Reinforced Concrete Beam-column Joints Research Articles

Prediction of Strength and Failure Modes in Reinforced Concrete Beam–Column Joints using Ensemble Machine Learning Techniques

The beam-column joint is one of the important structural elements that control the structural behaviour during seismic excitation. Moreover, failure of any of these components may lead to the partial or overall collapse of the entire structure. In view of the safety concern, there is a need to evaluate the response properties, such as failure modes and ultimate shear capacity of the beam-column joints. Conventional methods to evaluate the shear capacity of beam-column joints are usually time-consuming. Therefore, in this study, an attempt has been made to evaluate the joint shear capacity and mode of failure of an exterior beam-column joint using ensembled machine learning (ML) approaches like Decision Tree, Random Forest, AdaBoost, CatBoost, and LightGBM. The present study highlights the potential use of the CatBoost model as a useful tool that can assist in predicting the shear strength and failure mode. The results of the study reveal that this model has performed well in predicting the load-carrying capacity and shear strength with high correlation coefficients of 0.9836 and 0.9978, respectively. Moreover, it has been observed that the prediction model provided by the CatBoost algorithm exhibits the highest accuracy for classification in the failure mode of beam-column joints.

Seismic Performance of Highly Eccentric Reinforced Concrete Beam–Column Joints

Seismic Performance of Highly Eccentric Reinforced Concrete Beam–Column Joints

Numerical analysis of FRP retrofitting in RC beam-column exterior joints at high temperatures and predictive modeling using artificial neural networks

PurposeThe purpose of this study is to perform a numerical analysis of fiber-reinforced polymer (FRP) retrofitting in reinforced concrete (RC) joints at high temperatures and predict models using artificial neural networks (ANN). The aim was to gain insights into their structural behavior across a range of loading conditions from room temperature to 800°C. Additionally, the research assessed the efficiency of carbon fiber-reinforced polymer (CFRP), glass fiber reinforced polymer (GFRP) and aramid fiber reinforced polymer (AFRP) strengthening in enhancing the structural performance of the critical sections.Design/methodology/approachThe linear numerical simulations were conducted to evaluate the performance of RC beam-column joints using finite element modelling (FEM) analysis. The ANN model demonstrated exceptional effectiveness in predicting the stiffness of frames with openings, establishing itself as the premier machine learning algorithm for frame stiffness estimation. In the conventional model, 300°C was proven to be an effective temperature approach. Subsequently, maintaining a constant temperature of 300°C, an in-depth analysis of nearly 30 models of three retrofitting techniques was conducted under thermomechanical loading.FindingsThe CFRP retrofits yielded 15% less deflection and 30% more stress than the remaining FRPs, and the ANN models predicted the deflection, main stresses, bending moment and shear force. The ANN model results were compared with those of other frequently used models. The R thresholds (R = 0.954, 0.981, 0.986, 0.968, 0.978 and 0.936) for training, testing and validation indicated that the ANN model achieved data variability. The findings indicate that the ANN model is more accurate because of the strong connection between the numerical model and the prediction.Originality/valueTo identify the pinpoint of critical segments within the rehabilitation section and determine the most effective wrapping method among the three laminates.

Machine Learning Ensemble Methodologies for the Prediction of the Failure Mode of Reinforced Concrete Beam–Column Joints

One of the most critical aspects in the seismic behavior or reinforced concrete (RC) structures pertains to beam–column joints. Modern seismic design codes dictate that, if failure is to occur, then this should be the ductile yielding of the beam and not brittle shear failure of the joint, which can lead to sudden collapse and loss of human lives. To this end, it is imperative to be able to predict the failure mode of RC joints for a large number of structures in a building stock. In this research effort, various ensemble machine learning algorithms were employed to develop novel, robust classification models. A dataset comprising 486 measurements from real experiments was utilized. The performance of the employed classifiers was assessed using Precision, Recall, F1-Score, and overall Accuracy indices. N-fold cross-validation was employed to enhance generalization. Moreover, the obtained models were compared to the available engineering ones currently adopted by many international organizations and researchers. The novel ensemble models introduced in this research were proven to perform much better by improving the obtained accuracy by 12–18%. The obtained metrics also presented small variability among the examined failure modes, indicating unbiased models. Overall, the results indicate that the proposed methodologies can be confidently employed for the prediction of the failure mode of RC joints.

