An Analytical Investigation On Improving Ductile Reinforcement Of Exterior Beam Column Joints

Transcription

1 September 216 IJIRT Volume 3 Issue 4 ISSN: An Analytical Investigation On Improving Ductile Reinforcement Of Exterior Beam Column Joints JERIN SARA SEBASTIAN 1,VIVEK PHILIP 2 1 M.Tech. student, Saintgits college of Engineering, Pathamuttom 2 Assistant Professor, Saintgits college of Engineering, Pathamuttom Abstract Beam-column joints are considered as the critical zones of failure. In practical condition, extending the beam reinforcement into the column below the soffit and providing confining reinforcement at ends of beam and at the joint will lead to congestion of reinforcement. Inorder to avoid this congestion we should reduce the development length of the bars and increase the spacing of confinement bars such that no failure occurs. This project aims to check the feasibility of using steel plates for anchorages and also to investigate upon the feasibility of using steel fibres at the confining region to increase the capacity of the concrete thereby increasing the spacing of confining reinforcement without compromising the ductile capacity. Index Terms mechanical anchorages, steel fibres, development length,ductile capacity I. INTRODUCTION The beam column joint is defined as the portion of the column within the depth of the deepest beam that frames into the column. The function of a joint is to enable the adjoining members to develop and sustain their ultimate capacity. In a moment resisting frame, three types of joints can be identified viz interior joint, exterior joint and corner joint. Among the beam-column joint, exterior joint behaves more critically than the interior joint during the occurrence of earthquake. The failure of joints may be due to anchorage failure or shear failure or both. Both these failures are brittle in nature. Inorder to ensure the ductility of the structure brittle failure should be prevented and inorder to prevent these failures sufficient anchorage and confining reinforcement must be provided at the joints. Fig 1 shows the practice of providing anchorage length in exterior beam column joints and Fig 2 shows the ductile detailing of the beam column joint. Fig 2 Ductile detailing of beam column joints as per IS 1392:1993 Extending the beam reinforcement into the column below the soffit and providing confining reinforcement at ends of beam and at the joint will lead to congestion of reinforcement.this will lead to improper placing and compaction of the concrete in the joint and thus leads to the deterioration of the core strength of the joint. Reduction in core strength results in the brittle failure of the joints. Inorder to avoid this congestion we should reduce the development length of the bars and increase the spacing of confinement bars such that no failure IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 138

2 September 216 IJIRT Volume 3 Issue 4 ISSN: occurs. This project aims to reduce the development length of the beam reinforcement by providing steel plates for anchorages and using steel fibres at the confining region to increase the capacity of the concrete thereby increasing the spacing of confining reinforcement without compromising the ductile capacity. II. OBJECTIVES OF THE STUDY To obtain a converged finite element model of the exterior beam column joint to demonstrate its ductile behaviour. Investigate upon the feasibility of using welded plates with equivalent bond strength(mechanical anchorages), as an alternative to development length Investigate upon the ductile performance of mechanical anchorages with different configurations. To investigate the ductile performance of fibre reinforced beam column joint as an alternative to closely spaced confining reinforcement. III. SCOPE OF THE STUDY Analysis is carried out using ANSYS 16.2 Non linear static analysis of the joint models are carried out. The specimen is subjected to monotonic incremental loading at end of the beam. The specimen is detailed as per IS 1392:1993 The non linear static analysis of the exterior beam column joints are performed and the results are obtained. These results are compared with the I S specimen to reach into necessary conclusions IV. MODELING AND ANALYSIS USING ANSYS Material plays an important role in modeling. Correct values of material properties have to be given as input in finite element analysis. The data used for modelling the specimens are given in table 1.Concrete is modelled using solid 65 elements. BEAM 188 element is used to model reinforcement bars and steel plates are modelled as SHELL 181 elements. Stress - strain values of concrete is also given in the engineering data in for achieving better accuracy in results. Stress - strain values of M25 concrete and steel fibre reinforced concrete is given in table 2 Table 1: Data used for modelling exterior beam column joints Sl. No Particulars Value/ size/units 1 Grade of concrete M 25 2 Grade of steel Fe 5 4 Column size 2X15 mm 5 Column height 8mm 6 Beam size 2 x 15 mm 7 Length of cantilivered 6mm portion of beam 8 Modulus of elasticity of 25MPa concrete 9 Poisson's ratio of concrete.23 1 Modulus of elasticity of 2MPa steel 11 Poisson' s ratio of steel.3 12 Yeild strength of Fe5 5 MPa steel 13 Cube strength of steel fibre 39 MPa reinforced concrete 14 Modulus of elasticity of steel fibre reinforced concrete 15 Ultimate uniaxial tensile 3.5MPa strength of M25 concrete 16 Ultimate uniaxial tensile 4.37MPa strength of steel fibre reinforced concrete 17 Shear transfer coefficient.2 for open crack 18 Shear transfer coefficent for 1 closed crack 19 Tensile crack factor.5 Table 2. Stress strain values of concrete used Material Stress(MPa) Strain M25 concrete Steel fibre reinforced concrete The models prepared can be classified under the following sections IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 139

