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INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING
Volume 4, No 3, 2014
© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0
Research article ISSN 0976 – 4399
Received on October, 2013 Published on February 2014 248
Experimental investigation on behavior of reinforced concrete beam-
column joint Kaliluthin.A.K1, Kothandaraman.S2
1-Assistant Professor, Department of Civil Engineering, B.S.Abdur Rahman University,
Chennai, India
2-Professor Department of Civil Engineering, Pondicherry Engineering College, Pudhucherry,
India
kalil.ak786@gmail.com
doi:10.6088/ijcser.201304010025
ABSTRACT
An experimental investigation carried out on the reinforced concrete exterior beam-column
joint was subjected to static load is reported in this paper. The objective of this study is to
investigate the existing RC beam column joints which are designed as per BIS 456-
2000,which must be strengthened, since they do not meet the requirement given in the
ductility code IS 13920-1993 are inadequate and it needs to be upgraded regarding the
detailing of reinforcements an attempt has been made to introduced an additional
reinforcement called “core reinforcements” in the joint region to improve the reinforcement
detailing pattern and strength behaviour to ensure good performance of the joint as well. The
experimental study was conducted on full scale exterior beam column joint specimens
detailed in three different categories such as IS 456-2000 as reference joint (RJ), IS:
13920:1993 as ductile joint (DJ) and core reinforcements as core joint (CJ) were casted and
tested under static loading and the results were evaluated with respect to in terms of strength,
ductility and stiffness degradation. Test results showed that the use of core reinforcement
reduced the damages in the joint region and is one of the most effective techniques to
improve the seismic resistance of RC structures.
Keyword: Exterior Beam-Column Joint, Reference joint, Ductile joint, Core joint, Strength,
Stiffness, Ductility.
1. Introduction
A beam-column joint is a very critical part in reinforced concrete framed structures where the
elements intersect in all the three directions. Joints ensure continuity of a structure and
transfer forces that are present at the ends of the members. The beam column joint is a crucial
zone in reinforced concrete moment resisting frames. Reinforced concrete frames must
perform satisfactorily under severe load conditions to withstand large lateral loads preferably
without irreparable damage. It is commonly accepted that it is uneconomical to design
reinforced concrete structures for the greatest possible force or force combination. Therefore,
the need for strength and ductility has to be weighed against strength and economic
constraints. Ductility is an essential property of structures to respond elastically during the
action of devastating forces, in particular the seismic forces. Ductility is defined as the ability
of sections, members and structures to deform inelastically without excessive degradation in
strength or stiffness. The most common and desirable sources of inelastic structural
deformations are rotations in potential plastic hinge regions. An energy dissipation
mechanism should be chosen so that the desirable displacement ductility is achieved with
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
249
smallest rotation demands in the plastic hinges. Development of plastic hinges in frame
columns is usually associated with very high rotation.
For a given displacement in a structural frame system, the rotation demand in the plastic
hinges would be much smaller if they are developed in the beams. For getting an efficient
performance of beam at beam-column joints adequate anchorage is essential which will
provide proper dissipation of energy and hence ductility to the structure. Otherwise, the
failure may cause longitudinal beam bars pulling out of the joint. Current design philosophy
requires that beam column joints have sufficient capacity to sustain the maximum flexural
resistance of all the attached members. The mechanism of force transfer within beam column
joint of a rigid frame during seismic events is known to be complex involving bending in
beams and columns, shear and bond stress transfer in the joint core. For the structures under
lateral load, Indian Standard code IS 13920 (1993) recommends to continue the transverse
loops around the column bars through the joint region. The length of anchorage is about
Ld +10φ (development length + 10 times the diameter of bar) inside the joint. The primary
aim of joint design is to safely encounter the shear force. This often necessitates a
considerable amount of joint shear reinforcement, which may result in construction
difficulties and hence often compromised.
