Fang, Wang, Zhang, Sause, Ricles, Chen 2019
In the study, a new energy dissipating and self centering device in braced frames (BFs) was investigated for its seismic performance. The system comprises superelastic shape memory alloy (SMA) ring springs which play fundamental roles in the overall self-centering and energy dissipation capability of the device. To investigate the performance of the proposed device, quasistatic loading tests on two different sized specimens of the system were performed and observed for their stiffness, strength, self-centering capability and energy dissipation capacity. All specimens exhibited satisfactory mechanical behavior with no residual damage, except for some minor wearing scratches on the springs and a few degradations that should be considered in future investigations.
System Concept
The system consists of an external cylinder and internal cylinder and spring sets assembled in between. The spring system is a combination of super elastic SMA outer rings and high strength steel inner rings that are designed to be stiff. When load is applied, the outer set of spring rings expands while the inner rings contract. By constraining cover plates at the two ends with a tightening nut, the springs only undergo compression regardless of the direction of the load. When load is removed, restoring force is applied by the springs eventually causing the system to recover. Friction from this motion of springs and the damping of SMA facilitate energy dissipation of the system. Additionally, the nut attached to the cover plates can also allow application of preloads on the SMA springs when it is tightened. This bidirectional function of the proposed self-centering energy dissipative device would make integration into braced frames simpler and more efficient.
Experimental Study, Results and Discussion
Tests on two specimens of the proposed system with different sizes of SMA outer rings were performed. The specimens were arranged vertically on a strong floor beam with a servo-controlled actuator connected to the top. Incremental displacement controlled load was applied by the actuator in two rounds. The specimens were initially subjected to incremental loads of 5 mm level followed by a second round that includes two extra cycles for full compression of the springs.
Overall, the specimens demonstrated satisfactory characteristics supported by the results. The springs were able to exert restoring force at all times and exhibited excellent self-centering behavior generating flag-shaped load-deformation hysteretic curves. As anticipated, deformation was formed only in the SMA ring spring while the rest of the system remained elastic. The specimens were also able to achieve equivalent viscous damping (EVD) of up to 20% and it was found that the number of repeated loads had little effect on the energy dissipation capacity. No damage except from minor scratches was formed after tests although gradual yield degradation of the system was observed with increasing load cycles. While the system exhibited satisfactory seismic performance, the study suggested that seismic designs in future investigations should consider limitations possessed by the proposed self-centering energy dissipative device.
Reference
Fang, Cheng, et al. “Behavior and Design of Self-Centering Energy Dissipative Devices Equipped with Superelastic SMA Ring Springs.” Journal of Structural Engineering, vol. 145, no. 10, Oct. 2019, p. 04019109. DOI.org (Crossref)