Performance-Based Seismic Demand of Soil-Foundation-Structure Systems

Abstract eng:
Seismic demand of soil-foundation-structure systems is evaluated herein in the light of performance-based design approaches, accounting for soil-foundation compliance effects. Due to kinematic and inertial interaction, structural response may be substantially different from the traditionally calculated fixed-base-structure-on-free-field approach. When soilfoundation-structure interaction (SFSI) is accounted, seismic demand of a system is identical to the notion of performance point, the latter being unique for every linear elastic SFSI system. We evaluate seismic demand using three distinct approaches: a) the traditional fixed-baseon-free-field approach, b) a finite element numerical code and c) the analytical approach proposed in FEMA440. In the current study, we utilize the exact ground motion at the foundation level and we propose a practical method to account for soil-foundation-structure interaction effects on seismic structural performance. We then highlight and quantify the discrepancies between our approach and the traditional approach and FEMA440 methodology. We propose use of effective foundation motion (EFM), which is affected by both kinematic and inertial interaction. Until now, no such index has been proposed, since relevant notions, such as the foundation input motion proposed in FEMA440, do not take into account both inertial and kinematic interaction. Moreover, EFM is measurable directly from actual records and field measurements. Using the graphical capacity spectrum method, intersection of the spectrum, expressed in acceleration-displacement format, that results for the acceleration time-history at the foundation level with the radial, which refers to the SFSI structural period, gives good estimation of structural response, compared to the direct method. Finally, for systems affected by interaction, FEMA440 seems to become un-conservative compared to numerical solutions and the deviation of EFM compared to the free-field motion can be from -60% up to +35%.

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