Joint Shear Failure Research Articles

3D beam–column corner joints retrofitted with X-shaped FRP sheets attached via the EBROG technique

Beam-column joints are fundamental structural components of a stable structure. Corner joints are more likely to fail due to their small size and inferior confinement. This calls for three-dimensional (3D) strengthening of corner joints with fiber reinforced polymer (FRP) sheets to improve the load carrying capacity and ductility of beam–column connections. The present study investigates the shear strength of non-seismic 3D corner joints lacking adequate transverse reinforcement in their joint panel but strengthened with carbon fiber reinforced polymer (CFRP) sheets. The externally bonded reinforcement on groove (EBROG) technique is used in the joints to delay the debonding of CFRP sheets off the concrete substrate. Maximum expected moment resistance of a beam/connection is typically obtained at the column interface. This, therefore, requires sufficient bonding strength provisioned between the FRP sheet and the concrete substrate over an acceptable range of the bonding length. Practical limitations, however, do not allow for enough length of the column interface to be strengthened. A new anchorage system of FRP fans is, therefore, provisioned to attain the required bonding strength. The results are verified by studying the hysteretic response of the 3D corner joints strengthened via the proposed EBROG method and with the provisioned FRP fans. For this purpose, six half-scale 3D joint specimens were constructed and subjected to relevant tests. The specimens included a control, one reference specimen that satisfied the current building codes, and four FRP-retrofitted specimens with X-shaped strengthening patterns. The results revealed that the proposed strengthening pattern prevented joint shear failure. Furthermore, the proposed EBROG technique outperformed the externally bonded reinforcement (EBR) techniques as evidenced by the higher values of maximum load-carrying capacity and ductility recorded by the joint while CFRP debonding off the concrete substrate was also delayed. Maximum loading led to a drift ratio of 4.5%, indicating that application of the EBROG technique combined with FRP fans inhibited load reductions throughout the test period.

Shear Rate Effects on the Post-peak Shear Behaviour and Acoustic Emission Characteristics of Artificially Split Granite Joints

Rock joints may be sheared at different rates under quasi-static or dynamic loading. Understanding the mechanical response of rock joints at different shear rates is of great importance for the mitigation of dynamic geo-hazards such as earthquakes, fault slip rockbursts and landslides. In this study, direct shear tests at various shear rates (0.001–0.1 mm/s) under different normal stresses (3–40 MPa) are conducted on split granite joints, and the influences of shear rates on the shear strength, post-peak shear behaviour and acoustic emission (AE) characteristics are analysed and discussed. The research findings suggest that both peak and residual shear strengths tend to decrease with increasing shear rate. Stick–slip occurs on all the joints, during which stress drop values increase with increasing shear displacement and normal stress. The stress drop magnitudes during stick–slip decrease with shear rate, while the time intervals between stress drops during stick–slip increase with shear rate. Further, the energy rate tends to increase while the AE events decrease with increasing shear rate, which is caused by the time-dependent deformation behaviour. The AE b value decreases linearly with the shear rate on a logarithmic scale, and the influence is more significant under high normal stress conditions. The variations in the b value can reflect the evolution process (first loading at lower and then higher shear rates) of dynamic geo-hazards and can be used as an effective indicator to predict the dynamic shear failure of granite joints in a temporal sequence. The results of this study will encourage better understanding of the rate-dependent shear behaviour of rough granite joints, particularly under high normal stress, and will provide some references for the monitoring and prediction of dynamic geo-hazards with respect to the AE (or micro-seismic) technique.

Experimental Study on Novel Energy‐Dissipating Prefabricated Beam‐Column Joints

A new dapped‐end beam to column connection is designed in this paper. Its assembly connection zone changes from inside the joint to midspan of the beam. The proposed connection can not only provide good structural integrity but also ensure that the plastic hinge moves away from the column edge. The rotational capacity of the plastic hinge determines the internal force redistribution of the joint and the energy dissipation capacity. The high‐strength bolts and steel plates are used to realize connection, further enhancing the rotation of the plastic hinge and minimizing the cast‐in‐place concrete volume. Three full‐scale exterior beam to column joints are casted and then subjected to reversal cyclic loading. The finite element (FE) analyses are carried out to compare with experimental results and study the effect of connection position on the structural behaviours. The obtained results show that the plastic hinges of all three specimens are firstly developed to a distance from the column edge, thus revealing that this kind of joint can achieve beam hinge mechanism and prevent joint shear failure. And the connection position is the most disadvantaged when coinciding with the plastic hinge zone, which would result in the excessive deformation and the early failure of the steel bar anchor system. The new type of joint shows good seismic performance during earthquake if the connection can be properly designed, and thus this kind of structural form can be applied to actual engineering structures in seismic regions.

