High damping, rubber bearing isolators supplemented with Cu-Al-Be- and Ni-Ti-based shape memory alloy bars subjected to near-fault motions with fling step and forward directivity

Shape memory alloy (SMA) bars are currently preferred over elastomeric seismic isolators due to the elimination of degradation within effective damping and stiffness factors during the cyclic response of an isolation system. These bars could also be used to prevent the functionality of the isolator units from failing due to large deformations. This study aims to investigate the performance of a high damping rubber bearing (HDRB) isolator that is combined with two different types of SMA bars, i.e., Nickel-Titanium (Ni-Ti) and Copper-Aluminum-Beryllium (Cu-Al-Be), subjected to near-fault ground motions that are characterized with forward directivity and fling step effects. To achieve this objective, a self-centering material with flag-shape, force-deformation hysteresis was utilized to simulate the behavior of SMA bars in OpenSees. A single degree of freedom (SDOF) system representing an isolated one-story shear building was developed to conduct nonlinear analysis under selected ground motions. The SMA bars were introduced as an X-shape within the model and were connected diagonally to the top and bottom of the isolator. Results showed that the HDRB system’s hysteretic response under near-fault ground accelerations experiences significant degradation, especially when near-fault motions involve the fling step effect. It was demonstrated that SMA bars effectively reduce large displacement observed in HDRB systems under near-fault earthquakes. Comparing the results of the base-isolated HDRB and SMA-HDRB subjected to selected ground motions demonstrated that maximum displacement was found to be significantly reduced by an average of 79% when SMA bars were used. Incorporating SMA bars with a larger diameter significantly improves the efficiency of SMA HDRB systems, and a reduction in maximum displacements is more pronounced for fling step, near-fault ground motions.