Hysteretic response of a nine story reinforced concrete building during the San Fernando Earthquake

The Millikan Library on the campus of the California Institute of Technology was strongly shaken during the San Fernando earthquake of February 9, 1971. The building was not damaged structurally, but the observed E-W response of the building showed a fundamental period of about 1. 0 sec, significantly longer than the 0. 66 sec observed in preearthquake vibration tests. In this study, the response of the fundamental mode was treated as that of a single- degree- of- freedom hysteretic structure, and four simple models, two stationary and two with changing properties, were examined to see if they could describe the observed response. It was found that an equivalent linear model and a bilinear hysteretic model both could match the response, provided their properties were changed during the earthquake. (Four changes were used). A linear model with constant properties and a stationary, bilinear hysteretic model did not give nearly as good agreement as the nonstationary models. The results indicated, in general, a degrading of the stiffness and energy dissipation capacity of the building, but it could not be determined whether the changes were sudden or gradual.