Seismic Soil-Structure Interaction in Buildings. II: Empirical Findings

System identification analyses are used to evaluate soil-structure interaction effects for 77 strong motion data sets at 57 building sites that encompass a wide range of structural and geotechnical conditions. Kinematic interaction effects on the “input” motion at the bases of structures are found to be relatively modest in many cases, whereas inertial interaction effects on the structural response to these motions can be significant. To quantify inertial interaction effects, fixed- and flexible-base modal vibration parameters are used to evaluate first-mode period lengthening ratios $\tilde{\mathit{T}}/\mathit{T}$ and foundation damping factors $\tilde{\zeta }{}_{0\text{.}}$ The response of some structures is dominated by inertial interaction (e.g., $\tilde{\mathit{T}}/\mathit{T}$ ≈ 4, $\tilde{\zeta }{}_0$ ≈ 30%), whereas others undergo negligible soil-structure interaction (e.g., $\tilde{\mathit{T}}/\mathit{T}$ ≈ 1, $\tilde{\zeta }{}_0$ ≈ 0). Simplified analytical formulations described in the companion paper by Stewart et al. are used to predict inertial interaction effects. The predictions are found to be reasonably accurate relative to empirical results, with some limitations for deeply embedded and long-period structures. A collective examination of the empirical and predicted results reveals a pronounced influence of structure-to-soil stiffness ratio on inertial interaction, as well as secondary influences from structure aspect ratio and foundation embedment, type, shape, and flexibility.