Seismic performance and prediction equations of sandwich beam-column joints subjected to skew cyclic loads

To study the seismic performance of sandwich beam-column joints constructed with high strength concrete column and normal strength concrete floor system and joint regions, four specimens with different column to beam concrete strength ratios (α) were tested under skew cyclic loads. The performance indices of the specimens including the failure mode, ductility, energy dissipation were compared and analyzed. The results show that the failure modes of the sandwich joints are in form of joint shear failure after yielding of the beam, while the ductility coefficient is found to be greater than 3.0. Compared to the joints with low concrete strength ratios α, the specimen with a high concrete strength ratio α features larger deformation at the joint. Based on the softened strut-and-tie model, a set of shear strength prediction equations for the sandwich beam-column joint, taking into account the effects of the concrete strength ratio α, floor slabs and plastic region of beams, is proposed. Comparison of the present tests against the published literature confirms that the shear strength of the sandwich joints can be well predicted by the proposed model.

Shear Strength of Exterior Wide Beam-Column Joints with Different Beam Reinforcement Ratios

ABSTRACT Full-scale exterior reinforced concrete wide beam-column joints with beam reinforcement ratios of 0.84%, 1.07% and 1.28% were tested under reversed cyclic loading. Beam flexural failure was observed in the specimen with reinforcement ratio of 0.84%, while joint shear failure after beam yielding was detected in other two specimens. Detailed analysis was conducted to evaluate the seismic performance of specimens including hysteretic behavior, cracking patterns, joint shear distortion, strain of the reinforcement and energy dissipation capacity. Accuracies of the joint shear demand and capacity predicted by four seismic codes of practice are assessed by joint shear forces from the experimental results.

Cyclic testing of square tubed-reinforced-concrete column to RC beam joints

This paper aims to investigate the cyclic behavior of a steel and concrete composite joint system connecting reinforced concrete (RC) beams and square tubed-reinforced-concrete (TRC) columns. In the joint system, the discontinuous outer thin-walled steel tube of a TRC column can avoid the direct transfer of an axial load and thus provide better confinement to the core concrete. Six specimens, including five beam-column joints with internal diaphragms in the joint region and one reference specimen with closely spaced stirrups in the joint region, were tested up to a large drift ratio of 6.7% under combined constant axial compression and cyclic lateral loading at the TRC column tip. All the six specimens were designed according to the strong-column/weak-beam concept. The cross-sectional dimensions of the TRC columns were 300 mm × 300 mm that is about a 1/2 model of the prototype column. The dimensions of the beam cross section were 200 mm × 350 mm and high-strength steel reinforcing bars (600 MPa) were used in the RC beams. Three important parameters were studied, including the width-to-thickness ratio of the square joint tube, the thickness of the internal diaphragm, and the level of axial load applied to the column. Two types of failure modes, i.e., joint shear failure and beam flexural failure with bond failure, were observed. The both failure modes were ductile. The analysis of ductility, energy dissipation capacity and bond demonstrated that the joint system has good seismic performance.

Mechanism of shear deformation, failure and energy dissipation of artificial rock joint in terms of physical and numerical consideration

The physical and mechanical change processes of rock are closely related to energy transformation, and its deformation and failure is an instability phenomena driven by energy exchange. This study investigated mechanism of shear deformation, failure and energy dissipation of joint using both physical and numerical direct shear tests under constant normal load (CNL) condition. Three kinds of joint surface were artificially prepared. An acoustic emission system was employed to monitor acoustic emission in physical test, and rupture frequency was recorded in numerical test to represent micro-crack development. By research of numerical micro-crack development accompanied with physical acoustic emission results, mechanism of shear deformation and failure of joints were illustrated schematically. By definition of dissipation energy, captured using the particle flow code (PFC2D), energy releasing and dissipation were discussed with microscopic damage evolution of joints. Results showed that joints under shearing present a dissipation trend of four stages including a slow rise stage, a rapid rise stage, a shock rise stage and a rapid decline stage.

