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Experimental Study on Structural Performance of RC Exterior Beam-Column Joints Retrofitted by Steel Jacketing and Haunch Element under Cyclic Loading Simulating Earthquake Excitation

, Ngoc-Hieu Dinh
Published: 7 June 2017

Abstract: Several retrofitting methods for reinforced concrete (RC) beam-column joints in old buildings without seismic details were developed. Four half-scale RC exterior beam-column joints were fabricated and tested under cyclic loading simulating earthquake excitation. The control specimen was designed to fail in joint shear. Two practical retrofitting strategies were applied to the control specimen which consider the architectural characteristic in real buildings, including steel jacketing and haunch retrofit solution. The structural performance of the test specimens was investigated in terms of various factors including damage and failure, load-drift relationship, ductility, dissipated energy, and strain profiles of longitudinal reinforcement. Experimental results confirmed that the proposed retrofit methods were shown to enhance the seismic capacity of the joints in terms of the strength, deformation capacity, and energy dissipation capacity while the shear deformation in the panel zone significantly reduced in comparison with the control specimen.1. IntroductionExisting reinforced concrete (RC) buildings in many developing countries had been traditionally designed to resist mainly gravity loads and wind loads without properly considering the seismic effects that pose a significant risk to human beings. In those buildings, beam-column joints have nonseismic reinforcement details. According to a previous report [1], the beam-column joints without seismic reinforcement details have been found to be susceptible to failure due to earthquakes which could contribute to partial or entire collapse of concrete buildings. Therefore, to ensure the safety of the existing RC buildings, it is essential to improve the strength and ductility of beam-column joints. From the observations of structural failure due to earthquakes, corner and exterior concrete beam-column joints have been recognized as the most vulnerable parts of RC frames due to the discontinuity in beam, weak concrete confinement inside the joint, and unreliable load-transferring mechanism dependent on concrete tensile strength [1].According to the previous studies by Lee et al. [2] and Teraoka et al. [3], the failure modes of nonseismically beam-column joints could be classified into three groups: B-failure () indicates flexural yielding of beams undergoing large inelastic deformation until ultimate rotational capacity without shear failure in joints; BJ-failure indicates joint failure after initial yielding of beam reinforcement (); and J series indicate joint failure by shear force without yielding of beam reinforcement (), where is the ratio of joint shear capacity to joint shear demand evaluated based on the beam yield and hardening mechanism.For the RC beam-column joints without seismic reinforcement details, several repair and retrofitting methods have been proposed in recent years. Engindeniz et al. [4] grouped several repairing and strengthening techniques as follows: (i) epoxy repair; (ii) removal and replacement of concrete in damaged areas; (iii) concrete jacketing; (iv) concrete masonry unit jacketing; (v) steel jacketing and addition of external steel elements; and (vi) strengthening with fiber-reinforced polymeric (FRP) composite application.Shafaei et al. [6] investigated the performance of four nonseismically detailed beam-column joints retrofitted with steel angles, which were mounted using prestressed cross-ties. This technique prevented the slippage by increasing the joint area of the bottom longitudinal reinforcement of beam; moreover, the plastic hinge was relocated far from the column face, and the shear strength, stiffness, energy dissipated, and ductility capacity were significantly increased up to 50%, 120%, 220%, and 220%, respectively.In addition, El-Amoury and Ghobarah [7] performed the seismic tests on beam-column joints strengthened with glass fiber-reinforced polymers (GFRP). The retrofit strategy included two systems: the first system is used for upgrading the shear strength of the joint with two U-shaped GFRP layers, and the second system is used for upgrading the bond-slip of the steel bars. The test results showed that the use of GFRP jacketing significantly enhanced the ductility and the load-carrying capacity of the retrofitted beam-column joints. Besides, the brittle joint shear failure of the retrofitted specimens was also eliminated and the stiffness degradation of the joints was reduced. Particularly, the energy dissipation capacity was increased by up to six times compared to that of nonretrofitted joints.Moreover, the haunch retrofit solution (HRS) using haunch elements was proposed by Pampanin et al. [8] and several comprehensive tests were performed by Genesio [9] using postinstalled anchors for optimization of the HRS. The main principle of HRS was to relocate the plastic hinge away from the vulnerable panel zone thus enhancing the global response of RC beam-column joints