Energy dissipation capacity is a critical aspect of traditional seismic design, which largely depended on the inelastic behavior of the structural design members. Unfortunately, based on a comprehensive analysis of the past major earthquakes, ensuring sufficient energy dissipation cannot guarantee efficient post-earthquake recovery due to several limitations. For example, after the earthquake, the large residual deformation induced by the yielding behavior is challenging to address, both technically and financially. In most cases, as witnessed in the past, large residual deformation makes demolition a feasible choice than repair. Drawing lessons from the past post-earthquake survey and analysis, residual deformation is rapidly being accepted as a critical parameter influencing seismic performance.
Self-centering structures have drawn research interests as promising materials for improving seismic performance. Typically, design members with self-centering capabilities develop hysteresis behavior that suppresses the residual deformation. However, this capability is accompanied by undesirable structural responses that can further compromise their performance, e.g., low energy dissipation capacity, pronounced high mode effects, larger peak absolute floor acceleration, and limited ductility and elastic strain can be induced.
Therefore, despite the significant research progress, extensive investigation of self-centering solutions to improve seismic performance is still required. Motivated by the pioneering research findings, a group of researchers from Tongji University: Professors Cheng Fang, Yiyi Chen, Wei Wang together with the research students Yiwei Ping and Junbai Chen, in collaboration with Professor Michael Yam from The Hong Kong Polytechnic University developed a novel hybrid self-centering solution to overcome the shortcomings of the existing solutions and improve the seismic performance. The work is currently published in the journal, Journal of Earthquake Engineering.
Briefly, two types of hybrid braces comprising of integrated viscoelastic dampers and prestressed superelastic shape memory alloy elements were proposed. A series of prototype buildings designed according to the American Society of Civil Engineers standards, such as hybrid self-centering steel-braced frames (HBFs) and pure self-centering braced frames (SCBFs), were systematically analyzed. The structures were also evaluated considering the floor acceleration and residual/peak inter-story drift under near-fault and far-field ground motions. The influence of various parameters on seismic performance was also discussed. Finally, a probability-based model for the residual inter-story drift (RID) prediction was developed based on the design recommendations.
Results demonstrated that viscoelastic materials capable of reaching a moderate damping ratio (0.1) are highly effective for controlling the peak inter-story drift (PID) and residual deformation under both NF and FF earthquakes without compromising other seismic performances. Compared to BRBF, pure SCBF exhibited decreased energy dissipation capacity and detrimental high-mode effect leading to a larger PID response. Consequently, the SCBF exhibited a significant reduction in the RID response, which was more pronounced when the damping ratio was added. Increasing the damping ratio from 0.1 to 0.2 resulted in pronounced RID, particularly in NF earthquakes. Most importantly, the HBF solution effectively addressed the shortcomings of the existing systems, especially at a relatively lower damping ratio. In this case, the authors recommended an optimal ratio of 0.05 for PID, RID and PFA control.
In summary, the most important finding was that the use of SMA-viscoelastic hybrid control could suppress the PFA, RID and PID simultaneously. A larger damping ratio was unnecessary for the hybrid control solutions as a relatively smaller damping ratio was desirable for cost-effectiveness . Based on the parametric study, design recommendations and optimal damping ratio were proposed. In a statement to Advances in Engineering, Professor Cheng Fang explained that the study results would provide more opportunities for designing and developing effective solutions for enhanced seismic performance.
Fang, C., Ping, Y., Chen, Y., Yam, M., Chen, J., & Wang, W. (2020). Seismic Performance of Self-centering Steel Frames with SMA-viscoelastic Hybrid Braces. Journal Of Earthquake Engineering, 1-28.