Revisiting the Versatility of Seismic Load-enabled Reinforced Concrete Beam Column Linkage

Abstract

In the seismic load-facilitated (vertical and lateral) reinforced concrete beam-column (RCBC), strong linkages are vital to protect the integrity of any civil structure foundation. Compared to other structural components, these joints often collapse during an earthquake in the seismic locations due to plastic-hinge zone mechanism. Most of the earlier reports on these issues mainly focused on two-dimensional RCBC joints and seldom emphasized on the need of three-dimensional RCBC linkages. Based on this fact, we critically overviewed several recent-state-of-the-art studies carried out on the improvement of RCBC linkages subjected to seismic loading. A careful evaluation of these articles revealed that the factors like shear strength (SS), failure, processing and selection of materials, as well as methods of strengthening (MS) of these RCBC joints are responsible for the collapse. It is established that for low axial load the ductility and bending resistance of the column must be enhanced to strengthen the RCBC links, enabling the diagonal strips or bars more robust than vertical ones and vice versa. Briefly, this article may provide the taxonomy for navigating the researchers into this field, leading to future sustainable development in the civil engineering construction sectors.

Full text

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DOI:10.9756/IAJSE/V11I1/IAJSE1139
Received: 06 May 2024/Revised: 22 May 2024/Accepted: 02 June 2024/Published: 08 June 2024
Revisiting the Versatility of Seismic Load-enabled Reinforced
Concrete Beam Column Linkage
Noor Alhuda Sami Aljabbri a*, Abdulamir Atalla Karim b, Fareed Hameed Majeed c
a* Department of Civil Engineering, Faculty of Engineering, University of Basrah, Basrah, Iraq.
E-mail: samynwralhdy9@gmail.com
b Department of Civil Engineering, Faculty of Engineering, University of Basrah, Basrah, Iraq.
E-mail: abdulamir.karim@uobasrah.edu.iq
c Department of Civil Engineering, Faculty of Engineering, University of Basrah, Basrah, Iraq.
E-mail: fareed.majeed@uobasrah.edu.iq
Abstract
In the seismic load-facilitated (vertical and lateral) reinforced concrete beam-column (RCBC), strong linkages are
vital to protect the integrity of any civil structure foundation. Compared to other structural components, these joints
often collapse during an earthquake in the seismic locations due to plastic-hinge zone mechanism. Most of the
earlier reports on these issues mainly focused on two-dimensional RCBC joints and seldom emphasized on the need
of three-dimensional RCBC linkages. Based on this fact, we critically overviewed several recent-state-of-the-art
studies carried out on the improvement of RCBC linkages subjected to seismic loading. A careful evaluation of
these articles revealed that the factors like shear strength (SS), failure, processing and selection of materials, as well
as methods of strengthening (MS) of these RCBC joints are responsible for the collapse. It is established that for low
axial load the ductility and bending resistance of the column must be enhanced to strengthen the RCBC links,
enabling the diagonal strips or bars more robust than vertical ones and vice versa. Briefly, this article may provide
the taxonomy for navigating the researchers into this field, leading to future sustainable development in the civil
engineering construction sectors.
Keywords: RCBC, Strengthening, Seismic Loads, Joint Failure, Reinforcement.
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Introduction
Over the past decades, several experimental and analytical studies have been conducted to obtain a comprehensive
understanding regarding the behavior of seismic load-facilitated reinforced concrete beam-column (RCBC) joints
wherein their high strength are essential to protect the civil construction foundation from collapse. Most of these
works focused mainly on the construction and testing of the RCBC sub-assemblies to determine various factors
influence their performance. Characteristics including joint shear strength (JSS), reinforcement details, connection
geometry, and axial loading were examined. Given various types of connections like interior, exterior, and corner
(knee) that exhibit distinct behaviors it became necessary to analyze their individual attributes. In addition, the
seismic behavior of the connections was evaluated by diverse analytical methods. Different parameters in the
analytical models were altered to simulate the factors responsible for the RCBC joints failure under seismic loading
(Ünal, 2010; Samadi et al., 2022).
Generally, methods such as steel caging, fiber-reinforced polymers (FRPs), and concrete jacketing have been used to
enhance the structural integrity of the RCBC. Furthermore, some recent studies have used hybrid elements and
materials for strengthening the RCBC linkages. Nevertheless, in cases when enhancing a structure's ability to resist
the horizontal forces, column reinforcing should be sufficient. It becomes imperative to further address the
beam-column connection, since it experiences substantial forces inside a limited space. The joint behavior has
significant effects on the structural response, particularly for the structures built to maintain vertical load alone and
the linkages deficient of reinforcement in the transverse direction. These joints are considered to be one of the
weakest components of the RCBC frame when exposed to the seismic stresses. Numerous strategies have been
developed on the reinforcement process of these BC joints in which concrete, composite materials, steel plates, are
inserted (Ruiz-Pinilla et al., 2022; Nilson, 1997; Paulay & Priestley, 1992; Murty, 2005).
Types of Beam-Column Connection
Joints are the part of the column inside the deepest framing beam. Moment-resistant frames include internal,
external, and corner joints. Interior joints are formed when four timbers enclosed the vertical sides of a column. An
external junction is formed during the framing of one beam into a vertical column face as well as two extra beams
are framed from the upright orientations. Figure 1 displays the formation of corner RCBC joints when each beam
frame contacts two neighboring vertical faces of a column (Rajaram et al., 2010).
Figure 1: Types of Joints in Building (Murty, 2005)
Influential Factors for Joint Shear Strength
There is a long-standing debate among the researchers regarding the factors that can considerably affect the JSS of
the RCBC structure under loading. Thus, the JSS of the RCBC became the most crucial aspects. To get an in-depth
understanding, numerous scholars developed JSS models and made experimental studies, discussing their benefits
and limitations. Different components were used in the empirical JSS models. A comparison of these JSS models
showed that their prediction accuracy differed greatly. This was mainly due to that fact that everyone did not
consider the same JSS variables, therefore obtaining different results. The key elements that determine the JSS are
described briefly hereunder.
Concrete Compressive Strength (CS)
Murad, (2020) developed a model for predicting the JSS of RCBC connections. The model parameters were CS of
concrete, degree of transverse reinforcement, geometrical characteristics of the joints (width, height, length, and