Effect of corrosion damage on seismic behaviour of existing reinforced concrete beam-column sub-assemblages

Environmental degradation of reinforced concrete (RC) in the form of reinforcement corrosion poses greater threat to both strength and serviceability of existing RC structures. If this corrosion damaged structures are situated in the zones of higher seismicity, there would be a great threat to their safety under earthquakes and hence it is extremely important to understand their seismic performance in order to formulate suitable remedial measures. In view of this, in present study, an exterior beam-column sub-assemblage of a residential building is taken up and is designed for two levels, namely (i) designed for seismic load without ductility detailing (non-ductile specimen), and (ii) designed for lower seismic load and with ductility detailing (ductile specimen) to represent the existing RC structures of different design evolutions. Two specimens are cast for each level of design and one specimen from each of these two categories is subjected to accelerated corrosion. The experimental investigations are carried out on the uncorroded and corroded beam-column sub-assemblages under reverse cyclic loading. Corrosion of reinforcement changed the damage progression from joint shear failure to longitudinal and splitting cracking along the reinforcement bars followed by fracturing of corroded reinforcement bars. The corroded specimens showed poor hysteretic performance in the form of huge in-cyclic strength degradation. Furthermore, the cumulative energy dissipated by corroded non-ductile and corroded ductile specimens are respectively only 0.4 times and one-seventh of that of the corresponding uncorroded specimens. Thus, the seismic performance of corrosion affected existing structure is an alarming issue and warrants immediate retrofit intervention.

3D Particle-Based DEM Investigation into the Shear Behaviour of Incipient Rock Joints with Various Geometries of Rock Bridges

A 3D particle-based DEM model was established taking into account the geometries of rock bridges. The model was used to investigate the shear behaviour of incipient rock joints. Fifty-seven direct shear tests were conducted under constant normal load (CNL) boundary conditions using the established model, in which rock bridges with 19 different geometries and incipient joints with various areal persistence (between 0.2 and 0.96) were involved. Our results show that, for the cases having a single rock bridge, cracks often initiated around the edges of the rock bridges and coalesced first in the middle of the rock bridge areas. While for other cases containing multiple rock bridges, cracks initially appeared at the connection points (located in the middle of the joint planes) of the rock bridges and then propagated to the edges. High crack initiation stresses were measured, which were often more than 60% of the shear strength of the tested incipient rock joints. Sudden failures of the rock bridges subjected to shearing were observed, accompanying dramatic increases in the number of cracks. Another important conclusion derived from this research is that both joint areal persistence and rock bridge geometry played significant roles in the shear failure of the simulated Horton Formation Siltstone joints. The present study has shown that shear strength increased gradually when joint areal persistence was decreased. Interestingly, different shear strength values were measured for rock joints with the same areal persistence (e.g. K = 0.5). Shear velocity was also found to have a significant influence on the shear characteristics of the Horton Formation Siltstone joints. A higher shear strength was measured when the shearing velocity was increased from 0.01 to 1 m/s.

Mechanical performance of space sandwich joints under bidirectional cyclic loading

In high-rise buildings with high-strength concrete column and normal-strength concrete floor, the beams and slabs are usually cast in a continuous fashion through the beam–column joint to simplify construction, and this results in the lower strength concrete at the beam–column joint core (sandwich joint). It will influence the capacity of the joint. In this article, three groups of three-dimensional specimens consisting of sandwich joint specimens and corresponding traditional joint specimens were tested under bidirectional reversed cyclic loads to investigate seismic performance, including the failure mode, ductility, energy dissipation, and deformation. The test results show that the beam–column joint core can be cast with normal-strength concrete when the column concrete strength is less than 1.5 times that of the beam. However, when the ratio exceeds 1.5, the failure mode of the joint may change from beams flexural failure to joint shear failure and additional strengthening measures should be taken. Finally, the formula for calculating shear capacity of the three-dimensional sandwich joints is presented, and the predictions are compared to the experimental results.

