Reinforced Concrete Interior Research Articles

Experimental investigation of EBROG and bore-epoxy anchorage methods used for interior RC beam-column joints strengthened with CFRP sheets

In severe earthquakes, the energy dissipation and ductility capacities of the structures depend on the performance of the beam-column joints where the load transfer is performed. Experiences in past earthquakes have shown that damage occurs in the beam-column joints that do not have sufficient strength and rigidity. In particular, the shear failure observed in deficient detailed joints has caused the collapse of many structures. Joints are effectively retrofitted with carbon fiber reinforced polymer (CFRP) sheets to increase the earthquake safety of the structures. However, the debonding problem experienced in CFRP sheets significantly affects the efficiency of the applied retrofit and the earthquake behavior of the member. In this study, the retrofit of reinforced concrete beam-column joints with deficient shear strength was carried out with CFRP sheets by using externally bonded reinforcement on grooves (EBROG) and bore-epoxy anchorage methods. These two retrofit methods were applied to effectively utilize the full capacity of CFRP sheets by delaying the debonding. Six ½ scaled reinforced concrete (RC) interior beam–column specimens without transverse reinforcement in the joint core were constructed. One reference and five retrofitted joints were subjected to displacement-controlled cyclic loading. The shear failures in the specimens were delayed until advanced displacement levels. The use of EBROG and bore-epoxy anchorage methods improved the yield load of the specimens by 32 % to 69.05 % compared to the reference specimen. Additionally, significant increases were observed in the initial stiffness, load-carrying, and energy dissipation capacities of the retrofitted specimens up to 68 %, 64 %, and 104 %, respectively. The ductility of the retrofitted specimens increased by approximately 7 % to 26 %. The EBROG and bore-epoxy anchorage methods proved to be highly successful in preventing the early debonding of CFRP sheets. In specimens strengthened with EBROG and bore-epoxy anchorage methods, the displacement values at which the FRP sheets began to debonding approximately doubled. It was concluded that EBROG and bore-epoxy anchorage methods were more effective than the externally bonded reinforcement (EBR) method in improving the overall structural performance of the joints.

Efficiency of Flange-Bonded CFRP Sheets in Relocation of Plastic Hinge in RC Beam–Column Joints

Beam–column connection zones are high regions of interest in reinforced concrete (RC) structures, which are expected to respond elastically to seismic loads. Using carbon fiber-reinforced polymers (CFRP) to improve these connections, performance is critical in retrofitting deficient RC frames because existing slabs may pose numerous limitations in the design and wrapping of CFRP sheets in joints. The main aim of this research is to develop a new design for flange-bonded CFRP retrofit of frames, including slabs, for the relocation of plastic hinges of the connection area toward the beam and to develop beam–column joint capacity and building stability in cases of subjection to dynamic loads. The performance of these proposed retrofittings was explored both experimentally and numerically. Two full-scale fabricated interior RC joints of a real moment-resisting frame with moderate ductility were subjected to monotonic loads before and after retrofitting, and the results were used to detail the numerical progress and verify of the beam–column connection. Moreover, a parametric study was conducted on CFRP sheets’ optimal thickness to examine its influence on plastic hinge relocation in the connection region. Results show that the retrofitting method can efficiently relocate the plastic hinge to the mid-span of the beam, which, in turn, leads to improved capacity and achievement of the RC frame and guarantees better structural safety a lower cost.

A finite element study on the ultimate lateral drift capacity of interior reinforced concrete flat slab-column connections

Reinforced concrete (RC) flat slabs may be a part of the lateral load resisting system or may only serve as a gravity load bearing element in RC structures. In both cases, RC slab-column connections have a crucial role in the global seismic performance of RC buildings. Punching shear failure in flat slabs concentrated in the slab-column connection region is a common brittle failure mode which can adversely affect the global seismic behavior of structures. However, few studies have been done on the numerical simulation of brittle shear failure in slab-column connections under simultaneous gravity and lateral loads. In this paper, to evaluate the behavior of RC slab-column connections under simultaneous gravity and lateral loads and to examine the effects of various influential parameters on the lateral drift capacity of the connections, a numerical study is conducted on interior slab-column connections. The investigated influential variables are the flexural reinforcement in the slab, slab thickness, and the dimensions of the supporting column. The numerical study is implemented in the finite element software ABAQUS. First, the numerical model is verified through modeling three interior RC slab-column connections tested in other experimental studies. Then, all the connections are analyzed under the simultaneous effects of gravity and lateral loads and their lateral load-drift curves are determined. According to the analysis results, in addition to the gravity load intensity, other variables such as flexural reinforcement ratio, slab thickness, and the size of the supporting column affect the lateral behavior of the connection.