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depth), reinforcement ratios, and axial column loads. It was demonstrated that the CS of concrete to resist the shear
stress can considerably be lowered after the cracking. Six parameters model based on uniaxial gene expression
program (UGEP) was developed to calculate the JSS of RCBC connections. These parameters were square root of
the CS of concrete, transverse reinforcement (TR) of joints, depth of columns, panel widths of joints, ratios of beam
reinforcement and axial loads of column. In addition, 4 parameters biaxial GEP (BGEP) model was introduced.
These 4 parameters were square root of the CS of concrete, TR of joints, depth of columns, and panel widths of
joints. The model performance was assessed in terms of R2 values (R is the determination coefficient). The
calculated R2 value for the ACI, ASCE, UGEP and BGEP model was corresponded to 79, 79, 95, and 93%. The
GEP model revealed the highest value of R2, indicating its better accuracy to match with the experimental outcome
than others.
Pauletta et al., (2020) suggested an expansion to the existing model for predicting the SS of external joints during
seismic events. This model specifically focuses on RC inside beam-column connections. The required modifications
are implemented to account for the distinct physical configurations of the joints, wherein exterior joints have a
single beam while interior joints have two beams. The suggested model presents a formula for determining the
interior JSS. This formula considered the mechanical factors and contribution from the joint reinforcement and three
inclined concrete struts (horizontal stirrups of the column and in-between vertical bars). The CS of concrete was
found to affect the strength of the concrete struts. In contrast to the model used for external joints, the current
analysis considers three struts instead of two. Additionally, the impact of the axial load on the top column is taken
into consideration when determining the inclination of the concrete struts. The coefficients related to the the struts
and reinforcement contribution were adjusted from 69 recorded test datasets accessible in the existing reports. For
the calibration purpose, only the results of the cyclic test exhibiting the joint shear failure were chosen. In order to
validate the suggested model, the SS predicted from the proposed model was compared with the one achieved by the
models of Kim and LaFave, Kassem, and Wang et al. This comparison was conducted using the dataset consisted of
28 specimens. Furthermore, the results predicted by a designed were compared with the one achieved using
Eurocode 8 and ACI programs.
Beams-confined Joints
Karthik et al., (2020) developed a compatibility-strut and tie method (CSTM) for concrete beam specimens
undergoing testing. It was subjected to varying degrees of Alkali Silica Reaction (ASR) or Delayed Ettringite
Formation (DEF) deterioration as well as varying degrees of corrosion of the reinforcing bars. The simulation
accounted for age-modified cover in addition to the material properties of the core concrete. In addition, the resultant
passive pre-stress was exerted on the longitudinal as well as TR. With the increase of passive pre-stress both
strength and rigidity of the specimen was increased. The failure of very large beam-column joint was brought about
by the progression of nonlinear events. It was affected by varying degrees of ASR or DEF weakening that was
effectively monitored by the CSTM. Nevertheless, the anchorage length used by the system was insufficient.
Anchorage Systems for RC Beam-Column Joints
A total of 13 full-scale RCBC joints under reversed cyclic loading were investigated (Park & Paulay, 1973).
Important factors such as the way of securing the beam steel within the joints, existence of "U" bar, and degree of
transverse reinforcement were considered. Due to the fracture of diagonal stress and failure of anchoring, the joints
were found to degrade increasingly. In addition, the joints’ degradation was faster due to the cracks opening and
closing during the seismic stimulation. Consequently, an efficient anchoring and confinement in the coupling of
RCBC were indispensable to enhance their ant-seismic performance. Based on intensive studies they (Park & Paulay,
1973). proposed several strategies that could fulfill the core anchoring of joints, shear, and requirement of
confinement of the joints core. In these studies, mechanical bar with end anchoring, bend-up bar, and bend-up bar
inserted in end beam were utilized.
Transverse Reinforcement in Joint
For resisting the torsional and lateral forces, and confining the concrete, the TR within the region of joints must be
applied. Najafgholipour & Arabi, (2021) conducted a study to investigate the impact of the TR inside the joints’
panel on the structural response of RCBC moment resisting links. In order to achieve the intended objective, a
numerical investigation is carried out on various BC linkages using a verified model constructed in the finite
element (FE) program ABAQUS version 6.14. The results showed a simultaneous occurrence of the joints shear

