Experimental Study on behavior of Interior RC Beam Column Joints Subjected to Cyclic Loading P.Rajaram 1 A.Murugesan 2 and G.S.Thirugnanam 3 1 P.G.Student, Department of Civil Engineering, Institute of Road And Transport Technology, Erode 638316, Tamil Nadu, India.. 2 Research Scholar, Department of Civil Engineering, Sona College of technology, salem 636005, Tamil Nadu, India. 3 Assistant Professor and Head of Department of Civil Engineering, Institute of Road And Transport Technology, Erode 638316, Tamil Nadu, India.. gst_irtt@yahoo.co.in ABSTRACT Beam column joint is an important component of a reinforced concrete moment resisting frame and should be designed and detailed properly, especially when the frame is subjected to earthquake loading. Failure of beam column joints during earthquake is governed by bond and shear failure mechanism which are brittle in nature. Therefore, a current international code gives high importance to provide adequate anchorage to longitudinal bars and confinement of core concrete in resisting shear. Modern codes provide for reduction of seismic forces through provision of special ductility requirements. Details for achieving ductility in reinforced concrete structures are given in IS 13920. A two bay five storey reinforcement cement concrete moment resisting frame for a general building has been analyses and designed in STAAD Pro as per IS 1893 2002 code procedures and detailed as IS 13920 1993 recommendations. A beam column joint has been modeled to a scale of 1/5 th from the prototype and the model has been subjected to cyclic loading to find its behavior during earthquake. Non linear analysis is carried out in ANSYS software. Key Words: Beam Column Joint, Earthquake resistant Structures. 1. Introduction: The behavior of reinforced concrete moment resisting frame structures in recent earthquakes all over the world has highlighted the consequences of poor performance of beam column joints. Beam column joints in a reinforced concrete moment resisting frame are crucial zones for transfer of loads effectively between the connecting elements (i.e. beams and columns) in the structure. In the analysis of reinforced concrete moment resisting frames, the joints are generally assumed as rigid. In Indian practice, the joint is usually neglected for specific design with attention being restricted to provision of sufficient anchorage for beam longitudinal reinforcement. This may be acceptable when the frame is not subjected to earthquake loads. There have been many catastrophic failures reported in the past 49
earthquakes, in particular with Turkey and Taiwan earthquakes occurred in 1999, which have been attributed to beam column joints. The poor design practice of beam column joints is compounded by the high demand imposed by the adjoining flexural members (beams and columns) in the event of mobilizing their inelastic capacities to dissipate seismic energy. Unsafe design and detailing within the joint region jeopardize the entire structure, even if other structural members conform to the design requirements. Since past three decades, extensive research has been carried out on studying the behavior of joints under seismic conditions through experimental and analytical studies. Various international codes of practices have been undergoing periodic revisions to incorporate the research findings into practice. In RC buildings, portions of columns that are common to beams at their intersections are called beam column joints. Since their constituent materials have limited strengths, the joints have limited force carrying capacity. When forces larger than these are applied during earthquakes, joints are severely damaged. Repairing damaged joints is difficult, and so damage must be avoided. Thus, beam column joints must be designed to resist earthquake effects. Under earthquake shaking, the beams adjoining a joint are subjected to moments in the same (clockwise or counterclockwise) direction. Under these moments, the top bars in the beam column joint are pulled in one direction and the bottom ones in the opposite direction. These forces are balanced by bond stress developed between concrete and steel in the joint region. If the column is not wide enough or if the strength of concrete in the joint is low, there is insufficient grip of concrete on the steel bars. In such circumstances, the bar slips inside the joint region, and beams lose their capacity to carry load. Further, under the action of the above pull push forces at top and bottom ends, joints undergo geometric distortion; one diagonal length of the joint elongates and the other compresses. If the column cross sectional size is insufficient, the concrete in the joint develops diagonal cracks. 