A rigid mass driven by Coulomb friction from a randomly moving foundation is used as a simple model of a possible earthquake isolation system for a stiff compact structure. The structure is protected from accelerations greater than that produced by the limiting friction at the expense of a residual relative displacement with respect to the foundation. The RMS residual displacement is predicted approximately, both analytically and by computer simulation. The analytical approximation makes use of equivalent linearization applied to a nonstationary response problem. The computer simulation marches out exact response histories for an ensemble of independent earthquake realizations. There is good agreement between the two predictions. The trade-off between peak accelerations and residual displacements for typical earthquake intensities is displayed.

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