Dynamic analysis of several shear-type structures with base isolation indicates that the response of these structures follows their fundamental mode shape. Based on this observation, this paper uses an approximate procedure for computing the response of base-isolated structures. The procedure consists of modeling the structure and its base by a two-degree of freedom system, one representing the base and the other the structure. The response from the two-degree of freedom model and mode shapes of the structure are used to compute the response of the structure to earthquake excitation. The approximate procedure is simple, requires substantially less computational time than other methods, and gives results that are in excellent agreement with those from direct integration. Nonlinear properties and nonproportional damping are easily included in the model. Savings of approximately 54–77 percent in computational time result by using the approximate model.

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