1.1. Types of joints in frames: The joint is defined as the portion of the column within the depth of the deepest beam that frames into the column1. In a moment resisting frame, three types of joints can be identified viz. interior joint, exterior joint and corner joint (Fig.1). When four beams frame into the vertical faces of a column, the joint is called as an interior joint. When one beam frames into a vertical face of the column and two other beams frame from perpendicular directions into the joint, then the joint is called as an exterior joint. When a beam each frames into two adjacent vertical faces of a column, then the joint is called as a corner joint. The severity of forces and demands on the performance of these joints calls for greater understanding of their seismic behavior. These forces develop complex mechanisms involving bond and shear within the joint. (i) Interior joint (ii) Exterior joint (iii) Corner Joint Fig 1: Type of joints in frame 50
2. Experimental Investigation: The experimental study on interior beam column joint of a multistory reinforced concrete building in Salem Zone falling under the seismic Zone III has been analyzed using STADD.pro. The specimens were designed for seismic load according to IS 1893(Part I): 2002 & IS 13920: 1993. The structure is five storey two bay frames including 1.5 m foundation depth. The maximum moment is occurred at the ground floor roof level. We are considering that particular joint for the experimental study. Fig 2: Elevation and plan view of two bay frame (G+4) 2.1. Details of specimen: The test specimen was reduced to 1/5 th scale to suit the loading arrangement and test facilities. Prototype specimen having beam dimension of 305 X 460 including slab thickness and column dimension of 305 X 460. For testing model the dimension of beam was 120 X 170 mm without slab thickness and beam length of 450mm and that column size was 120 X 230 mm. Height of the column was 600mm. 2.2. Description of the formwork and reinforcement: Reinforcement for test specimen Fig 2: Formwork and 51
2.3. Reinforcement details: The reinforcement details of beam column joint are shown in fig.2. Main reinforcement provided in the beam was 10 mm diameter bars, 3 No s at top and 3 No s at bottom. The stirrups are 6 mm diameter bars at 30 mm c/c for a distance of 2d, i.e. 300 mm from the face of the column and at 60 mm c/c for remaining length of the beam. The longitudinal reinforcement provided in the column was 8 No s of 8 mm diameter bars equally distributed along four sides of column. The column confinements are 6 mm diameter bars at 30 mm c/c for a distance of 150 mm from the face of the column and at 60 mm c/c for remaining length of the column. 2.4. Casting and curing: The mould is arranged properly and placed over a smooth surface. The sides of the mould exposed to concrete were oiled well to prevent the side walls of the mould from absorbing water from concrete and to facilitate easy removal of the specimen. The reinforcement cages were placed in the moulds and cover between cage and form provided was 20 mm. Concrete mix designed for M30 (1:1:2.5) and water cement ratio is 0.40. Cement mortar block pieces were used as cover blocks. The concrete contents such as cement, sand, aggregate and water were weighed accurately and mixed. The mixing was done till uniform mix was obtained. The concrete was placed into the mould immediately after mixing and well compacted. Control cubes and cylinders were prepared for all the mixes along with concreting. The test specimens were remolded at the end of 24 hours of casting. They were marked identifications. They are cured in water for 28 days. After 28 days of curing the specimen was dried in air and white washed. 230 6mm dia lateral ties @ 30mm c/c 450 215 6mm dia 2 legged stirrups @ 30mm c/c 3 Nos of 10 DIA 300 120 170 215 3 Nos of 10 DIA 6mm dia 2 legged stirrups @ 60mm c/c 120 ALL DIMENSION IN mm 230 8 Nos of 8 DIA Fig 3: Ductile Detailing of Beam Column Joint as per IS 13920; 1993 52
Fig 4: Casting Stage of Beam Column Joint 2.5. Test setup and instrumentation: The specimen was tested in a reaction frame. The test setup is shown in figure 4. A hydraulic jack was used to apply the axial load for column. To record the load precisely a proving ring was used. The load is applied forward cyclic and reverse cyclic and deflection measured from every 3 KN by using LVDT. The deflection was measured at the beam free end tip. Loading is applied gradually such as 3,6,9,12,15 KN respectively for forward direction and 3, 6, 9, 12, 15 KN respectively for reverse direction. 2.6. Load sequence diagram: Fig 5: Load Vs cycle number chart 53
