#99-24
"Assessing the Benefits and Costs of Earthquake Mitigation"
Summary: Over the past decade, as the costs of natural disasters has skyrocketed in the United States, much emphasis has been placed on mitigating these hazards. However, to date, limited published data exists on the vulnerability of structures to ground shaking, and quantitative cost-benefit studies on earthquake mitigation are nearly non-existent. As "earthquake loss estimation" tools becomes more and more commonly used by insurance firms, reinsurance firms, and government, reducing uncertainties in these variables becomes more important
This paper presents the survey results gathered from professsionals in earthquake mitigation in California. Two different populations were surveyed, structural engineers and mitigation contractors. The structures chosen for the study were pre-1940, wood-frame, single family residences. The mitigation chosen was the bolting of cripple walls to the foundation followed by the bracing of those cripple walls with plywood. (Bolting and bracing must be done together to be effective. A "cripple wall" is the wood structure between the lowest occupied floor and the masonry or concrete foundation)
Three questions were asked:
- How valid are the initial vulnerability or damage curves utilized in earthquake loss analysis? Is the use of Modified Mercalli Intensity (MMI) appropriate for damage estimates?
- How valid are the assumptions on the reduction in vulnerability after mitigation? How beneficial is "bracing and bolting" the cripple wall in reducing damage for various levels of ground shaking?
- How valid are the estimates of the costs of mitigation? Since the costs are "upfront" and the benefits are seen over the life of the structure, an accurate assessment of the cost is important.
In general, the survey results can be summarized as follows. First, the use of the MMI scales was not well received by the engineers, who are more familiar with the Richter scale or measure of peak ground acceleration. Second the importance of quality of construction, the weight and shear capacity of the structure above ground, and non-structural damage should not be underestimated. Third, existing conditions such as rot and termite damage can greatly effect costs and effectiveness. Fourth, survey damage values were less that comparable values found in the "ATC-13" and FEMA 227/228 guidelines. Finally, there was wide variation in both damage and cost estimates in both survey groups.
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