Prediction of the shear strength of RC beam-column joints using new ANN formulations

Beam-column joint is a key part for reinforced concrete (RC) frame building structures under the impact of severe lateral actions of seismic or wind loading. Prior works indicate that there is lacking in the direction of formulation of new mathematical models to predict RC joints strength by artificial neural network (ANN) considering influential design parameters. Present endeavor is devoted to establish new empirical ANN models to estimate shear strength and failure mode for RC exterior and interior beam-column joints. These models were relied on training, testing and validation of a database of over 200 experiments, for the performance of RC joints, using ANN. More than 20 parameters, related to the geometry and loading of the joints, were selected as inputs for these models; but four effective inputs were used in the formulation of the final ANN models. The outputs for ANN models were compared to those counterparts of the commonly utilized models given in codes and other recent studies. This calculation confirms the reliability of current ANN mathematical models and their outstanding performance. Moreover, comparison shows that the proposed models were precise in including the effective independent variables in these models and avoiding the shortcoming of previous models.

Development of an Analytical Model for the FRP Retrofitted Deficient Interior Reinforced Concrete Beam-Column Joints

Beam-column joints (BCJs) constructed until the 1970s carry a low shear capability due to the absence of shear reinforcement. Fiber-reinforced polymers (FRP) are more reliable than other materials to strengthen a weak BCJ. To date, plenty of analytical models have been developed to analyze the actual contribution of the FRP to the shear strength of RC BCJs. However, the models developed are either too complex in computational efforts or based on empirical coefficients that result in compromised results. The models that formulate the contribution of FRP to the shear strength of the FRP-strengthened deficient interior BCJ are very limited, and such models are too complex. An adequate BCJs’ FRP strain equation must still be developed to address these issues. Therefore, the FRP effective strain equation and contribution of FRP to RC BCJs are derived in this research work using an updated database of the appropriate BCJs. The initial analytical model of Bousselham, which Del Vecchio later improved, is further extended to FRP-strengthened deficient interior BCJs. For this purpose, an updated database of the 32 tests around the world of FRP-strengthened interior BCJs deficient in seismic reinforcement is prepared. Firstly, the experimental effective FRP strain is derived using the experimental database. Then, a power-type equation is derived for the effective FRP strain by considering the crucial parameters of the FRP-strengthened interior BCJs. Finally, the experimental shear strengths and those determined with the proposed equation of the FRP-strengthened joints are compared. The average ratio between the experimental and analytical (proposed model) joint shear strengths of the considered specimens ensured the accuracy of the suggested model. The suggested approach makes computing the FRP enhancements required to avoid shear failure in interior joints easy and reliable for researchers and field engineers interested in seismically reinforcing existing structures.

Seismic Performance of Interior Reinforced Concrete Beam–Column Joint with Corroded Reinforcement

This study presented an experimental and analytical investigation on the seismic behavior of a reinforced concrete (RC) interior joint with corroded reinforcements. A total of eight RC interior joints with similar reinforcing detailing were tested and subjected to lateral cyclic loading. The variables studied in this research were the corrosion level of reinforcements in terms of mass loss and the column axial force ratio. The cyclic performance of all the joints including crack pattern, lateral loading capacity, energy dissipation, and lateral displacement decomposition were examined. The corroded joint constitutive model was calibrated and validated using the finite-element method. An analytical strut-and-tie model (STM) was developed for explaining the internal force flow and further parametric investigations. Results indicated that corrosion of reinforcements had a substantial effect on the strength and lateral drift capacity of the joints. In addition, based on the STM model, the axial force ratio exceeding 0.3 and longitudinal reinforcement corrosion level around 6–10% were detrimental toward joint behavior and can contribute to joint shear failure.

