* (Research Scholar), Lecturer (SG)/ Civil Engg. (Associate Professor)/ Civil Engg., Govt. Polytechnic College,Coimbatore Coimbatore Institute of Technology, Coimbatore.

*Revathi, R. and *Sri Santhi, V. G.

ABSTRACT

The current design recommendations for reinforced concrete beam column joints are based onexperimental investigations on the Seismic behavior, the effect of slab on the behavior of beam columnjoints in the design has not been completely resolved. The effective width of the slab to be used for thecase of seismic loading has not been explicitly addressed in design code. Ignoring the slab contributionto the flexural capacity of beam leads to significant underestimation in structural strength and it alsoleads to failure mechanism. The effect of presence of slab on the behavior of beam column joint isnecessary. Numerous studies have underscored the significance of the presence of slab on the beam –column joints. Therefore to focus the effective participation of slab on the behavior of joints anexperimental investigation on Beam – Column joints with slab (BCJS) and Beam – Column jointswithout slab (BCJ) under monotonic loading was carried out.

Keywords: Beam column joint, Beam column joint with slab, Exterior column, Energy dissipation,and Ductility factor.

INTRODUCTION

Under loading condition, the slab acts as an integral part of the main beam, increases boththe positive and negative flexural capacities of the beam. The contribution of slab to thepositive flexural capacity of the beams in terms of equivalent effective slab width, asrecommended by ACI 318-83 is well accepted and is commonly used in gravity load designof buildings. The contribution of slab to the negative flexural capacity of beam ( ie, when aslab is in tension whether in terms of an effective slab width or otherwise) has not yet beensettled. The primary reason is that due to lack of adequate and conclusive test data nospecific recommendations on the effective width of slab included. It is also difficult tocompare the results from tests of specimen and correlating the response of connectionstested in the laboratory with the performance of a real building.

BACKGROUND INFORMATION

Hikmat E. Zerbe et al. (1990) studied the seismic response of connections in two bayreinforced concrete frame subassemblies with floor slab. It was found that the presence of

I J C E4(1) (2012): pp. 85-92

slab increases the shear in the joints and was not affected by the energy dissipations capacityin continues subassembly. Lateral load resistance increased by as much as 30 percent at 3percent lateral drift and degradation of stiffness. Based on the results, the suggestion is madeto include the effect of a floor slab in the procedure for designing beam column connections.

Gilson N.Guimaraes et al. (1992) studied the evaluation of joint-shear on four interiorbeam-column slab connections using combination of normal and high-strength (concreteand reinforcement) materials. It was concluded that both the normal and high strengthtransverse reinforcement provided adequate confinement for joints.

Myoungsu Shin et al. (2004) conducted experiment on reinforced concrete eccentricbeam-column-slab subassemblies (edge connections with flush exterior edge-beam andcolumn faces) under large lateral displacement reversals. It was concluded that the floorslabs diminished differences between seismic performances of the specimens and increasedjoint shear strengths of the specimens when compared with other eccentric connectionswithout floor slabs.

Bindu. K. R. et al. (2008) investigated the effect of cross inclined bars at the joints asconfining reinforcement on the behaviour of exterior reinforced concrete beam-to- columnconnections subjected to earthquake loading. The test results indicated that the cross inclinedbars as confining reinforcement improve the seismic performance of the joints.

Burcu B. Canbolat et al., (2008) an experimental study was conducted that focused onthe effect of eccentricity of spandrel beams and floor slab with respect to the column. Testresults indicated that including the floor system significantly reduced the negative influenceof eccentricity and the damage was reduced. It was observed that the deterioration of jointshear stiffness and strength were delayed.

Thomas H., K. Kang et al. (2008) conducted an experimental study to provide acomparative assessment of the seismic performance of the shear reinforcement for reinforcedconcrete slab – column connections. It was concluded that both forms of shear reinforcementstudied effectively increased the punching shear strength and ductility of the slab-columnconnections.