3 September 216 IJIRT Volume 3 Issue 4 ISSN: Validation model or IS model - designed and detailed as per IS 1392 Models for investigating the feasibility of using mechanical anchorages instead of providing development length. Models for investigating the feasibility of increasing the spacing of confining reinforcement. A. Validation model or IS model Kaliluthin A. K et al(214) conducted an experimental investigation based on the reinforced concrete exterior beam column joints. The ductile joint specimen designed as per IS 1392:1993 used in the experimental study is considered for validation. The details of the specimen is given in Fig 3 Fig 3 Details of IS specimen The FEM model of the IS specimen was developed using ansys software and the FEM is shown in fig 4. The model thus created is then discretised into finite elements by meshing. The mesh size adopted was 25mm. Fig 5 shows the beam column joint after meshing. Fig 5 Meshing of the specimen B. Models for investigating the feasibility of using mechanical anchorages instead of providing development length. Models for this investigation are created by using plates of different dimension inorder to optimise the ideal plate configuration and also by changing the location of the plates to find out the ideal location of the plates. The models created inorder to find out the ideal plate dimension are as follows 1. Specimen with 25x25x6mm plates used as mechanical anchorages( Fig 6) 2. Specimen with 3x3x6mm plates used as mechanical anchorages( Fig7) 3. Specimen with 25x35x6mm plates used as mechanical anchorages( Fig 8) 4. Specimen with 2x2x6mm plates used as mechanical anchorages( Fig9) The ductile behaviour of the exterior beam column joints with the above mentioned mechanical anchorages are studied and the dimension of mechanical anchorage is idealised. The idealised specimen is used for further analysis. The modelled specimens are shown in figures given below fig 4 FEM model of IS specimen IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 14

4 September 216 IJIRT Volume 3 Issue 4 ISSN: Fig 6 Specimen with 25x25mm plates as mechanical anchorage Fig 7 Specimen with 3x3mm plates as mechanical anchorage The next objective of this investigation is to find out the ideal location of the plates. The models created to meet this objective are as follows. 1. Specimen with mechanical anchorages placed at end of the longitudinal reinforcement of beam(fig 1) 2. Specimen with mechanical anchorages placed at centre of the column width(fig 11) 3. Specimen with mechanical anchorages placed at a distance of one fourth the width of the column from the face of the joint(fig 12). The wireframe model of the above mentioned specimens are shown in the following figures. Fig 1 Specimen with plates at end Fig 8 Specimen with 25x35mm plates as mechanical anchorage Fig 11 Specimen with plates at centre of the column Fig 9 Specimen with 2x4 mm plates as mechanical anchorage IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 141

5 September 216 IJIRT Volume 3 Issue 4 ISSN: Fig 12 Specimen with plates at one fourth the column width C. Models for investigating the feasibility of increasing the spacing of confining reinforcement. Inorder to investigate the feasibility of increasing the spacing of confining reinforcement steel fibres are used in the confining zone. Past experimental results shows that steel fibre reinforced concrete enhances the strain capacity and ultimate load carrying capacity. For this investigation steel fibres along with mechanical anchorages are used with increased stirrup spacing and the ductile behaviour of the joint is compared with the IS model. In this feasibility study three specimen are modelled and they are 1. Specimen with stirrups at 1 mm spacing with mechanical anchorage and fibres 2. Specimen with stirrups at12 mm spacing with mechanical anchorage and fibres 3. Specimen with stirrups at 15mm spacing with mechanical anchorage and fibres. The FEM models of the above mentioned specimens are given in Fig 13,14 and 15 Fig 14 Specimen with stirrups at 12mm spacing Fig 15 Specimen with stirrups at 15mm spacing All the above mentioned models were subjected to monotonic loading at the edge of the beam and fixed support were assigned at the faces of the column. The loading and the bounadry conditions are shown in fig 16 and fig 17. Fig 13 Specimen with stirrups at 1mm spacing Fig 16 Loading of the specimen IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 142

