A Full‐Scale Shake Table Test on the Torsional Responses of Base‐Isolated Structures by Friction Pendulums

ABSTRACT Torsional response remains an important concern in the seismic performance of base‐isolated structures. In this study, a full‐scale shake table test was conducted on a three‐layer FPB‐isolated structure, together with numerical simulations and bearing component tests, to investigate its torsional behavior and three potential sources of torsion: superstructure flexibility, pressure dependence of the friction coefficient, and bearing‐to‐bearing variability in the friction coefficient. The results show that, compared with the transient eccentricity ( ε k ), the proposed effective transient eccentricity ( ε s ) provides a clearer indication of torsional response because it directly represents the effective lever arm producing torsion. Superstructure flexibility increases ε s by altering the instantaneous distribution of inertia forces, although the resulting torsional displacement amplification remains limited. The torsional response observed in the shake table tests is mainly associated with variability in the friction coefficient, whereas the pressure dependence of the friction coefficient has a relatively limited influence. These findings provide a mechanical interpretation of the torsional response of FPB‐isolated structures and highlight the importance of considering variability in individual bearing properties in torsional assessment.