UCLA

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. Conventional engineering practice typically ignores soil-structure interaction (SSI) during evaluation of the seismic demand on buildings based on the perception that consideration of SSI will reduce demands on structures and ignoring SSI effects will cause seismic demands to be conservatively biased. I show that it is not always conservative to ignore SSI effects. 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. These data are interpreted to evaluate strengths and limitations of engineering analysis procedures for SSI.

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. We utilize these general expressions with impedance solutions for rectangular-footprint foundations to: (1) compare predicted foundation damping levels with those from field case histories and (2) develop new foundation damping relationships for application in the building code (NEHRP Provisions).

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.

To expand the data inventory of response histories to evaluate SSI, we performed forced vibration testing of a well-instrumented steel and reinforced concrete structure that has removable bracing. The testing was performed at three sites with varying soil conditions. I describe testing at one of the sites located in Garner Valley, California. During testing at this site an adjacent structure and local concrete slab were also instrumented in addition to the test structure. I describe the testing setup, schemes, instrumentation, and data processing techniques. The data are analyzed to evaluate the stiffness and damping associated with the foundation-soil interaction, revealing linear-elastic behavior at low forcing levels characterized by smaller stiffnesses and both lower and higher damping than is predicted by classical models. Nonlinear behavior at stronger shaking levels includes pronounced reductions of stiffness and changes in damping. Interestingly, kinematic interaction effects observed on an adjacent slab excited principally by surface waves were of a similar character to expectations from analytical models for body wave excitation from earthquakes. If verified, these results could lead to site- and foundation-specific test methods for evaluating kinematic interaction effects.