A Novel Approach to Monitoring the Performance of Carbon-Fiber-Reinforced Polymer Retrofitting in Reinforced Concrete Beam–Column Joints

A Novel Approach to Monitoring the Performance of Carbon-Fiber-Reinforced Polymer Retrofitting in Reinforced Concrete Beam–Column Joints

Image-based assessment of seismic damage in RC exterior beam-column joints

Seismic damage to reinforced concrete (RC) beam-column joints in reinforced concrete (RC) structures significantly impacts their stability and safety after an earthquake. Cracks caused by earthquakes weaken these joints, reducing their ductility, strength, and stiffness. Therefore, a systematic method for assessing damage in RC beam-column joints is crucial. This study introduces a new method that uses image analysis, along with ranking algorithms and machine learning, to assess seismic damage in RC exterior beam-column joints. The study identified three key indicators of severe damage in joints: drift ratio, strength, and stiffness. The method uses image analysis to extract features from preprocessed, binary images of cracks. These features capture both the texture and geometry of the cracks. The chosen features, including crack area, aspect ratio, and distribution of line widths, along with structural parameters like geometry and bond index, were selected using F-test and MRMR algorithms. Seismic damage assessments were conducted on 115 images from 37 specimens using four machine learning models (Regression Trees, Support Vector Machine, Gaussian Process Regression, and Gradient Boosting). The study achieved a high R-squared value of 0.80, indicating strong accuracy, for assessing seismic damage based on drift ratio using the image-based method. This demonstrates that image analysis techniques can effectively link surface crack patterns to quantifiable image features, leading to a more precise assessment of seismic damage.

Failure mechanism and plastic hinge of RC joints under impact load acting on column ends

Impact loads often act on columns end during accidents such as vehicle impacts and terrorist attacks. The resulting deformations lead to a redistribution of internal forces in the member as well as a reduction in the load carrying capacity. The integrity of the core area of the joint is threatened, which could trigger a progressive collapse. This study numerically investigated the damage and deformation of reinforced concrete (RC) beam-column joints when they are impacted on the column end. The resistance mechanism of joints and the influence of impact location as well as impact velocity was explored. Besides, the deformation capacity of RC beam-column joints under impact loading was quantified using equivalent plastic hinges. It was found that the damage and plastic deformation is concentrated at the impacted column end. The horizontal reaction force at the beam end provides resistance and plays the role as a bearing. The column bottom reaction force is in the same direction as the impact force. However, it is much smaller than the beam end reaction force. The resistance mechanisms within the joints can be categorized into strut-and-tie near the loading point and arches in the mid-span of the beam and column. The core area rotates and has a significant lateral displacement towards the impact location, whether it is loaded at the beam end or the column end. As the impact location moves away from the core aera, the impacted column end exhibit more bending failure. With increasing impact velocity, the impact force, reaction force and total energy absorption of the joint increase. The deformation increases, with more damage occurring in the core area and spreading to the beam. In addition, the plastic curvature was used to describe the actual plastic deformation of the joints, whose maximum occurring at the interface where the impacted column end meets the core area. A formula for the equivalent plastic hinge length of RC joints was proposed, taking into account the effects of impact location and impact velocity.

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.

Analytical and experimental investigation on performance of ENTA damper systems for RC moment frame under cyclic loading

An external ENTA damper system or steel frame seismic retrofit approach was investigated for historic reinforced concrete (RC) buildings. RC frame specimens were evaluated under cyclic lateral loading to confirm the external retrofit approach. The investigation involved an examination and comparison of hysteresis-based behaviors, such as strength, ductility, and energy dissipation, with the aim of gaining a deeper comprehension of the distinct impacts of external retrofit systems on seismic performance. The experimental findings indicate that the implementation of the external ENTA damper system and steel frame effectively enhanced the stiffness and durability of vulnerable RC moment frames, while maintaining their deformation capacity. The enhancement of the energy-dissipation capacity was achieved by the use of measures to prevent premature failure of the RC beam-column joints. On the basis of the experimental results, the failure mechanism of the RC frame was discussed, considering the shear connection behavior. In order to further validate the experimental results, analytical modeling of the undamaged specimens was carried out using the LS-DYNA program. The test strengths of the frame specimens were evaluated and compared to the anticipated values derived from a simple plastic mechanism.

