research paper behavior of exterior beam column joints with … · evaluated seismic resistance of...

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Int. J. Struct. & Civil Engg. Res. 2015 Dhake Pravinchandra D et al., 2015

BEHAVIOR OF EXTERIOR BEAM COLUMN

JOINTS WITH DIAGONAL CROSS BARS

AND HEADED BARS

Dhake Pravinchandra D1*, Jape Anuja S1, Patil Hemant S2 and Patil Yogesh D2

1 K.K.Wagh Institute of Engineering Edu. & Research, Nashik, 422003, India.

2 Sardar Vallabhabhai National Institute of Technology, Ichchhanath, Surat, 395007, India.

*Corresponding author: Dhake Pravinchandra D � [email protected]

ISSN 2319 – 6009 www.ijscer.com

Vol. 4, No. 1, February 2015

© 2015 IJSCER. All Rights Reserved

Int. J. Struct. & Civil Engg. Res. 2015

Research Paper

INTRODUCTION

The all reaction forces of columns and beamsin RC structures subjected to strong groundmotions concentrate in the joint, because ofthat beam-column joints are crucial regions ofstructures. Joints are point of weakness due

Beam column joints are one of the most critical components of a reinforced concrete structure,especially if the structure is likely to be subjected to lateral loads. Failure of beam column jointduring earthquake is governed by bond and shear failure mechanism, which is brittle in nature.Unsafe design and detailing within the joint region jeopardizes the entire structure, even if otherstructural members conform to the design requirements. Use of standard 900 and 1800 hookedbars up to required development length often results in steel congestion, difficult fabrication andconstruction, as well as poor concrete placement. Use of the headed bar can offer a potentialsolution for these problems and may also ease fabrication, construction, and concrete placement.This paper presents the experimental work carried out on four different arrangements ofreinforcement of beam column joints. The aim of the research is to investigate the pull-outbehaviors such as strength, failure mode, and crack patterns of different arrangements ofreinforcement in exterior beam column junctions. All joints were tested by using reversed cyclicloading. In the first arrangement, the beam bars are extended in the column for distanceLd+(10xDia) from the inner face of column. In the second arrangement the beam bars arecrossed diagonally in the beam column junction. In the third arrangement headed bars areprovided with all heads in two parallel planes, whereas in the fourth arrangement, the heads areprovided in two orthogonal planes.

Keywords: Beam-column joints, Headed bars, Hysteresis loops, Cyclic loading

to lack of adequate anchorages for barentering the joint from the beam and column.In the design and detailing of beam columnjoints, it is desired to prevent the brittle shearfailure of the joint so that the integral capacityof the connecting beams and columns can be

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developed. It is also necessary to provideproper confinement to the joint core to maintainthe integrity of the joint core and to reduce thestiffness degradation. Proper anchorage ofreinforcement is essential to reinforcedconcrete structures to ensure composite actionbetween reinforcement and concrete to resistthe member design forces. In general,anchorage is achieved by a combination ofbond and bearing on hooks. Failure of beamcolumn joint during earthquake is governed bybond and shear failure mechanism, which isbrittle in nature. Unsafe design and detailingwithin the joint region jeopardizes the entirestructure, even if other structural membersconform to the design requirements (Uma andPrasad, 2006).

In conventional practice to reducedevelopment length of bar, bends are requiredfor effective transfer of load. In normal practice900 and 1800 hooks are provided. These aregenerally known as conventional anchorages.

Bend reduces length of bar because ofincrease in frictional resistance at the bend dueto confinement of concrete inside the bend byradial components of bar in tension. Hansonet al. (1972) tested corner joint, side joint andinterior joint specimens with conventionalanchorage system. Alva et al. (2007) studiedcyclic behavior of RC connectionsexperimentally and concluded that concretecompressive strength is the major factor thatgoverns the joint shear capacity. Theexperimental results also indicated that jointtransverse reinforcement affects the load-displacement response of such connections(Alva et al., 2007). Scott investigated straindistribution of beam column junctions withconventional anchorage system of

reinforcement. The author also focused onlocation of plastic hinge formation (Scott,1991). Asha and Sunderrajan (2007)evaluated seismic resistance of exterior beamcolumn joints with detailing as per IS 13920-1993.

