Time-variant dynamic characterization and seismic response identification of base-isolated buildings

This study introduces the Mod-ξ(var) approach for identifying time-variant modal parameters of base-isolated (BI) structures subjected to seismic excitations. The approach combines a modified Newmark integration scheme with recursive optimization over short data windows, ensuring continuity of modal responses while capturing nonlinear effects such as stiffness degradation and damping variability. Theoretical formulation and validation are first demonstrated using a Finite Element (FE) model of a base-isolated single-degree-of-freedom (BI-SDOF) system with Bouc–Wen hysteretic behavior subjected to harmonic seismic excitations, revealing frequency-dependent damping and deformation-driven stiffness changes. The method's accuracy is confirmed by comparison with damping and secant stiffness estimates from a traditional hysteresis-based analysis. The methodology is then applied to a full-scale five-story BI-MDOF building tested on a shake table, successfully tracking time-variant natural frequencies, damping ratios, and mode shapes. Beyond modal identification, the Mod-ξ(var) approach enables the derivation of relevant engineering quantities such as the Empirical Response Spectrum (ERS), the estimation of the hysteretic loop of the seismic isolation layer, the evaluation of modal contributions to the response, and the disaggregation of modal energy terms, highlighting the dominance of first-mode energy dissipation. Results show strong agreement with experimental data and consistency with the BI-SDOF benchmark, underscoring the robustness of Mod-ξ(var) for assessing the seismic performance of base-isolated buildings.