Effect of isolation pads and their stiffness on the dynamic characteristics of bridges

In this paper, the effect of the stiffness of the rubber pads on the dynamic characteristics of a base-isolated bridge is examined using transfer functions in the frequency domain. A three dimensional structural model that accounts for continuous mass distribution along each member is used. Results are obtained for a model of the bridge without isolation pads and for various values of the shear modulus of the rubber. This allows comparing the values of the predominant frequencies and the dynamic amplification of the motions over an extended range of frequencies. The results are then compared to the power spectra of the motions recorded at three points of an instrumented pier (the base, the top of the pier, and the same location on the deck) under an actual earthquake. The model can explain some of the observed behavior very well although there are still some points that cannot be resolved due to lack of sufficient information on the spatial variability of the motions.