Experimental study on behavior of Retrofitted with FRP wrapped RC Beam- Column Exterior Joints Subjected to cyclic loading

Transcription

1 Experimental study on behavior of Retrofitted with FRP wrapped RC Beam- Column Exterior Joints Subjected to cyclic loading E. Senthil kumar 1 A.Murugesan 2 G.S.Thirugnanam 3 1 Post Graduate student, 3 Assistant Professor and Head Department of Civil Engineering, Institute of Road and Transport Technology, Erode , Tamil Nadu, India. 2 Research Scholar, Department of Civil Engineering, Sona College of technology, salem senthilstructures@gmail.com doi: /ijcser ABSTRACT In the last few decades, moderate and severe earthquake have struck different places in the world, causing severe damage to reinforced concrete (RC) Structures. Retrofitting of existing structures is one of the major challenges that modern civil engineering structures has demonstrated that most of them will need major repairs in the near future. Up gradation to higher seismic zones of several cities and towns in the country has also associated in evolving new retrofitting strategies. One of the techniques of strengthening of the RC structural members is through confinement with a composite enclosure. This external confinement of concrete by high strength fiber reinforced polymer (FRP) composite can significantly enhance the strength and ductility and will result in large energy absorption capacity of structural members. FRP material, which are available in the form of sheet, are being used to strengthen a variety of RC elements to enhance the flexural, shear, and axial load carrying capacity of these elements. An experimental investigation of the behavior of retrofitted FRP wrapped exterior beamcolumn joints with detailing as per IS : 1993 under seismic conditions is presented. The experimental study on exterior beam-column joint of a multistory reinforced concrete building (G+ 4 storey) 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: The test specimen is reduced to one fifth model of beam column joint from prototype specimen. Column confinement and beam stirrups are provided closely in joint region according to IS : Three Specimens were cast and tested to failure during the present investigation. One is Control specimen test up to post ultimate load, and another two Specimen test up to 70% of the ultimate Load. The test specimens were evaluated in terms of loaddisplacement relation, ductility, stiffness, load ratio and cracking pattern. Test results are compared with analytical modeling of beam column joint performed in STAAD.Pro. and ANSYS Software. Keywords: FRP, Retro fitting, STADD Pro 64

2 1 Introduction Recent Earthquake have exposed the Vulnerability of existing reinforce concrete (RC) beam- column joints to seismic loading. Until early 1990s, concrete jacketing and steel were the two common methods adopted for strengthening the deficient RC Beam column Joints. Concrete jacketing results in substantial increase in the cross section are and self jackets are poor in resisting weather attacks. Both methods are however labor intensive and sometimes difficult to implement at site. A new technique has emerged recently which uses fiber reinforced polymer sheet to strengthen the beam- column joint. FRP materials have a number of favorable characteristics such as ease to install, immunity to corrosion, high strength, availability in sheets etc. The simplest way to strengthen such joints is to attach FRP sheets in the joint region in two orthogonal directions. 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. Problems of diagonal cracking and crushing of concrete in the joint region can be controlled by two mean, namely providing large column sizes and providing closely spaced closed-loop steel ties around column bars in the joint region. The ties hold together the concrete in the joint and also resist shear force, thereby reducing the cracking and crushing of concrete. Providing closed-loop ties in the joint requires some extra effort. Indian Standard IS: recommends continuing the transverse loops around the column bars through the joint region. In practice, this is achieved by preparing the cage of the reinforcement (both longitudinal bars and stirrups) of all beams at a floor level to be prepared on top of the beam formwork of that level and lowered into the cage. However, this may not always be possible particularly when the beams are long and the entire reinforcement cage becomes heavy. The gripping of beam bars in the joint region is improved first by using columns of reasonably large cross-sectional size. As explained in Earthquake Tip 19, the Indian Standard IS: requires building columns in seismic zones III, IV and V to be at least 300 mm wide in each direction of the cross-section when they support beams that are longer than 5m or when these columns are taller than 4m between floors (or beams). The American Concrete 65