EXPERIMENTAL SETUP

In order to study the behavior of BCJS an experimental study was conducted on reinforcedconcrete Beam -column joint (BCJ) and Beam- column joint with slab (BCJS) of a fourstoried reinforced concrete building. The building was analyzed with seismic loading andits joints were designed according to IS 1893 (Part-A) and IS 13920- 1993. From the analysisand design results an edge beam column joint was taken for the experimental investigation.The original size of the exterior beam and column was 360mm X 360mm. An one thirdscale exterior beam – column joint with and without slab were cast and tested undermonotonic loading. The beam - column joint specimen was three dimensional and consistsof top and bottom columns, one exterior beam to the right and another to the left of columnin the transverse direction and one middle beam in the perpendicular direction to the rightand left beam.

Figure 1: Test Setup for Beam Column Joint Specimen

The column was subjected to constant compressive load at the top with the help ofhydraulic jack and the bottom end was fixed. The ends of the right beam (RB) and leftbeam (LB) were simply supported, because the moment developed at the left end will bethe same as that developed at the right end. Monotonic loading was applied at the end ofmiddle beam (MB) using hydraulic jack, the deflections at the junction of LB,RB and MBwere noted with LVDT’s placed at the top of the beams near the joints and at the bottom ofmiddle beam. Strain in concrete was observed nearby the beam and column joint regionusing strain gauges. Proving ring was used to note the incremental value of loading exactly.Deflection, strain and crack were noted for each increment of loading carefully.

Same procedure was followed for the beam-column with slab. The sizes of the beamand column were the same and the centre line of beam coincided with the centre line of thecolumn to avoid eccentricity of the column. Beam – Column joint with slab consist of theabove beam with slab covering left , right and middle beams as shown in fig. 2.

MATERIAL PROPERTIES

Size of Specimen

BCJ and BCJS specimens were reduced to one third scale to suit the loading arrangement,size of both beam and column were 120 mm X 120mm. Length of column was 1120mm,length of beam on right and left of column was 630mm each and length of middle beamwas 830mm.The dimensions of slab was 1440mm X 830mm X 50mm.

Portland pozzolana cement conforming to IS 1489 – 1991 was used. The results ofpreliminary tests of cement were specific gravity 3.15, Standard consistency 30% , InitialSetting time - 120 minutes, 7 days compressive strength was 19.40 N/mm2 and 28 dayscomp. strength was 28.72 N/mm2, splitting tensile strength was observed as 2.83 N/mm2

at 28 days and tensile strength of concrete was 6.53 N/mm2. Sand conforming to zone IIIof IS 383 -1970 was used as fine aggregate. Its fineness modulus was 2.2 and specificgravity was 2.7. Crushed granite aggregate particles passing through 10 mm and retainedon 4.75 mm IS Sieve was used as coarse aggregate which met the grading requirement ofIS 383 – 1970. Its specific gravity was 2.72 and water absorption was 0.15%.Potablewater which satisfies drinking standards was used for concrete mixing and its subsequentcuring. Concrete mix was designed for M20 grade of concrete as per the procedure in IS10262-1982. The mix proportion for M20 grade of concrete was 1 : 1.48 : 3.33 with watercement ratio of 0.5.

EXPERIMENTAL RESULTS

From the experimental study the following parameters were investigated.

1. Deflection

2. Energy dissipation

3. Ductility factor.

Load Vs deflection curve is drawn. The deflection was very minimum if the slab wasconsidered with the Beam column joint. Fig. 3 shows Load Vs Deflection curve (BCJ),Fig. 4. shows Load Vs Deflection of Beam column joint with slab (BCJS).

Figure 2: Test setup for Beam Column Joint with Slab Specimen

Graph is drawn between Energy dissipation and Deflection. Energy dissipation capacitywas also greater in the Beam–Column joint with slab (BCJS). Fig 5 and Fig 6 show theenergy dissipation Vs Deflection curve of BCJ and BCJS specimens.

Cracks were developed at the junction of beam – column joint at 7.7 kN , where as inthe beam – column joint with slab, yield line cracks were formed only on the surface of theslab at 30 kN. Fig. 7 and Fig. 8 show the crack pattern of BCJ and BCJS Specimens.

