• Earthquakes and Structures
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• v.8 no.3
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• pp.665-679
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• 2015

The strain rate of reinforced concrete (RC) structures stimulated by earthquake action has been generally recognized as in the range from $10^{-4}/s$ to $10^{-1}/s$. Because both concrete and steel reinforcement are rate-sensitive materials, the RC beam-column joints are bound to behave differently under different strain rates. This paper describes an investigation of seismic behavior of RC beam-column joints which are subjected to large cyclic displacements on the beam ends with three loading velocities, i.e., 0.4 mm/s, 4 mm/s and 40 mm/s respectively. The levels of strain rate on the joint core region are correspondingly estimated to be $10^{-5}/s$, $10^{-4}/s$, and $10^{-2}/s$. It is aimed to better understand the effect of strain rates on seismic behavior of beam-column joints, such as the carrying capacity and failure modes as well as the energy dissipation. From the experiments, it is observed that with the increase of loading velocity or strain rate, damage in the joint core region decreases but damage in the plastic hinge regions of adjacent beams increases. The energy absorbed in the hysteresis loops under higher loading velocity is larger than that under quasi-static loading. It is also found that the yielding load of the joint is almost independent of the loading velocity, and there is a marginal increase of the ultimate carrying capacity when the loading velocity is increased for the ranges studied in this work. However, under higher loading velocity the residual carrying capacity after peak load drops more rapidly. Additionally, the axial compression ratio has little effect on the shear carrying capacity of the beam-column joints, but with the increase of loading velocity, the crack width of concrete in the joint zone becomes narrower. The shear carrying capacity of the joint at higher loading velocity is higher than that calculated with the quasi-static method proposed by the design code. When the dynamic strengths of materials, i.e., concrete and reinforcement, are directly substituted into the design model of current code, it tends to be insufficiently safe.

• Journal of the Korea Concrete Institute
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• v.24 no.4
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• pp.399-406
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• 2012

Beam-column joints are generally recognized as the critical regions in the moment resisting reinforced concrete (RC) frames subjected to both lateral and vertical loads. As a result of severe lateral load such as seismic loading, the joint region is subjected to horizontal and vertical shear forces whose magnitudes are many times higher than in column and adjacent beam. Consequently, much larger bond and shear stresses are required to sustain these magnified forces. The critical deterioration of potential shear strength in the joint area should not occur until ductile capacity of adjacent beams reach the design demand. In this study, a method was provided to predict the deformability of reinforced concrete beam-column joints failing in shear after the plastic hinges developed at both ends of the adjacent beams. In order to verify the deformability estimated by the proposed method, an experimental study consisting of three joint specimens with varying tensile reinforcement ratios was carried out. The result between the observed and predicted behavior of the joints showed reasonably good agreement.

• Journal of the Korea Concrete Institute
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• v.18 no.3 s.93
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• pp.321-330
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• 2006

In order to remodel old aged reinforced concrete buildings, it is often required to extend the residence area of the buildings by increasing the slab area. The slab area is usually extended by attaching a new slab to the existing slab with hinged joint or rigid joint. Transmission of the loads of the attached slabs to the existing slabs depends on the connecting methods, such as hinged or rigid connection. In this research, 8 specimens and 24 RC slabs connected by rigid joints were tested. The new slab was connected to the existing slab by three types of rigid joints using dowel bars and longitudinal tensile bars. Main parameters of the slabs were three types of the rigid joints, anchor length of steel bars(0, 50, 60, 100, and 120mm), development length of steel bars(100, 200, and 300mm), and the spacing of the steel bars(150, 200, 300, and 450mm). The test results indicated that the flexural strength of the RC test slabs having various types of rigid joints was approximately the same to that of the slab without any connections.

• Steel and Composite Structures
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• v.19 no.1
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• pp.209-221
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• 2015

Results from an experimental study on the seismic response of six composite reinforced concrete column-to-steel beam interior joints are presented. The primary variable investigated is the details in the joint. For the basic specimen, the main subassemblies of the beam and column are both continuous, and the steel beam flanges extended to the joint are partly cut off. Transverse beam, steel band plates, cove plates, X shape reinforcement bars and end plates are used in the other five specimens, respectively. After the joint steel panel yielded, two failure modes were observed during the test: local failure in Specimens 1, 2 and 4, shear failure in Specimens 3, 5 and 6. Specimens 6, 3, 5 and 4 have a better strength and deformation capacity than the other two specimens for the effectiveness of their subassemblies. For Specimens 2 and 4, though the performance of strength degradation and stiffness degradation are not as good as the other four specimens, they all have excellent energy dissipation capacity comparing to the RC joint, or the Steel Reinforced Concrete (SRC) joint. Based on the test result, some suggestions are presented for the design of composite RCS joint.

• Structural Engineering and Mechanics
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• v.35 no.1
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• pp.1-18
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• 2010

Bonding of carbon fiber reinforced polymer (CFRP) composites has become a popular technique for strengthening concrete structures in recent years. The bond stress between concrete and CFRP is the main factor determining the strength, rigidity, failure mode and behavior of a reinforced concrete member strengthened with CFRP. The accurate evaluation of the strain is required for analytical calculations and design processes. In this study, the strain between concrete and bonded CFRP sheets across the notch is tested. In this paper, indirect axial tension is applied to CFRP bonded test specimen by a four point bending tests. The variables studied in this research are CFRP sheet width, bond length and the concrete compression strength. Furthermore, the effect of a crack- modeled as a notch- on the strain distribution is studied. It is observed that the strain in the CFRP to concrete interface reaches its maximum values near the crack tips. It is also observed that extending the CFRP sheet more than to a certain length does not affect the strength and the strain distribution of the bonding. The stress distribution obtained from experiments are compared to Chen and Teng's (2001) analytical model.