Current code guidelines for ductile joints are having very low level of acceptance by the
construction and structural engineers because of its installation difficulties and the hardships
in placing and consolidating the concrete in the beam column joint regions.
Numerous attempts have been made to improve the detailing of joints to overcome the
practical difficulties. Use of headed reinforcement reduced the difficulties to a considerable
extent (Wallace 1997; Berner and Hoff 1994). ACI Report 352R-02 (2002) provides
guidelines for the use of headed bars. Chun et al (2007) studied on the reversed cyclic
behavior of beam- column joints with hooked bars and headed bars. It is reported that the
headed bars with moderate transverse reinforcement are effective for both ACI 352 Type 1
and 2 joints. It also reported that the large drift levels to develop the bar with ratios of the net
head area to the bar area (Anh/Ab) of 3 or 4 was found to be adequate.
Addition of steel fibre generally enhances the mechanical properties of concrete including
fatigue strength, impact strength and ductility. In exterior beam column joints steel fiber
imparts high ductility to the joints which is the most desirable property for the beam column
joints. Addition of fibers to the joints decreased the rate of stiffness degradation appreciably
when compared to joints without fibres (Ganesan et al, 2007).
Kumar and Shamim (1999), conducted studies on the effect of column axial load, shear and
tension reinforcements of the beam on the performance of joints. They reported that, increase
in shear reinforcement did not affect the ultimate strength at low axial load levels (upto 20%)
but at higher axial load levels (upto 80%) the ultimate strength of joint increased with
increased shear reinforcement. Further, increase in shear reinforcement decreased the
ultimate rotation of beam-column joint and this reduction was significant at higher percentage
of tensile reinforcement in beams.
From the experimental study on the external beam column joints reinforced with inclined
(lateral) bars Tsonos et al (1992) reported that the joints acquired high strength and no
appreciable deterioration noticed after reaching their maximum capacity. Also, low joint
shear stresses in the presence of high flexural strength resulted in satisfactory performance of
exterior beam column joints reinforced with inclined bars.
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
250
Chidambaram and Thirugnanam (2012) studied the beam column joint by introducing a small
projection beyond the column face as shown in Figure 1. They reported that the load carrying
capacity of their proposed joint carried 45% more load than the conventional reinforced joint.
Further, the cumulative energy absorption enhanced four times that of
conventional beam column joint.
Li and Kulkarni (2010) have studied the behavior of wide beam column joints. They
concluded that wide joints performed well. They concluded that due to larger cross section
the reinforcement may be relaxed without affecting the performance of joints.
Figure 1: Ductile Detailing of Special Anchorage Beam Column Joint
(Source: Chidambaram and Thirugnanam, 2012)
1.1 Summary
In every structural system the most attention is required to the joints both during the design
and construction stages. There are practical difficulties involved in the construction of
reinforced beam-column joints. In Indian context in most of the construction RC joints are
not given adequate attention to proper reinforcement detailing. In particular, joints are often
executed without adequate shear reinforcement and development length. Of course it is also a
global phenomenon, which is evidenced by numerous brittle failures of structures due to joint
failure under devastating forces.
Engineers and Scientists have been continuously working to overcome this problem. ‘Headed
bars’ is one such technique to overcome this practical difficulty. In addition, fibre reinforced
joints, inclined bars at joints and joint enlargements, both laterally and longitudinally are tried
by researchers. Though every technique has certain specific technical advantage, they have
certain inherent disadvantage too. Under such circumstances an attempt has been made to
improve the reinforcement detailing pattern in order to improve the acceptable level by the
field engineers and of course obviously to ensure enhanced performance of the joint as well.
To improve the load carrying capacity of RCC beam-column joints, additional reinforcement,
called ‘Core Reinforcement’ has been introduced in the present study. It is kept in mind that
while using ‘Core Reinforcement’ the bent beam bars need not be extended into the column
for full development length.
