The 2001 Nisqually earthquake, which had a moment magnitude of 6.8, damaged at least 78 bridges in western Washington State. Reports of damage sustained by bridges during this earthquake were used to correlate the likelihood of damage with the following parameters: distance to the epicenter, estimated peak ground acceleration, estimated spectral acceleration at periods of 0.3 s, 1.0 s, and 3.0 s; year built; and type of bridge. This goal was accomplished by collecting reports of bridge damage from state and local agencies, and comparing them with the population of bridges listed in the Washington State Bridge Inventory. The level of ground shaking at each bridge site was estimated from ShakeMaps, which were developed from data from the Pacific Northwest Seismic Network.
Of the four ground-motion parameters considered, the likelihood of bridge damage was best correlated with spectral acceleration at a period of 0.3 s. For a given level of spectral acceleration, bridges constructed before 1940 were the most likely to be damaged, while those constructed after 1975 were the least vulnerable. Although the number of movable bridges was small, this type of bridge was particularly vulnerable. Bridges with a steel main span were more likely to be damaged than those constructed of reinforced concrete. However, the number of steel bridges was small, and the most common type of damage to
steel-span bridges was actually damage to the reinforced concrete substructure.
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