High-seismic-performance framed structures enabled by novel shape memory alloy elements

Abstract

The extensive devastation caused by earthquakes has prompted a heightened global awareness regarding the improvement of structural resilience against earthquakes. To develop high-seismic-performance structural system, the self-centering (SC) concept has arisen as a promising solution to minimize residual deformation after earthquakes, improving the overall resilience and performance of structures. Shape memory alloys (SMAs) possess inherent features making them promising candidates for use in seismic-resisting SC structures. This thesis systematically investigates the applications of SMA-based devices in various high-seismic-performance structural systems. Although existing studies have demonstrated the potential of SMA-based SC structures, relevant research remains in the early stages with many unaddressed questions, such as seismic performance of flexural-type SMA-based devices, applications of SMA-based devices in structures other than steel structures, the effect of varying outdoor temperature in practical applications, and global performance of structural systems incorporating SMA-based devices. For instance, the effects of ambient temperature, particularly low temperature, on the mechanical and superelastic behaviors of flexural-type SMA elements have been rarely studied since its deformation mechanism is quite different from existing axial-type SMA elements. Moreover, few studies have investigated the implementation of flexural-type SMA devices in structural components. Research into timber systems utilizing SMA components is also limited in scope. Exploration of the seismic-resilient performance of multi-story frame structures integrating flexural-type SMA-based isolators subjected to earthquakes can be rarely found. This thesis aims to address some critical research gaps through combined numerical and experimental studies at both component and system levels. The thesis consists of three parts. The first part mainly investigates the behavior of flexural-type SMA elements. The second part focuses on the investigation of SMA-based devices at component level. The third part focuses on the global performance of structural systems incorporating SMA-based devices. In the first part, the effects of varying temperature (particularly low temperature) on the mechanical and superelastic behaviors of one flexural type of SMA elements, i.e., SMA-based U-shaped dampers (SMAUDs), were explored via systematical experiments. The thermo-mechanical properties of SMAUDs were characterized. Cyclic tests were conducted on SMAUDs under a wide range of ambient temperatures, ranging from −40 °C to 20 °C. The variance of hysteretic properties of SMAUDs, including strength, stiffness, SC, and energy dissipation capabilities, was investigated under different ambient temperatures. Additionally, the effect of thermal cycling on the hysteretic behavior of SMAUDs was investigated. In the second part, the performance of SC timber beam-column connection incorporating SMA bars was investigated through experiments and numerical analyses. For comparison, timber connection incorporating SMA angles was also experimentally investigated. Based on the cyclic behaviors of the timber connection, the performance of steel beam-column connection integrating flexural-type SMA angles was further investigated through experiments and numerical analyses. The comparative experimental and numerical investigations provided insights into the potential benefits and limitations of incorporating SMA-based devices in various beam-column connections for earthquake engineering applications. In the third part, the seismic performance of SMAUD isolators in protecting multi-story steel frames was evaluated through systematical shake table tests as well as numerical investigations. A steel frame equipped with conventional steel U-shaped dampers (SUD) isolators was also experimentally tested on the shake table for comparison. Finally, a novel strong backup (SB) system was proposed for the mitigation of the weak behavior observed in SMA-braced frames, in which its efficacy was assessed via nonlinear pushover and dynamic time-history analyses. This research utilized a combination of numerical and experimental investigation to explore the potential of SMA-based devices in high-seismic-performance structural systems. Based on results of this research, SMA-based devices are proven to exhibit excellent SC and energy dissipation abilities. Integration of SMA-based devices enhanced the performance of timber structures in terms of SC and energy dissipation abilities. For flexural-type SMA-based devices, they maintained their stiffness, strength, and energy dissipation under low ambient temperature, differing from existing axial-type SMA-based devices. SMA angles are proven to be easily installed on the steel connection to provide SC and energy dissipation abilities without complicated PT configuration. SMAUD-based isolators exhibited excellent SC ability and effectively protected MRF from seismic damage. The proposed SB system can mitigate the weak behavior observed in SMA-braced frame. Some challenges are also discussed based on the research findings.