Seismic Response of RC Beam-Column Joints Strengthened with FRP ROPES, Using 3D Finite Element: Verification with Real Scale Tests

A 3D-finite element analysis within the numerical program ABAQUS is adopted in order to simulate the seismic behavior of reinforced concrete beam-column joints and beam-column joints strengthened with CFRP ropes. The suitability of the adopted approach is investigated herein. For this purpose, experimental and numerical cyclic tests were performed. The experiments include four reinforced concrete (RC) joints with the same ratio of shear closed-stirrup reinforcement and two different volumetric ratios of longitudinal steel reinforcing bars. Two joints were tested as-built, and the other two were strengthened with CFRP ropes. The ropes were applied as Near Surface Mounted (NSM) reinforcement, forming an X-shape around the joint body and further as flexural reinforcement at the top and bottom of the beam. The purpose of the externally mounted CFRP ropes is to allow the development of higher values of concrete principal stresses inside the joint core, compared with the specimens without ropes, and also to reduce the developing shear deformation in the joint. From the results, it is concluded that X-shaped ropes reduced the shear deformation in the joint body remarkably, especially in high drifts. Further, as a result of the comparisons between the yielded outcome from the attempted nonlinear analysis and the observed response from the tests, it is deduced that the adopted method sufficiently describes the whole behavior of the RC beam-column connections. In particular, comparisons between experimental and numerical results of principal stresses developing in the joint body of all examined specimens, along with similar comparisons of force displacement envelopes and shear deformations of the joint body, confirmed the adequacy of the applied finite element approach for the investigation of the use of CFRP-ropes as an efficient and easy-to-apply strengthening technique. The findings also reveal that the connections that have been strengthened with the FRP ropes demonstrated improved performance, and the crack system preserved its load capacity during the reversal loading tests.

Seismic behaviour of Exterior RC beam-column joints repaired and strengthened using post-tensioned metal straps

Exterior beam-column joints are the most vulnerable part of substandard reinforced concrete (RC) buildings and are often the first to be damaged during earthquakes. This article presents an experimental and numerical investigation into the behaviour of exterior RC beam-column joints repaired and strengthened using Post-Tensioned Metal Straps (PTMS) for active confinement. The study focused on full-scale beam-column joints with an inadequate core zone detailing, thus emulating the deficiencies found in existing substandard RC buildings. Initially, four “bare” joints were subjected to cyclic tests to induce substantial damage within the core zone. Subsequently, the damaged core of the joints was repaired and recast with new concrete, and PTMS were applied to strengthen the joints, followed by another round of cyclic testing. The experimental findings were compared with predictions generated through established models from existing literature. The results revealed that ASCE/SEI 41–17 guidelines accurately predict the shear capacity of the bare joints. It is shown that recasting the core with new concrete significantly increased the joint’s shear capacity by up to 42% compared their bare counterparts. The use of PTMS strengthening further enhanced the capacity by up to 25%. A “scissors model” was employed for numerical simulations of both bare and PTMS-strengthened joints using DRAIN-2DX, which proved effective at predicting their nonlinear load-displacement envelope response. This article contributes towards the development of new cost-effective post-earthquake strengthening techniques for beam-column joints, with the potential to reduce the vulnerability of substandard RC buildings in developing countries.

External Reinforced Concrete Beam-Column Joints: Experimental Investigation, Analytical Prediction, and Simple Design Suggestions

External Reinforced Concrete Beam-Column Joints: Experimental Investigation, Analytical Prediction, and Simple Design Suggestions

Accurate measurement of the bond stress between rebar and concrete in reinforced concrete using FBG sensing technology

Recent earthquakes in several developing countries have shown that reinforced concrete (RC) buildings with improper structural detailing experience severe damage under seismic motions. Using low-quality construction materials such as brick aggregates, resulting in low-strength concrete, significantly impacts the bond between rebar and concrete. Accurate evaluation of the bond performance of such low-strength concrete is one of the key issues for seismic safety assessment of RC buildings, especially in Bangladesh; thus, the bond performance is usually evaluated through laboratory tests. However, conventional measurements of bond stress based on rebar strains measured by electrical resistance strain gauges are likely to negatively impact the bond behavior/performance because of the reduced total contact area between rebar and concrete as well as the changing rebar surface boundary conditions. Under the above social and academic backgrounds, in this study, a new measurement technique that applies fiber Bragg grating (FBG) sensors embedded in optical fiber to rebar strain measurements is developed, and its effectiveness is investigated to realize more accurate measurements of the bond stress between rebar and concrete. Two 70% scaled RC beam-column joint specimens in which the beam rebar was anchored in a straight manner were constructed with identical detailing, except for the beam rebar strain measuring methods. The specimens were then subjected to cyclic lateral loading until failure. By comparing the experimental data acquired by the above two different devices (the FBG sensors and conventional strain gauges), it was found that the experimental bond strength on the beam rebar based on the strain data measured by the FBG sensors was much higher than that from the data measured using conventional strain gauges. Which negatively impacted the test data on the beam-column joint’s capacity in the specimen applied the conventional measuring method, indicating the necessity of the presented method not only for accurate evaluation of the bond stress between rebar and concrete but also for seismic safety assessments of RC buildings.

