A two stage approach for the sesmic analysis of base-isolated structures

Base Isolation System (BIS) is a very effective strategy for reducing the effects of earthquakes on building structures. The classical Response Spectrum Method (RSM) contin-ues to be the most common approach for the design of base-isolated buildings. This paper of-fers a new strategy of seismic analysis and design for such structures in conjunction with the RSM. The main advantages of the proposed approach are: first, reduced computational effort with respect to an exact complex-valued modal analysis, which is obtained through a two-stage transformation of coordinates, both involving real-valued eigenproblems; second, accu-rate representation of the damping, which is pursued by consistently defining different viscous damping ratios for the modes of vibration of the coupled dynamic system made of superstruc-ture and BIS; third, ease of use, since a convenient reinterpretation of the combination coeffi-cients leads to a novel Damping-Adjusted Combination (DAC) rule, in which just a single response spectrum is required for the reference value of the viscous damping ratio.