Dynamic Soil-Structure Interaction of Instrumented Buildings and Test Structures

The effects of soil-structure interaction (SSI) are investigated through careful interpretation of available data from instrumented buildings and recently performed forced vibration experiments on instrumented buildings and test structures. Analysis of field performance data is undertaken to provide deeper insights into SSI phenomena ranging from kinematic effects on foundation ground motions to mobilized foundation stiffness and damping across a wide range of frequencies and loading levels.;Foundation damping incorporates the combined effects of energy loss due to waves propagating away from the vibrating foundation in translational and rotational modes (radiation damping), as well as hysteretic action in the soil (material damping). Previous foundation damping models were developed for rigid circular foundations on homogenous halfspace and were often expressed using confusing or incomplete functions. Starting from first principles, we derive fundamental expressions for foundation damping in which foundation impedance components representing radiation damping and the soil hysteretic damping ratio appear as variables, providing maximum flexibility to the analyst.;Ground motions at the foundation levels of structures differ from those in the free-field as a result of inertial and kinematic interaction effects. Inertial interaction effects tend to produce narrow-banded ground motion modification near the fundamental period of the soil-structure system, whereas kinematic effects are relatively broad-banded but most significant at high frequencies. Kinematic interaction effects can be predicted using relatively costly finite element analyses with incoherent input or simplified models. The simplified models are semi-empirical in nature and derived from California data. These simplified models are the basis for seismic design guidelines used in the western United States, such as ASCE-41 and NIST (2012). We compile some available data from building and ground instrumentation arrays in Japan for comparison to these two sets of models. We demonstrate that the model predictions for the sites under consideration are very similar to each other for modest foundation sizes (equivalent radii under about 50 m). However, the data show that both approaches overestimate the transfer function ordinates relative to those from a subset of the Japanese buildings having pile foundations. The misfit occurs at frequencies higher than the first-mode resonant frequency and appears to result from pile effects on kinematic interaction that are not accounted for in current models.;A complete model of a soil-foundation-structure system for use in response history analysis requires modification of input motions relative to those in the free-field to account for kinematic interaction effects, foundation springs and dashpots to represent foundation-soil impedance, and a structural model. The recently completed NIST (2012) report developed consistent guidelines for evaluation of kinematic interaction effects and foundation impedance for realistic conditions. We implement those procedures in seismic response history analyses for two instrumented buildings in California, one a 13-story concrete-moment frame building with two levels of basement and the other a 10-story concrete shear wall core building without embedment. We develop three-dimensional baseline models (MB) of the building and foundation systems (including SSI components) that are calibrated to reproduce observed responses from recorded earthquakes. SSI components considered in the MB model include horizontal and vertical springs and dashpots that represent the horizontal translation and rotational impedance, kinematic ground motion variations from embedment and base slab averaging, and ground motion variations over the embedment depth of basements. We then remove selected components of the MB models one at a time to evaluate their impact on engineering demand parameters (EDPs) such as inter-story drifts, story shear distributions, and floor accelerations. We find that a "bathtub" model that retains all features of the MB approach except for depth-variable motions provides for generally good above-ground superstructure responses, but biased demand assessments in subterranean levels. Other common approaches using a fixed-based representation can produce poor results. (Abstract shortened by UMI.).