FRP-CONFINED CAPSULE-SHAPED COLUMNS UNDER AXIAL AND LATERAL LOADINGS TAMALI BHOWMIK NATIONAL UNIVERSITY OF SINGAPORE 2011 FRP-CONFINED CAPSULE-SHAPED COLUMNS UNDER AXIAL AND LATERAL LOADINGS TAMALI BHOWMIK (B. Eng., Jadavpur University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL & ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 ACKNOWLEDGEMENTS This thesis represents the hard work of many individuals to whom I could never express the extent of my gratitude. First and foremost, I express my sincere and profound gratitude to my supervisors, Prof. Tan Kiang Hwee and Prof. Balendra for their erudite and invaluable guidance throughout the study. Their analytical and methodical way of working has inspired me and under their guidance I have learned a lot. Prof. Tan and Prof. Balendra’s assistance and valuable comments during the preparation of this thesis are also appreciated. I would also like to acknowledge NUS for providing all necessary financial and academic support without which my Ph.D. would have been a distinct dream. Grateful thanks are extended to the staffs of the Structural Engineering Laboratory, National University of Singapore for their assistance rendered throughout the study. Mr. Koh Yian Kheng, Mr. Choo Peng Kin, Mr. Ishak Bin A Rahman, Mr. Ang Beng Oon, Mr. Yip Kwok Keong, Mr Kamsan Bin Rasman, Mr Ow Weng Moon and Mr Wong Kah Wai, Stanley had been extremely helpful during the experiments. Special acknowledgements are given to Mr. Choo Peng Kin, Mr. Koh Yian Kheng and Mr. Ishak Bin A Rahman who had assisted and guided me tremendously from the fabrication up to the testing of the specimens. Special thanks are given to Madam Annie Tan, Mr. Ang Beng Oon and Mr. Yip Kwok Keong for their kind help whenever I need. Last but not the least, I sincerely thank our laboratory manager Mr Lim Huay Bak for his invaluable guidance throughout the experiments. i The friendship and collaboration with a number of my fellow graduate students has also been invaluable. Special thanks are given to Du Hongjian, Aziz Ahmed and Patria Kusumaningrum for their help and great encouragement. Words are not enough to thank my family for the support they have given me during this long and sometimes difficult journey. I can only grab this opportunity to remember their endless support and unconditioned love during my pursuit of higher education. ii TABLE OF CONTENTS Acknowledgements Table of Contents Summary i iii viii List of Tables x List of Figures xii List of Symbols xviii CHAPTER 1: INTRODUCTION 1.1 Background 1.2 Retrofitting by Fiber reinforced polymer systems 1.3 FRP-confined concrete columns 1.3.1 Reprofiling of the rectangular sections 1.4 Research objectives 1.5 Organization of the thesis CHAPTER 2: LITERATURE REVIEW 2.1 General 10 2.2 FRP-confined columns under axial loading 11 2.2.1 Effect of FRP amount and concrete strength 12 2.2.2 Effect of aspect ratio and shape of column 13 2.2.3 FRP-confinement models 14 2.2.4 Stress-strain models for FRP- confined columns 22 2.3 FRP- confined columns under lateral loading 23 iii 2.3.1 Types of column failures during earthquake 24 2.3.2 Seismic retrofitting by CFRP confinement 26 2.4 Summary 28 CHAPTER 3: ANALYSIS AND TESTING UNDER AXIAL LOADING 3.1 Proposed Confinement Models 40 3.1.1 Model “A” 40 3.1.2 Model “B” 44 3.2 Test Program 48 3.3 Material Properties 49 3.4 Test Preparation 50 3.4.1 Preparation of column specimens 50 3.4.2 Installation of Carbon FRP Systems 51 3.5 Test Instrumentation, Setup and Test Procedure 51 3.6 Test Results and Discussion 52 3.6.1 Overall Behavior 52 3.6.1.1 Series I columns 52 3.6.1.2 Series II columns 54 3.6.2 Failure characteristics 55 3.6.3 Stress-Strain Response 56 3.6.3.1 Series I columns 56 3.6.3.2 Series II columns 58 3.7 Comparison with predictions of proposed confinement models 59 iv 3.8 Summary 60 CHAPTER 4: ANALYSIS AND TESTING UNDER LATERAL