Soil-pile-structure interaction effects on high rises under seismic shaking

Soil-structure interaction of high-rises was investigated in both theory and field measurement. The instrumented structure motions and free-field ground motions are available so that the recorded structure and ground motions can be used to validate the theory and program developed for this study. Currently, the design code only suggests the use of advanced methods to analyze the high-rise building performance. No details are provided in the code. The effects of soil structure interaction are only available for low rise buildings with simple analysis procedures and are not available for high rise buildings.;The computer code SSI3D is developed to evaluate the nonlinear soil-pile-structure interaction of high-rises. Besides several soil models for static load, two models are modified and implemented for cyclic load: Modified Hyperbolic and Modified Ramberg-Osgood models. SSI3D is validated by comparing the analysis results and the measured seismic performance of an actual high-rise structure with soil-structure interaction effects.;In fully nonlinear soil-pile-structure time history analysis, the unknown bed rock motion to be used as input motion must be determined from free-field motion by using deconvolution procedures. The resulting bed rock motion is then propagated to the ground surface. In this analysis, the calculated ground surface motion is compared to the ground surface motion used to calculate bed rock motion to check the validity of the de-convolution and motion propagation procedures. Unfortunately, there are differences between the deconvolution and convolution analyses described above because of the use of soil viscous damping in convolution and soil damping ratio in deconvolution. A new method is recommended to determine the appropriate soil viscous damping from the soil damping ratio by matching transfer functions from soil layer to soil layer. The excellent agreement between the calculated free-field motions using soil viscous damping and the measured free field motion confirms the validity of this method.;Soil-structure interaction includes both kinematic and inertial interactions. In design code, two interactions are considered separately. In this study, the transfer function from free-field motions to base foundation motions of buildings, and equivalent stiffness, and damping of soil-pile system represented kinematic and inertial interaction are recommended for use in the design code. When the nonlinear analyses are performed, the nonlinear pile stiffness is recommended.;As high rises are built or designed for areas of high seismic activity, it is critical to examine their seismic responses using an analysis code that reveals their realistic behavior under strong seismic shaking. To better understand the seismic soil-structure interaction effects, two 20-story hypothetical buildings and one 30-story actual building were subjected to seismic response analyses using SSI3D. To reflect the evolution of SSI effects, analyses were performed for the cases with rigid base, flexible base with linear foundation springs, flexible base with linear soil, flexible with nonlinear springs, and the full SSI analysis of flexible base with nonlinear soils for two hypothetical buildings. The last case depicts the most realistic SSI responses of high rises. Results of analyses presented include the comparison of natural periods, base shears, and the displacements at the top floor of the buildings. It was observed that the natural periods increase and the base shears decrease as the base become more flexible.