Some researchers used diagonallycrossed bars in exterior beam columnjunctions and found that diagonal bars hadimproved the ductility and energy absorptioncapacity than the specimens with arrangementof 900 hooks (Bindhu and Jaya, 2010). Evenfor interior beam column junctions diagonal bararrangement had shown improved results(Jing, 2003).

There are lots of disadvantages of suchconventional anchorages. Use of standardhooks often results in steel congestion, difficultfabrication and construction, as well as poorconcrete placement. Use of the headed baroffers a potential solution for these problemsand may also ease fabrication, construction,and concrete placement.

Headed bars are formed by attachments ofplate at the end of straight reinforcing bar. Suchbars are anchored by combination of bondalong straight bar length and direct bearing atthe head. Like hook bars they can developsufficient anchorage strength within shortdistance, but, they do not create muchcongestion. However headed bars have notbeen widely used in other structure such asbridges, building, or other traditional structures.Chun et al. tested some beam column junctionswith headed bars and found that headed barhas enough anchorage capacity in the exteriorbeam-column joints (Chun et al., 2007; Chunand Kim, 2004). In their study all heads are

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kept in one plane. There is little guidancecurrently available for the design of headed baranchorage either in the form of code provisionsor published research.

This study attempts to investigate the pull-out behaviors such as strength, failure mode,and crack patterns of different arrangementsof reinforcement in exterior beam columnjunctions. In the first arrangement, the beambars are extended in the column for distanceLd+(10xDia) from the inner face of column. Inthe second arrangement the beam bars arecrossed diagonally in the beam columnjunction. In the third arrangement headed barsare provided with all heads in two parallelplanes, whereas in the fourth arrangement, theheads are provided in two orthogonaldirections.

EXPERIMENTAL

INVESTIGATIONS

Material Properties and ConcreteMix Design

The materials required for the experimentalwork were tested in the laboratory to getnecessary data for mix design. 53 gradePozzolana Portland cement was used. Naturalriver sand with specific gravity 2.69 andfineness modulus 3.5 which conforms tograding zone II was used as fine aggregate.Crushed basalt with maximum size of 20 mmand specific gravity 2.79 is used as courseaggregate. Concrete mix design is carried outfor concrete grades M30 for mediumworkability. The mix proportions are finalizedafter taking some trials for target strengthdetermined by considering standard deviationequal to 5.

Reinforcement: Thermo mechanicallytreated ribbed bars of diameter 12 mm wereused (TMT-TISCON). Three bars were testedfor mechanical properties. For all the barsultimate stress was in the range 650 to 665 N/mm2 and 0.2% proof stress was in the range515 to 525 N/mm2.

Details of Specimen

Exterior beam column joint was considered forexperimental work. In the test model thedimension of beam was 200 x 165 mm withlength of 400 mm and column size was 220 x165 mm with total height of 800 mm.

Reinforcement Details

In all the specimens main reinforcementprovided in the beam was 3-#12 at top and 3-#12 at bottom. In column 4-#12 + 4-#10reinforcement was provided. In beam #6 @75 mm C/C stirrups were provided whereasin columns #6@ 75 mm C/C ties wereprovided.

• Specimen S1: The reinforcement details ofbeam column joint are shown in Figure 1.The arrangement of the reinforcement isprovided according to IS 13920-1993(1993). The beam bars are extended in thecolumn for distance Ld+(10xDia) from theinner face of column.

• Specimen S2: The reinforcement details ofbeam column joint are shown in Figure 2.All four corner bars of the beam areextended in the column for distanceLd+(10xDia) from the inner face of column.For 12 mm diameter this length is 660 mm.The top and bottom middle bars of the beamare extended diagonally in the column.

