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.

This study investigates the response of a shape memory alloy (SMA)-based isolation system that combines multiple groups of SMA cables and a lead rubber bearing (LRB). The isolation device, named as multi-level SMA/lead rubber bearing (ML-SLRB), is designed such that it maintains its efficiency under frequent, design and extreme levels of seismic events. Two large-size ML-SLRB isolation devices were designed and fabricated. The response of the proposed isolation systems was evaluated together with a conventional LRB under increasing amplitudes of cyclic loads. The effects of loading rate and vertical pressure on the response of the ML-SLRB isolator were evaluated. Finite element models of the fabricated ML-SLRB isolators were developed and analyzed to assess the response of the different SMA cable groups at different stages of the loading. The test results, supported by the finite element analyses, revealed that the SMA cable groups used in a loop configuration in the ML-SLRB isolator are prone to stress concentrations and early damage. The ML-SLRB isolators that employed its main SMA cable groups in a straight configuration successfully achieved a multi-level performance where the stiffness of the isolator increased as the demands of the displacement increased. The developed isolator also exhibited lower residual drifts compared to the LRB isolator.

To prevent unseating failures and pounding problems in simply supported bridges, steel restrainers that can limit relative displacements of adjacent spans have commonly been used. This study proposes a shape memory alloy (SMA)-based restrainer that can serve as both energy dissipater and restrainer under cyclic tension and compression loading. The proposed device named as, long-stroke SMA restrainer (LSR), consists of a superelastic SMA bar, a steel tube, and a filler material. The SMA bar provides re-centering force and dissipates energy, while steel tube and the infill material prevent buckling of SMA bar under compression. Taking advantages of large strain capacity of SMAs, the LSRs are designed to accommodate large displacement demands for bridge restrainers. The mechanical response of NiTi SMA bars with a diameter of 12 mm was first studied through uniaxial tension and compression testing. Buckling and post-buckling response of the SMA bars were also characterized. Then, a high-fidelity finite element (FE) model was developed and validated to represent the behavior of SMA bars. Next, the buckling mechanism of proposed LSR was studied through FE simulations and a design methodology for the LSR was developed to prevent buckling. Results shows that the LSR can exhibit stable energy dissipation capabilities with excellent self-centering ability both under tension and compression loading and can serve as an effective restrainer in simply supported bridge structures.