6 Load(N) September 216 IJIRT Volume 3 Issue 4 ISSN: Fig 17 Boundary condition of the specimen fig 19 Deflection of IS specimen under ultimate load V. RESULTS AND DISCUSSIONS The results obtained from the non linear static analysis of the models are given below. The analysis was done with the objective of finding out the feasability of using steel plates as mechanical anchorages and increasing the spacing of confinement reinforcement with steel fibres. Load deflection graphs for each specimen was plotted and the ultimate load, ultimate deflection was found out. Ductility factor was found out by taking the ratio of ultimate deflection to the deflection under first crack A. Validation model/is specimen The model was anlysed and the load deflection graph, ultimate load, ultimate deflection, first crack load deflection under first crack etc are obtained and compared with the experimental results Deflection IS specimen fig 18 load deflection graph of IS specimen Fig 2 Initial cracks developed at the joint of IS specimen Table 3 Comparison of analytical and experimental results Method First Deflection Ultimate Deflection Crack Under First Load(kN) Under Load Crack Ultimate (kn) Load Experiment Analysis From the results it was found that the finite element model created has an accuracy of 91% in computing deflection and 93 % in computing load. Therefore the model can be effectively used to represent the ductile behaviour of exterior beam column joint B. Specimen with mechanical anchorages 1.Changing the plate dimensions Non linear static analysis of the specimen with varying the plate dimensions are performed. The load defelction graph of the specimens are shown in figure IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 143

7 September 216 IJIRT Volume 3 Issue 4 ISSN: to fig 23 and the results are obtained are tabulated as shown in table x25 plate Fig 21 Load deflection graph of specimen with 25x25 mm plate Fig 22 Load deflection graph of specimen with 3x3 mm plate x35 Fig 22 Load deflection graph of specimen with 25x35 mm plate x Fig 23 Load deflection graph of specimen with 2x4mm plate Table 4. The results of Specimen with varying plate dimensions Model First Deflecti Ultimat Deflectio Ductilit Crac on e n Under y factor k Under Load(k Ultimate Load First N) Load (kn) Crack 25X x x x IS specime n According to the results obtained it was found that the ductility of the joint can be increased by using plates as anchorages. It was found that by using 3x 3 x 6mm plate as mechanical anchorages the limit state capacity, ultimate load capacity and ductility increases. Therefore 3x 3 mm plate was used as mechanical anchorages in further models. It was also found that the as bearing area of the plates increases the ductility also increases. 2. Changing the location of plates Load deflectiongraphs of models analysed are given in fig 24 to fig 26 and the summary of results is given in table 5 IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 144

8 LOAD (N) September 216 IJIRT Volume 3 Issue 4 ISSN: plate at end Fig 24 Load deflection graph of plate at end plate at centre Fig 25 Load deflection graph of plate at centre 12 8 Model Plate at end Plate at centre Plate at One fourth IS specime n C. Specimen with steel fibres and mechanical anchorages with increased stirrup spacing The load deflection graph of specimens are given in Fig 27 to Fig 29. The results of analysis is summarised in table First Crac k Load (kn) Deflecti on Under First Crack Ultimat e Load(k N) DEFLECTION Deflecti on Under Ultimate Load Ductilit y factor Fig 27 Load deflection graph of specimen with 1 mm stirrup spacing 6 4 plate at one fourth Fig 26 Load deflection graph of plate at one fourth mm spacing Table5. The results of Specimen with varying the loaction of plates Fig 28 Load deflection graph of specimen with 12 mm stirrup spacing IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 145

