Effect of soil structure interaction on the dynamic responses of base isolated bridges and comparison to experimental results

In this paper, the effect of soil structure interaction and base isolation on the dynamic characteristics of an instrumented bridge is examined using transfer functions and measured motions in the frequency domain. A three dimensional structural model in the frequency domain which accounts for continuous mass distribution along each member and the effect of axial forces as well as rotary inertia is adopted. The dynamic stiffness of pile foundations and surface mat foundation underneath piers, determined separately using appropriate numerical methods, is incorporated into the structural model and the global stiffness matrix. Results are obtained for models of a bridge both without and with isolation pads for various values of the equivalent shear stiffness. This allows one comparing the values of the predominant frequencies and the dynamic amplification of the motions over the frequency range of interests. The transfer functions are also obtained at the bottom of the piers to evaluate the impact and importance of soil structure interaction effect on the dynamic behavior of the system. Results are then compared to the power spectra of the motions recorded at various points of an instrumented bridge (the base, the top of the pier, and the same location on the deck) from an actual earthquake. The method and results can explain many of the observed behavior very well although there are still some points that cannot be resolved due to lack of accurate input information and limitation of the method.