PSI - Issue 44

Massimiliano Ferraioli et al. / Procedia Structural Integrity 44 (2023) 974–981 Massimiliano Ferraioli et al./ Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction The new generation of seismic codes has been giving increasing importance not only to traditional parameters such as stiffness, strength, and ductility but also to new design concepts such as robustness and resilience which is the ability of the structures to quickly resume their use following an earthquake. Consequently, the main objective of the seismic design is being shifted from collapse prevention to damage control aimed at limiting the economic losses due to earthquakes. Many types of self-centering (SC) passive dampers have been developed that proved to significantly limit the residual drifts after a seismic event. New materials have been proposed and tested. Among them, shape memory alloys (SMAs) are earning more and more relevance for their application in buildings (Casagrande et al. 2021). NiTi (nickel and titanium) based SMAs have proved high fatigue and corrosion resistance and good energy dissipation performance. Moreover, their superelastic behavior allows them to develop flag-shape hysteretic loops (with deformations up to 8%) with negligible residual deformations. These properties have been usefully employed not only to produce seismic isolation and energy dissipation devices (i.e., energy-dissipating dampers, base isolation systems and concentric braces, bridge restrainers) but also to make the structure usable and repairable even after strong earthquake ground motions. Different shapes and operating principles were developed: helical springs (Zhuang et al. 2016; Fang et al. 2015), wires to make braces, restrainers, and dampers (Qiu et al. 2017, Fang et al. 2015), bars (Speicher et al. 2009), tensile tendons (Abolmaali et al. 2006), concentrically braces (Qiu et al. 2017), isolation devices (Dolce et al., 2000; Ding et al., 2011). 2. Recentering SMA damper (RSMAD) The self-centering systems can be grouped into three categories: 1) braced frames; 2) rocking systems; 3) post tensioned (PT) steel systems. Among them, the SMA-based bracing systems seem to be the most promising for practical applications for the advantages of steel bracing (i.e., ease and speed of construction, ability to accommodate openings, minimum disruption to the occupants of the building). A NiTi-based re-centering SMA damper (RSMAD) is considered in this paper. The mechanical scheme of the RSMAD tested and validated by Qian et al. (2013) is shown in Fig. 1a. The push-pull rod is used to connect the RSMAD damper to the structure using a diagonal brace (Fig. 1b). Under seismic actions, the push-pull rod moves in both directions, and the SMA cables dissipate energy when subjected to axial tension and at the end, they return the device to its initial position (re-centering capability). The SMAs exhibit very specific behavior that is uncommon in materials traditionally used in structural engineering. Their stress-strain behavior also depends on the temperature (Fig. 2a). It may exist in two different states, called phases (i.e., austenite phase and martensite phase). If the working temperature ( T ) exceeds the maximum temperature M d at which martensite occurs ( T ≥ M d ), the SMA shows an elastic-plastic behavior. If the working temperature exceeds the austenite finish temperature ( T ≥ A f ), the SMA shows a flag-shaped hysteresis loop that is characterized by a good energy dissipation capacity but also by the ability to recover its initial shape even after large strains. This property is called “pseudo-elasticity” or “super-elasticity” (SE). If the working temperature is lower than the austenite start temperature ( T < A s ) a residual strain occurs after unloading, but it may be fully recovered if the SMA is heated beyond the austenite finish temperature ( A f ). This phenomenon is universally known as Shape Memory Effect (SME).

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Fig. 1. (a) Mechanical scheme of a recentering SMA damper (RSMAD); (b) Damped brace for retrofitting the frames structures.