RC Connections Research Articles

Rebar-concrete bond behaviour in grouted sleeve connectors

Grouted sleeve connectors (GSCs) are commonly used in precast/prefabricated concrete structures. To ensure force transfer, connectors tend to be overdesigned, and this has a detrimental effect on the plastic deformation capacity of the connection region and the displacement ductility of the precast structure. To enable improvements in the design of precast RC connections involving GSCs, an accurate analysis of the bond behaviour of GSCs is required. This paper presents an analytical model for the local bond behaviour under the particular sleeve confinement. A representative cross-section of a GSC is treated as interconnected hollow circular tubes. Three stages are identified according to the state of the grout, namely elastic, partially cracked, and fully cracked. The confining pressure is formulated within the cross-section, and the bond stress is subsequently evaluated by a modified bond stress-slip law accounting explicitly for the confinement. Compared with traditional bond-slip laws, the model is capable of representing the evolution of confining stress in the loading process. Comparison with relevant experimental results show good agreement. A subsequent parametric study demonstrates the trends of influence of various parameters, and notably, within allowable limits, a reduction of the grout thickness tends to increase the bond strength in the connectors.

Presenting a strategy to prevent FRP debonding for strengthening RC connections

Defects in the beam-column connections of old RC frames significantly increase the vulnerability of exterior connections in an earthquake. One of the methods for improving the performance of defective connections in the previous studies was to use FRP. The noteworthy point in the past studies is the focus on 2D connections without considering the lateral beams and slabs. These elements in 3D connections simultaneously increase the confinement and increase the shear demand in the connection core. Therefore, considering the limited studies about the improvement of 3D weak connections using FRP, the aim of this study is to improve defective connections. On the other hand, the main challenge in using FRP is debonding. In this paper, a plan for the installation sequence of FRP sheets on 3D geometric connections presented, while avoiding the creation of holes or grooves on the concrete surface. For this purpose, five experimental specimens, including one connection with seismic details, two connections with defects, and two strengthened connections are tested in the laboratory under cyclic loading. The results show improvement in the behavior of the strengthened connections, moreover, FRP debonding was not observed.

Cyclic behavior of non-seismically detailed full-scale exterior beam-column subassemblies with setback in columns

Discontinuity in cross-section depth may exist in RC columns. If present in a non-seismically detailed frame, such a discontinuity (termed herein setback in columns, SBC) may trigger unexpected seismic damage. However, this typology of irregularity within connections has long been overlooked. This paper offers initial experimental evidence showing how vulnerable these irregular, non-conforming RC connections would be under earthquake-like loading. Four full-scale exterior beam-column subassemblies with SBC were constructed and cyclically tested, including three “as-built” units and a control one improved with seismic detailing. Results indicated that all the units succumbed to joint shear failure, and exhibited distinctly asymmetric responses in different loading directions. Particularly, when subjected to a positive (sagging) beam moment, the “as-built” units displayed a very steep joint cracking pattern and, hence, remarkably lower peak loads as compared to the case where a negative (hogging) beam moment was applied. Possible causes inducing such a salient failure mode are discussed, and a detailed account of the cyclic behaviors of the four units presented. Moreover, based on the concept of wide and narrow joint panels, existing major code provisions and analytical procedures for regular connections were adapted to evaluate the joint shear capacity of the current irregular subassemblies. A practical modeling approach was also developed through ABAQUS user-defined subroutine for capturing the observed global hysteretic responses. Overall, the outcomes of this study highlight the vulnerability of non-seismically detailed exterior connections with SBC, and provide a reference for future related research.

Performance of new CFST square column-to-foundation connections for cyclic loads

Although the structural behavior of circular Concrete Filled Steel Tube (CFST) columns-to-foundation connections has been studied substantially, there are limited studies on connections for square CFST columns, which are more in demand in high-rise buildings. The acceptable behavior of these columns for their use in high-rise buildings require sufficient stiffness for controlling the drift, safe and efficient transfer of imposed loads, and adequate ductility for withstanding earthquake loads. In the present study, authors aim to develop two such column-to-foundation connections for the square CFST columns. The performance of the two square CFST column-to-foundation connections was assessed experimentally as well as numerically under cyclic lateral loading. The comparison of the connections was made with one of the prevalent square CFST column-to-foundation connections and a conventional RC connection to study the inelastic deformation capacity of connections under cyclic loads. The test results illustrate the efficient and improved behavior of the suggested connections. Numerical models were also developed, and the experimentally obtained results were predicted and compared. The developed numerical models were capable of predicting the experimental response of connections to large drift levels with acceptable accuracy. The numerical results were reasonably close to the experimental response.