Cyclic behavior of interior RC beam-column joints strengthened with NSM-CFRP ropes

Beam-column joints in existing concrete structures could become unsafe for many reasons, such as design error and seismic loading. Thus, in this study, the effectiveness of near surface mounted carbon reinforced polymer (NSM-CFRP) ropes for enhancing interior reinforced concrete (RC) beam-column joints was investigated. Five interior one-third scale RC beam-column joints without transverse reinforcement (stirrups) at the beam-column joints were tested under cyclic loading. The first specimen was tested as a control beam-column joint, while the four remaining beam-column joints were strengthened using NSM-CFRP ropes with a variety of CFRP rope configurations. Results were drawn based on hysteretic curves, envelope curves (load vs. drift ratio), energy dissipation, stiffness, ductility index and failure modes. The results showed that the stiffness and yield load of strengthened beam-column joints was improved compared with the control beam-column joint. The use of NSM-CFRP ropes resulted in an increase in the yield load of the strengthened beam-column joints ranging from 118.7% to 164.1% than the control beam-column joint depending on the NSM-CFRP ropes’ number and its position. The use of X- shaped rope with two ropes on each side of beam and two on each side of column imparted the best effectiveness on stiffness and yield load, for instance, the stiffness increased by 236.7% compared to control beam-column joint. Finally, the bond slip occurred between the rope and concrete; therefore, the rope was only effective before the drift ratio reached 1%-1.5%, therefore, the maximum load is only affected by the RC.

Seismic retrofit of large-scale interior RC beam-column-slab joints after standard fire using steel haunch system

Steel haunch system was proposed to retrofit fire-damaged reinforced concrete (RC) beam-column-slab joints and ensure their seismic safety. Five RC beam-column-slab joints were designed, including one control specimen without fire damage, one control fire-damaged specimen, three fire-damaged specimens retrofitted with different types of steel haunch system, i.e., separate steel haunches, a combination of steel haunches and carbon fiber-reinforced polymer (CFRP) sheets, and a combination of steel haunches and bolted side plates (BSP). The fire-damaged RC joints were subjected to ISO 834 standard fire. After fire tests, low-cycle reversed loading tests were conducted on all the control and retrofitted specimens. The influence of fire damage and retrofit methods on the hysteretic behavior, load capacity, stiffness, ductility factor, and energy dissipation capacity of the joints was analyzed. Experimental results showed that fire exposure significantly reduced the seismic performance of RC beam-column-slab joints. The plastic hinges of spatial joints were transferred from the beam-column interface to the zone outside steel haunches after retrofitting. Compared with the control fire-damaged specimen, the hysteretic curves of the retrofitted specimens were more stable and plumper. All the three strengthening methods effectively enhanced the ultimate load capacity (increased by 24–70%), stiffness, ductility and cumulative hysteresis energy (increased by 107–362%) of the post-fire joints. CFRP sheets and bolted side plates show excellent compatibility with the steel haunches. The preload degradation of bolts used to fix the steel haunches reduced the retrofit efficiency of the steel haunch system. The numerical results of the sequentially coupled thermal-stress analysis model for retrofitted post-fire RC joints based on ABAQUS were consistent with the experimental results.

Seismic performance evaluation of retrofitted RC beam-column joints reinforced by plain and deformed bars

The seismic retrofit of existing reinforced concrete (RC) beam-column joints in the case of the interior reinforcement of deformed bars and plain bars, as well as the decision regarding convenient retrofit strategies in the joints panel, are of major interest throughout the seismically active areas of the world. The efficiency of the proposed retrofit method called ‘joint enlargement’ in RC exterior and interior beam-column joints reinforced with deformed and plain bars, which have been performed under two different experimental setups in same university laboratories, is investigated. Seven half-scale beam-column joint reinforced by plain bars and four half-scale beam-column joint reinforced by deformed bars were tested. All specimens were subjected to a cyclic quasi-static loading to observe different levels of structural damage. The tested specimens comprised of five control specimens and six retrofitted specimens with various reinforcement detailing and varying dimensions of joint enlargement. The test results show significant improvement of the retrofitted specimens of both deformed and plain bars in terms of protecting the joint region, showing more ductile response and higher hysteresis energy capacity. The ultimate lateral load capacity was increased for all retrofitted specimens. The tests also show that the retrofit strategy for specimens reinforced with plain bars was performed well. Accordingly, the proposed retrofit strategy may be recommended for similar cases in existing old buildings reinforced with plain or deformed bars.