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failure followed by flexural reinforcement yield in the beam. This research aims to evaluate the impact of two
primary factors, namely the quantity of TR (tensile reinforcement) and its arrangement, on the seismic performance
of connections. Based on the findings of the investigation, it can be concluded that the implementation of TR does
not provide a statistically significant enhancement in the ultimate strength of connections that are mostly susceptible
to joint shear failure. However, the TR has the potential to improve the nonlinear distortion ability of the
connections, particularly during the simultaneous occurrence of joints shear failure with the beam’s flexural
reinforcement yield.
Strengthening Methods for Reinforced Concrete Joints
In order to guarantee the optimal performance of RCBC structures during the earthquake, it is important that the
various components of the building possess a certain degree of flexibility. The incorporation of seismic
considerations into construction regulations did not occur until the 1970s, thereby rendering the majority of
structures constructed before this era ill-equipped to withstand earthquakes. The main thing that makes a building
fall down is damage to the columns and the joints between beams and columns. Retrofitting buildings to protect
them from earthquakes is now a fairly usual thing to do. There is already a lot of knowledge about how to use
different techniques to strengthen individual elements like beams and columns. However, treating the beam-column
joint is more difficult because of high amount of concentrated load over a tiny region that is hard in achieving in the
accessible constructions (Ruiz-Pinilla et al., 2014). There exists a wide range of diverse methods for strengthening
joints in concrete structures like Ferrocement laminates, Jacketing, post-tension, steel plate bonding, and FRP
composites (Khlef et al., 2021).
Ruiz-Pinilla et al., (2014) conducted an experiment to ascertain the behavior of steel jacket used as a seismic
reinforced element inserted into RCBC frames. Twenty full- scale interior beam-column connections were subjected
to testing. Geometry and reinforcements were chosen based on extant structures, which were designed solely for
gravitational loading using the concept of strong beams and feeble columns. All specimens underwent column
strengthening, and four distinct forms of column-joint connection strengthening were evaluated. Two types of beam
reinforcements were used in the designed experiment wherein the specimens were subjected to the cyclic stress and
gravity for testing. The results showed that the specimens’ failure modes can significantly be affected by the
reinforcing methods and applied axial stress on the column, thus influencing the joints seismic behaviors.
Qazi et al., (2013) performed some experiments using various types of carbon fiber reinforced polymer (CFRP)
anchoring systems to improve the strength of RCBC joints. This work presented four novel anchoring approaches to
tackle the aforementioned problem. The results for each technique were analyzed to determine the failure modes,
load-displacement curves, longitudinal strain distribution curves, hysteresis loops, and ultimate capacity. It was
established that the use of CFRP-based anchoring systems can effectively improve the overall seismic performance
of RCBC joints. A new model for the seismic strengthening of the exterior RCBC joints was developed (Qazi et al.,
2013) and their joint SS was evaluated (Figure 2). Under cyclic loading, four RC exterior joints without subjecting
to any transverse reinforcement were examined, wherein the first joint acted as control sample and other joints were
modified to circular section from the square ones followed by enclosure by various CFRP proportions (Figure 3). A
model for the SS of the joints was devised conceptualized on the mean plane stress. Then, the dataset enclosing 32
joints reinforced by conventional FRP approach and 3 joints reinforced by the new model was used for their
performance evaluation. The model simulation results displayed a considerable improvement in the shear and
seismic capability of the reinforced joints for all the studied failure modes. In addition, the changes in the CFRP
proportion were found to produce distinct failure modes in the joints. It was surmised that the proposed new
reinforcing procedure can be advantageous to enhance the resistance of FRP with the concrete against the SS of the
joints. In short, the SS of the CFRP-reinforced joints was accurately predicted by the new model.
The outperforming nature of the model suggested its practical applications in strengthening RCBC joints against
seismic loads. In addition, this approach can be beneficial in strengthening the RCBC joints for RC frame or bridge
of constructions without transverse beam’s direct link to the BC joints and main beam’s linkage at a certain distance
from the column face, implying the presence of opening in the slab or the absence of direct connectivity of the
edge-side column of the buildings to the transverse beam. The hysteresis behavior of the reinforced specimens
obtained in different manner (TS, TS1 and TS2) showed an appreciable improvement in the seismic behavior
compared to the control sample (TS0). The stiffness of the joints was used to judge the performance. In the
difference of failure modes, the results proved that the proposed strengthening method enhance the shear strength

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and the seismic performance of the connections. These findings indicated a possibility that CFRP can significantly
contribute the RCBC to resist the shear loads. Control specimen (TS0) exhibited shear failure like TS1 but there was
an improvement in the dissipation energy which was assigned to the strengthening of the BC due to CFRP inclusion.
The specimens TS and TS2 exhibited beam flexural failure which desired for the structure in the seismic location.
Figure 2: Images of a Strengthened Beam-Colum Join (Hadi & Tran, 2016)
Figure 3: Seismic Improvement of Strengthen Specimen (Hadi & Tran, 2016)

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Yurdakul & Avşar, (2016) examined the effectiveness of the newly developed reinforcing approach, wherein
post-tension rods was applied externally in the RCBC, without conformity of any code requirement (Figure 4). Five
samples were tested. These specimens were deficient due to the absence of tensile reinforcement in the joint, as well
as inferior material properties, such as low-strength concrete and the presence of round plain reinforcement rods. As
a local retrofitting technique, two diagonally mounted post-tension rods on each side of the joint are utilized. The
control sample was brittle wherein most of shear fracture occurred at the joints, whereas the rest of the RCBC
specimens showed nearly elastic behavior (Figure 5). The final lateral load capacities of all the retrofitted specimens
were significantly increased. However, in three retrofitted specimens, the brittle failure mechanisms like shear
failure of joints or joints-beams collapse were observed. After testing all specimens, it is determined that the
proposed retrofitting procedure can enhance the lateral force capacities of the beam-column assemblies to meet code
requirements, which demonstrated numerically (Ali et al., 2023). To observe varying degrees of structural
degradation, a cyclic load up to 8% of the drift proportion was applied to all specimens. As a local retrofitting
technique, two diagonally mounted post-tension rods on each side of the joint are utilized. The reference specimen
exhibited a brittle behavior, with the majority of shear fractures occurring in the joint, when the beam column joint
in the elastic range. The ultimate lateral load capacity of all retrofitted specimens was significantly increased. In the
three retrofitted specimens, brittle failure mechanisms like shear failure of the joints or beams-joints failures were
observed. The specimen with transverse beam exhibited a relatively ductile response. The axial stress in the rods
after tension was identical to those devoid of transverse beams. The test results of all samples showed that the
proposed retrofitting procedure can be useful to strengthen the capacity of the lateral force of the RCBC assemblies
to meet the regulations.
Figure 4: Detail of Evaluated Beam-Column Join (Yurdakul & Avşar, 2016)