3 Institute recommends a column width of at least 20 times the diameter of largest longitudinal bar used in adjoining beam. In exterior joints where beams terminate at columns, longitudinal beam bars need to be anchored into the column to ensure proper gripping of bar in joint. The length of anchorage for a bar of grade Fe415 (characteristic tensile strengths of 415MPa) is about 50 times its diameter. This length is measured from the face of the column to the end of the bar anchored in the column. In columns of small widths and when beam bars are of large diameter, a portion of beam top bar is embedded in the column that is cast up to the soffit of the beam, and a part of it overhangs. It is difficult to hold such an overhanging beam top bar in position while casting the column up to the soffit of the beam. On the other hand, if column width is large, the beam bars may not extend below the soffit of the beam. Thus, it is preferable to have columns with sufficient width. Such an approach has been used in the American practice [ACI1318M, 2002]. 2 Experimental investigation The experimental study on exterior 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: 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 consider that particular joint for the experimental study. Fig.1. Two bay 5 storey Frame with seismic force X & Z Directions. 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 66

4 with out slab thickness and beam length of 450mm and that column size was 120 X 230 mm. Height of the column was 600mm. Fig 2: Ductile Detailing of Beam Column Joint as per IS 13920; Description of the formwork and reinforcement Fig 3: Formwork and Reinforcement for Test specimen Fig 4 : Casting Beam-Column Joint 67

5 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 Nos at top and 3 Nos 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 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. Fig 5: Test Setup for Cyclic Loading for Control Specimen 68

6 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. Fig 6 : Load Vs Displacement curve subjected to cyclic loading for Control Specimen Fig 7 : Failure due to Ultimate Load for Control Specimen 69

7 Fig 8 : Maximum Stress and Crack Pattern due to ultimate load 2.6 Comparison between experimental study and FEM analysis Experimental results are compared with FEM model analysis in stadd.pro, the behavior of the exterior beam column joint are similar. Maximum stresses are occurred at the junction for the ultimate loading at the beam tip. The crack pattern are formed and clearly visible in the model as shown in fig: 7. 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 as shown in fig: 8. Fig : 9 Plastic hinge formation at junction after ultimate load 70

8 2.7 Preparation of Retrofitted Specimens Fig 10: Preparation of Retrofitted Specimens The Exterior beam column joint specimen named as SL-1 (Single Layer) & DL-2 (Double Layer) was tested subject to quasi- static cyclic loading simulating earthquake loads. The Load was applied by using screw jack totally 5 cycle were imposed. The beam-column joint was gradually loaded by increasing the load level during each cycle. The load sequence consists of 3kN, 6kN, 9kN and upto 70% ultimate load. The deflection measured at tip during the cycle of loading. As the load level was increased in each cycle. 2.8 Load Carrying Capacity The first crack was witnessed during 4 th cycle at the load level of 17.0kN. As the load level was increased, further cracks were developed in other portions. The 70% of ultimate load carrying capacity of the SL-1 & DL -2 specimens was 20.0kN recorded at 5 th cycle. After the testing of the two specimens the cracks are filled with the fiber glass paste mixed with unsaturated polyester resin. The surface area was covered with help of fiber glass paste in all direction as shown in Fig. The Spalling of the concrete portion are filled with this fiber glass paste. 2.9 Wrapping of GFRP mat with Resin Glass fiber mat class-e grade 300 (chopped Fiber) was used to wrapping the specimens the mat are cut into desired shape. The Unsaturated Polyester resin with accelerator of Cobalt 6% and catalyst of.01% was added with resin mix thoroughly. The Fiber glass mat placed over the specimen and the resin was applied uniformly in all direction with help of brush. Single Layer thickness was applied to the specimen of EXT-1, Double Layer Thickness was applied to the Specimen of EXT -2. The specimens are allowed to dry in the room temperature up to 16 Hr. 71

9 Fig 11 : Load setup for the retrofitted specimens for SL-1 & DL Loading and load deflection behavior The GFRP wrapped specimen was subjected to quasi-static cyclic loading simulating earthquake loads. The history of load sequence followed for the test was presented in figure 12. The load was applied by using screw jack Totally 14 cycles were imposed. The cyclic load versus deflection was presented in figure Fig 12: Load sequence Diagram 72