without seismic reinforcement details by altering the hierarchy of strength suitably. These tests had proved the efficiency of the HRS to the hierarchy of strength in beam-column joints in order to prevent brittle joint shear failure and induce the ductile beam failure at much higher lateral loads.In this study, to develop the seismic retrofitting techniques to beam-column joints in existing concrete buildings in Korea, four half-scale RC exterior beam-column joints were tested. A control specimen is designed to be failed in J-failure, which refers to joint failure before plastic hinges formed at the ends of adjacent beams. Thus, this specimen is associated with low displacement and ductility. Then, two different retrofit strategies were applied to the control specimen: steel jacketing and haunches retrofit solution. The retrofitting methods used in this study emphasize the practical details, architectural characteristics of real buildings, and construction ability of retrofitting methods. All specimens were tested under simulated seismic loading. Based on the test results, the structural performance of control and retrofitted specimens is assessed in terms of various factors: failure mode, hysteretic behavior, drift capacity, and energy dissipation capacity.2. Experimental Program2.1. Test SpecimensFigure 1 shows a typical RC 10-story building in Korea with 3900 mm in story height and three bays of 8000 mm. This building was built with a nonseismic reinforcement details, which is generally designed and constructed in Korea during 1970s and 1980s. As reported by Korea National Emergency Management Agency [10], the beam-column joints of old buildings constructed during 1970s and 1980s in Korea do not have seismic reinforcement details; stirrups or ties had standard 90-degree hooks and large spacing, and the anchorage of top longitudinal reinforcing bars of beams was bent down inside the joint regions while the anchorage of bottom longitudinal reinforcing bars was bent down away from joint regions, which might decrease strength and deformation capacity of the joints [10]. All specimens in this study were designed to simulate the exterior beam-column joints in second floor of the building with a half scale.Figure 1: A typical RC 10-story building.The control specimen (specimen J) has nonseismic reinforcement details inside the joint region. In this study, to achieve J-failure mode of the control specimen, the design top reinforcement ratio of beams was 2.9% corresponding to a ratio of joint shear capacity to joint shear demand, , of 0.73. For all specimens, for positive bending moment of beams, the bottom longitudinal reinforcement ratios of beams had the same value of 0.43%. The configurations and details of specimen J were presented in Figure 2.Figure 2: Configurations and details of control specimen.Figure 3 illustrates the schematic drawings of retrofitted specimens. It should be noted that, in test specimens, only a portion of the joint panel zones was retrofitted with retrofit materials to consider the existence of transverse beams and floor slabs in real structures. However, the effect of transverse beam and slab confinement on seismic performance of exterior RC beam-column joints is not clarified so far [11, 12]. Moreover, in this study, the authors consider the worst circumstance of joints subjected to lateral load without confinement in joint region for rehabilitation purpose. Hence, in this study, the concrete transverse beams and slabs were not included in test specimens.Figure 3: Details of retrofitted specimens.Figures 3(a) and 3(b) show the details of specimens J-S1 and J-S2, which were strengthened in joints with steel jackets (SS400) having a thickness of 16 mm and specified yield strength of 400 MPa. In specimen J-S1 (Figure 3(a)), two sides of column were attached with steel plates having a length of 1100 mm, which were installed with anchor bolts HILTI HSL-3 M12 (the diameter of 12 mm and the length of 120 mm). Meanwhile, in specimen J-S2, three sides of column were installed with steel plates with bolts as shown in Figure 3(b). It is noted that the surfaces of concrete were covered by a layer of epoxy grout having a thickness of 5 mm before steel plates were installed. The details of steel jacket design procedure are summarized in Appendix A.Figure 3(c) shows the details of specimen J-H, which was strengthened using haunch retrofit solution. In this retrofitted technique, a haunch element was only installed at the bottom part of the beam, due to considering the architectural characteristic in real buildings. The haunch element used in this test consisted of three steel plates welded together. The design length of haunch element is 424 mm, which is 0.2 times the length of the beam (2100 mm), according to the recommendation by Sharma et al. [13]. To connect the haunch element with beam and column, five anchor bolts HILTI HSL-3 M12 (the diameter of 12 mm and the length of 120 mm) at each side of beam and column were used. The details o
Keywords: reinforced concrete / Shear strength / Load Drift / four half / Half Scale / Simulate Earthquake / Joints Retrofitted / Exterior Beam

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