• Magazine of the Korea Concrete Institute
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• v.6 no.3
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• pp.162-172
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• 1994

The flexural and horizontal shear behavior of overlaid concrete slabs with polymer interface is investigated in the present study. An experimental program was set up and several series of overlaid concrete slabs have been tested to study the effect of different surface preparations and dowel bars between old slab and overlay under service and ultimate loads. 'The cracking and ulti mate load behavior for various cases including acryl emulsion treatment and doweled joints has been studied. The present study indica.tes that the overlaid concrete slabs behave integrally with existing bottom slabs up to ultimate range for rough and doweled joints with polymer interface. The pres ent study provides a firm base for the realistic design of two-layered RC slabs in bridges.

• Journal of the Korea institute for structural maintenance and inspection
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• v.7 no.2
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• pp.231-238
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• 2003

Reinforced concrete buildings damaged by earthquake which can be reused can ensure the stability in its structure by repair-restrengthening, but when such a repair-restrengthening is conducted inappropriately or its structural strength is greatly reduced by earthquake again, it should have repair-restrengthening. This study selects beam-column joints which are vulnerable to earthquake as the object of experiment, performs repair-restrengthening after applying the first and the second dynamic loading to the objects of experiment, examines the capacity of restrengthening according to structural characteristics and loading velocity and verifys the validity of repair-restrengthening.

• Proceedings of the Korean Society for Rock Mechanics Conference
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• 1995.03a
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• pp.1-26
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• 1995

Key aspects of the Norwegian Method of Tunnelling (NMT) are reviewed. These include a predictive method of support design using the six-parameter Q-system of rock mass characterisation. The rock mass rating or Q-value is updated during tunnel driving. The designed tunnel support generally consists of wet process, steel fibre reinforced shotcrete combined with fully grouted, untensioned rock bolts, Even in poor rock conditions S(fr) + B usually acts as the final rock reinforcement and tunnel lining. Since it is a drained lining, it is very economic compared to cast concrete with membranes. Light, free-standing steel liners are used to prevent water affecting the runnel environment. Rock mass conditions, and hence lining design and cost estimation can be assessed by careful use of seismic surveys. Relationships between the P-wave velocity, the rock mass deformation modulus and the Q-value have recently been established, where tunnel depth, rock porosity and the uniaxial compression strength of the rock are important variables. The rock mass modulus estimate, and simple index testing of the joints, provide the key input which joints are discretely represented (either in two dimensions with the UDEC code or in three dimensions with the 3DEC code) is generally favoured compared to continuum analysis. The latter may give a misleading impression of uniformity and deformations tend to be understimated. Q-system NMT designs of S(fr) + B (fibre reinforced shotcrete and bolting) are numerically checked and adjustments made to bolt capacities and shotcrete thickness if overloading is evident around the modelled profile.

• Structural Engineering and Mechanics
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• v.10 no.6
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• pp.589-601
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• 2000

A simplified analysis procedure utilizing the strut-tie modeling technique is developed to take a close look into the post-elastic deformation capacity of beam-column connections in ductile reinforced concrete frame structures. Particular emphasis is given to the effect of concrete strength decay and quantity and arrangement of joint shear steel. For this a fan-shaped crack pattern is postulated through the joints. A series of hypothetical rigid nodes are assumed through which struts, ties and boundaries are connected to each other. The equilibrium consideration enables all forces in struts, ties and boundaries to be related through the nodes. The boundary condition surrounding the joints is obtained by the mechanism analysis of the frame structures. In order to avoid a complexity from the indeterminacy of the truss model, it is assumed that all shear steel yielded. It is noted from the previous research that the capacity of struts is limited by the principal tensile strain of the joint panel for which the strain of the transverse diagonal is taken. The post-yield deformation of joint steel is taken to be the only source of the joint shear deformation beyond the elastic range. Both deformations are related by the energy consideration. The analysis is then performed by iteration for a given shear strain. The analysis results indicate that concentrating most of the joint steel near the center of the joint along with higher strength concrete may enhance the post-elastic joint performance.

• Steel and Composite Structures
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• v.29 no.5
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• pp.635-645
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• 2018

Beam-column joints play a significant role in static and dynamic performances of reinforced concrete frame structures. This study contributes a numerical approach of topologically optimal design of carbon fiber reinforced plastics (CFRP) to retrofit existing beam-column connections with crack patterns. In recent, CFRP is used commonly in the rehabilitation and strengthening of concrete members due to the remarkable properties, such as lightweight, anti-corrosion and simplicity to execute construction. With the target to provide an optimal CFRP configuration to effectively retrofit the beam-column connection under semi-failure situation such as given cracks, extended finite element method (X-FEM) is used by combining with multi-material topology optimization (MTO) as a mechanical description approach for strong discontinuity state to mechanically model cracked structures. The well founded mathematical formulation of topology optimization problem for cracked structures by using multiple materials is described in detail in this study. In addition, moved and regularized Heaviside functions (MRHF), that have the role of a filter in multiple materials case, is also considered. The numerical example results illustrated in two cases of beam-column joints with stationary cracks verify the validity, benefit and supremacy of the proposed method.