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(a) Control Specimen
(b) Strengthen Specimens
Figure 5: Difference between the Hysteresis Response (Yurdakul & Avşar, 2016)
Bansal et al., (2016) studied the strength behavior of RCBC with external joints. These joints were subjected to an
initial load equivalent to a predetermined proportion of the ultimate load. Additionally, the joints were retrofitted
using ferrocement jacketing using two distinct wrapping techniques. In the retrofitting plan RS-I, wire mesh was
installed in an L shape at the top and bottom of the beam-column connection. In contrast, scheme RS-II included not
only wire mesh in an L shape at the top and bottom, but also diagonal wire mesh at the union. A comparison was
made between the outcomes of the retrofitted beam-column joints and the controlled joint specimens. The findings
indicated a significant enhancement in both the ultimate load-bearing capacity and yield load of the modified
specimens. Nevertheless, there was no discernible improvement in the ductility and energy absorption. The ultimate
load of specimens is maximally improved when they are wrapped diagonally with mesh wire. Bindhu et al., (2016)
proposed a novel reinforcing model for the jacketing of brittle external RCBC connections. The study included
conducting experimental examinations on five different kinds of joint specimens under the application of reverse
cyclic loading. It was shown that the retrofitting of brittle external RCBC connections via the innovative jackets can
result in a noticeable improvement in performance compared to the control specimen. Also, the inflexible RCBC
links revealed the development of diagonal fractures in the area of joints that led to overall collapse of the
construction. The results also indicated that the specimen subjected to conventional retrofitting had significant
improvement in the final load, capacity to dissipate energy, and dislocation in the brittleness than the control sample.
Specifically, the specimen with jacket demonstrated nearly 40% increase in the ultimate load than the control
sample.
An experiment was performed to examine strength performance of the joints between RCB and column during
earthquakes (Attari et al., 2019), wherein the results showed a considerable improvement in the strength
performance. Ten reinforced concrete beam-column joints on a scale of one to three (1/3) were test under reverse
cycle loading with axial loading to simulate an earthquake. Herein, FRP systems were used to improve the strength

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of the joints. These systems were made up of carbon and fiberglass fabric, as well as a mixture of FRP fabric. Four
samples were reinforced using various FRP systems and two samples were utilized as reference. Four damaged
samples were fixed with FRP system and then tested again. The results displayed a considerable improvement of the
structures’ strength, flexibility, and energy dissipation capacity. It was affirmed that glass fiber GFRP is just as
effective as carbon fiber CFRP when it comes to fixing damage beam-column joints. In addition, it was claimed that
the use of a combined sheet (glass-carbon) can greatly improve the RC joints' ductility and energy absorption at a
very reasonable cost.
Golias et al., (2021) did some tests to determine the effectiveness of FRP bands composed of CF placed outside in
an X-shape as well as both sides of the RCBC joints region. Under reverse cycle load, six full-size samples of RC
beam–column joints are studied. Carbon-fiber-reinforced plastic (CFRP) ropes have been put crosswise in the three
joints to make them stronger and protect against stress. The experimental findings showed that the performance of
the X-shaped CFRP ropes-mediated RCBC joints sub-assemblies was better than the reference specimens in
addition to superior hysteretic behavior. They also emphasized the benefits and simplicity of the developed
reinforcement approach for practical on-field applications. Figure 6 shows various installation stages including the:
(a) arrangement of U-shaped notch for encapsulating the CFRP rope; (b) inclusion of A-type epoxy resin at the
notch; (c) epoxy resin interpenetration into rope; (d) use of the interpenetrated CFRP rope in the notch and hole;
(e) type B resin sealing of notch and hole; and (f) ultimate appearance of the reinforced sample.
Figure 6: Various Installation Stages of the Externally Applied CFRP Ropes (Golias et al., 2021)
A novel retrofitting specification was introduced (Kim & Lee, 2021), wherein they provided a strategy to enhance
the structural and constructional ability of the RCBC for retrofitting purposes. The retrofitting process involved the
use of prefabricated components, which are promptly assembled on site via the use of bolts and chemical anchors. In
order to assess the structural efficacy of concrete beams that have undergone retrofitting using the recommended
details, a series of tests were conducted on a total of five concrete beams. These beams were examined both with
and without retrofitting. The retrofitting approach that was presented demonstrated a substantial improvement in
both highest loading capability and flexibility of the RCBC. The experimental findings indicated nearly 3 times and
2.5 times improvement of the corresponding flexural strength and ductility of the RCBC. Furthermore, the ability of
energy dissipation was increased by 7 times compared to the control specimen. Ebanesar et al., (2022) showed
experimental studies of beam-column joints strengthened with steel plates and steel plates with shear connections
under cycle load. Six external beam-column joints made to a smaller size were cast for the study. As a point of
comparison, two examples with closed stirrups were used. Two of the rest were given steel plates to make them
stronger, and the other two were given steel plates and shear connections to make them stronger. The results showed
that the steel plate with shear connections had a better performance. It was concluded that the proposed method can
be used as a good way to improve the ability of RC buildings to resist the earthquakes.
Hejazi et al., (2022) introduced a computer model based on nonlinear fracture mechanics to represent the crack
spreads through CFRP. Two RCBC joints were made and test to prove that the suggested model was correct. When

