Effects of foundation soil stiffness on the 3-D modal characteristics and seismic response of a highway bridge

This paper presents the main results of an investigation into the influence of foundation soil stiffness on the modal characteristics and seismic response of a highway bridge with spread foundations. A sensitivity study including six types of soil profiles is first performed to examine the effects of foundation soil stiffness on critical dynamic response parameters of the bridge. 3-D modal characteristics of lateral, vertical, longitudinal and torsional vibrations modes are identified and compared for various soil and rock conditions. The study is then extended to estimate the time history nonlinear seismic response of coupled foundation soil-bridge models utilizing an extension of the Fast Nonlinear Analysis (FNA) algorithm. Two different approaches for modeling soil foundation interaction are considered: a PGA consistent linear soil model and an advanced non linear soil model. Results show, among others, the major influence of soil flexibility effects on the modal characteristics and seismic response behavior of the bridge. Moreover, the results illustrate clearly the importance of soil nonlinearity on coupled foundation soil-bridge response. It follows, from a design perspective, that analytical models used for the seismic analysis of bridge structures should explicitly consider the effects of soil-structure interaction including nonlinear soil behavior.