Πειραματική και θεωρητική διερεύνηση ορισμένων θεμελιωδών θεμάτων της δυναμικής αλληλεπίδρασης εδάφους-θεμελίωσης-ανωδομής

Abstract

The present thesis was fulfilled in the framework of the European research project “Seismic hazard assessment, site effects and soil structure interaction studies in an instrumented basin EUROSEISRISK, EVG1-CT-2001-00040)”. The part of the study concerning soil-foundation-superstructure interaction includes free and forced vibration experiments that were conducted on a bridge-pier model structure, which was erected at the EUROSEISTEST site (http://euroseis.civil.auth.gr). The aim of the thesis is to study the specific fundamental issues of the dynamic soil-foundation-superstructure interaction using experimental records of a bridge-pier model. The main targets of the present thesis concern the analysis of the free field wave field due to the vibration of the structure, the experimental and numerical estimation of the dynamic impedances of surface foundations and the investigation of the importance and influence of damping on the structural dynamic response. The first chapter of the present thesis presents the synoptic structure of the full text whereas the second chapter refers to the main characteristics of the problem. The mechanics and the effects of the soil-foundation-superstructure phenomenon in the structural design are presented. Furthermore, follows a comprehensive description of the methods for the study of the phenomenon and some of the most important research works that have been published till present. The experimental results, concerning the stress-field on the interface between the soil and foundation as well as the time history recordings of the free-field motion are presented in the fourth chapter. The study of the recordings in the time domain provides valuable information concerning the soil damping, whereas the study in the frequency domain provides valuable information concerning the fundamental period of the bridge-pier model. The pattern of the spatial attenuation of the soil movement can also be detected taking into account the different instrumentation schemes. The finite element method, as a second method for the study of the phenomenon is applied in the fourth chapter. Various experiments are simulated. The numerical results of the 3D soil-foundation-superstructure model are compared with the corresponding experimental records in the frequency as well as in the time domain. The satisfactory comparison of the results enables the use of the finite element method on the further study of the SSI phenomenon. The spatial extend of the soil structure creates many problems in the simulation of the soil-foundation-superstructure mechanism. Therefore the spring analog is applied in several cases in order to reduce the number of the degrees of freedom of the finite element model. The spring stiffness represents the soil stiffness whereas the value of the damper is related to the damping mechanisms developing during the phenomenon, such as spatial and hysteretic damping. The fifth chapter includes the results of various analytical and numerical studies concerning the estimation of the foundation dynamic impedances. Various cases concerning the surface foundation shape and the soil profile are presented. The static and dynamic impedances of the foundation of the bridge pier model are calculated in the sixth chapter by the combination of the experimental and the finite element method. The difference in the stiffness and inertial characteristics of the bridge pier model enables the study of various dynamic systems preserving the same geometry but having different dynamic characteristics. Therefore, various pairs of real and imaginary values of dynamic impedances relating to different frequencies of vibration are calculated. The results of the hybrid method are compared with analytical results and some important conclusions are derived concerning the influence of the superstructure and the assumed soil profile characteristics such as stiffness and damping. The phenomenon of soil-foundation-superstructure interaction is directly related to the wave propagation in the soil medium. Therefore, in the first part of the seventh chapter the basic Lamb’s problem is studied by the implementation of the finite element method. The satisfactory concordance of the numerical and the fundamental analytical results confirms the correctness of the wave propagation simulation which is related to the spatial attenuation of the soil movement. The second part of the study refers to the numerical calculation of the dynamic impedances of a surface, rigid foundation which is based on various soil profiles. The comparison between the numerical and analytical results clarifies different matters concerning the wave propagation mechanism and especially the influence of soil damping. The aforementioned parameter which characterizes the soil dynamic behavior is further studied in the eighth chapter. The radiation pattern as well as the viscous soil damping are two mechanisms that function simultaneously during the soil-foundation-superstructure phenomenon. The first part of the study refers to the compatibility between the function of the Rayleigh damping mechanism which is used in the finite element method and the elastic-viscoelastic correspondence principle which is used in the analytical studies. The concordance of the results in terms of dynamic impedances verifies the correct use of the Rayleigh damping. The second part of the study investigates the influence of viscous damping to evaluate foundation displacement characteristics. The vector approach of the displacements applied herein relates the modification of their values, due to the viscous mechanism, and thus enables the estimation of the corresponding damping coefficient. The ninth and last chapter of the present thesis includes the comprehensive summary of the conclusions of the study. Further extensions of the study refer to a flexible foundation, a stratified soil profile or the use of a nonlinear relationships concerning soil behavior.