Effect of joint angle in coal on failure mechanical behaviour of roof rock–coal combined body

Geological dynamic hazards during coal mining can be caused by the structural instability of a composite system of roof rock and coal layers, and joints in coal play a vital role in this structural instability. In this paper, uniaxial compression simulation tests on roof rock–coal combined bodies with a single joint at different angles in coal were conducted using PFC2D software. In particular, the effects of joint angle on the uniaxial compressive strength (UCS), acoustic emission (AE) and failure characteristics in a combined body were analysed. The results show the following. (1) The joint in coal causes a decrease in the UCS and crack initiation stress, and has promoting effects on the AE occurrence of the combined body. With an increase in the included angle α between the loading direction and joint plane direction, the UCS, AE occurrence time and crack initiation stress first decrease and then increase, presenting ‘V-shaped’ curves. The UCS and crack initiation stress at α of 45° are the lowest, and the AE occurrence time at α of 30° is the earliest. (2) The AE characteristics at α of 0°, 75° and 90° are basically consistent with those of the intact combined body, whereas at α between 15° and 60°, the intensity and frequency of AE events in the macro-failure stage are reduced by the joint in coal. (3) The failure of the combined body mainly occurs within the coal, whereas no apparent failure is observed for the roof rock. Only when α is between 15° and 45°, the roof rock near the bedding plane is destroyed by crack propagation in the coal. (4) Three typical failure modes (i.e. V-shaped shear failure cutting across the coal, shear failure partly along the joint plane direction and shear failure along the joint plane direction) are found for combined bodies with a single joint in coal.

Soft Computing-based Approach on Capacity Prediction of FRP Strengthened RC Joints

Shear failure of the RC beam-column joints is a brittle failure which has no priorwarning and can induce tremendous damages because of collapse of column and joint before theconnected beam. This paper is focused on one particular method of strengthening the RC joints,that is, the use of FRP composites as confining element. The results of previous studies have shownthat strengthening the RC beam-column joints with FRP composites can improve their shearcapacity. In this study, the data collected from the existing standards and studies regarding the FRp strengthened RC joints were used to develop an artificial neural network model for predicting theshear strength contribution of FRP jacket. The developed model was then used to evaluate the roleof different parameters on this contribution, and finally derive a formula for contribution of FRp jacket to the shear strength of the RC beam-column joints.

Experimental Behavior of Square Tubed-Reinforced-Concrete Column to RC Beam Joints with Internal Diaphragms under Cyclic Loading

This paper describes experimental investigation on three square tubed-reinforced-concrete (TRC) column to reinforced concrete (RC) beam joints with internal diaphragms subjected to cyclic loading, where width-to-thickness ratios of the steel tube in the panel zone and levels of axial load were selected as test parameters. Two failure modes, namely, joint shear failure and beam failure + bond failure were observed in the current test. Test results showed that the proposed composite joints exhibited favorable seismic performance.

Geophysical Signatures of Shear‐Induced Damage and Frictional Processes on Rock Joints

AbstractIn this study, ultrasonic waves recorded during direct shear experiments on rock joints were employed to investigate the shear failure processes. Three types of wave attributes were systematically observed prior to the shear failure of the rock joints: (a) maximum in the amplitude of the transmitted wave, (b) maximum in the dominant frequency of the transmitted wave, and (c) maximum in the velocity of the wave. Different processes occurring during both frictional sliding and stick‐slip oscillations were identified in this study: (a) interseismic phase and (b) preseismic phase. The interseismic phase is associated with elastic loading, very small local slip rate, and increasing ultrasonic transmission along the contact surfaces. The rock joint is considered locked, and the increase in ultrasonic transmission represents an increase in the real (true) area of contact because of interlocking and contact aging. The start of the preseismic phase is marked by the onset of precursors for different regions of the rock joint. Following the interseismic and preseismic phases, coseismic phase occurs. The coseismic phase begins with the reduction in the applied shear stress and is associated with an abrupt increase in the local slip rate. The reductions in transmitted amplitude, wave velocity, and dominant frequency all indicate the preseismic phase when the asperity contacts begin to fail before macroscopic frictional sliding. The observation of the preseismic phase in both the loading phase leading to stable sliding and stick‐slip failure modes suggests that microphysical processes of fault weakening may share key features for these two failure modes.