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the suggested model was used, the outputs of the CFRP-strengthened joints agreed well with the results of the
experiments (8–12%). It was also observed that the CFRP sheets inclusion could decrease the cracks proliferation in
the beam. Compared to the control specimen, the mean length of the cracks was reduced by 36.9%. The results
showed that cracks formed in the control specimen's link area, while in the strengthened CFRP sheets specimen's the
crack occurred in the beam resulting in checking the failure mode. Ruiz-Pinilla et al., (2022) analyzed the design of
two reinforced joints. They examined a total of eight RCBC with steel cages under the cyclic and gravity loading.
The joint panels in the presented structure were difficult to access wherein the joints reinforcement was consisted of
external solution like vertical or diagonal rods and column capitals (Figure 7). The proposed approach was found to
substantially improve the RCBC joints strength. The failure was observed to transfer to the joints in an undesirable
manner. In this investigation, the column-joint interface was protected from bending failure by vertical bars, but
failure occurred at the column-joint interface. Additionally, diagonal rods can prevent joint failure (Figure 8).
Except the reference sample (A.W.L0), other specimens were reinforced in two ways using the vertical or diagonal
bars. Like earlier investigation, the current study demonstrated a remarkable improvement of all factors related to
the seismic behavior (as shown in Figure 9).
Figure 7: Strengthening of Beam-column Joint Before Test (Ruiz-Pinilla, et al., 2022)
Figure 8: Joint Damage After the Last Load Cycle (Ruiz-Pinilla, et al., 2022)

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Figure 9: Shear Versus Drift Ratio of Tested Beam-Column Joint (Ruiz-Pinilla et al., 2022)

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Using the nonlinear finite element method, (Al-Rousan et al., 2023) investigated the efficacy of FRP composites
with an external bonding to enhance the quality of damaged RCBC links and alter their failure modes. At the start,
the modeled RCBC joints were compared with the earlier obtained experimental outcomes. Afterward, the model
was extended to determine the effects of axial column loads and CS of concrete on the FRP model without and with
reinforcement. The experimentation was performed for various applied axial column loads (0%, 25%, 50%, and
75%) in which the CS values of concrete were degraded to 0%, 25%, and 50%. The structural performance of all
models was evaluated in terms of the failure modes, stress distributions, and ultimate ability in both loading
orientation with their corresponding displacement. The use of FRP to externally RCBC connections was found to
improve largely their cyclic performances wherein the all quantities like ultimate load capacities, horizontal
displacements, flexibility of displacements, and dissipated energy were increased. In addition, when the axial
column load exceeded 25%, the FRP transformed the failure modes of the area amid joints and columns from the
non-ductile to ductile. This transformation was mainly ascribed to the plastic hinges generation only on the beam
sideways. For the applied axial load of the column less than 25%, the ultimate axial load capacity and resulting
deflection are increased. Nonetheless, some findings demonstrated that with the increase the applied axial load by
25% the resultant stiffness of the column can degrade more nearly by 3% and 16% for the undamaged and damaged
joints, respectively. In contrast, axial loading results in a 170% increase in dissipated energy, an increase of 25%.
Due to their precision, the resultant observations assist specialized engineers in retrofitting B-C connections in
existing structures.
Fiber-reinforced Polymer (FRP) Composites
The FRP or composite plate-mediated strengthening of RCBC was mainly inspired by the idea of steel plate bonding
techniques. FRP-based materials are outstanding alternatives to steel because they can prevent the interfacial
corrosion. The material FRP is mainly obtained by joining together several tiny fibers within the resin matrices. The
resins (for example different types of polyester, epoxy vinyl esters) are the most frequent resins utilized) are used in
saturating, fixing, and providing load paths to these reinforced tiny fibers, thereby enabling the transfer of effective
loads amid these fibers. The physicochemical properties of these fibers-based composites are decided by the
attributes of individual component together with their relative ratios and orientations of the fibers in the resin matrix.
The FRP materials are classified in 3 categories like GFRP, CFRP, and aramid FRP (AFRP). Figure 10 shows the
stress-strain relationship of these fibers compared to mild steel. The linear elastic properties of FRP show their
higher tensile strength (ranged from 2400 - 3400 MPa) than mild steel (Valerio, 2009). The generated shear stresses
amid the fibers get confined within the matrix, thus limiting the applied forces normal to the fibers.
Figure 10: Relationship of Stress-Strain of Fibers and Steel (Valerio, 2009)
The CFRP has several benefits than the traditional building materials like steel and concrete. There is an increasing
demand of these construction components (CPRF-based composites) for retrofitting and reinforcement in the
concrete sectors. Characteristically, these construction materials are used as "externally bonded" agents to enhance
the axial, torsional, flexural, and shear load capacity of various structural components of the RCBC, improving the
structural performance, durability, and serviceability via the mechanism of extra confinement. Typically, the surface
of concrete is coated by CFRP composites for reinforcing externally in a simple way. New structural components