Seismic performance of reinforced concrete interior beam-column joints with novel reinforcement detail

Horizontal stirrups are normally required in reinforced concrete (RC) beam-column joints (BCJs) for resisting shear forces in seismic design. For RC moment-resisting frames subjected to a high lateral load, a large number of stirrups are needed in joint cores. This may cause reinforcement congestion, leading to construction difficulty and insufficient concrete compaction, which can result in poor seismic performance. In this study, a novel reinforcement detail in the form of unbonded diagonal bars mechanically anchored at beam ends is proposed for RC interior BCJs. The detail alleviates the reinforcement congestion through partially replacing horizontal stirrups, and improves the seismic performance of BCJs by plastic hinge relocation and input shear force reduction mechanisms. Four 2/3-scale RC interior BCJ specimens were prepared and tested under quasi-static cyclic load, including one specimen designed with the current code and three specimens adopting the proposed reinforcement detail. Test results show that the proposed reinforcement detail is able to relocate the plastic hinges away from beam-joint interfaces as well as improve the loading capacity, energy dissipation capacity, stiffness, and bonding condition of beam reinforcements within the cores of BCJs. The combined use of stirrups and the proposed reinforcement detail significantly enhances the cracking resistance and reduces joint distortion, while additional amount of stirrups results in a marginal improvement. Moreover, an analytical model considering plastic hinge relocation and input joint shear force reduction is proposed for BCJs with the novel reinforcement detail. The model can adequately predict the failure mode and loading capacity of BCJ specimens.

Prediction of Joint Shear Deformation Index of RC Beam–Column Joints

In the analysis of reinforced concrete (RC) buildings, beam–column joints are regarded as rigid nodes. In fact, joint deformation may make a significant difference in the lateral response of RC buildings if joints are not properly designed and detailed. To consider joint flexibility, several types of joint models have been proposed. However, these models require complicated computations, consequently making them challenging to apply in engineering practice. This paper proposed a simple approach for predicting the contribution of the joint deformation to the total deformation of RC interior beam–column joints under critical structural deformations. To develop such a simple and accurate approach, experimental and analytical studies were performed on RC interior beam–column joints. In this study, eight half-scale joint specimens were tested under reversed cyclic loading, and 39 full–scale FE models were constructed, varying the selected key parameters. The experimental and analytical results showed that the “joint shear” is a useful index for the beam–column joints with high shear stress levels of vj>1.7 fc′ but is unsuitable for defining the failure of beam–column joints with medium or low shear stress levels of vj≈1.25–1.7fc′ and vj≈1.0fc′. Based on the results, three equations were developed to predict the joint shear deformation index (SDI) of RC interior beam–column connections corresponding to three different types of failure (i.e., joint failure before beam yielding, joint failure after beam yielding, and beam flexural failure). SDI predictions of the proposed equations correlate well with 50 test results of beam–column joints available from the literature.

Nonlinear Response Study and a Rotational Spring Model for Degrading Cyclic Behavior of Interior Reinforced Concrete Connections

ABSTRACT A parametric study is performed on the cyclic behavior of reinforced concrete connections. A number of 64 analytical cases are developed being different in the value of the axial force of column, volume of the transverse reinforcement in the connection, ratio of the bending capacity of column and beam, and the continuity index of the longitudinal bars of beam in the connection. The stiffness, dissipated energy, ductility, and ultimate shear stress and strain of the connections are calculated. Finally, a cyclic model for the nonlinear behavior of the connection is proposed that includes stiffness and strength degradation and longitudinal bar slip.

Finite Element Analysis of Reinforced Concrete Interior Beam Column Connection Subjected to Lateral Loading