Classification of failure mode and prediction of shear strength for reinforced concrete beam-column joints using machine learning techniques

Beam-column joints are one of critical components that control the oveerall performance of reinforced concrete building frames under seismic loadings. To identify the response mechanism, including the classification of failure mode and the prediction of associated shear strength, of beam-column joints, this paper introduces the application of machine learning techniques. The efficiency of various machine learning techniques is evaluated using extensive experimental data from 536 experimental tests, all of which exhibited either non-ductile joint shear failure prior to beam yielding or ductile joint shear failure after beam yielding. It has been seen from the comparison that lasso regression has a better efficiency and reasonable accuracy in the classification and prediction. The suggested formulations as a function of influential input variables can be easily used by structural engineers to provide an optimal rehabilitation strategy for existing buildings and to design new structures.

Effect of post-tensioned spandrel beam on wide beam–column connections

Reinforced concrete wide beam flooring systems are used increasingly in seismic regions, despite little information being available on their performance under seismic actions. This paper presents the results of a set of experiments performed on two full-scale exterior wide beam–column connections, which were tested under lateral quasi-static reversed cyclic loading. The control specimen had both longitudinal and transverse reinforcement in its spandrel beam. The second specimen was built in the same way but provided with post-tensioning force. The specimens were tested under a combination of axial force and quasi-static reversed cyclic lateral displacements. Results show that the control specimen reached its expected beam flexural capacity at a drift ratio of 3%, and then the joint shear failure and spandrel beam torsional failure controlled the behaviour. The seismic performance of the specimen with a post-tensioned spandrel beam was considerably improved compared to the control specimens. The specimen sustained a 5% drift without any significant drop in strength. No signs of joint shear failure or spandrel beam torsional failure were observed in this specimen. The results indicate that adopting a post-tensioned spandrel beam can not only prevent torsional failure, but can also improve the joint shear capacity and displacement ductility.

NONLINEAR FINITE ELEMENT ANALYSIS OF NONSEISMICALLY DETAILED INTERIOR RC BEAM-COLUMN CONNECTION UNDER REVERSED CYCLIC LOAD

This paper presents a nonlinear finite element analysis of non-seismically detailed RC beam column connections under reversed cyclic load. The test of half-scale nonductile reinforced concrete beam-column joints was conducted. The tested specimens represented those of the actual mid-rise reinforced concrete frame buildings designed according to the non-seismic provisions of the ACI building code. The test results show that specimens representing small and medium column tributary area failed in brittle joint shear while specimen representing large column tributary area failed by ductile flexure though no ductile reinforcement details were provided. The nonlinear finite element analysis was applied to simulate the behavior of the specimens. The finite element analysis employs the smeared crack approach for modeling beam, column and joint, and employs the discrete crack approach for modeling the interface between beam and joint face. The nonlinear constitutive models of reinforced concrete elements consist of coupled tension-compression model to model normal force orthogonal and parallel to the crack and shear transfer model to capture the shear sliding mechanism. The FEM shows good comparison with test results in terms of load-displacement relations, hysteretic loops, cracking process and the failure mode of the tested specimens. The finite element analysis clarifies that the joint shear failure was caused by the collapse of principal diagonal concrete strut.

Capacity estimation of FRP strengthened RC beam-column joints using hierarchy of strength assessment

In this paper, the fiber reinforced polymer (FRP) beam-column joint strengthening layout adopted by the researchers at University of Canterbury has been modified for real life structural engineering applications. A simplified analysis and design procedure has been proposed. The reported procedure has been validated against the past experimental work. Additional capacity by the FRP sheets has been assessed using the strength hierarchy assessment method. The local response of as-built and strengthened reinforced concrete beam-column joints has been related to the global base shear demand, resulting in base shear values corresponding to each damage state (e.g. joint shear failure, beam and column plastic hinging). It is showed that the analysis procedure for FRP joint strengthening used together with the strength hierarchy assessment method results in good agreement with the experimental work utilizing similar FRP strengthening layouts.

