Development and Evaluation of Superelastic SMA Gap Dampers for Pounding Mitigation in Seismically Base-Isolated Structures

A seismic base isolation system effectively mitigates superstructural responses by increasing the structural natural period and providing additional damping to dissipate seismic energy during strong earthquakes. Deformations of the system are primarily concentrated in the isolation layer due to the flexibility of isolation bearings. Although a wide isolation clearance is necessary to accommodate these large deformations, practical and architectural constraints may make it insufficient. As a result, moat wall (MW) pounding may occur in base-isolated structures during extreme earthquakes, compromising seismic isolation effects. Hysteretic gap dampers (GDs) have been developed in isolation systems to dissipate seismic energy when the threshold deformation occurs, mitigating MW pounding. However, the effectiveness of the isolation may be limited during immediate aftershocks or future earthquakes due to the deficient self-centering (SC) capability of conventional hysteretic GDs. To address these issues, this study develops an adaptive seismic isolation system incorporating shape memory alloy (SMA) GDs to provide additional damping and SC capability during extreme earthquakes, wherein the core elements of the GDs are U-shaped dampers (UDs). The hysteretic responses of SMA- and steel-UDs were experimentally investigated through quasi-static cyclic tests. The test results confirm that the SMA-UDs exhibit flag-shaped hysteresis loops with excellent SC capabilities under cyclic loading, whereas the steel-UDs show sufficient energy dissipation but are accompanied by considerable residual deformations. Subsequently, typical isolation systems incorporating different GDs (i.e., SMA-, steel-, and combined-GDs) were designed based on a displacement-based design procedure. The seismic performances of the systems with MWs and GDs were evaluated systematically through comparative studies on seismically base-isolated reinforced concrete frame buildings. The comparison results demonstrate that, unlike the conventional MWs, the SMA-GDs significantly reduce the responses of the isolation layer and the accelerations of the superstructural floors. Compared with the steel-GDs that experienced unrecoverable cumulative damage, the SMA-GDs with excellent SC capabilities demonstrate stable hysteretic behavior in the isolation layer. The combined-GDs provide optimal response mitigation between the isolation system and the superstructure.