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
251
2. Objective of the present study
The fundamental objective of this study is to ensure either equivalent or improved ductile
behavior of the joints compared to the standard ductile joints and make the proposed detailing
technique should be practically feasible and acceptable. In order to overcome the practical
difficulties in the execution of beam-column joint reinforcement detailing in a modified form,
called ‘Core Reinforcement’ is proposed in the present work.
3. Experimental programme
In order to accomplish the objectives of this work the following three different types of RCC
beam-column joints were prepared, tested and their performance are compared.
1. The beam rods were bent 90° and terminated in the joint within the depth of the beam.
Such joints were called ‘Reference joint’ and designated as ‘RJ’.
2. The beam rods were bent 90° and terminated with adequate development length as
stipulated in IS 13920 (1993) to ensure ductile behavior of joints. Such joints were
called ‘Ductile Joint’ and designated as ‘DJ’.
3. The beam rods were terminated as in case 1, but in addition “Ⱶ” type cage
reinforcement was introduced in the joint region; such joints were called ‘Core Joint’
and designated as ‘CJ’.
The above joint details are shown in Figures 2-4.
3.1 Joint details
The cross section of the column was 200mm × 150mm and its length was 800mm. The beam
size was 150mm x 200mm (depth) and its length was 6000mm, measured from the face of the
column. The high yield strength deformed bars (HYSD) Fe500 grade bars conforming to IS
1786 (2008) were used in this test program. The columns were reinforced with 4 No of 12
mm diameter and the beams were provided with No of 12 mm diameter bars at top and
bottom. For transverse reinforcement 8 mm diameter bar was used for both the columns and
beams. For core reinforcement 6mm diameter main rods and lateral reinforcement were used.
The above detail may be checked with the Figures 2-4.
Figure 2: Reference joint (RJ)
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
252
Figure 3: Ductile joint (DJ)
Figure 4: Core joint (CJ)
3.2 Materials
Ordinary Portland Cement 53 grade conforming to IS: 12269 (1987) was used for the
investigation along with naphthalene based superplasticizer. The fine aggregate used was
river sand passing through 4.75 mm IS sieve and having a fineness modulus of 2.83 Crushed
granite stones passing 12.5 mm and retained on 4.75 mm and having a fineness modulus of
6.95 were used. Steel reinforcement (Fe-500) HYSD bars conforming to IS 1786 (2008) were
used for the present study.
3.3 Mix proportions
Mix proportions for M25 grade concrete were obtained based on the IS 10262-2009. The
details of mix proportions thus obtained are given in Table 1. Same mix proportions were
maintained for all the specimens. The 28 day average compressive strength from 150 mm
cube test was 34.55 N/mm2.
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
253
Table 1: Concrete mix proportions
Cement
(kg/m3)
Fine aggregate
(kg/m3)
Coarse aggregate
(kg/m3)
Water
(l/m3)
Superplasticizer
(l/m3)
350 670 1195 158 7
3.4 Casting
Wooden moulds were used for casting the specimens. Required quantities of cement, sand
and coarse aggregate were mixed thoroughly with the superplasticizer. Reinforcement were
fabricated and placed inside the moulds. Mixing was done manually till a homogenous
mixture was obtained. The concrete mixes were poured into moulds in layers, and the moulds
were tamped for thorough compaction. After casting, the specimens were covered with wet
gunny bags to prevent loss of moisture. After 24 hours, specimens were demoulded and cured
under damp curing till testing.