The beam column connection is the most critical zone in a reinforced concrete frame. The strength of connection affects the overall behavior and performance of RC framed structures subjected to lateral load and axial loads. The study of critical parameters that affects the overall joint performances and response of the structure is important. Recent developments in computer technology have made possible the use of Finite element method for 3D modeling and analysis of reinforced concrete structures. Nonlinear finite element analysis of reinforced concrete interior beam column connection subjected to lateral loading was performed in order to investigate joint shear failure mode in terms of joint shear capacity, deformations and cracking pattern using ABAQUS software. A 3D solid shape model using 3D stress hexahedral element type (C3D8R) was implemented to simulate concrete behavior. Wire shape model with truss shape elements (T3D2) was used to simulate reinforcement’s behavior. The concrete and reinforcement bars were coupled using the embedded modeling technique. In order to define nonlinear behavior of concrete material, the concrete damage plasticity (CDP) was applied to the numerical model as a distributed plasticity over the whole geometry. The study was to investigate the most influential parameters affecting joint shear failure due to column axial load, beam longitudinal reinforcement ratio, joint panel geometry and concrete compressive strength. The Finite Element Model (FEM) was verified against experimental test of interior RC beam column connection subjected to lateral loading. The model showed good comparison with test results in terms of load-displacement relation, cracking pattern and joint shear failure modes. The FEA clarified that the main influential parameter for predicting joint shear failure was concrete compressive strength.

Numerical study on shear failure and the corresponding size effect of interior RC beam-column joints

The shear failure of the core zone of reinforced concrete (RC) beam-column joint has brittle characteristics, so the size effect of shear strength can be caused. Based on the existing experimental research results, the numerical test method is used to analyze the structural size, axial compression ratio, and stirrup ratio on the effect of failure mechanism and failure mode of the RC beam-column joint, and reveal their influence on the size effect of shear strength. The maximum cross-sectional dimension of the joint is 900 mm × 900 mm. The results show that: (1) under the monotonic loading, the beam-column nodes exhibit brittle shear failure in the core region, and the nominal shear strength has a significant size effect; (2) the increase in the axial compression ratio can enhance the shear capacity of the middle joint and also strengthen the size effect of shear strength; (3) an increase in the stirrup ratio can increase the shear capacity of the joint, but it can also weaken the size effect of the shear strength. Lastly, the classical size effect law proposed by Bažant can reflect the size effect of shear failure of the middle node, but it cannot describe the quantitative influence of the axial compression ratio and stirrup ratio.

Experimental evaluation of seismic performance of interior RC beam-column joints strengthened with FRP composites

Lack of joint confinement for the majority of pre-1970 reinforced concrete (RC) frame construction has resulted in weakening the link between the column and the beam and collapse of the whole structure. The main focus of this research study is based on four interrelated tasks: (i) design and development of innovative repair and retrofit techniques for reinforced concrete (RC) beam-column joints using advanced FRP composite laminates and pre-cured composite connectors; (ii) experimental evaluation of the different techniques using full-scale testing; (iii) comparison in behavior between as-built and different retrofit specimens; and (iv) conclusions and recommendations for future research.Experimental results confirmed the superiority and success of the proposed strengthening protocols, not only in restoring the original strength capacity, but also in enhancing the overall seismic performance of the deficient joints evaluated in this study including strength and ductility. For example, the use of carbon/epoxy wet layup composite laminates resulted in an appreciable increase of both strength and ductility up to 1.34 and 3.04 times, as compared with as-built specimen, respectively. Also, the proposed technique for enhancing shear strength and rebar bond slippage of the joints using high-strength carbon/epoxy FRP composite laminates and a hybrid composite connectors (HCC) achieved significant results. The novel proposed technique improved the shear strength of the joint 2.5 times the control deficient specimen. The use of the hybrid composite connector (HCC) succeeded in relocating the hinging mechanism to form in the beam span away from the joint region. The ductility of the retrofitted specimen was 2.18 times the control (as-built) specimen. It is anticipated that the results of this pioneering study will provide alternative innovative reinforcing and strengthening methodologies to enhance the construction and repair methods for reinforced concrete moment frame structures. These innovative techniques contribute to higher reliability and safety, as well as lower construction and repair costs of RC moment frame structures. It is also anticipated that the proposed strengthening protocol can also be applied to cover similar details such as base-column, pile-cap as well as T- and knee bridge joints.

Analytical and numerical modeling of RC beam-column joints retrofitted with FRP laminates and hybrid composite connectors

This paper summarizes results on an analytical and numerical simulation of structural behavior of reinforced concrete (RC) beam-column joints retrofitted with different types of fiber-reinforced-polymeric (FRP) composite laminates and hybrid connectors. In this study, non-linear numerical simulations for the behavior of reinforced concrete (RC) beam-column joints that were evaluated in another phase of this study. The behavior of a total of eight full-scale interior RC beam-column specimens were numerically evaluated. The interior reinforced concrete (RC) beam-column joint specimens were subjected to both simulated gravity and low-frequency full-cyclic reversal loads. For repair and external strengthening applications, three systems are evaluated including high-strength carbon/epoxy composite laminates, high-modulus carbon/epoxy laminates and E-glass/epoxy external laminates. For bond-slip retrofit, the light-weight hybrid composite connector, developed by the principal author, was evaluated through large-scale tests and is molded numerically. Good correlation between numerical and experimental results is achieved. A comparison between the numerical and experimental results are also presented.