Seismic Behavior of Demountable Reinforced Concrete (RC) Beam-to-Column Joints with Damage-Control Fuses

In this paper, a new type of assembled RC beam–column joint with a beam-end steel cover-plate connection is proposed to achieve seismic toughness and damage control of the joint. Energy-dissipation plates with different structural forms are proposed, and a series of seismic performance indexes of the joints are calculated and analyzed by using the finite element method. The energy-dissipation plate with an arc notch can reach the yield condition faster, and the ultimate bearing capacity of the joint reaches the maximum. The bending design of energy-dissipation plates is carried out by calculating the demand bending moment, and energy-dissipation plates of different structural forms are simulated and verified. The results show that the proposed design formula can ensure that the bending moment at the beam end still maintains elastic deformation when the energy-dissipation plate yields. The important parameters affecting the bending moment of the weakened part in the middle of the energy-dissipation plate are analyzed. Finally, this paper also analyzes the important parameters affecting the seismic performance of the joints. The results show that the seismic performance of the newly assembled RC beam–column joints proposed in this paper is better than that of cast-in-place joints. Increasing the longitudinal reinforcement ratio appropriately can greatly improve the ultimate bearing capacity and ductility of the joints. Increasing the thickness of the energy-dissipation plate, increasing the strength of the energy-dissipation plate, increasing the axial compression ratio of the column, increasing the strength of the concrete, and increasing the strength of the shear web can improve the ultimate bearing capacity of the joints but also reduce their ductility. Under different axial compression ratios, the strain in the core area of the joints is low, and the compressive damage of the concrete is zero, which verifies the effectiveness of the damage-control design of the proposed model.

Failure mechanism and dynamic response of reinforced concrete joints under impact load acting on beam ends

Beam-column joints are crucial to guarantee the integrity and ductility of reinforced concrete (RC) frame structures. Considering the strain rate effect, the failure mechanism of RC beam-column joints under impact load acting on the beam near the core area of joints was numerically investigated. Based on the verified numerical model, the differences between failure modes of joints under static and dynamic loads were compared. The deformation and damage of joints were analyzed. In addition, the effects of impact location and impact velocity were discussed. The results show that the joints that meet the shear demand under static load have shear failure under impact load. The impact load causes greater deformation and damage to the joints. The transfer of the shear force and the development of the bending moment is different as the impact location changes, resulting in various dynamic responses. As the impact location approaches the core area, the joint evolves from bending failure to shear failure. The maximum shear force only appears at the impact location when the distance between the impact location and the core area is more than 3.5 times the width of the core area (b). However, the rotation of the joint causes the yield of the stirrup in the core area. It is worth noting that when the distance is less than or equal to 2b, the maximum shear force is transmitted to the core area. With the decrease of the distance, the time history curves of impact force have multiple peaks with increased peak values while the displacement and duration decrease. The damage range of the joint is expanded when impacted with higher velocities. Meanwhile, the magnitude and duration of impact force, as well as displacement are increased. With the decrease of distance or impact velocity, more energy is consumed by concrete rather than rebars due to the smaller rotational deformation of the joint.

