Influence of Seismic Isolation Level on a Building Response

Abstract eng:
Base isolation systems are applied in building systems to improve their seismic performance by isolating the structure from full intensity of the seismic input at the ground level. In some instances, due to the economic or environmental constraints, it is not possible to apply the isolation devices at the ground level. Instead, isolators may need to be placed at the top of ground floor columns. This is common when isolation is used in existing structures as a mitigation measure. The columns may then be subjected to altered internal forces below the isolation level. To study the consequences of this application, a simple five-story existing residential building was employed. An isolation system consisting of friction pendulum devices has been designed as a retrofitting alternative to improve the projected poor seismic performance of the building to resist effects given by the current Turkish seismic design code. Three dimensional model of the building is analyzed under the effect of recorded ground motion at the building region and the code specified design spectrum. In addition to the isolator displacements, the internal force demands from time history analyses have compared with the member capacities both before and after isolation. Results reveal that the increased force demands in the columns below the isolation level make these members critical and the isolation becomes ineffective. In this type of applications, either top of the columns should be restrained to obtain diaphragm action or the columns need to be retrofitted.

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