Seismic performance of a novel self-sustaining beam-column connection for precast concrete moment-resisting frames

In this paper, a novel prefabricated reinforced concrete (PC) self-sustaining beam-column connection for moment-resisting frames was developed to achieve the targets of short erection time, high construction efficiency, low-cost and satisfactory seismic performance. The connection design eliminates the need of temporary supports for the PC beams and slabs during the assembly process in site, and reduces the amount of lateral supports for PC multi-storey columns and formwork for cast-in-place concrete. As the designed thickness of PC U-shells at the beam ends was about 1/3 of the beam width, there could be a marked effect on the achieved integrity of such connections, especially under seismic loading. To investigate the seismic performance of this PC connection, five large-scale PC self-sustaining beam-column connections specimens and one reference conventional RC connection were designed and tested under reverse cyclic loading. The test parameters included the length and area of the flexural reinforcing bars placed at the bottom of PC U-shells, and the anchorage measures (stirrups) inside the PC U-shell. The five precast specimens exhibited similar crack distributions and failure patterns due to the gap-opening between the PC beams and column surface, which was attributed to the reduced effective width and depth of beam cross-section. The test results showed that the use of longer flexural reinforcing bars had little influence on the load-carrying capacity, but contributed to the initial stiffness and energy dissipation capacity. The load-carrying capacity increased by 24% when the area of flexural reinforcing bars increased by 50% in the U-shell region. The incorporation of stirrups in the overlapping region of beam flexural reinforcing bars and longitudinal rebars improved their bond-slip behaviour in specimen PC-S. Compared with specimen PC-C, the energy dissipation capacity of specimen PC-S was improved by 16.5%. Finally, the failure pattern and load-carrying capacity of the PC specimens were analysed and discussed using a simplified mechanical model.

Seismic retrofitting of severely damaged RC connections made with recycled concrete using CFRP sheets

An experimental and numerical program is carried out in this research to investigate the influence of CFRP sheets on the cyclic behavior of unconfined connections made with recycled concrete. Cement is partially replaced by silica fume, iron filling and pulverised fuel ash using two different percentages: 15% and 20%. Each specimen is partially loaded at the first stage and then specimens are repaired using CFRP sheets. The repaired specimens are then laterally loaded until failure. In addition, a finite element model is built in ABAQUS and verified using the experimental results. The experimental results have shown that the repaired specimens have regained almost double the capacity of the un-repaired specimens and hence the adopted repair configuration is recommended for retrofitting seismically vulnerable RC connections. Increasing cement replacement percentage by silica fume, fuel ash or iron filling from 15% to 20% has reduced joint carrying capacity and weakened the joint. It is recommended using 15% pulverised fuel ash or silica fume as cement partial replacement to enhance the strength and ultimate drift of beam-column joints under cyclic loading. Iron filling concrete is also recommended but the enhancement is relatively less than that found with pulverised fuel ash concrete and silica fume concrete.

Re-evaluation of existing tests on RC connections using post-installed reinforcing bars

Post installed rebars offer a convenient and robust solution for rehabilitation, retrofit and strengthening of reinforced concrete structures, whenever additional structural members might be required. Although certain post installed rebars have significantly higher bond strength as compared to cast in systems, presently they are considered equivalent to corresponding cast-in systems. The present paper focuses on the comparison of behavior of RC connection using post-installed and cast-in reinforcement. The experimental investigations carried out so far have been based on a direct comparison between cast-in and post-installed reinforcing bars used to form structural connections such as column-foundation or beam-column connections. Such a comparison can be significantly affected by the assumptions made in the design and the boundary conditions used in the experiments. In this work, the existing experiments are critically reviewed and numerically re-evaluated to bring out the findings in a more rational framework. To this end, 3D finite element simulations are carried out where first the numerical model is validated against the experimental results and then the validated model is used to explore the influence of the assumptions and boundary conditions used in the tests on the original conclusions. It has been shown that the conclusions and inferences by the researchers who performed the tests were suitable for the test setup and test conditions used by them but may not be generalized to every case.