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can be reinforced by CFRP composites in 3 ways such as internally by CFRP bars, permanently by CFRP
formworks for RCBC, and pre-stressed concrete structures by CFRP tendons (Mostofinejad & Hajrasouliha, 2018).
Joints Subjected to Seismic Loading
The seismic activity associated with an earthquake encompasses both horizontal and vertical ground movements,
with the vertical component often exhibiting lower intensity. Furthermore, the factor of safety against gravitational
loading has the capacity to account for extra stresses resulting from vertical acceleration generated by earthquakes.
The most significant influence on the structure is due to Earth’s movement horizontally, causing the foundation to
shake backward and forward. In fact, this shaking is restricted by the mass of the structures through the
establishment of inertia. Figure 11(a) displays the force (F = Ma) development (with acceleration a) in building of
mass M during earthquake. For a structure with rigid walls and foundation, it accelerates alike ground during the
seismic vibration. Most of the civil structures in practice are flexible to some extent thereby can absorb some energy
via minor deformation wherein the force will be less than Ma (Figure 11(b)). However, a highly flexible foundation
can withstand much larger force when Earth’s movement occurs repetitively (Figure 11(c)). Clearly, it demonstrates
that the impacts of lateral forces on the civil foundations depend on both Earth’s acceleration and types of structures.
The dynamical response of the civil structures is significant which is determined by the effectual loads. The seismic
load can be quantified using either the seismic coefficient or response spectral technique, wherein the later considers
structural dynamics in addition to ground motion (Rajmani & Guha, 2015).
Figure 11: Force Developed by Earthquake (Rajmani & Guha, 2015)
The structural requirement on any RCBC joint (interior, exterior, or corner) is significantly affected by the type of
loading system and loading path. Consequently, it is crucial to employ design procedures that properly account for
the severity of each form of load. For example, continuous reinforced concrete structures subjected to only gravity
loads will be designed according to the strength under monotonic loading without stress reversals. In other instances,
the design of joints will be determined by both the strength and ductility of the contiguous members under reversed
loading, such as in the case of a rigidly joined multistory frame subjected to seismic loading. As a result of repetitive
reversal loading, the strength of the concrete will degrade, necessitating a substantial quantity of joint reinforcement
in the second case (Lu et al., 2012; Sharifianjazi et al., 2022). Pampanin et al., (2002) tested the structural properties
(subjected to the simulated earthquake loading) of six 2/3-scaled RCBC subassemblies in the buildings of Italy
constructed during the 1950s and 1970s. Both internal and external tees and knees joints’ were tested under
increasing level of cyclic loads of the inter-story drifts. These joints had smooth bars, poor reinforcement detailing
(without any transverse reinforcements in the joints area), weak anchorage (bars with hooks end), and without any
capacity design principles. The test results showed how vulnerable the joint panel zone is and there is important bar
slipping happens when smooth bars are used, and there aren't enough anchors. A particular brittle failure mechanism
was seen. This was caused by the shear cracks and stress development at the position of hooks.
The behavior of two knee samples having various reinforcements was administrated by flexural damage
accumulation at the interfaces of the columns. The external tee-joint samples displayed a comparable mechanism of
non-ductile hybrid failure composed of joints shear damage unified with longitudinal beam bar slipping in the joints
area with the accumulation of CS at the anchorage of end-hook. The evidenced mechanism presented some

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attractive results than the ones characteristically predictable for external tee-joint specimen. In fact, the beam bar
was bent away from the joints or into the joints area (Figure 12) based on the nature of anchorage use. Figure 13
shows the early age development of failure mechanism because of bonds deterioration and beam bars slippage
wherein an extra localized accumulated stress was acted at the compressed bar edges, after first joint diagonal cracks
combined with ineffective strut mechanisms that led to the rupture of concrete specimen.
Figure 12: Unconventional Mechanism of Damages for External Tee-Joints: (a) Beam Bars Bent Away from
the Joints Area; (b-c) Beam Bars Bent in the Joint Area; (d) End-Hook Anchorage “Concrete Wedge” Process
(Pampanin et al., 2002)
Figure 13: Development of Failure Mechanism (Sharifianjazi et al., 2022)
Li et al., (2009) investigated experimentally and analytically seismically loaded beam-column junctions made of
light reinforced concrete. The seismic behavior of five 3/4-scale reinforced concrete beam-column junctions was
investigated through testing. Parameters like the columns orientation and existence of slabs on the top of beams
were determined. To simulate earthquake loads, quasi-static load reversals were applied to the specimens. Included
in the experimental outcomes are joint SS, joint shear strain, cracks, and early rigidity. The obtained findings
presented the seismic behavior of the specimens. Due to uniqueness of the samples, it was impossible to alter a
number of crucial parameters. In order to additional analyze, a numerical study was made using 3D nonlinear finite
element model. Subsequently, this model results were compared with the experimental data. Finally, the parametric
studies were conducted to determine the impact of several critical parameters on the joints characteristics. These key
parameters include axial loads at the column, depths of column to beam reinforced bar diameters ratio, and effective
width of slabs. Li & Kulkarni, (2010) examined experimentally and numerically the RC wide beam-column
connections under seismic loads. Three full-scale wide exterior beam-column specimens were subjected to seismic
loads for in the experimental investigation. The specimens were subjected to simulations of earthquake loads via
quasi-static load reversals. On the basis of the experimental findings regarding the overall behavior of joints,
hysteresis curve, and strain response of the longitudinal reinforcement in the specimen, seismic performance
analysis was performed on the joints. The obtained outcomes were compared with the ones generated from 3D
nonlinear finite element model simulation. Parametric studies were conducted to determine the joints behavior
subjected to the column axial loads, transverse beams, and various beam bars anchorage ratio. It was shown that
with the increase of column axial loads the seismic behavior was improved without increasing the total story shear
force by increasing of transverse beam reinforcement. Furthermore, the study demonstrated that an increase in the
anchorage length can enhance the joints shear.

