بررسی رفتار زمین لرزه‌ای اتصالات تیر-ستون(BCJها) تقویتی با آلیاژ حافظه‌دار شکل فوق الاستیسیونی بصورت تجربی

ABSTRACT Experimental Investigation on the Seismic Behavior of Beam-Column Joints(BCJs) Reinforced with Superelastic Shape Memory Alloys

Beam-column joints (BCJs) in RC moment resisting frames are usually considered the weakest link in such a structural system [Park and Paulay, 1975]. Since the 1970’s, design codes started enforcing stricter seismic provisions for the detailing of reinforcing bars in BCJs. However, BCJs remain extremely vulnerable during earthquakes [Saatcioglu et al., 2001]. It has been emphasized that earthquake resistant structures need to be sufficiently ductile as it is difficult and costly to build structures that can perform elastically under strong ground motion. In conventional seismic design of RC structures, reinforcing bars are expected to yield in order to dissipate energy resulting in permanent deformations due to the post-yield plastic properties of steel reinforcing bars. If superelastic (SE) SMAs could be used as reinforcing bars, such elite materials can undergo large inelastic deformations and recover their original shape by stress removal mitigating the problem of permanent deformations. This unique property is known as superelasticity. Hence, when used as reinforcement in critical structural elements along with conventional steel, SMA can undergo large inelastic strains caused by seismic loads, but potentially recover deformations at the end of the earthquake [Saiidi and Wang, 2006]. Their high strength, large energy hysteretic behavior, full recovery of strains up to about 8%, and high resistance to corrosion and fatigue make them strong contenders for use in earthquake resistant structures [Wilson and Wesolowsky, 2005]. In particular, Ni-Ti alloy has been found to be the most promising SMA for seismic applications.

SMAs have gained increased usage in structural applications [Alam et al., 2007a]. For instance, Dolce et al., [2004] used SMA bracings for the seismic retrofitting of existing frames. Maji and Negret [1998] used SMA wires/tendons in prestressed concrete. Indirli et al., [2001] utilized SMA rods for strengthening structures through the application of corrective post-tensioning forces. Inaudi and Kelly [1994], Clark et al., [1995], and others contributed significant analytical and experimental studies on structural displacement control using SMAs. Recent research in the application of SMAs in vibration control includes amongst others the work of DesRoches and Delemont [2002] and Wilde et al., [2000].

Manufacturing of SMA involves overcoming many difficulties, which increases its cost. Adding the fact that the behaviour of large diameter SMA specimens is not well documented in the literature, it was not until 2004 that it found its way as reinforcement in RC structures. Wang [2004] used SMA rods in the plastic hinge area of RC columns and evaluated the seismic performance of these columns. Two 1/4-scale spiral RC columns with SMA longitudinal reinforcement in the plastic hinge area were designed for laboratory shake table testing. Each specimen was subjected to a series of scaled motion amplitudes. It was observed that the SMA RC columns were superior to the conventional steel RC columns in limiting relative column top displacement and residual displacements. Also, they withstood larger earthquake amplitudes than that of the conventional ones. The shake table data showed that SMA RC columns were able to recover nearly all of the post-yield deformation, thus requiring minimal repair. Alam et al., [2007b] demonstrated the potential of developing smart RC bridges utilizing SMAs as reinforcement and/or prestressing tendons.

In this article, it is proposed to use SE SMAs as reinforcement in conjunction with steel in RC beam-column joints. Two BCJ specimens have been designed and constructed according to current seismic design standards, and tested under reversed cyclic loading. The prime objective of this study is to investigate the behavior of concrete BCJ reinforced with SMA in its plastic hinge zone under reversed cyclic loading and compare its performance to that of conventional steel-RC BCJ.