Cyclic Behavior of RC Beam-Column Joints Made with Sustainable Concrete

This research investigates the behavior of unconfined RC connections made with sustainable concrete under cyclic loading. Different types of sustainable concrete are used where cement is partially replaced by silica-fume, iron filing and pulverized fuel ash using two different percentages 15% and 20%. The numerical finite element model, which simulates the cyclic behavior of RC connections, is verified using the experimental results and can reasonably predict joint response and its damage pattern. Test results have shown that beam-column connections made with iron filings-concrete are significantly damaged when exposed to cyclic loading while others made with silica fume concrete and fuel ash-concrete are stronger and experience minor cracks. Increasing cement replacement percentage from 15% to 20% for any waste material reduces sustainable concrete compressive strength and weakens the joint.

Experimental assessment of fire-exposed RC beam-column connections with varying reinforcement development lengths subjected to column removal

This paper describes an experimental investigation into the behaviour of RC beam-column connections under a column removal scenario induced by fire. A purpose-built hybrid heating furnace is employed to carry out the fire tests on five RC connections with varying reinforcement development lengths, with and without cooling effects. The thermal response of the RC connection specimens, including the temperature field and the axial force-furnace temperature curves, is firstly described. Subsequently, push-down tests are carried out on the beam-column connections, and the horizontal support reactions are closely monitored using a specially-designed sensor system. Based on the experimental results, the joint vertical load-displacement and bending moment-rotation relationships are presented, together with an account of the failure modes observed. The mechanical behaviour is discussed in detail, including the tying and rotation capacity provided by the connection specimens, with emphasis on the effect of the reinforcement development length as well as the heating regime. The experimental capacity interaction curves are also compared with the theoretical prediction for ambient condition, and employed to carry out a detailed examination of the underlying failure mechanisms. Finally, the findings are used to provide practical recommendations for enhancing the structural robustness of structural configurations of the form considered in this study.

PERFORMANCE OF REINFORCED CONCRETE BEAM COLUMN CONNECTIONS EXPOSED TO FIRE UNDER CYCLIC LOADING

ABSTRACTFire could dramatically reduces the characteristic strength of the reinforced concrete elements [1,2]. A lot of efforts has been devoted to study the effect of fire on the reinforced concrete elements during past decades. These previous studies focused on evaluating fire resistance for concrete elements, [3] and provide some recommendations of structural fire-resistance [4] such as cover thickness, reinforcement ratio and coating materials. A lot of research focused on RC columns [5,6,7], beams [8] and slabs [9] which has been exposed to fire, where less efforts aimed to study the effect of fire on RC beam column connections [10,11,12].The main objective of the present work is to study the structural behavior of reinforced concrete beam-column connections exposed to fire under cyclic loading [13,14] and to evaluate the reduction in concrete strength during fire process. To achieve these goals, this research consists of two parts, the first part is experimental program, and the second is the theoretical analysis. The experimental program include 12 one- third scale specimens half of them casted from normal concrete and other half from SCC. The effect of some parameters on the connection's behavior like reinforcement ratio, type of concrete (normal or self-compacted concrete) [15] and exposure fire time were studied. Experimental program extended also to study heat distribution inside RC beam–column connections by measuring temperature at 65 point Distributors at 5 sections all over the RC connection. Theoretical analysis was done by using finite element program (ANSYS12.1) and a comparison between experimental program and theoretical analysis was done SCC showed good properties in hardening state when exposed to high temperature compared with normal concrete [16]. Increasing reinforcement ratio of RC connections increases its fire resistance Good agreement between analytical and experimental work was observed.

Configuration and Size Effects on Bond Stress-Slip and Failure Modes of RC Connections

AbstractBond slip of RC structures occurs wherever primary cracks are present. These cracks usually appear at the beam-column and column-foundation joints, for example, when structures are subjected to seismic loadings. As a result, it is imperative to consider bond slip in performing an accurate evaluation of the response of RC structures. The distributions of stress and strain in an embedded rebar vary depending on the geometry, such as concrete cover and embedment length, which are referred to here as configuration effects. Because bond stress and bond slip can be defined as functions of stress and strain of the embedded rebar, respectively, the relationships of bond stress and bond slip also differ. This, in turn, leads to different crack patterns: splitting cracks, shearing-off failures, or no cracks in the concrete. These differing crack patterns also cause distinct governing failure modes involving splitting failure, shearing-off failure, and bar fracture, respectively. Consequently, the relationsh...