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Sharma et al., (2011) proposed a new model to simulate the shear features of RCBC joints under seismic loading.
This model used the limiting major tensile stresses in the joints as the failure criterion in order to account for the
column's axial load. The spring properties were determined by the real deformation that occurred in the
subassemblies as a result of joint shear distortion. The model is readily implementable in any commercial nonlinear
analysis software and requires no special components or subroutines. This model was more realistic compared to the
rotational multiple-spring model and was easy to analyze. The formulas for obtaining spring characteristics are
provided. At present, this model can be applied for the evaluation of the nonlinear static and dynamical properties of
RCBC links as well as hysteretic performance. I addition, the accuracy of model was compared with other existing
models, displaying a great promise. The focus of this investigation is the modeling of exterior connections. The
obtained results from the proposed RCBC joints model were in good agreement with the experimental findings.
Hosseini et al., (2018) studied engineered cementitious composite (ECC) in 3D external RCBC joints to enhance the
seismic properties of the buildings. ECC is the subclass of cementitious composite reinforced with fibers that show
excellent performance than traditional concretes. It shows distinct characteristics like excellent tensile ductility, high
absorption of energy, high resistance against shear, and enhanced bond strength when reinforced by steel (rebar). To
understand the behavior of external RCBC joints subjected to compound loads, some 3-D samples were designed
and characterized at seismic loading. For enhancing the RCBC joints performance the traditional RC was replaced
with reinforced-ECC that could extend from the panel area to the neighboring beam and column, allowing the
occurrence of a latent plastic hinge region. The experimental results indicated that an effective implementation of
ECC in the potential plastic hinge zones of RCBC joints can appreciably increase their damage resistance against
seismic movement. Shear cracks were observed in the RC sections of the specimens while the flexural cracks
behavior of the ECC sections was insignificant due to its high shear strength.
Zhao et al., (2019) introduced a novel macro beam-column joint element model that incorporates the effects of the
joints inelastic deformation specifically for the internal joints containing stirrups. The macro-RCBC joints were
formulated by considering the force transmission mechanisms and inelastic response mechanisms. This was
achieved by using axial springs to represent the bar-slip mechanism of the longitudinal reinforcement together with
concretes and reinforcements at the joints core and interfacial shear. The joints core was comprised of a total of 8
concrete and 8 reinforced elements, facilitating the occurrence of joints shear distortion. The constitutive relations
were formulated in order to establish the behavior of the suggested joints model, taking into account its dimensions,
geometric characteristics, and material qualities. A suitable model was used to simulate the decrease in strength of a
constrained concrete strut as a result of joint deterioration and cyclic load history. Adjustments were made to
enhance the accuracy of the anchoring zones simulation responses. In order to verify the validity of the developed
macro-RCBC joints model, a comparison between numerical and experimental results were made based on 6 RCBC
sub-assemblages. It was observed that the developed model can effectively replicate the hysteric responses, joints SS,
and joints shear deformations of the RCBC sub-assemblages. Furthermore, the simulation outcomes of the suggested
joints model when compared with the existing joint models demonstrated its excellent accuracy and efficiency.
Joint Failure
The joints’ regions are the crucial and vulnerable components of construction. The RCBC junction is prone to
experiencing five distinct modes of failure (Meinheit & Jirsa, 1981). The first kind of connection failure is the beam
hinge (Figure 14(a)) which is caused by the generation of plastic hinge at the extremity of beam within the joints’
region. This situation arises whenever the beam is unable to withstand a more significant strain, and the
reinforcement fails, resulting in numerous fractures. Despite withstanding the strain by the joints zone the failure of
beams can result in the collapse of the joints core. As depicted in Figure 14(b), the other type of failure can be
utilized to symbolize the failure of column hinges, which is emerged during the breakage of the plastic hinges of
column due to the action of shear or compressive forces. Together with the beam failure, column failure can be
characterized by numerous fractures, indicating the inability of the columns reinforcement in withstanding the loads.
It is very important to avoid this type of failure as it can sway the frame that is difficult to restore. As shown in
Figure 14(c), the third form of failure is induced by the concrete’s cover spalling at the joints area. This occurs due
to the fractures formed at the face of joints in which the fractured concrete explodes under increasing loads. The
concrete spall cover must be avoided because it can decrease the compressive strain in the column. Figure 14(d)
depicts the failure of the anchorage bar within the joint as the fourth form of joint failure. This failure is a column
joint on the exterior. It is essential to anchor the beam-column along its anchorage length for resisting the negative
moment by the reinforcement. This failure will be caused by insufficient anchorage length or poor detailing. A

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346
narrow curvature of the bend bar can generate very high bearing stress, leading to the anchorage collapse. Any
inability of the frame structure in transmitting the biaxial shear can reduce its energy absorption capacity. Figure
14(e) illustrates the joints shear failure.
(a) Failure of Beam Hinge
(b) Failure of Column Hinge
(c) Failure of Spalling at Concrete Cover
(d) Failure of Ancorage
(e) Failure of Joints Shear
Figure 14: Various Failure Modes in RCBC Joints (Meinheit & Jirsa, 1981)

