Illustrating a New Possibility for the Estimation of Floor Spectra in Nonlinear Multi-Degree of Freedom Systems

Abstract eng:
The adequate consideration of acceleration demands in the seismic design of acceleration-sensitive nonstructural components, although challenging, has shown to be a key area for improvement within the field of earthquake engineering. Moderate and strong seismic events in recent decades have reinforced the importance of these nonstructural systems as a large source of monetary loss, disruption and occupant hazard. However, numerous recent studies have shown that modern design codes and guidelines fall short of incorporating critical parameters necessary to estimate spectral floor acceleration demands. Further, recent trends in performance-based earthquake engineering have moved towards conveying seismic demands in a probabilistic manner; with the median seismic demand to be paired with an estimate of dispersion to allow better design decisions to be made in terms of building importance, occupancy and other performance objectives. This paper describes a novel procedure to combine important factors affecting spectral floor acceleration response in terms of both central tendency and uncertainty from ground motion variability (record-to-record dispersion) while maintaining a reasonable level of simplicity in implementation. The study focuses on the influence of both structural and nonstructural damping ratios on the amplification of floor response spectra demands as well as the effect of nonlinear response in the primary structure. The fundamental frequencies and mode shapes are considered via a modal superposition method to incorporate modal characteristics of individual structures. The proposed technique for estimating floor response spectra targets the median floor spectra for given modal and damping characteristics as well as the expected ductility demand of the main structure for a given input spectrum. The consideration of nonlinear demands and damping ratios is incorporated through empirical relationships derived from linear single degree of freedom systems and nonlinear multiple degree of freedom (MDOF) systems, analyzed with a large suite of ground motions at various intensities. Distinctions are made in the procedure with respect to the type of structural system based on the preliminary findings of the study. The approach is applied to a reinforced concrete cantilever wall and a steel momentresisting frame building as case studies. The proper consideration of higher mode response of the primary structure and nonstructural damping ratio are shown to have the greatest influence in central tendency across all periods. The good correlation between predicted results and those obtained via nonlinear time-history analyses suggests that the new procedure could represent a useful development for the assessment of nonstructural components within a performance-based earthquake engineering framework.

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