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The goal of this study is to determine experimentally the inelastic response modes of seismically isolated structures. A steel specimen is designed and constructed to simulate the dynamic behavior of a Single-Degree-Of-Freedom (SDOF) system. The specimen is seismically isolated with four friction pendulum bearings and excited by a group of earthquake ground motion records using the shaking table of ETH Zurich Laboratory. A mechanical clevis connection consisting of two hinges and two replaceable steel coupons is designed and constructed to facilitate the parametric investigation of the inelastic response of the isolated structure for varying values of its strength. Two fundamentally different inelastic response modes have been observed during the excitation of the structure: A response mode dominated by large sliding displacement demand in the isolators and a response mode characterized by large displacement ductility demand in the isolated structure. This paper shows the effect of the design parameters of the isolation system and the isolated structure on the manifestation of these two response modes, thus paving the way for the understanding of the inelastic response of seismically isolated structures subjected to extreme earthquake ground motions.

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