Seismic soil-structure interaction in buildings. II: Empirical findings

System identification analyses are used to evaluate soil-structure interact ion effects for 77 strong motion data sets at 57 building sites that encomp ass a wide range of structural and geotechnical conditions. Kinematic inter action effects on the "input" motion at the bases of structures are found t o 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 par ameters are used to evaluate first-mode period lengthening ratios (T) over tilde/T and foundation damping factors <(zeta)over tilde>(0). The response of some structures is dominated by inertial interaction (e.g., (T) over til de/T approximate to 4, <(zeta)over tilde> approximate to 30%), whereas othe rs undergo negligible soil-structure interaction (e.g., (T) over tilde/T ap proximate to 1, <(zeta)over tilde>(0) approximate to 0). Simplified analyti cal formulations described in the companion paper by Stewart et al. are use d to predict inertial interaction effects. The predictions are found to be reasonably accurate relative to empirical results, with some limitations fo r deeply embedded and long-period structures. A collective examination of t he empirical and predicted results reveals a pronounced influence of struct ure-to-soil stiffness ratio on inertial interaction, as well as secondary i nfluences from structure aspect ratio and foundation embedment, type, shape , and flexibility.