Extracting Empirical Capacity Curves: a Case Study of the Seven-Story Hotel in Van Nuys, California

Abstract eng:
After strong earthquakes, damage detection methods can be used on instrumented buildings to provide quick feedback on the status of the structural system of a building. One such damage detection method is the empirical capacity curve extraction method. An empirical capacity curve is a force-displacement curve developed from recorded response data of a particular event. The information from an empirical capacity curve is used to quantify damage to the structural system; depending on the type of empirical capacity curve obtain, damage quantification parameters could include stiffness degradation, strength degradation, ductility ratios, and changes to the fundamental period. For a case study, the empirical capacity curve extraction method has been applied to the seven-story hotel located at 8244 Orion Avenue in Van Nuys, CA. This building is instrumented with accelerometers at the ground, roof, and some intermediate floors. Historical response records are available for the 1971 San Fernando and 1994 Northridge earthquakes. Using these response records, a fundamental mode empirical capacity curve can be extracted for each of the two events by using only the respective roof and ground recorded acceleration records. For fundamental mode empirical capacity curves, stiffness degradation, changes in the fundamental period and if the yield point can be estimated, ductility ratios are the best parameters for damage quantification. For the 1971 earthquake, only the east/west acceleration response records are available. For the 1994 earthquake, both the east/west and north/south response records are available. After the 1971 earthquake, the east/west direction fundamental period appears to decrease by half yet after the event, the building was deemed to only suffer nonstructural damage. For the 1994 earthquake, the initial fundamental period was close to that of the fundamental period after the 1971 earthquake; at the end of the 1994 earthquake, the fundamental period was less than half of the post 1971 earthquake fundamental period. Substantial structural damage was noted on the fourth floor columns. Many researchers such as Freeman, Lepage, and Luna have developed their own capacity curves for this building using the response data. The resulting empirical capacity curve has been compared with the results of the other researchers. While generally similar, there are differences between each researcher’s capacity curves for this building. If the empirical capacity curve extraction method was employed at the time immediately after the events, an alert could have been triggered to notify first responders had the fundamental period or stiffness of the system decreased below a certain threshold.

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