SENSITIVITY OF MODAL PARAMETERS OF MULTI-SPAN BRIDGES TO SSI AND PIER COLUMN INELASTICITY AND ITS IMPLICATIONS FOR FEM MODEL UPDATING

Abstract

MODAL PARAMETERS ARE IMPORTANT PROPERTIES OF A BRIDGE THAT ARE CENTRAL TO THE STRUCTURAL HEALTH MONITORING (SHM) PROCESS. MODAL PARAMETERS DETERMINED THROUGH FORCED VIBRATION TESTING, AMBIENT VIBRATIONS OR SEISMIC EXCITATIONS HAVE BEEN USED TO CALIBRATE NUMERICAL MODELS THROUGH FEM MODEL UPDATING. MASS, STIFFNESS AND BOUNDARY CONDITIONS ARE THE THREE MAIN PARAMETERS THAT ARE FINE-TUNED DURING THE FEM MODEL UPDATING PROCESS FOR OBTAINING A HIGH FIDELITY NUMERICAL MODEL BY MATCHING NUMERICAL MODAL PARAMETERS WITH THE OBSERVED ONES. THIS STUDY INVESTIGATED THE SENSITIVITY OF MODAL FREQUENCIES AND MODE SHAPES TO CHANGES IN BOUNDARY CONDITIONS (SOIL-STRUCTURE INTERACTION (SSI) EFFECT) AND PIER COLUMN INELASTICITY (STIFFNESS EFFECT) THROUGH NON-LINEAR FEM MODELING AND MORE THAN 450 NON-LINEAR DYNAMIC TIME-HISTORY ANALYSIS OF AN ORDINARY MULTI-SPAN HIGHWAY BRIDGE. THE BRIDGE WAS FOUNDED ON SHALLOW FOUNDATIONS IN FIVE ROCK PROFILES AND ON PILE-GROUP FOUNDATIONS IN FIVE SOIL PROFILES. THE BRIDGE SYSTEM WAS SUBJECTED TO 21 SEISMIC GROUND MOTIONS VARYING IN INTENSITY FROM 0.036G TO 0.61G. IT WAS FOUND THAT MODAL FREQUENCIES WERE MORE SENSITIVE TO THE SSI EFFECT FOR LOW LEVELS OF SEISMIC EXCITATIONS BUT WERE AFFECTED MORE BY CHANGES IN THE PIER COLUMN STIFFNESS FOR HIGHER LEVELS OF SEISMIC EXCITATIONS. MODE SHAPES, ON THE OTHER HAND, WERE FOUND TO BE RATHER INSENSITIVE TO SSI AS WELL AS CHANGES IN PIER COLUMN STIFFNESS FOR ALL LEVELS OF EXCITATIONS.