A frequency domain dynamic analysis is needed to obtain the response of structures constructed on a system of base isolators on flexible soil when subjected to earthquake ground motion. In such systems, the lateral stiffness of the base isolators is so small, compared to the structural stiffness, that in an earthquake attack the fundamental vibration mode dominates the response. With this in mind, a simple equivalent model is introduced for the assembled structure and its flexible isolators. Next, this simple model is modified to include the effect of soil flexibility. This final model is believed to be appropriate for the frequency domain analysis of the whole system. Frequency independent dynamic characteristics of the model are determined and several response spectra pairs are prepared for both the exact and developed models. The results are in good agreement while the computational efforts for the simplified model is less.

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