Bridges, Tunnels and other Transportation Structures
Accelerated bridge construction (ABC) has gained attention worldwide due to its potential in reducing on-site construction time, traffic disruptions, environmental impacts, and improving work-zone safety and construction quality. Although many ABC techniques have been successfully implemented in low seismic areas, the ABC applications are very limited in regions of moderate-to-high seismic activity. Connection details are the most critical issue facing the use of precast elements in seismic regions, especially if they are located where inelastic deformations occur. They must not only be easy to construct but also must be robust enough to maintain the integrity of component elements under seismic loading.
In this study, a novel shape memory alloy (SMA) based energy dissipation device called Confined Superelastic Damper (CSD) is developed and experimentally evaluated for use in connections of prefabricated bridge components. The CSD consists of a superelastic Nickel-Titanium (NiTi) SMA bar encased in a steel tube filled with grout. The bar carries axial load while the outer tube, via the grout, provides confinement to the core and prevents global buckling in compression. A comprehensive experimental program is carried out with varying geometrical and mechanical parameters to examine the cyclic behavior of CSDs under tension-compression loading. Mechanical properties of prime interest for the intended application such as energy dissipation and self-centering capability are evaluated with various test conditions. Test results revealed stable flag-shaped hysteresis loops associated with good energy dissipation capacity.
Successful implementation of CSDs for accelerated bridge construction in seismic regions is demonstrated through numerical simulations. The CSDs are installed across the column-footing or column-bent cap interfaces of precast concrete piers to form dissipative controlled rocking (DCR) system. In a conventionally controlled rocking bridge pier, collapse prevention is dependent upon the ductility capacity of the plastic hinge at the base, which in turn depends upon the dissipaters and post-tensioning. Typically, metallic hysteric dissipaters are used for such purpose and they may undergo into inelastic range after multiple cycles, with the possible need of replacement. Not only the replacing is a drawback but also the dissipaters are designed at a set level of earthquake loading, and all have the same capacities. Consequently, if few dissipaters fail and yielding of the post-tensioning occurs, the pier would lose significant stiffness and would be prone to P-∆ effect which would eventuate in the collapse of the structure.
The proposed DCR system mitigates the shortcomings mentioned above. Owing to SMAs unique material property of undergoing large deformation without any strength degradation, the ductility capacity at the rocking interface is improved substantially. Furthermore, the CSDs provide self-centering capability due to the inherent flag-shaped hysteretic behavior of SMAs. The dampers eliminate the inelastic action, thereby ensuring the capacity protection of the connection regions. They are designed, detailed, and attached to the pier in such a way that they are activated hierarchically under increasing levels of earthquake intensities. Thus, the onset of collapse is delayed. In general, the proposed DCR system is a promising technique in realizing construction of resilient bridge structures faster than using traditional methods.