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Estimation of floor acceleration spectra for inelastic buildings

Non-structural elements play an important role in performance-based earthquake engineering. In the paper a practice-oriented method for direct determination of floor acceleration spectra, which are used for the seismic design and evaluation of acceleration sensitive equipment, is proposed. The method can be used for inelastic multi-degree-of-freedom (MDOF) structures and equipment modelled as an elastic single-degree-of-freedom (SDOF) oscillator. It is based on an extension of the procedure for inelastic SDOF structures developed previously by the authors. The structural absolute accelerations at different floors are determined by the pushover-based N2 method. Floor acceleration spectra are derived directly from an inelastic spectrum for the structure and an elastic spectrum for the equipment. In the resonance region, where the natural period of the equipment is close to the natural period of the structure, the spectral values are based on empirical values obtained in a parametric study, whereas the floor spectra in the pre- and post-resonance regions are based on the principles of dynamics of structures. Generally, higher mode effects cannot be neglected. It is assumed that the modal superposition approach is applicable, as an approximation, also in the nonlinear range of response, so the results obtained for different modes of vibration are combined by a special combination rule. In majority of cases, the inelastic behaviour influences the response in the fundamental mode of vibration, whereas the higher mode response is elastic. The proposed method has been validated through several test examples and the results show a good agreement with the results obtained using the classical approach for the determination of floor spectra with response history analyses. The proposed method represents a useful tool in the stage of conceptual design, as well as for checking results of more elaborate analyses. Also, it can be used as a basis for simplified code procedures.

Author(s):

Vladimir Vukobratovic    
University of Novi Sad
Serbia

Peter Fajfar    
University of Ljubljana
Slovenia