Providing a strategy to restore damaged RC beam-column joints with the combination of CFRP and HPFRCC

This paper endeavors to assess the seismic performance of 3D reinforced concrete (RC) beam-column joints situated on the exterior of buildings, but lacking special ductility requirements. The joints have been rehabilitated by High-Performance Fiber-Reinforced Cement Composites (HPFRCC) and retrofitted using Carbon Fiber Reinforced Polymer (CFRP) wrap methodology. In this context, five beam-column joints located on the exterior of a structure have been subjected to cyclic lateral loading, while being scaled down to half their original size. The present study entailed an investigation into the cyclic behavior of three control specimens in its initial stage. Following the initial examination, it was observed that the control specimens, lacking any designated ductility standards, experienced significant core damage. During the subsequent phase of experimentation, the impaired concrete sections of the control samples were remediated through substitution with HPFRCC matter. Additionally, CFRP sheets were implemented for retrofit purposes. The modified specimens were then exposed to comparable cycles of loading. An analysis is conducted on the empirical findings of the specimens under investigation. The findings of this study indicate that the utilization of HPFRCC materials and CFRP sheets in the restoration and enhancement of deteriorated joints has led to a notable enhancement of their strength and displacement capacity. Specifically, this approach has facilitated the creation of a flexural plastic hinge and impeded the occurrence of shear failure within the core, leading to an improved overall behavior of the repaired damaged joint, which is comparable to that of undamaged seismic joints. The application of this technique demonstrates a high degree of efficacy in the rehabilitation and enhancement of edifices with deficient seismic details that have sustained damage subsequent to an earthquake event.

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.

Torsional Modeling of Reinforced Concrete Beam–Column Joint Retrofitted by Aramid Fiber—Experimental and Numerical Analysis

The performance of structural composites during loading has always been a concern for the designers and construction industry since the reinforced concrete structure was discovered. In this study, lateral load–displacement behavior of beam–column joints wrapped with aramid fiber is evaluated using both experimental and numerical analysis subjected to torsional moment (beam-eccentric loading). Three categories of reinforcement concepts are adopted for the preparation of the beam–column joints, where members are wrapped with aramid fiber at the joints, and others are not fortified with aramid fibers. Prior to testing, the structural composites are cured for maximum 28 days into water. The beam–column joints are subjected to lateral load at a point near the column end of the beam–column connection, and the corresponding deflections are measured until the member fails. Based on the test results, ductility and energy absorption capacity are evaluated. The findings of the numerical investigation of beam–column joint show there is not much variation in the experimental and numerical analysis; it is clearly found that aramid fiber wrapping provided large rigidity in the joint, and it is also prolonged the final failure of the joints. This study shows that in addition to the conventional reinforcement, providing the hanger reinforcement and the diagonal reinforcement improves the rigidity of the beam–column joints during severe loadings, as this study described.

Seismic performance evaluation and strengthening of RC beam-column joints adopted in Nepal

Beam-column joints are a crucial component of the reinforced concrete (RC) moment-resisting frame (MRF) as the behavior of the joints significantly influences the strength and ductility of the overall framing system. This study is performed to explore the seismic behavior of interior and exterior beam-column joints designed based on NBC 205:1994 (NBC1), NBC 205:2012 (NBC2), and the revised seismic design standard NBC 105:2020, defined as a well-designed joint (NBC3) which are the building standards used for designing low-rise RC buildings in Nepal and compares the results from the numerical simulations quantitatively. Three possible retrofit options (CFRP, steel and RC jacketing) were explored. Finally, retrofitting options were suggested for the joint of NBC1 and NBC2 with consideration of a well-designed joint (NBC3) as the currently required capacity of the joint based on the demand of revised seismic design standard NBC 105:2020.

Experimental cyclic response of heat-damaged RC beam-column joints strengthened with CFRP strings

Reinforced concrete (RC) buildings may be subjected to high temperatures during their service life, which causes significant deterioration in the mechanical and physical characteristics of several structural members (i.e., joints), making it necessary to upgrade their overall performance to withstand the additional stresses. Therefore, the effectiveness of carbon fiber reinforced polymer (CFRP) strings used as near-surface mounted (NSM) reinforcement in restoring the structural performance and minimizing the failure mechanism of heat-damaged RC beam-column (B-C) joints is examined in this study through experimental work. The experimental research consists of twelve RC beam-column joint sub-assemblages divided into three groups: four joints were kept at room temperature, four were heated to 400 °C, and four were heated to 600 °C. Three joints from each batch were tested under a quasi-static cyclic load after adding one, two, or three strings of CFRP reinforcement. The cyclic response of joints was evaluated by determining the ultimate load capacity in pulling and pushing and its corresponding lateral displacement, energy dissipation, displacement ductility, and stiffness degradation. The experimental results showed that the upgraded joints with CFRP strings experienced a significant increase in energy dissipation, initial stiffness, displacement ductility, and ultimate load capacity in pulling and pushing over reference joints.