings are sometimes renovated to look new, although this is not very common in houses. Knowing the age of your house will help engineers and architects decide what additional earthquake safety measures may be needed.

Foundations are often an area of weakness in older houses. Sometimes older houses do not have foundations at all, or they have weak ones. Most houses have perimeter wall foundations, which are continuous at the ground around the edge of the house. Many newer houses have concrete slab foundations that perform well in earthquakes. The most common problem in wood frame houses is in the area between the floor of the house and the top of the foundation, either this part of the structure is poorly braced or not well-bolted to the foundation. Over 23,000 homes were damaged in the Loma Prieta earthquake of October, 1989, and most of the damage was due either to lack of anchor bolting to foundations or in adequately braced cripple walls.

Even in recently constructed houses, there are features that have proven to be vulnerable to earthquake damage; most of them relate to the building configuration Particular configurations have been associated with damage in past earthquakes:

1) house over garage (see section III.F.)

2) many large windows or doors (particularly at building corners)

3) large overhangs

4) split levels and complex geometry

5) stilts supporting the structure (as on a hillside site)

Unusual configurations are not necessarily hazardous, but if they are poorly designed or constructed, they can be particularly vulnerable to earthquake damage.

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