Seismic retrofit of beam–column joints using prestressing wire

An effective external prestressing technique is presented for the seismic retrofit of reinforced concrete beam–column joints with prestressed high-strength steel wires. This method is to improve the seismic performance and enhance the shear strength of beam–column joints with non-seismic design as well as to overcome the shortcomings of traditional seismic retrofitting techniques for the joints. Six large-scale interior beam–column joints were tested under reversed cyclic loading, where five were strengthened by the proposed technique and one was un-retrofitted as a control specimen. Particular emphasis was placed on the effects of the ratio of prestressed high-strength steel wires, prestressing level of wires and polymer grout layer. Test results showed that the joint shear capacity was increased significantly and cracks in beam–column joints were controlled effectively after retrofitting. All retrofitted joint specimens failed in a ductile mode of beam flexural failure, whereas the control specimen failed in a brittle mode of joint shear failure. In addition, the energy dissipation capacity as well as ductility of the retrofitted joint specimens was enhanced significantly compared with the un-retrofitted specimen.

Finite Element Analysis of Reinforced Concrete Beam-Column Connections with Governing Joint Shear Failure Mode

REINFORCED CONCRETE (RC) BEAM-COLUMN CONNECTIONS ESPECIALLY THOSE WITHOUT TRANSVERSE REINFORCEMENT IN JOINT REGION CAN EXHIBIT BRITTLE BEHAVIOR WHEN INTENSIVE DAMAGE IS CONCENTRATED IN THE JOINT REGION DURING AN EARTHQUAKE EVENT. BRITTLE BEHAVIOR IN THE JOINT REGION CAN COMPROMISE THE DUCTILE DESIGN PHILOSOPHY AND THE EXPECTED OVERALL PERFORMANCE OF STRUCTURE WHEN SUBJECTED TO SEIS-MIC LOADING. CONSIDERING THE IMPORTANCE OF JOINT SHEAR FAILURE INFLUENCES ON STRENGTH, DUCTILITY AND STABILITY OF RC MOMENT RESISTING FRAMES, A FINITE ELEMENT MODELING WHICH FOCUSES ON JOINT SHEAR BEHAVIOR IS PRESENTED IN THIS ARTICLE. NONLINEAR FINITE ELEMENT ANALYSIS (FEA) OF RC BEAM-COLUMN CONNECTIONS IS PERFORMED IN ORDER TO INVESTIGATE THE JOINT SHEAR FAILURE MODE IN TERMS OF JOINT SHEAR CAPACITY, DEFORMATIONS AND CRACKING PATTERN. A 3D FINITE ELEMENT MODEL CAPABLE OF APPROPRIATELY MODELING THE CONCRETE STRESS-STRAIN BEHAVIOR, TENSILE CRACKING AND COMPRESSIVE DAMAGE OF CONCRETE AND INDIRECT MODELING OF STEEL-CONCRETE BOND IS USED. IN ORDER TO DEFINE NONLINEAR BEHAVIOR OF CONCRETE MATERIAL, THE CON-CRETE DAMAGE PLASTICITY IS APPLIED TO THE NUMERICAL MODEL AS A DISTRIBUTED PLASTICITY OVER THE WHOLE GEOMETRY. FINITE ELEMENT MODEL IS THEN VERIFIED AGAINST EXPERIMENTAL RESULTS OF TWO NON-DUCTILE BEAM-COLUMN CONNECTIONS (ONE EXTERIOR AND ONE INTERIOR) WHICH ARE VULNERABLE TO JOINT SHEAR FAILURE. THE COMPARISON BE-TWEEN EXPERIMENTAL AND NUMERICAL RESULTS INDICATES THAT THE FE MODEL IS ABLE TO SIMULATE THE PERFORMANCE OF THE BEAM-COLUMN CONNECTIONS AND IS ABLE TO CAPTURE THE JOINT SHEAR FAILURE IN RC BEAM-COLUMN CONNECTIONS.