Shape Memory Alloy Restrainers Research Articles

Quantifying the effect of pulse-like ground motions on the seismic design of SMA restrained isolation bridges using probabilistic repair cost method

The shape memory alloy (SMA) restrainer serves as an effective but expensive bridge restraining device. However, the impact of pulse effect on SMA design approach under pulse-like ground motions (PLGM) has not been fully quantified in previous studies, which hinders its application in near-fault regions. Therefore, based on the risk probability assessment method throughout the entire life cycle, by accounting for the comprehensive repair cost of each component damage, this paper introduces a parameter design method for SMA restrainer of near-fault bridges that considers structural parameters, pulse parameters, and economic indicators. Firstly, the repair cost ratio (RCR) of bridge system, which means the expenses for repairs expressed as a proportion of bridge replacement costs, was regarded as the life-time optimization goal and overall performance indicator. Secondly, by accounting for near-fault effects, the relationship between RCR and SMA design parameters was established by convolution algorithm of probabilistic seismic hazard analysis, demand analysis and capacity analysis. A novel probabilistic seismic demand model was utilized to quickly determine the RCR of the bridge system under PLGM. Finally, the influence of pulse period on the rational design parameter of SMA restrainer was comprehensively investigated by RCR-based method. A seismic isolation arch bridge was selected as the illustrated case in this article. The results indicate that the rational design parameters of SMA exhibit a pattern of initially rising, then falling as the pulse period increases, reaching the peak value when the pulse period approaches bridge fundamental period. Moreover, the price parameter of SMA significantly affect the optimal design parameters, and the effective range is also recommended.

Seismic performance assessment of a multispan continuous isolated highway bridge with superelastic shape memory alloy reinforced piers and restraining devices

AbstractThe objective of this study is to analytically determine the effectiveness of a novel bridge system with superelastic (SE) shape memory alloy (SMA) reinforced concrete piers. The bridge is also equipped with SE SMA cable restrainers to prevent the bridge spans from a large displacement that can potentially cause span unseating. In the concrete bridge piers, the conventional steel reinforcements in the plastic hinge regions are replaced with SE SMA rebar to avoid large plastic deformation and improve its self‐centering capacity. A typical three‐span continuous highway bridge is modeled with SMA‐reinforced piers and SMA restrainers. Numerical simulations of the bridge are conducted under destructive near‐fault ground motions. The seismic responses and fragility curves of the novel bridge (Bridge IV) are assessed and compared with the reference bridge (Bridge I), the bridge with only SMA‐reinforced piers (Bridge II), and the bridge with only SMA restrainers (Bridge III). The results revealed that the SMA‐reinforced pier can successfully reduce the residual deformation and damage probability of the bridge; however, the bridge with only SMA‐reinforced piers is less efficient in preventing a large displacement. The use of SMA restrainers can efficiently limit the displacement of the bridge spans but increase the damage probability of the bridge piers. The proposed novel bridge having SMA‐reinforced piers equipped with SMA restrainers (Bridge IV) is more efficient than the bridge with only SMA‐reinforced piers (Bridge II)) or the bridge with only SMA restrainers (Bridge III).

Long-stroke shape memory alloy restrainers for seismic protection of bridges

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.

Sensitivity analysis and multi-criteria optimization of SMA cable restrainers for longitudinal seismic protection of isolated simply supported highway bridges