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Figure 2: Reinforcement Arrangement with Diagonally Crossed Bars

220

A300

B

165

200

2#12 barsat top

2#12 barsat bottom

4#12 at corner

BEAM

1#10 at the centre of column on all faces

200

300

COLUMN

S2

B

6mm stirrups@ 75mm A-A B-B

2 #12 at top

2 #12 at bottom

4#10

220

2 #12 top

2 #12

2 #10

All dimensions are in mm

A

R/F detail is same as S1, onlycentre face bars from thebeam have been provideddiagonally.

400

165

Figure 1: Reinforcement Arrangement as per IS 13920

220

A300

B

165

200

2#12 bars at top

2#12 bars at bottom

4#12 at corner

BEAM


200

300

COLUMN

S1

B


2 #12 at top

2 #12 at bottom

4#10

220

2 #12 top

2 #12

2 #10


A

400

165

2#12

2#12

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• Specimen S3: The reinforcement details ofbeam column joint are shown in Figure 3.All the beam bars were provided withheads of diameter 50.4 mm and thickness12 mm. The head was drilled centrally. Thebar was inserted in the hole and weldedfrom both the faces. All six head plates werekept in two parallel vertical planes. Thehead plates of the corner bars are touchedto the column bars of outer face. The headplate of the middle bar was kept 50 mmabove the head plates of the corner bars.

• Specimen S4: The reinforcement details ofbeam column joint are shown in Figure 4.All the beam bars were provided withheads of diameter 50.4 mm and thickness12 mm. The head plates of the corner barsare touched to the column bars of outer face.These four plates are kept in vertical plane.

The remaining middle top bar and middlebottom bar were bent through 90º for 100mm length. At the end of these ‘L’ bent headplate were welded. Now these two headplates were in horizontal plane.

CASTING AND CURING

The mould is arranged properly and placedover a smooth surface. The sides of the mouldexposed to concrete were oiled well to preventthe side walls of the mould from absorbingwater from concrete and to facilitate easyremoval of the specimen. Concrete mixdesigned for M30 was used. The concrete wasplaced into the mould immediately after mixingand well compacted. Control cubes and wereprepared for all the mixes along withconcreting. The moulds were removed after24 h from casting. All the specimens were

Figure 3: Reinforcement Arrangement with Headed Bars(Head Plates in Parallel Planes )

220

A300

B

165

200

2#12 bars at top

2#12 bars atbottom

4#12 at corner

BEAM


200

300

COLUMN

S3

B


2 #12 at top

2 #12 at bottom

4#10

220

2 #12 top

2 #12

2 #10


A

R/F detail is same asS1, only all bars fromthe beam are headedbars upto outer face ofthe column.

400

165

2#12

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cured in water for 28 days. After 28 days ofcuring the specimen were dried in air and whitewashed.

Test Setup and Instrumentation

The specimen was tested in a reaction frame.The test setup is shown in Figure 5. A 1000

Figure 5: Test Setup

Figure 4: Reinforcement with Headed Bars (Head Plates in Orthogonal Planes)

220

A300

B

165

200

2#12 bars at top

2#12 bars atbottom

4#12 at corner

BEAM


200

300

COLUMN

S4

B


2 #12 at top

2 #12 at bottom

4#10

220

2 #12 top

2 #12

2 #10


A

R/F detail is same asS3, only top andbottom centre headedbars are bent incolumn up to 100mm.

400

165

2#12

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kN capacity calibrated hydraulic jack mountedvertically on the frame was used to apply axialload on the column. A constant load of 100 kN,which is about 15% of the axial capacity of thecolumn, was applied to the columns for holdingthe specimens in positions and to simulatecolumn axial load. Two ends of the columnwere given an external axial hinge support inaddition to lateral hinge support provided atthe top and bottom of the column. Another two500 kN capacity hydraulic jacks were used toapply reverse cyclic load. The load was appliedat distance 50 mm from free end of the beamface. The load was measured by inserting loadcell in between the jack and the beam face.Loading was applied gradually such as 5, 10,15, 20, …70, 75 kN respectively for forwarddirection and 5,10,15, 20,…70, 75 kN,respectively for reverse direction. Figure 6shows the loading history in terms of appliedcycles versus load. Two LVDTs were used tomeasure deflections. The deflections weremeasured at the beam free end tip (at loadingpoint) and at distance 175 mm from columnface along beam.

