Far-field long-period ground motions were featured by long duration and rich low frequency components. Particularly, multiple cycle pulses similar to harmonic vibration were generated at the late stage of some seismic waves. These may cause the adverse influence on the seismic performance of the long-period structures including the isolation building, which needed to be investigated deeply. To this end, the characteristics of far-field long-period ground motions were discussed. Then, six far-field long-period earthquake records and three common earthquake records were selected as the input of ground motions, and the seismic responses of a RC mid-story isolated frame structures were obtained by nonlinear analysis. Also, the impact of the long-period components of seismic waves, especially harmonic-like components on shock absorption performance were investigated. Further, a combined isolation scheme by the addition of viscous dampers in the isolation layer was proposed, and its effects on limiting deformation of the isolation layer and reducing nonlinear responses were explored. The results show that the effectiveness of the mid-story isolation subjected to far-field long-period ground motions on reducing nonlinear seismic responses are not as good as those subjected to common ground motions. Particularly, the nonlinear responses of the mid-story isolated structure are seldom reduced or significantly enlarged under the far-field harmonic-like strong ground motions when compared with those of the seismic structure, and the maximum displacement of isolation bearings is far beyond the allowable displacement. It is also indicated that the combined isolation can effectively control nonlinear responses of the mid-story isolated structure under far-field long-period ground motions, especially harmonic-like ground motions. In particular, the maximum displacement of the isolation bearings can be significantly decreased, preventing the destruction of the isolation bearing.
Key words
Far-field long-period ground motion /
Mid-story isolation structure;Combined isolation /
Nonlinear response;Limiting deformation of isolation layer
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