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In performance-based seismic design, bridges are expected to satisfy specific performance objectives under several levels of seismic hazard. In this paper, a multi-level SMA/lead rubber bearing (ML-SLRB) isolation system was proposed to ensure both isolation efficiency and capability to limit excessive bearing displacements under different levels of earthquake excitations. The ML-SLRBs also offer advantages such as the ability to provide re-centering forces and good fatigue and corrosion-resistant. The ML-SLRB isolation system consists of three groups of SMA cables, each is designed to be activated at a certain seismic hazard level, and a conventional lead rubber bearing. First, the design and working mechanism of this new isolation system were described in detail. Then, a design procedure was proposed for seismic isolation of bridge structures with ML-SLRBs. Next, the hysteretic response of ML-SLRBs was simulated in a general-purpose structural engineering software. A four-span continuous box-girder bridge was designed and modeled with different isolation systems including ML-SLRBs. Nonlinear dynamic analyses of the isolated bridges were conducted under both far-fault and near-fault earthquakes. Results show that compared to isolations systems that do not adapt their stiffness according to increasing seismic demand, e.g. the isolators with a bilinear force-displacement response, the proposed isolation system exhibits high isolation efficiency at small or moderate earthquakes, while effectively limits the bridge displacements to avoid pounding and girder unseating under extreme earthquakes.

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To enhance the bridge resilient performance, a novel re-centering seismic isolator (RSI) is developed by incorporating damping enhanced (DE)-sliding-lead rubber bearing (LRB) with superelastic shape memory alloy (SMA), which functions with the yielding-sliding hysteretic mode. The numerical model of the novel RSI is developed in OpenSees platform and validated by comparing the numerical and experimental hysteretic loops of the superelastic SMA and DE-sliding-LRB. A parametric design procedure is proposed to determine the optimum parameters of the novel RSI system. A three-span continuous girder bridge is selected to investigate the response mitigation efficiency of the novel RSI system compared with LRB system under near-fault ground motions. A systematic parametric study is conducted to design the optimal parameters of the novel RSI system for bridges. Case study is conducted to investigate the effectiveness of the proposed design procedure and the novel RSI system for response mitigation of bridges. Results show that the damping capability is effectively enhanced by designing the friction coefficient of the sliding element. The novel RSI system can achieve dual mitigation of the displacement responses and base force in piers for bridges. Case study demonstrates the effectiveness of the novel RSI and parametric design procedure.

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