Considering the flexibility of isolation bearings, isolated bridges are susceptible to unseating problems or even collapse due to excessive displacement under strong earthquakes. Shape memory alloy (SMA) wires can be utilized as seismic restrainers to confine this displacement. Recently, a design procedure for SMA restrainers has been proposed based on a linearized two degree-of-freedom analytical model. The key design parameters include three geometric factors (the ratio of the allowable displacement to the seat width, Δa/Lsw, horizontal angle, θ0, and normalized elongation parameter α), and three factors related to material properties of SMA (the maximum applied strain, εmax, the elastic modulus, E0, and the austenite to martensite start stress, fy,SMA). The objective of this study is to optimize the design procedure by evaluating the effect of each design factor on the seismic response of an isolated bridge which is retrofitted by SMA restrainers. After conducting a one-half fractional factorial analysis, it was found that the material factors have slight impact on the relative displacement between the pier and the girder, and the base shear of the pier, with percentage contributions smaller than 4% and 13%, respectively. Then, a full factorial analysis is followed through a multi-criteria decision making process in order to optimize the SMA restrainer. The results show that the optimal value of Δa/Lsw, which effectively reduces the relative displacement and simultaneously control the base shear of the piers, is 0.4; the optimum value of θ0 ranges from 0 to π/4; and the most effective value of α is 1.0.

Effectiveness of using rubber bumper and restrainer on mitigating pounding and unseating damage of bridge structures subjected to spatially varying ground motions

Pounding and unseating damages to bridge decks have been observed in almost all the previous major earthquakes. Recent studies have highlighted that adjusting the fundamental periods of adjacent structural elements close to each other, the only method suggested by the codes to mitigate pounding and unseating damage, is not sufficient to prevent such damages owing to the relative displacement induced by spatially varying ground motions. As pounding and unseating damage could lead to significant loss of economy and life owing to inability to quickly access the damaged area immediately after an earthquake, it is important to protect lifeline bridge structures. Past earthquakes have revealed that the commonly used steel cable restrainers have limited effectiveness. Additionally, only limited research has focused on mitigating pounding forces on the bridge joints that lead to localized damages and disruptions of the serviceability of the bridge after strong shakings. This study presents an extensive investigation on the effectiveness of combining rubber bumpers as a shock absorbing device along with Shape Memory Alloy (SMA) or steel cable restrainers to mitigate pounding and unseating damages on multiple-span bridges subjected to spatially varying ground motions. The responses of bridge structures with different restraining devices acting alone and in combination with rubber bumpers subjected spatially varying ground motions are compared and discussed. The result indicates that the SMA restrainers combined with rubber bumpers could lead to better performance in terms of reduction of joint opening and mitigation of large pounding forces.

Erratum to: Seismic fragility assessment of SMA-bar restrained multi-span continuous highway bridge isolated by different laminated rubber bearings in medium to strong seismic risk zones

This study analytically determines the seismic fragility of a three-span continuous highway bridge fitted with laminated rubber bearings and shape memory alloy (SMA) restrainers. Fragility function, which expresses the likelihood of exceeding a damage state conditioned at a given earthquake intensity, has been derived based on SeismoStruct’s nonlinear incremental dynamic analysis results of the bridge subjected to medium to strong earthquake excitation records. A total of 20 excitation records with peak ground acceleration values ranging from 0.45 to 1.07 g, are used in the nonlinear dynamic analysis of the bridge. A 2-D finite element model scheme is used in this study considering nonlinearity in the bridge piers and the isolation bearings. Two types of laminated rubber bearings are used in the bridge system in addition to the SMA restrainers: high damping rubber bearings and lead rubber bearings. The fragility curves are constructed for two bridge components (i.e. piers and isolation bearings), and the system as well. The component fragility curves are combined to evaluate the fragility curves for the entire bridge system at different damage states. The bridge system, for simplicity, considers the bridge deck, isolation bearings with SMA restrainer and bridge piers but excluding the bridge foundations and the abutments. The numerical results show that the failure probability of the bridge system is dominated by the bridge piers over the isolation bearings. Moreover, the inclusion of SMA restrainers in the bridge system exhibits high probability of failure, especially, when the system is isolated with lead rubber bearings.

Experimental study of a highway bridge with shape memory alloy restrainers focusing on the mitigation of unseating and pounding

This paper presents an experimental study to investigate the performance of shape memory alloy (SMA) restrainers for mitigating the pounding and unseating of highway bridges when subjected to seismic excitations. Mechanical property tests of the SMA wire used in the restrainers are conducted first to understand the pseudo-elastic characteristics of the material. Then, a series of shaking table tests are carried out on a highway bridge model. The structural responses of the highway bridge model equipped with SMA restrainers, installed in the form of deck-deck and deck-pile connections, are analyzed and compared with the uncontrolled structures. The test results of this study indicate that the SMA restrainers are not only effective in preventing unseating but also in suppressing the seismic-induced pounding of the highway bridge model used in this study.

