The mid-story isolation design method is recently gaining popularity for the seismic protective design of buildings particularly located at highly populated areas. In a mid-story isolated building, the isolation system is incorporated into the mid-story rather than the base of the building. In this paper, the dynamic characteristics and seismic responses of mid-story isolated buildings are investigated using a simplified three-lumped-mass structural model for which equivalent linear properties are formulated. It is found that the nominal frequencies of the superstructure and the substructure respectively above and below the isolation system have significant influences on the isolation frequency and equivalent damping ratio of a mid-story isolated building. The mass and stiffness of the substructure are of greater significance than the superstructure in affecting the dynamic characteristics of the isolated building. Moreover, based on the response spectrum analysis, it is noted that the higher mode responses may contribute significantly to the story shear force of the substructure. The adverse effect arising from the coupling of higher modes on the acceleration responses of the superstructure is presented numerically and experimentally. A simple method to guarantee the mid-story isolation design against the modal coupling effect attributed to the improper design of the substructure and superstructure is proposed. Consequently, the equivalent lateral force procedure of design codes should carefully include the effects of higher modes.
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