Experimental evaluation of supplemental friction dampers for torsional control of step-terrace mountain isolated structures

Step-terrace mountain (STM) isolated structures are susceptible to torsional response when eccentricity exists in the isolation layers, owing to dropped stories, multi-level foundations, and discontinuous stiffness distribution. This study experimentally investigates the use of supplemental friction dampers for torsional mitigation in eccentric STM isolated structures. Shaking-table tests were conducted using a 1:10-scale reinforced concrete STM isolated structural model. Comparative tests were performed for isolated and isolation–damping configurations under different isolation-layer eccentricity cases and seismic intensity levels. Torsional rotation, torsional displacement ratio, and relative torsional effect were adopted to evaluate the control performance of the proposed system. The results show that the supplemental friction dampers reduced the torsional response of the STM isolated structure under all investigated eccentricity configurations. Across all three tested eccentricity configurations, the supplemental friction dampers consistently reduced torsional response, with the most pronounced mitigation observed in case 2. Under increasing seismic intensity, the isolation–damping structure consistently exhibited lower torsional response than the corresponding isolated structure, while the fundamental frequency remained nearly unchanged. The supplemental dampers therefore improved torsional control without noticeably altering the global dynamic characteristics of the isolation system. The tested damper configuration corresponded to a total friction capacity of ∼13% of the total isolation-system yield force, while its equivalent secant stiffness contribution was only about 3% of the total horizontal stiffness of the isolation system. These findings provide experimental evidence for the application of supplemental friction dampers in torsional control of eccentric STM isolated structures.