10 Fig 13. Load Vs Deflection curve for Retrofitted Specimen Fig 14 : Ductility Factor for Forward and reverse Cycle for Control Specimen 73

11 Fig: 15 Cumulative Energy Absorption for Forward and Reverse Loading for Control Specimen Fig: 16 Relative Energy Absorption for Forward and Reverse Loading for Control Specimen 74

12 Fig: 17 Ductility Factor for Forward and Reverse Loading for Control Specimen Fig: 18 Stiffness Degradation for Forward and Reverse Loading for Control Specimen 75

13 Fig: 19 Comparison of Load Carrying Capacity for Control & Retrofitted Specimens Fig: 20 Comparison of Ductility factor for Control & Retrofitted Specimens 76

14 Fig: 21 Comparison of Cumulative Ductility for Control & Retrofitted Specimens Fig 22 : Comparison of Relative Energy Absorption for Control & Retrofitted Specimens 77

15 Fig 23 : Comparison of Cumulative Relative Energy Absorption for Control & Retrofitted Specimens 3 Conclusion Beam column joint in the moment resisting frame have traditionally beam neglected in design process while the individual connected elements, that is, beam and column, have received considerable attention in design. Research on beam column joint of reinforced concrete moment resisting frame was started only in the 1970s. The 1993 version of IS incorporated some provision on the design of beam column joints. However, the Provisions are inadequate to prevents shear and bond failure of beam column joint in severe seismic shaking. Therefore, these provisions need to be upgraded substantially with inclusion of explicit provisions on shear design and anchorage requirement. This article proposes provision for shear design of beam column joint and anchorage requirements of tension beam bars in the joint area. It also suggests provisions for the confinement of wide beam and column connections. The plastic hinge formation at the joint after the ultimate load. The structural behavior of RCC beam column joint exterior type has been studied analytically by using standard software packages STAAD Pro and ANSYS. Experimental investigation has been carried out and the test results show that the structural behavior of exterior beam column joint model has been similar to that of the analytically predicted one. The Loading Carrying capacity of the retrofitted Specimen is 60% More than that of the Control Specimen. The Load Deformation characteristics also improved to large extent in the case of the Retrofitted specimen over the control specimen. The enhancement in the energy absorption capacity of the retrofitted specimen was in the rang of 30% - 60% over the control specimen. The Failure was in the column Portion of the joint for the control specimen which is to be avoided. In case of the Retrofitted specimen the failure was noticed in the beam portion only and the column was intact and this is the most preferred type of failure under seismic load which will prevent progressive collapse of the structure. 78

16 4 Reference 1. Hung-Jen Lee and Si-Ying Yu Cyclic Response of Exterior Beam-Column Joints with Different Anchorage Methods The ACI structural Journal, Title No.106-S32, May- June A.G.Tsonos, I.A.Tegos and G.Gr.Penelis Seismic resistance of Type 2 Exterior Beam column joints reinforced with inclined bars The ACI structural Journal, Title No.89- S1, Jan-Feb G. Appa Rao, M.Mahajan, M.Gangaram and Rolf Eligehausen Performance of nonseismically designed RC beam column joints strengthened by various schemes subjected to seismic loads, Journal of structural engineering, V.35, No.1, Apr-May 2008, pp Murthy C.V.R, Durgesh C.Rai, K.K.Bajpai and Sudhir.K.jain, Anchorage Details and Joint Design in Seismic RC Frames, the Indian Concrete Journal, April 2001, pp G.S.Thirugnanam Ductile Behavior Of FRP Strengthened R.C Beams Subjected To Cyclic Loading IRTT Erode. 6. A.Murugesan and Dr.G.S.Thirugnanam Ductile Behavior of Steel Fiber 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 A.Murugesan and Dr.G.S.Thirugnanam Ductile behavior Reinforced Concrete Beam - Column joints Subjected to Cyclic loading, National Conference on Recent Advances in Concrete, Steel and Composite Structures, I.R.T.T., Erode (2009),pp