9 September 216 IJIRT Volume 3 Issue 4 ISSN: Fig 28 Load deflection graph of specimen with 15 mm stirrup spacing As per the results obtained it was found that by increasing the spacing of confining reinforcement the ductility of the beam column joints also increases. For the model having 1mm stirrup spacing there is an increase in limit state capacity, ultimate load capacity and ductility by 5%,47% and 1% For the model having 12mm stirrup spacing there is an increase in ultimate load capacity and ductility by 24.3%and 44%.The limit state capacity is also better since the deflection under the first crack was lesser than IS model VI. CONCLUSION 15 mm strp Table 6. Results of specimen with steel fibres and mechanical anchorages with increased stirrup spacing Model First Deflecti Ultimat Deflecti Ductilit Crac on e on y factor k Under Load(k Under Load First N) Ultimate (kn) Crack Load Strp Strp Strp IS specime n The results are carefully studied and the conclusion are made and concluded as given below According to the results obtained it was found that the ductility of the joint can be increased by using plates as anchorages. As the bearing area of the plates used as mechanical anchorage increases the ductility also increases. When plates are placed at the centre of column width of the beam column joint, it was found that the ductility increases. Also there is considerable reduction in the ultimate load carrying capacity and limit state capacity from the IS specimen. The performance of the specimen anchoraged with steel plates at one fourth position of the column width is very poor when compared to the IS model. The perfect location of providing plate mechanical anchorage is at the end of the longitudinal beam reinforcement. It was found that the limit state capacity, ultimate load capacity and ductility increases by 2.5%,1.5%, 22%. The performance of exterior beam column joint can be improved by providing both plates as mechanical anchorages and steel fibres at the confining zone By providing steel fibres and mechanical anchorages with a stirrup spacing of 1mm the limit state capacity, ultimate load capacity and ductility improves the performance of exterior beam column joint by 5%,47.5% and 9.69% when compared to the IS model The spacing of confining reinforcement can be increased from 1 mm spacing to 12mm spacing without compromising the performance of exterior beam column joints. REFERENCES [1] P.Rajaram A.Murugesan and G.S.Thirugnanam "Experimental Study on behavior of Interior RC Beam Column Joints Subjected to Cyclic Loading" ; International Journal Of Applied Engineering Research, Dindigul Volume 1, No1, 21 [2] B. Bayhan,J.P. Moehle S. Yavari & K.J. Elwood,S.H. Lin, C.L. Wu & S.J. Hwang,"An Experimental and Analytical Study in Reinforced Concrete Frames with Weak Beam-Column Joints ";WCEE 212 [3] Akanshu Sharma, G.R. Reddy, R. Eligehausen, K.K. Vaze "Modelling Beam-column joints in performance analysis of Reinforced Concrete Frame Structures " [4] Romanbabu M. Oinam, Choudhury.A.M, And Laskar A I, "Experimental Study on Beam- Column Joint with Fibres under Cyclic Loading"; IOSR Journal of Engineering (IOSRJEN) e-issn: , p-issn: Vol. 3, Issue 7 (July. 213), V2 PP IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 146

10 September 216 IJIRT Volume 3 Issue 4 ISSN: [5] Siva Chidambaram.K.R, Thirugnanam.G.S "Comparative Study on Behaviour of Reinforced Beam-Column Joints with Reference to Anchorage Detailing"; Journal of Civil Engineering Research 212, 2(4): [6] Kaliluthin.A.K, Kothandaraman.S "Experimental investigation on behavior of reinforced concrete beam column joint"; International Journal Of Civil And Structural Engineering,Volume 4, No 3, 214 [7] S. Rajagopal,S. Prabavathy "Investigation on the seismic behavior of exterior beam column joint using T-type mechanical anchorage with hairclip bar"; Journal of King Saud University Engineering Sciences (215) 27, [8] S.R.Uma,Sudhir K. Jain "Seismic Design Of Beam-Column Joints In Rc Moment Resisting Frames Review Of Codes " [9] J. Shafaie, A. Hosseini, M. S. Marefat "3d Finite Element Modelling Of Bond-Slip Between Rebar And Concrete In Pull-Out Test" [1] G. Metelli, F. Messali, C. Beschi, P. Riva "A model for beam column corner joints of existing RC frame subjected to cyclic loading"; Engineering Structures 89 (215) [11] Bilal S. Hamad, M.ASCE; and Elias Y. Abou Haidar "Bond Studies of High-Strength Concrete Joints Confined with Stirrups, Steel Fibers, or Fiber-Reinforced Polymer Sheets"J. Struct. Eng., 216 [12] Sangjoon Park, Ph.D; and Khalid M. Mosalam, Ph.D Experimental Investigation of Nonductile RC Corner Beam-Column Joints with Floor Slabs, J. Struct. Eng., 213 [13] Mohamed Hasaballa and Ehab El-Salakawy Shear Capacity of Exterior Beam-Column Joints Reinforced with GFRP Bars and Stirrups; Journal Of Composite Structures 215 [14] S M Kularnia, Y D Patil, "A Novel Reinforcement Pattern for Exterior Reinforced Concrete Beam-Column Joint"; Procedia Engineering 51 ( 213 ) [15] Rahmani Kadarningsiha,, Imam Satyarno, Muslikh, Andreas Triwiyono"Proposals of beam column joint reinforcement in reinforced concrete moment resisting frame : A literature review study"procedia Engineering 95 ( 214 ) IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 147