Fig 6: Test Setup for Forward & reverse Cyclic Load 2.7. Comparisons of experiment study and FEM analysis Experimental results are compared with FEM model analysis in ANSYS, the behavior of the interior beam column joint are similar. Maximum stresses are occurred at the junction for the ultimate loading at the beam tip. The crack patterns are formed and clearly visible in the model as shown in fig: 7&8. The maximum stress are developed in the FEM model at junction the tensile stress at top exceed the maximum tensile stress and compressive stress occurred at bottom in the forward cyclic loading. Fig 7: Failure at joints due to ultimate load (23 KN) 54
Fig 8: crack pattern of post ultimate load (backside of model) 2.8. Load Deflection curve: 55
Right side Loading Left side Loading Fig 9: Load deflection curve left and right side of beam under cyclic loading 3. Parameters have been studied: 1. Ductility, 2.Energy absorption, 3.Stiffness. 3.1. Ductility: Ductility may be broadly defined as the ability of a structure to undergo inelastic deformations beyond the initial yield deformation with no decrease in the load resistance. A quantitative measure of ductility has to be with reference to a load deformation response which is nearly horizontal. Then the ratio of ultimate deformation to the deformation at the beginning of the horizontal path (of first yield) can give a measure of ductility. Fig 10: Ductility factor Vs Load Cycles 56
Fig 11: Cumulative ductility factor for forward and reverse loading 3.2. Energy absorption: When the beam column joint is subjected to reverse cyclic loading, such as those experienced during heavy wind or earthquake, some energy is absorbed in each load cycle. It is equal to the work in straining or deforming the structure to the limit of deflection. The relative energy absorption capacities during various load cycles were calculated as the sum of the areas under the hysteric loops from the versus load deflection diagram. The relative energy absorbed during the first cycle of loading was calculated as 1.676 KN mm and during 7 th cycle 123.75 kn mm. Fig 12: cumulative energy absorption for forward and reverse cycle 3.3. Stiffness: Stiffness is defined as the load required to causing unit deflection of the beam column joint. The procedure for calculating stiffness was as follows: a) A tangent was drawn for each cycle of the hysteric curves at a load of P=0.75 Pu where Pu was the maximum load of that cycle. b) Determine the slope of the tangent drawn to each cycle, which gives the stiffness of that cycle. In general, with the increase in the load there is degradation of stiffness 57
Fig 13: Stiffness degradation for forward and reverse cycle 5. Conclusion: The structural behavior of RCC beam column joint interior type has been studied at analytically by using standard software packages STAAD Pro and ANSYS. Experimental investigation has been carried out and test results show that the structural behavior of interior beam column joint model has been similar to that of the analytically predicted one. From test results, important parameter has been worked out such as ductility, energy absorption, stiffness degradation etc., in order to access the seismic behavior of the beam column joint when earthquake comes. 6. Reference: 1. A.G.Tsonos, I.A.Tegos and G.Gr.Penelis[1992]. Seismic resistance of Type 2 Exterior Beam column joints reinforced with inclined bars The ACI structural Journal, Title No.89 S1, Jan Feb 1992. 2. A.Murugesan and Dr.G.S.Thirugnanam[2009] Ductile Behavior of Steel Fibre Reinforced Concrete beam column joints subjected to Cyclic loading, National Conference on Advances and Innovations in civil Engineering, Mepco Schlenk Engineering College,Sivakasi (2009),pp 27 33. 3. G.A.Lakshmi, Anjan Dutta and S.K.Deb[2008] Numerical studies of strengthening of beam column joint under cyclic excitation using FRP composites Journals of structural Engineering, Vol 35,No 1, Apr May 2008, pp 59 65. 4. Preeda chaimahawan & Amorn pimanmas [2008] Seismic retrofit of substandard beam column joint by planer joint expansion, Materials and structures, DOI 10, June May 2008, 5. Ramesh.S [2008] Experimental study on behavior of Exterior RC beam column joints subjected to cyclic loading M.E. thesis report, Anna University, Chennai, July 2008. 6. Thirugnanam.G.S [2001] Ductile behavior of SIFCON structural member Journals of structural engineering, April June 2001, Volume 28, No.1. 7. Yung Chih Wang and Kai Hsu[2009] Shear Strength of RC Jacketed Interior Beam Column Joints without Horizontal Shear Reinforcement The ACI structural Journal, Title No.106 S24, Mar Apr 2009. 58
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