Models for behaviour analysis of monolithic wall and precast or monolithic floor slab connections

This paper presents the comparative analyses of experimental and numerical research results of RC connections between the precast slab and monolithic wall (Type 1), and both monolithic slab and wall elements (Type 2). Type 1 was applied in prefabricated building system developed in Tuzla, Bosnia and Herzegovina, as the most sensitive part of the structure. In order to provide a favourable response of the bearing structure connections under seismic load (Type 1) it is dislocated in the span. In order to compare the behaviour of listed structure connection types, three specimens made of precast slabs and monolithic wall as well as three specimens made of monolithic slabs and walls were tested under quasi-static load. Thus, the mathematical models are proposed in order to analyse both types of connection, based on the exact method of displacement and FEM. Furthermore, the stiffness matrix is modified by introducing the stiffness parameter (semi-rigid) connection. The approximate Strut and Tie model is proposed according to the stress field analysis obtained by FEM.

RC造柱梁部分架構の柱曲げ強度増大を図るための外付け補強法とその実験検討

In this paper, new seismic retrofitting methods by external reinforcement is proposed. Main objectives of this technique is to increase the flexural strength of existing RC columns detailing weak column and strong beam joint type in RC connection and to ensure the seismic performance subjected to large deformation under cyclic lateral loads. The proposed methods herein is the combined the steel plates and continuous fiber sheets with grout mortar layer. Furthermore, additional external longitudinal bar through the beam-column joint which is encased in mortar layer. These members are connected with the existing RC columns together with bonded anchor.In order to investigate the increasing flexural strength, the lateral cyclic loading tests were conducted for the five specimens with two different joint type of exterior and interior columns which the different mechanical properties of beam were planned, and respectively, compared them with the original RC frame. Test results shows that this method was very effective in enhancing the flexural strength of column. But its strength for all specimens of interior columns were almost similar, because hysteresis characteristics were hardly affected by mechanical properties of beam.

Study on Shear Strength of SRC Beam-Column Connections

This paper proposes a new model to predict the horizontal shear strength of SRC, especially SRHC/SRUHC, beam-column connections on the basics of previous models developed for RC and RCS connections. The model divides the contributions to shear strength of a connection into three parts, the contribution of steel web, inner strut and outer strut. The predicted values coincide with experiment results very well, which proves that the model is suitable for SRC connections. It is found that the inner strut plays a dominant role in the strength mechanisms in SRC joints, so adequate web stiffeners in joints should be provided. It is the stirrups in a joint, rather than concrete strut, that usually determine how much contribution the outer strut mechanism provides to the joint.

Mesoscale models of ac conductivity in composite polymeric electrolytes

The presented paper shows a possibility of creation of a useful and efficient model of conductivity for composite polymeric electrolytes. It deals with a more elaborate and versatile mesoscale approach (in comparison to the already applied Effective Medium Theory approach for which each new composite microstructure forces a new mixing rule development) based on the Random Resistor Network method. The introduced model can predict the ac conductivity values for a whole spectrum of frequencies for different composites with a wide range of possible microstructures. It is based on complex current iteration procedure for a randomly generated three-dimensional network of impedances constituting a digital image of the electric properties of the virtual sample studied. A spatial unit of the material present in the sample is defined as a parallel RC connection.

Hysteretic behavior of exterior post-tensioned flat plate connections

The purpose of this study is to evaluate hysteretic behavior of exterior post-tensioned flat plate slab–column connections (PT connections) designed to resist only gravity loads. For this purpose, experimental studies were conducted using three approximately two-thirds-scale test specimens; two exterior PT connection specimens having different tendon layouts and one exterior reinforced concrete flat plate slab–column connection (RC connection) specimen. Quasi-static cyclic loading was applied to the specimen with a constant gravity load. All specimens had bottom bonded reinforcement around the column as required by ACI 352.1R-89. This study collects previous test results to draw a general conclusion for the hysteretic behavior of the PT exterior connections. This study observed that the tendon layout influenced the hysteretic behavior of PT exterior connections, which means that lateral drift capacity, dissipated energy, failure mechanism, and ductility vary with respect to tendon layouts. Moreover, this study shows that the amount of bottom reinforcement specified by the ACI 352.1R-89 is sufficient for resisting positive moments resulting from moment reversal under the cyclic loading. Shear strength of the test specimens is more accurately predicted by the shear strength equation considering average compressive strength ( f p c ) due to post-tensioning tendons than that without considering f p c .

