Experience from the October 17, 1989 Loma Prieta earthquake has confirmed what previous engineering studies have stressed: Almost all serious damage to wood frame houses has been a result of either 1) insufficient connections between frame and foundation and/or 2) insufficient lateral bracing (shear-strength) in the load bearing, supporting walls.
Without adequate frame-to-foundation connections, a house can be displaced completely from its foundation in a strong quake. Insufficient lateral bracing typically results in either a partial or complete collapse of the perimeter cripple walls, which form the basement and crawl space areas. Either form of damage can amount to a total loss scenario for the homeowner.
The good news is that properly reinforced houses in the hardest-hit areas of Santa Cruz and Watsonville performed very well during the Loma Prieta quake, suffering little or no damage. The overall pattern of residential performance not only mirrored the experience of previous California earthquakes, but also corroborated what various engineering reports have predicted for years.
There is no question that the Bay Area is due for another quake far worse than the Loma Prieta earthquake. The impact on homeowners could range from minimal to disastrous depending on whether appropriate reinforcing measures are taken.
Photos courtesy of The Bancroft Library, USGS and FEMA.
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