Seismic response control of high arch dams including contraction joint using nonlinear super-elastic SMA damper

In this study, the shape memory alloy (SMA) restrainer bars utilized to reduce the seismic response of arch dams with vertical contraction joints were investigated. The SMA damper model and the nonlinear behavior of arch dams affected by contraction joint opening/closing during earthquakes were simulated in ANSYS. Moreover, the nonlinear damping control principle and method were discussed. The effectivity of the SMA restrainer bars in arch dams was assessed through comparing with a traditional measure of reinforced steel across contraction joints. The SMA restrainer bars were effective in limiting the relative openings of the contraction joints and reducing the seismic acceleration. In addition, the new SMA vibration damper devices of arch dams are simple and easy to install.

Large scale testing of nitinol shape memory alloy devices for retrofitting of bridges

A large scale testing program was conducted to determine the effects of shape memoryalloy (SMA) restrainer cables on the seismic performance of in-span hinges of arepresentative multiple-frame concrete box girder bridge subjected to earthquakeexcitations. Another objective of the study was to compare the performance of SMArestrainers to that of traditional steel restrainers as restraining devices for reducing hingedisplacement and the likelihood of collapse during earthquakes. The results of thetests show that SMA restrainers performed very well as restraining devices. Theforces in the SMA and steel restrainers were comparable. However, the SMArestrainer cables had minimal residual strain after repeated loading and exhibited theability to undergo many cycles with little strength and stiffness degradation. Inaddition, the hysteretic damping that was observed in the larger ground accelerationsdemonstrated the ability of the materials to dissipate energy. An analytical study wasconducted to assess the anticipated seismic response of the test setup and evaluatethe accuracy of the analytical model. The results of the analytical simulationillustrate that the analytical model was able to match the responses from theexperimental tests, including peak stresses, strains, forces, and hinge openings.

Development, Shake Table Testing, and Design of FRP Seismic Restrainers

The development and performance of fiber-reinforced polymer (FRP) fabrics as an alternative to steel for restrainers to reduce bridge relative movements at hinges during earthquakes was explored. Glass, carbon, and hybrid (glass/carbon) restrainers were developed and tested on a representative in-span hinge using a shake table at the large-scale structures laboratory at the University of Nevada, Reno. The components of the study presented in this article are: (1) the FRP restrainer development and testing; (2) comparisons among FRP, steel, and shape memory alloy restrainers; and (3) development and evaluation of a simple restrainer design method and a numerical example. Important findings of the study were that compared to steel restrainers, the FRP restrainers were effective in substantially reducing the relative hinge displacements and pounding at hinges. The method to develop the flexible portion of the FRP restrainers and the bond to superstructure was successful in accomplishing the target performance. The proposed restrainer design method provides a rational yet simple tool to design FRP or other types of restrainers.

Seismic retrofit of simply supported bridges using shape memory alloys

Recent earthquakes in the United States and Japan have highlighted the vulnerability of bridges to collapse due to excessive movement at the intermediate hinges and abutments. This study investigates the effectiveness of shape memory alloy restrainer bars to reduce the seismic vulnerability of bridges. Full-scale tests of shape memory alloy (SMA) restrainer bars are conducted to determine their force-deformation and energy dissipation characteristics. The 25.4 mm diameter bars are subjected to cyclical strains up to 8%, with minimum residual deformation. The effectiveness of the SMA restrainer bars in bridges is assessed through nonlinear analyses of a typical multi-span simply supported bridge. The SMA restrainer bars are effective in limiting the relative displacement at the piers and abutments. In addition, the bars are shown to be very effective for near-field ground motion.