Dynamic Responses of Flat Plate Systems with Shear Reinforcement

Earthquake-resistant designs often use a lateral-force resisting system to resist the seismic forces and a nonparticipating system to support vertical loads. Use of flat plate floor systems consisting of a conventional or post-tensioned reinforced concrete slab-column system incorporating shear reinforcement within the slab-column connection region has become increasingly popular as a lateral-force resisting system. There is little information available to assess the dynamic responses of such systems, particularly for post-tensioned systems. This article reports on a study that consisted of shaketable tests of a conventional reinforced-concrete slab-column frame (RC specimen) and a post-tensioned slab-column frame (PT specimen). Lateral drift ratios of approximately 3 and 4% were achieved during testing for the RC and PT system frames, respectively, with relatively little loss of lateral load capacity. The damage observed at the RC connections was more significant and more widely distributed than those observed for the PT specimen. The test results indicate that use of slab shear reinforcement substantially reduces the extent of damage and prevents the dropping of the slab observed in reinforced concrete connections where shear reinforcement is not provided. The authors note that this may be an important factor in post-earthquake repairs.

Experimental study on the dynamic response of gravity-designed reinforced concrete connections

This paper reports an experimental programme aiming to shed some light on the response of non-seismic RC beam–column joints to excitations of different frequencies. The RC connections tested were designed only for gravity loads, thus rendering the joint cores weaker than the adjoining members when subjected to a lateral load. Altogether, six tests were conducted on full-scale specimens, which were subjected to reversed cyclic displacements applied at different speeds varying from slow quasi-static loading to high-speed dynamic loading as fast as 20 Hz. Although all specimens as expected suffered joint shear failure, the maximum joint shear stresses observed in the tested specimens, despite lacking transverse hoops inside the joint cores, were more than the horizontal shear stresses allowed in ductile RC joints with the same grade of concrete according to the existing seismic design codes. The damage patterns and failures of the specimens showed a better correlation with the residual storey shear stiffness than with the loss of storey shear strength during the repeated cycles. By analysing the test results, this paper also discusses how an inadvertent inertial force develops during high-speed displacement reversals.

靭性型接合金物を用いた鉄骨架構の実大実験

When steel or RC connections are designed, these connections had to have a sufficient strength and stiffness so far. However, many response control structures are developed recently. It is mean that specific member and the control device absorb the seismic energy and main members are not damaged much. For the steel structures, semi-rigid connections as easy connections are studied by many researchers. This is expected that seismic energy is absorbed by the earlier yield of angle members and possibility that seismic response of steel frame are reduced. However many reports are only presented concerned with member test and plane frame analysis so far and these semi-rigid connections are not apply actual design yet. This paper present that ductile-type connection has a large plastic deformation capacity. Ordinary steel and low yield steel are used by angle connection.

High‐Strength RC Connections Subjected to Inelastic Cyclic Loading

The experimental results from reversed cyclic loading tests on 12 corner-reinforced concrete beam-column subassemblies are presented. The primary variables were the concrete compressive strength, ranging between 8.1 and 13.6 ksi (55.8 and 93.8 MPa); the joint shear stress, which was either 1,100 or 1,400 psi (7.6 or 9.7 MPa); and the joint transverse reinforcement, which consisted of either four or six sets of hoops and ties placed within the depth of the joint. The provided joint reinforcement was lower than that obtained by inserting the higher concrete-compressive-strength values into the current recommendations. A small axial load was applied to the column portion of the subassembly and held constant during the test. The free end of the beam was subjected to cyclic displacements representing a wide range from elastic to severe inelastic loading. It is concluded that in spite of the brittle nature of plain high-strength concrete, properly detailed frames constructed with high-strength concrete, exhibit ductile hysteretic response similar to those for ordinary-strength connections.