Experimental investigation on behavior of reinforced ... beam-column joint is a very critical part...

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

[email protected]

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.





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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.





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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)





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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.





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.





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





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





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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





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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





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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)





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:





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.





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7. IS 456-2000 – Plain and reinforced concrete - code of practice.

8. Josef Hegger, Alaa Sherif and Wolfgang Roeser., (2003), Nonseismic design of

beam-column joints, ACI structural journal, pp 100-S68.

9. Murthy C.V.R., Durgesh C. Rai, bajpai.K.K, and Sudhir K. Jain., (2003),

Effectiveness of reinforcement details in exterior reinforced concrete beam column

joints for earthquake resistance, ACI structural journal, 100(2), pp 149 -156.

10. Pradip Sarkar, Rajesh Agarwal and Devdas Menon., (2007), Design of RC beam-

column joints under seismic loading – a review, Journal of structural engineering,

33(6), pp 449-457.

11. Sathish Kumar S.R., Vijaya Raju B. and Rajaram G.S.B.V.S., (2002), Hystertic

Behavior of lightly reinforced concrete exterior beam-to-column joint sub-

assemblages, Journal of structural engineering, 29(1), pp 31-36.

12. Shyh-Jiann Hwang, Hung-Jen Lee, Ti-Fa Liao, Kuo-Chou Wand and Hsin-Hung Tsai,

(2005), Role of hoops on shear strength of reinforced concrete beam-column joints,

ACI structural journal, pp 102-S45.

13. Siva Chidambaram K.R, Thirugnanam G.S., (2012), Comparative study on behavior

of reinforced beam-column joints with reference to anchorage detailing, Journal of

civil engineering research, 2(4), pp 12-17.

14. Sung Chul chun, Sung Ho Lee, and Thomas H.-K. Kang, Bohwan Oh, and John W.

Wallace., (2007), Mechanical anchorage in exterior beam-column joints subjected to

cyclic loading, ACI structural journal, pp 104-S12.

15. Tsonos A.G.,Tegos I. A.,and Penelis G. Gr., (1992), Seismic resistance of type 2

exterior beam-column joints reinforced with inclined bars, ACI structural journal, pp

89-S1.

16. Uma S.R. and Sudhir k. Jain., (2006), Seismic design of beam column joints in

reinforced concrete moment resisting frames–review of codes, Structural engineering

and mechanics, 23(5), pp 579_597.

17. Veerendra Kumar and Mohammed Shamim., (1999), Influence of beam reinforcement

on exterior beam – column joints, Journal of structural engineering, 26(2), pp 123 –

127,

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