3.5 Testing of specimens
The specimens were tested in a loading frame of capacity 1000kN. The specimens were
shifted from casting yard to the loading frame and mounted on the loading frame centered to
the hydraulic jack. The column was centered accurately using plumb bob to avoid
eccentricity. Bearing plates are provided at top and bottom surface of the column. The
exterior beam column joint specimens were tested for monotonic loading. The point load was
applied to the beam end in the upward direction. A nominal axial load of 5kN was applied
through the hydraulic jack to hold the column firmly. The hydraulic jack of 200 kN capacity
was used to apply load without shock at the free end of the beam as shown in Figure 5. A
load cell of 500 kN capacity was used to measure the applied load. Two numbers of linear
variable differential transducers (LVDTs) were used to measure the deformations, at two
different locations. The gauge length of each LVDT was 200 mm. One LVDT placed near the
free end of the beam, and the other LVDTs were paced near the joint region. The load cells
and LVDT were connected to a data acquisition system. A careful visual inspection was
made to observe the cracks during the test. The crack pattern and the crack growth at
different stages of loading are marked on the specimen at every increment of loading. The
photograph of the test setup is shown in Figure 5. Under each category three joint specimens
were tested to ensure repeatability of test results.
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
254
Figure 5: Test setup
4. Results and discussions
4.1 Load carrying capacity
During testing the deflection measurement at the free end of the beam and the crack
formation was carefully noted. Load corresponding to the formation of first crack and the
ultimate load were noted and presented in Table-2.
Table 2: Load test results
Specimen
Designation
Load (kN)
at first crack at Ultimate stage
RJ 7.2 , 7.6 & 7.3
(7.36)
11.25 , 11.30 & 11.20
(11.25)
DJ 7.4 , 7.2 & 7.2
(7.27)
14.40 , 14.20 & 14.0
(14.20)
CJ 8.7 , 8.5 & 8.6
(8.60)
14.9 , 15.2 & 15.1
(15.10)
Note: The average values are given in bracket.
The average values are in Figure 6
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
255
7.36 7.27
8.6
11.25
14.215.1
0
2
4
6
8
10
12
14
16
Lo
ad
(k
N)
RJ CJ DJ
First crack load
Ultimate load
Figure 6: Comparison of load carrying capacity
4.2 Behavior of specimens
The typical crack patterns of the three joints are given in Figures 7- 9. The reference joint did
not show much propagation of crack after the first crack. The crack width of the first was
widening as the load increased and eventually reached the ultimate load. This joint did not
show much ductile behavior. However, in the ductile joint the propagation of crack took
place after the formation of first crack (Figure 8). In the core joint the propagation was better
compared to the other joints. Two cracks were formed as the deflection was much smaller in
this joint the propagation did not go grow into the depth of the beam. Core joint specimen
showed improved ductile behavior.
Figure 7: Reference Joint (RJ) after failure
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
256
Figure 8: Ductile Joint (DJ) after failure
Figure 9: Core joint (CJ) after failure
4.3 Load - Deflection behaviour
The ultimate load and the corresponding deflection values were obtained for various
specimens are listed in Table 3. A typical load-deflection plot is shown in Figure 10, it can be
seen that the specimen with core joint showed, low strength degradation in the load-
deflection plot. The performance of core joint was found to be better than the reference and
ductile joints.
Table 3: Comparison of load – deflection
Designation Ultimate Load (kN) Deflection(mm) @ Ultimate Load
RJ 11.25 19.0
DJ 14.20 18.0
CJ 15.10 16.0
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
257
Figure 10: Typical Load-Deflection plot
4.4 Stiffness behavior
Stiffness is the essential variable controlling safety against stability. Stiffness factor is
defined as the load required to cause unit deflection of the beam column joint. The stiffness
factor for the various joints has been estimated based on the average deflection noted at
ultimate load, which is presented in Table-4. The stiffness factor is progressively increasing
from the reference joint to the core joint. A mathematical model has been fitted to a sample
specimen for each type of joint. A close relationship (R2>0.9) has been fixed. Stiffness factor
estimated based on these models (Table-4) also confirm this trend of stiffness factor among
the type of joints.
