Calibration of simple models for seismic soil-structure interaction from field performance data

A significant amount of strong motion data has recent become available from sites with instrumented structures and free-field accelerographs that afford the opportunity to empirically evaluate seismic soil-structure interaction (SSI) effects and to calibrate SSI analysis procedures. This research utilizes this data and limited numerical analyses to investigate kinematic and inertial SSI effects. Insights gained from these analyses are used to develop recommendations for simplified analysis procedures.; The work on inertial interaction focuses on analysis of complex-valued, frequency-dependant impedance functions, which define the stiffness and damping characteristics of foundation-soil interaction. While simple closed-form expressions are available for calculation of the impedance of a rigid disk foundation on a homogenous visco-elastic halfspace, there is significant model uncertainty associated with the use of these equations for realistic (and much more complex) site conditions. A two-pronged approach is taken to address this problem. First, impedance functions are estimated from strong motion recordings from sites with instrumented structures and free-field seismographs using system identification procedures. These results provide a “direct” quantification of impedance functions during earthquake shaking, and are compared to predictions from simple models. Second, design procedures in the NEHRP provisions (BSSC, 2000) for first-mode period and damping of a structure on a compliant base are calibrated against field performance data. The procedures are found to produce predictions that are biased relative to the field data. Revisions to the impedance function calculation procedures are proposed that reduce the bias.; The work on kinematic interaction investigates variations between foundation-level and free-field ground motions using strong motion recordings from 31 sites with instrumented structures and free-field seismographs. The foundation/free-field ground motion variations are quantified in terms of frequency-dependent transmissibility function amplitude, |H|. Procedures are developed to fit to |H| analytical models for base slab averaging (a kinematic soil-structure interaction effect) for the assumed conditions of a rigid base slab and a vertically propagating, incoherent incident wave field. A widely used ground motion incoherence parameter (κ) is estimated with these fitting procedures. A strong correlation is found between κ and parameters describing site geology and foundation flexibility.