RESULTS AND DISCUSSION

Figure 7 to 10 show the crack pattern and theloads at which cracks appeared.

Figure 11 to 14 show the load Vs. deflectiongraphs-hysteresis loops for joints S1, S2, S3and S4, respectively.

The diagonal first crack at the joint regioninitiated at 25 kN load in joint S1 and S2whereas in joints S3 and S4, it initiated at 35kN load. The first crack at beam interfaceinitiated at 25 kN load in S1, 20 kN in S2, 15kN in S3 and S4. With further increase inloading, the cracks propagated further and

initial cracks started widening. At each cyclesome new cracks formed. In joint S1 the cracksare distributed on entire beam column joint. Inspecimen S2 major only five cracks occurred,two diagonal ‘x’ crack in the joint region, onealong the beam interface and two ‘x’ cracks inthe beam portion. In Joint S3 only four majorcracks appeared, one along beam interface,two ‘x’ cracks in the beam portion and onediagonal crack in the joint region. The diagonalcrack in the joint region which initiated in 7th

cycle, propagated further directly in the 13th

cycle. The crack pattern of S4 is almost sameas that joint S2 with the main difference thatthe diagonal cracks propagated in parallellines. This difference in the crack pattern is duethe arrangements of heads in orthogonalplanes. The cracks in joint S3 had not widenedmuch more as compared to other joints. Afteroccurring initial cracks, increase in load wasobserved. It may be due to the followingreasons. During cyclic loading, when unloadingtakes place, tip of the crack becomes blunt,and during reloading the specimen, moreenergy is required to propagate the crack orto change the direction of propagation fromthe blunt crack tip. This in turn increases theultimate load (Ganeshan, 2007). The strengthparameters and energy dissipation amongdifferent joint specimens are compared andreported in Table 1.

The seismic design philosophy relies onproviding sufficient ductility to the structure bywhich the structure can dissipate seismicenergy. The structural ductility essentiallycomes from the member ductility wherein thelatter is achieved in the form of inelasticrotations. In reinforced concrete members, theinelastic rotations spread over definite regions

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Figure 6: Load Sequence Diagram

Figure 7: Crack Pattern of Joint S1

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Figure 11: Load - Displacement Hysteresis Loops of Joint S1

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Figure 15: Cumulative Energy Dissipation Curves

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called as plastic hinges. During inelasticdeformations, the actual material propertiesare beyond elastic range and hence damagesin these regions are obvious. The plastichinges are “expected” locations where thestructural damage can be allowed to occur dueto inelastic actions involving largedeformations. Hence, in seismic design, thedamages in the form of plastic hinges areaccepted to be formed in beams rather thanin columns. Mechanism with beam yielding ischaracteristic of strong-column-weak beambehavior in which the imposed inelasticrotational demands can be achievedreasonably well through proper detailingpractice in beams (Uma and Prasad, 2006).In the present study from the crack pattern, itis observed that the failure mode for joints S1,S2 and S4 is combined joint and beam modefailure, whereas in joint S3, it is beam modefailure.

The load-displacement hysteresis loop forjoint S1, S2 and S4 exhibited almost sameultimate strength, Joint S3 failed at 70 kNultimate load, but the failure was not at thejunction or beam interface but in shear at theload point. The loops are much closely spacedfor Joint S3. Joint S4 had shown largedisplacements during downward cycles.

Energy Dissipation

The energy dissipated at the beam column jointspecimens through plastic deformation wasthe sum of the area in the beam tip load-displacement hysteresis loop as shown inFigure 15. The energy dissipated by Joint S3is lowest, which is due to early failure. The bestenergy dissipation potential was exhibited byJoint S4.