Experimental and Numerical Model of Interior Reinforced Concrete Beam–Column Joints Strengthened with Carbon Fiber-Reinforced Polymer Sheets

The present study involves full-scale experimental test and numerical model of interior reinforced concrete beam–column joints strengthened by carbon fiber-reinforced polymer (CFRP) sheets. The present study proposes a novel strengthening technique for interior beam–column joints. The experimental test intends to achieve a fundamental understanding of the behavior of interior joint strengthened by CFRP wrap in columns region with L-shape overlays on the top and bottom of beams. The purpose of implementing this system is to transfer the failure of the columns regions to the beams regions. This technic is a feasible economic solution. Hence, two beam column joints were made and tested. One interior joint was tested in an unstrengthened condition to act as the control joint. A numerical simulation based on plastic damage model by ABAQUS software was carried out to validate the experimental results. The CFRP wrap mechanism prevented the development of cracks in the joint. The length of cracks decreased because CFRP sheets were applied. The average decrease was approximately 37% of the crack length of the control beam–column joint. It is observed that the compression strut zone was increased by the application of CFRP wrap in the column zone.

Size effect tests on shear failure of interior RC beam-to-column joints under monotonic and cyclic loadings

The main objectives of this study are, on one hand to present an experimental campaign on the shear failure behavior of reinforced concrete (RC) beam-to-column joints having different structural sizes, especially full-scale joints, and on the other hand to evaluate the possibility of the existence of size effect on the corresponding nominal shear strength of the RC joints. The test results of geometrically similar interior RC beam-to-column joints with the joint width ranging from 300 mm to 900 mm under both monotonic and cyclic vertical loadings are reported in this study. The effects of stirrup ratio and loading type on the seismic shear failure behavior at the core region of the joints are studied. Moreover, the size effect of the corresponding nominal shear strength of the RC beam-to-column joints is also explored. The experiment results demonstrate that the failure modes of RC beam-to-column joints with different structural sizes are similar. There is a significant size effect, namely, the larger the structural size of the joint is, the smaller the nominal shear strength at the core region of the joint is. Increasing the structural size causes poor ductility capacity, fast degradation of stiffness and more dissipation energy. Moreover, cyclic loading makes the size effect on shear strength of RC joints stronger. The stirrups weaken the size effect on shear failure since the tested shear bearing capacity is mostly carried by the concrete part.

Seismic Performance of the King Cross Steel Profile in Interior Reinforced Concrete Beam-column Joint

Seismic Performance of the King Cross Steel Profile in Interior Reinforced Concrete Beam-column Joint

Strut-and-tie model for interior RC beam-column joints with substandard details retrofitted with CFRP jackets

A softened strut-and-tie model (STM) is developed for interior reinforced concrete (RC) beam-column joints without any steel hoops in the joint or intermediate longitudinal column steel reinforcement, and discontinuous bottom beam flexural steel reinforcement. The STM model is extended to identical joints retrofitted with Carbon Fiber-Reinforced Polymer (CFRP) composites. The STM model is compared to experiments of two full-scale RC beam-column interior joints, one of which was retrofitted with CFRP composites. Failure modes for the original joint included anchorage failure of bottom steel beam bars, crushing of concrete nodal zones, diagonal joint shear failure, and for the retrofitted joint CFRP laminate delamination and crushing of joint core concrete. The STM model is based on the joint reinforcement details and experimental performance, including column axial load effects and the contribution of CFRP horizontal and diagonal laminates as tension ties. STM model assumptions were verified with strain gauge measurements. The STM model was successful in estimating the ultimate shear strength of the original and retrofitted joints. Recommendations are presented for evaluating the strut width for both original and retrofitted joints that include the quality of the bond of beam steel reinforcement to concrete in the beam-column joint.