Coupled Nonlinear Torsional Seismic Behaviour of a Triple-Friction-Pendulum-Isolated Five-Storey RCC Building Under Nonstationary Earthquake Excitation

Triple Friction Pendulum (TFP) bearings are widely employed as seismic isolation systems to reduce seismic force and deformation demands in structures. However, the combined influence of vertical mass-distribution irregularity, soil–structure interaction (SSI), structural eccentricity, and nonstationary earthquake excitation on the nonlinear torsional response of TFP-isolated buildings remains insufficiently understood. This study investigates the seismic response of a five-storey reinforced concrete (RCC) building equipped with TFP bearings subjected to nonstationary spectrum-compatible horizontal earthquake ground motions. The investigated structure represents a low-to-medium-rise isolated building with a superstructure period Ts ≤ 0.5 s, and the conclusions are applicable within the examined structural configuration, adopted TFP properties, and equivalent SSI modelling assumptions. A nonlinear time-history analysis framework combined with Monte Carlo-based spectrum-compatible ground-motion simulations is employed to evaluate the effects of SSI, structural eccentricity, and vertical mass-distribution irregularity. Validation against OpenSees benchmark simulations demonstrates strong agreement in the predicted structural responses. Parametric analyses show that increasing the soil stiffness ratio from 0.5 to 2.0 reduces the normalized isolation displacement by approximately 46.5% and the corner rotational response by approximately 36.7%. Conversely, increasing the vertical mass-irregularity ratio from 1.0 to 1.5 increases the inter-storey drift ratio by approximately 60% and corner-displacement magnification by approximately 48.6% within the investigated cases. The results demonstrate that seismic response is governed not only by the magnitude of vertical mass irregularity but also by its location due to changes in inertia-force distribution, modal participation, and lateral–torsional coupling. These findings highlight the importance of considering SSI and vertical mass distribution simultaneously when assessing the seismic performance of torsionally asymmetric TFP-isolated RCC buildings. Further studies incorporating taller structures, multidirectional excitation, detailed soil–foundation interaction models, and experimentally calibrated TFP properties are required to extend the applicability of the findings.