The arrangement of reinforcement withheaded bar performed equally well and gavegood results as compared to bent up bars anddiagonal bars under static cyclic loading. In thisstudy the provided diameter of the head wasevaluated from the possible ultimate concretestress immediately behind the head, whereonly the head area minus the bar area iseffective. But, if the pressure cone behind headis considered, then required area of head canbe reduced considerably. The effect ofconfining reinforcement and the bond stresssupplied by the embedded length of the ribbedbar will further reduce the required area of thehead.

CONCLUSION

1. The failure mode for joints S1, S2 and S4is combined joint and beam mode failure,whereas in joint S3, it is beam mode failure.It shows that head plate attached to straight

Table 1: Ultimate Strength and Energy Dissipation of Joints

Joint First crack load First crack load Ultimate Cumulative Energy Mode of

Specimen at joint region at beam Interface Load kN Dissipation kN mm Failure

S1 25 kN 25kN 75 1739 J-B

S2 25kN 20kN 75 1025 J-B

S3 35kN 15kN 70 912 B

S4 33kN 15kN 75 2166 J-B

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bar forms compression strut in front of theplate which reduces the diagonal cracks.

2. Headed bars with orthogonally orientedplates provide best energy dissipationpotential to the external beam column joint.

3. The arrangement of reinforcement withheaded bar performed equally well andgave good results as compared to bent upbars and diagonal bars under static cyclicloading.

REFERENCES

1. Alva G M S, El Debs A L D C and El DebsM K (2007), “An Experimental Study onCyclic Behaviour of Reinforced ConcreteConnections”, Canadian Journal of CivilEngineering, Vol. 34, No. 4, pp. 565-575.

2. Asha P and Sundarajan R (2007),“Evaluation of Seismic Resistance ofExterior Beam-Column Joints withDetailing as per IS 13920:1993”, TheIndian Concrete Journal, pp. 29-34.

3. Bindhu K R and Jaya K P (2010),“Strength and Behaviour of Exterior BeamColumn Joints With Diagonal CrossBracing Bars”, Asian Journal of CivilEngineering (Building and Housing),Vol. 11, No. 3, pp. 397-410

4. Chun SC, Lee S, Kang T H K, Oh B andWallace J W (2007), “MechanicalAnchorage in Exterior Beam-Columnjoints Subjected to Cyclic loading,” ACIStructural Journal, Vol. 104, No. 1, pp.102-112.

5. Chun S C and Kim D Y (2004),“Evaluation of Mechanical Anchorage of

Reinforcement by Exterior Beam-Columnjoint Experiments,” 13th WorldConference on Earthquake Engineering,Vancouver, B.C., Canada, Paper 0326,2004.

6. Ganesan N, Indira P V and Abraham R(2007), “Steel Fibre Reinforced HighPerformance Concrete Beam-ColumnJoints Subjected To Cyclic Loading,”ISET Journal of EarthquakeTechnology, Vol. 44, No. 3-4, pp. 445-456.

7. Hanson N W and Conner H W (1972),“Tests of Reinforced Concrete Beam-Column joints Under Simulated SeismicLoading”, Research and DevelopmentBulletin RD012, Portland CementAssociation, pp. 1-11.

8. IS 13920, “ductile detailing of reinforcedConcrete structures subjected to Seismicforces,” BIS, 1993.

9. Jing L (2003) “ Effects of Diagonal SteelBars on Performance of Interior Beam-Column Joints Constructed with HighStrength Concrete”, Research Thesis,University of Hong Kong.

10. Scott RH (1991), “A PreliminaryInvestigation of Strains in ReinforcedConcrete Beam-Column Connectionsdue to Seismic Loading”, Magazine ofConcrete Research, Vol. 43, No. 154, pp.59-64.

11. Uma S R and Prasad A M (2006),“Seismic Behaviour of beam column jointsin RC moment resisting frames A review”,The Indian Concrete Journal, pp. 33-42.

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