Table 4: Stiffness factor
Specimen Details Stiffness factor (kN/mm)
RJ 0.59
DJ 0.79
CJ 0.95
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
258
Figure 11: Stiffness Factor for Reference Joint (second specimen)
Figure 12: Stiffness Factor for Ductile Joint (Second specimen)
Figure 13: Stiffness Factor for Core joint (Second specimen)
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
259
4.5 Ductility behavior
Ductility of a structure is its ability to undergo deformation beyond the initial yield
deformation, while still the load being sustained. Ductility can be defined as its ability to
sustain inelastic deformation without loss in load carrying capacity, prior to collapse. The
ductility was evaluated for core joint (CJ) and compared with ductile joint (DJ) and reference
joint (RJ). The ductility factor µ, a measure of ductility of a structure, is defined as the ratio
between ∆u and ∆y, where ∆u and ∆y are the respective deflections at the end of the elastic
range and when the yield is first reached. Thus,
yu ∆∆= /µ
The value of ∆u for various specimens has been determined from the load deflection curve.
After the formation of first crack, the load-deflection curve takes/trying to take a plateau. The
deflection corresponding to that condition has been considered as ∆u and based on this
condition ductility factors have been determined. With reference to the Figure 11, ∆y=13 and
∆u= 22 for RJ, with reference to Figure 12, ∆y=10 and ∆u=25 for DJ and with reference to
Figure 13, ∆y=6 and ∆u=20. Based on the average of three such ∆ys and ∆us the ductility
factors have been estimated and presented in Table-5.
Table 5: Ductility factor
Specimen Designation Ductility factor (µ)
RJ 1.69
DJ 2.50
CJ 3.35
Figure 14: Comparison of Ductility factor (RJ, DJ and CJ)
5. Conclusion
The experimental study on the external beam-column joint with core reinforcement resulted
in the following conclusions:
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
260
1. The fundamental conclusion is that the proposed core joint has performed better than
the other two types of joints studied.
2. The first crack load for the core joint was 15.29 % more than the reference joint and
ductile joint. Whereas, the load at first crack remained same for reference and ductile
joints.
3. The ultimate load carrying capacity of core joint was found to be 25.5% and 6% more
than reference joint and ductile joint respectively.
4. The load deflection behavior was found to be similar between reference joint and core
joint and the ultimate deflection in reference joint was 19mm, 18mm in ductile joint
and 16mm in core joint.
5. The stiffness factor of core joint has exhibited a significant increase of 38 % when
compared to Reference joint and increase of 17% when compared to ductile joint.
6. The ductility factor of core joint was higher by 50 % compared to reference joint and
25 % compared to ductile joint.
List of symbols and abbreviations
ACI - American Concrete Institute
BIS - Bureau of Indian Standards
RJ - Reference Joint
CJ - Core Joint
DJ - Ductile Joint
Ld - Development length
Φ - Diameter of bar
Anh - Area of head
Ab - Area of bar
LVDT - Linear Varying Differential Transducer
6. References
1. ACI committee 352:2002, Recommendations for design of beam column joints in
monolithic reinforced concrete structures. ACI report 352R-02.
2. Ahmed Ghobarah and A.Said., (2002), Shear strength of beam-column joints,
Engineering structures, 24, pp 881-888,
3. Bing Li and Sudhakar A. Kulkarni., (2010), Seismic behavior of reinforced concrete
exterior wide beam-column joints, Journals of structural engineering, pp 26 – 36.
4. Ganesan, N. Indira, P.V. and Ruby Abraham., (2007) Steel fibre reinforced high
performance concrete beam-column joints subjected to cyclic loading, ISET journal
of earthquake technology, 44(3-4), pp 445–456.
5. IS 13920:1993 ductile detailing of reinforced concrete structures subjected to seismic
forces-code of practice.
6. IS 1893(Part 1):2002 criteria for earthquake resistant design of structures.
Experimental investigation on behavior of reinforced concrete beam-column joint
Kaliluthin.A.K, Kothandaraman.S
International Journal of Civil and Structural Engineering
Volume 4 Issue 3 2014
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7. IS 456-2000 – Plain and reinforced concrete - code of practice.
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Wallace., (2007), Mechanical anchorage in exterior beam-column joints subjected to
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