Figure 3: Load Vs Deflection of BCJ

Figure 4: Load Vs Deflection of BCJS

Figure 5: Energy Dissipation Vs Deflection of BCJ

Figure 6: Energy Dissipation Vs Deflection of BCJS

Figure 7: Shows the Crack Pattern of Beam Column Joint

Ductility Factor

Ductility is an important characteristic of any structural element. It is described as thecapacity of a structural element to undergo beyond yield without loosing much of the loadcarrying capacity. It has generally been measured by a ratio called ductility factor. It isusually expressed as a ratio of deflection (�) at failure to the corresponding property atyield.

Ductility factor µ� =

�

u / �

y

where �u – Ultimate displacement, �y – yield displacement

Ductility factor was considerably increased in beam column joint with slab (BCJS).

Displacement

S. No. Name of the speciemen Yield �y

Ultimate- �u

Ductility factor(mm) (mm) µ��=

�u / �

y

1 Beam column joint 11.28 42.28 3.748without slab (BCJ)

2 Beam column with slab (BCJS) 5.2 31.5 6.057

SUMMARY AND CONCLUSION

An experimental investigation was carried out on BCJ and BCJS specimens under monotonicloading and performance of BCJ and BCJS were analysed.The effect of slab in Beam columnwith slab was effective when compared with Beam column joint.

Figure 8: Shows Crack Pattern of Beam Column Joint with Slab

1. The yield load carrying capacity of BCJS was 4.70 times more than BCJ specimen.

2. The ultimate load carrying capacity of BCJS specimen was 2.69 times more thanBCJ specimen.

3. The energy dissipation capacity of BCJS was 2.32 times more than the BCJspecimen.

4. Initial crack was observed at the slab in BCJS specimen and it was propagatedtowards the beam , but Initial crack was formed at the junction of the beam columnjoint in BCJ specimen.

5. The ductility factor of BCJS was 1.62 times more than the BCJ specimen.

From the experimental investigation it is quite evident that the effect of slab in beamcolumn joint enhances the yield load carrying capacity, ultimate load carrying capacity andenergy dissipation capacity. Hence it is inevitable that the effect of slab should be takeninto consideration while designing beam- column connections.

References

[1] Hikmat E. Zerbe and Ahmad J. Durrani, (1990), “Seismic Response of Connections in two-bay R/CFrame Subassemblies”, Journal of Structural Engineering, 115(11).

[2] Gilson N. Guimaraes, Michael E. Kreger, and James O. Jirsa (1992), “Evaluation of Joint-shear Provisionsfor Interior Beam Column Slab Connections using High-strength Materials”, ACI Structural Journal,89(1).

[3] Ahmed Ghobaraah and A.Said (2002), “Shear Strength of Beam Column Joints”. Journal of EngineeringStructures, 24, 881-888.

[4] Myoungsu Shin and James M. LaFave (2004), “Seismic Performance of Reiforced Concrete EccentricBeam-Column Connections with Floor Slabs”, ACI Structural Journal, Title no.101-S41, Pg 403-412.

[5] Ahmed Ghobaraah and A.Said (2002), “Shear Strength of Beam Column Joints”. Journal of EngineeringStructures, 24, 881-888.

[6] Devados Menon, Pradip Sarkar and Rajesh Agrawal (2007), “Design of RC Beam Column Joints UnderSeismic Loading – A Review”. Journal of Structural Engineering, 33, Pg 449-457.

[7] Bindu, K. R. and Jaya K. P. (2008), “Performance of Exterior Beam Column Joints With Cross InclinedBars Under Seismic Type Loading”, Journal of Engineering and Applied Science, 7, Pg 591-597.

[8] Burcu B. Canbolat and James K.Wight (2008), “Experimental Investigation on Seismic Behavior ofEccentric Reinforced Concrete Beam-Column-Slab Connections”, ACI Structural Journal, Title no. 105-S16, Pg 154-162.

[9] Thomas H., K. Kang and John W. Wallace, (2008), “Seismic Performacnce of Reinforced Concrete Slab-Column Connections with Thin Plate Stirrups” ACI Structural Journal, Title no. 105-S58, Pg 617-625.

[10] IS:13920-1993, “Indian Standard Code of Practice for Ductile Detailing of Concrete Structures Subjectedto Seismic Forces, Bureau of Indian Standards, New Delhi, 1993.