رفتار زلزله در چارچوب های بتن مستحکم شده آلیاژ حافظه شکل-پلیمر مستحکم فیبری تحت خطر لرزه ای متوالی Seismic behavior of SMA–FRP reinforced concrete frames under sequential seismic hazard

Traditional structural seismic design philosophy currently adopted by most seismic design codes is based on preventing the collapse of the structure during an earthquake through introducing the feature of ductility, which allows some level of damage to occur at specific components or regions (e.g. plastic hinges) in the structure. In the case of reinforced concrete (RC) structures, the damage is due to the crushing of concrete and the plastic deformation of steel reinforcement, which has been considered recently as one of the drawbacks of steel rebars. Excessive steel deformation (beyond yielding) in RC moment resisting frames (MRFs) for example often results in permanent residual drifts, which not only cause overall capacity degradation but also pose safety issues for the occupants even under gravity loads [1]. Several recent studies have focused on improving the post-earthquake functionality of RC structures through introducing the feature of re-centering. Various techniques have been proposed to introduce this feature to structures including the use of post-tensioned steel bars [2], enhancing post yield stiffness using steel fiber composite bars [3] and the use of superelastic shape memory alloy (SMA) rebars [4,5]. Although using SMA materials to provide RC structures with the ability to re-center is quite promising, it is faced with some challenges. For example, using large diameter SMA rebars that are not available commercially makes it cost prohibitive. In addition, research have shown that large diameter SMA rebars exhibit reduced hysteretic area and damping capability compared to small diameter wires [4,6]. This is primarily due to the accumulation of more distorted martensite crystalline structure and inherent deficiencies which exist in larger diameter rods as compared to small diameter wires. To address the previously discussed limitations in steel reinforcing bars using SMA wires instead of bars, the second author participated in a study which proposed the idea of using a new type of fiber reinforced polymer (FRP) composite known as shape memory alloy-FRP (SMA–FRP) as reinforcement for concrete structures [7]. The proposed reinforcement was sought as a mean to introduce the features of ductility and re-centering to RC structures. A schematic of the newly proposed composite rebar is shown in Fig. 1a. As illustrated in the figure, the proposed SMA–FRP reinforcing bar comprises polymeric resin reinforced with small diameter NiTi superelastic SMA fibers with or without supplementary conventional reinforcing fibers (e.g. glass, carbon, etc.). The nonlinear, yet pseudo-elastic behavior shown in Fig. 1b typical of superelastic SMA fibers [8] will allow SMA–FRP composite reinforcement to exhibit hysteretic and ductile behavior with minimal damage to the RC structure. The flag-shape hysteretic behavior of superelastic SMA is a direct result of a reversible stress-induced phase transformation between austenite and martensite phases. A more recent study by Zafar and Andrawes [9], explored in depth the manufacturing procedure and experimental testing behavior of the proposed SMA–FRP composite. In these previous studies, SMA–FRP has proven to be a promising alternative to both steel and FRP rebars, which deserves further investigation. This paper focuses on investigating analytically, the performance of SMA–FRP rebars in RC MRF structures subjected to main shock–aftershock earthquake sequences and compare it with that of conventional steel rebars.