Abstract
A rational methodology to evaluate behaviour factors for base isolated structures (BIS) code design is presented. The method used in this study is to directly derive q-factors by the nonlinear response analysis of 2 d.f. isolated models subject to strong seismic excitations artificially generated according to EC8 design spectra. The influence of several parameters on the ultimate behaviour of BIS has been analysed and collapse spectra have been derived for base isolated structures. Having defined theoretical collapse conditions for a 2 d.f. system subject to a set of artificial excitations, the minimum strength spectra needed for the structural survival have been evaluated. By dividing the elastic design spectra by the inelastic response ones, reduction factors for base isolated structures can be determined. A comparison with the UBC-91 new requirements for isolated structures shows a good agreement between the recommended behaviour factors and the ones calculated in the present paper.
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