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347
Comparative Performance Evaluation of Various Reported RCBC Joints
Table 1 illustrates a comparison among strengthened RC joint researches through a number of samples,
strengthening method and material, type of failure, and percentage of enhancement after strengthening.
Table 1: A Comparative Evaluation of the Performance of Various Reported RCBC Joints
Ref.
No. of
samples
Strengthening
method
Strengthening
materials
Type
of
joint
Type of failure
% of enhance-
ment in joint
ductility
% of enhance-
ment in struc-
tural strength
(Shannag &
Alhassan, 2005)
10
External sheet
High-performance
fiber-reinforced
2D
Brittle failure
(joint-column regions)
or Ductile fail-
ure(beam)
-
30
(Lee et al.,
2010)
3
External sheet
CFRP
2D
Shear failure or
de-bonding
-
36
(Le-Trung et al.,
2010)
8
External T-shape,
L-shape, X-shape,
and strip combina-
tion
CFRP
2D
Shear failure or beam
flexural failure
361
31.7
(Mady et al.,
2011)
5
External bars and
stirrups
GFRP
2D
Concrete crushing
followed by bar rup-
ture or Joint degrada-
tion
-
20
(Sharbatdar et
al., 2011)
3
Internal grids and
bars
CFRP
2D
Shear failure of the
joint
-
21
(Ruiz-Pinilla et
al., 2014)
20
External steel angles
with stiffeners
(capitals) at the
corners with chemi-
cal anchor
Steel stiffeners
2D
Tensile failure of
external bars or
Pull-out failure of
chemical anchor
-
35
(Hadi & Tran,
2016)
4
External concrete
covers and wrapping
Concrete and FRP
2D
Formation of the beam
flexural hinge at a
distance from the joint
face or by joint shear
failure
-
142
(Yurdakul &
Avşar, 2016)
5
External
post-tension rods
Steel rods
2D
Joint shear failure or
beam-joint failure
49
21
(Mostofinejad &
Hajrasouliha,
2018)
5
External bonding
reinforcement on the
groove (EBROG)
technique
CFRP
3D
Flexural failure mode
at the beam-column
junction
69
38
(Ercan et al.,
2019)
6
External sheets and
internal steel bars
CFRP sheets and
steel bars
2D
Strain softening of the
joint
-
27
(Attari et al.,
2019)
10
External FRP sheets
Hybrid sheet (Glass
and Carbon)
2D
Shear failure or prem-
ature failure of the
carbon sheet
88
89.5
(Kim & Lee,
2021)
5
External modular-
ized steel plates
Steel Plates
2D
Concrete crushing at
the top
150
200
(Golias et al.,
2021)
6
Grooves with
X-shaped Ropes
CFRP Ropes
2D
Joints’ shear
-
33
(Ebanesar et al.,
2022)
6
External steel plate
with shear connect-
ors
Steel plate with
shear connectors
2D
Joints' shear damage
98.6
47.2
(Ruiz-Pinilla et
al., 2022)
8
External vertical or
diagonal bars and
capitals connecting
Steel bars and
capitals
2D
Joints’ interface failure
-
85.7

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348
Conclusions Further Outlook
T This article provided a comprehensive overview on the RCBC joints subjected to cyclic loads wherein the current
trends, main challenges, and future drifts of this research field are highlighted. Because the RC joints failure due you
do insufficient strength and ductility is important corner of frame since this causing the loss of strength and failure
of structures. During seismic action, the lower strength, stiffness and dissipate energy ductility is a major concern,
using CFRP surrounding concrete can be improved those parameters during earthquake and change the failure mode.
For this reason, research conducted to study the effects of seismic parameters using different types and configuration
of CFRP. Research on reinforced concrete joints subjected to seismic loading and strengthening of reinforced
concrete joints has resulted in a number of positive outcomes. It has been demonstrated comprehensively that the
beam-column joint is a critical zone in reinforced concrete framed structures subjected to seismic load. Over the
years’ numerous strategies have been adopted to enhance the seismic resistance of RCBC links wherein the novel
composite material like FRP became prominent due to various exotic attributes. In addition, various numerical
models have been proposed to understand the behavior of RCBC joints in buildings under seismic loads. Finite
element code like ABAQUS was applied to determine the performance of RCBC joints subjected to seismic loading
and compared with experimental outcomes. The main conclusions of this review paper are:
1. Most of the reported studies focused on the design and characteristics of 2D RCBC joints and seldom pre-
sented the behavior of 3D RCBC joints seismic performance.
2. Many researchers used FRP composites to strengthen the RCBC joints because of the immense benefits of
this material.
3. When the axial load is low and what is required from strengthening the RCBC linkages is to increase its
ductility and increase the bending resistance of the column, then diagonal strips or bars are more appropriate
than vertical ones and vice versa.
4. The RCBC connections exhibited shear failure and slipping of the steel bars in the beam due to insufficient
anchoring in the lower section of the beam.
5. The use of a U-shaped jacket of any material effectively preserved the structural integrity of the RCBC links
by confinement. Furthermore, this technique notably enhanced the ductility of the repaired connection.
6. The incorporation of suitable transverse reinforcements inside the beam-column joints is a viable strategy for
enhancing the ductility of RCBC joints.
7. The use of steel capitals or steel plates with shear connector results in an enlargement of the RCBC joints,
hence improving the columns bending strength and seismic resistance.
Supplementary Materials: “No supplementary Materials”.
Author Contributions: The following statements should be used “Conceptualization, N.A.S.A. and A.A.K.;
methodology, N.A.S.A.; software, F.H.M.; validation, A.A.K. and F.H.M.; formal analysis, N.A.S.A.; investigation,
N.A.S.A.; resources, N.A.S.A.; data curation, N.A.S.A.; writing—original draft preparation, N.A.S.A.;
writing—review and editing, A.A.K.; visualization, F.H.M.; supervision, A.A.K.; project administration, N.A.S.A.;
funding acquisition, A.A.K. All authors have read and agreed to the published version of the manuscript.
Funding: Please add: “This research received no external funding”.
Institutional Review Board Statement: “Not applicable”.
Informed Consent Statement: “Not applicable”.
Data Availability Statement:
Acknowledgments: Authors thankful of Basrah University for them support.
Conflicts of Interest: “The authors declare no conflicts of interest.”.
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