Previous studies conclude that additional capacities of isolation system are needed beyond minimum demand required in minimum design loads for buildings and other structures (ASCE 7) to achieve the targeted level of reliability for seismically isolated structures. To implement this concept for practical design, a probabilistic design method with minor computational cost is proposed to obtain the isolation system capacities based on the targeted failure fragility. As an essential statistical parameter for determining the fragility curve shape, total system uncertainty value (βTOT) is needed. Among different sources of βTOT, the record to record uncertainty (βRTR) contributes most to the isolator deformation demand and is challenging to obtain for a specific design case. A simplified procedure with minor computational cost has been proposed for estimating βRTR when failure is governed by the isolators. Main assumptions and simplifications in the proposed design method have been investigated and justified. Finally, a step-by-step procedure for implementing the proposed probabilistic design method has been introduced. A design example was given, and the three-dimensional incremental dynamic analysis was performed to validate the design method.

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