A considerable number of reinforced concrete box-girder highway bridges are located in seismically active zones. This type of bridge bore a major portion of the damage inflicted during the 1994 Northridge, 1989 Loma Prieta, and 1971 San Fernando earthquakes in the western area of the United States. Most of the existing box girder viaducts were built before the advent of computers. Earthquake loads were calculated by the modified seismic factor method. The strong correlation between the ground motion frequency of seismic events, structure frequency, and structure damage is well established and has shown the criticality of structure frequency in the determination of structure seismic damage. In this study, a methodology to assess the seismic vulnerability of concrete box-girder bridges by correlating the fundamental natural frequencies analytically and experimentally has been developed.;The analyses carried out in this study proceeded as follows: (1) Field ambient vibration measurements were conducted on three reinforced concrete box-girder bridges in the Pacific Northwestern region of the U.S. Laboratory data analyses were conducted to derive the fundamental natural frequencies and other modal properties of the bridges. (2) A procedure to build finite element models using space frame, spring, and mass elements and to analytically compute the fundamental natural frequencies via modal analysis was developed. (3) An identification method in a matrix form was developed to identify the sensitive parameters in this type of bridge and to reach a best agreement between the analytical and experimental natural frequencies. (4) Finally, the seismic responses of this type of bridge was assessed by performing linear time-history transient analyses.
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