LOADING 4.1 Introduction 83 4.2 Theoretical Considerations 84 4.2.1 Ultimate lateral load capacity 84 4.2.1.1 Shear Capacity 84 4.2.1.2 Flexural Capacity 86 4.2.2 Inelastic displacement 4.3 Numerical Modeling in SEISMOSTRUCT 90 94 4.3.1 Overview of SEISMOSTRUCT 94 4.3.2 FEA modeling 95 4.3.3 Limitation of SEISMOSTRUCT 98 4.4 Test program 4.5 Material properties 98 100 4.5.1 Internal steel reinforcement 100 4.5.2 Concrete 101 4.5.3 FRP system 101 4.6 Fabrication of specimens 101 4.7 Test setup, instrumentation and procedure 103 4.7.1 Instrumentation 104 4.7.1.1 Strain Gauges 104 4.7.1.2 Displacement transducers 104 4.7.2 Test procedure 105 v 4.8 Test results and discussions 4.8.1 Behavior and mode of failure 106 106 4.8.1.1 Pushover tests 106 4.8.1.2 Cyclic test 108 4.8.2 Lateral load-displacement response 109 4.8.3 Effect of reprofiling 111 4.8.4 Effect of CFRP systems 112 4.8.5 Moment-curvature response 113 4.9 Comparison of theoretical, numerical and experimental results 114 4.10 Summary 115 CHAPTER 5: CASE STUDIES 5.1 Introduction 146 5.2 Modeling of FRP-confined column 147 5.2.1 Calibration of FEA model 150 5.3 Case study 1: Sub-frame of a 4-storey residential building 150 5.3.1 FEA modeling of sub-frame 151 5.3.2 Member properties 152 5.3.3Boundary conditions 153 5.3.4 Vertical and lateral loads 153 5.3.5 Failure identification 155 5.3.6 FEA results and discussion 155 5.4 Case study 2: Northridge bridge pier 157 5.4.1 Case study 2(a): Pier with section aspect ratio of 3.0 157 vi 5.4.1.1 FEA modeling of pier 157 5.4.1.2 Failure identification 158 5.4.1.3 Pushover analysis 160 5.4.1.4 Seismic demand and capacity 162 5.4.2 Case study 2(b): Pier with section aspect ratio of 4.4 168 5.4.2.1 Pushover analysis 168 5.4.2.2 Evaluation of seismic adequacy 169 5.5 Summary 170 CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS 6.1Conclusions 194 6.2 Recommendations for further research 197 REFERENCES 198 vii SUMMARY The application of externally bonded fiber reinforced polymer (FRP) composites has achieved enormous popularity in recent times for the strengthening and repairing of old concrete bridges and buildings. Specifically, concrete columns and bridge piers in existing structures, which generally have inadequate transverse reinforcement as per old design codes, are vulnerable to seismic attacks and thus they need to be strengthened to enhance their strength and ductility. The externally bonded transverse FRP systems provide additional confinement which lead to an increase in the compressive strength, shear capacity and ductility of the confined columns and piers. Previous studies on FRP-confined columns indicated that the confinement effect is most significant in circular column sections and, to some extent, in square sections rather than in rectangular sections with large aspect ratios. But, in reality, many building columns and bridge piers are rectangular in shape with an aspect ratio of more than 1.5, and as large as 7, which may well be termed “wall-like” columns. To enhance the confinement effect from FRP systems, reprofiling of rectangular columns is proposed herein by adding two circular concrete segments at the shorter sides and thus forming a capsuleshaped section before applying the transverse FRP reinforcement. The current thesis presents the details and results of a study on the FRPconfined capsule-shaped columns subjected to axial and lateral loads. The main parameters of the study were the effect of section geometry and the number of ply of transverse FRP sheets. viii Chapter Case studies Sa api dpi Sd Area enclosed by hysteresis loop = 4*area of shaded parallelogram = ED (a) A single hysteresis loop (b) ED = 4*area of the parallelogram= 4(apidpi-2A1-2A2-2A3) = 4(aydpi-dyapi) Figure 5.21 Derivation of energy dissipated by damping (ED) 189 Chapter Case studies 1.4 1.2 Spectral acceleration (g) Seismic demand curve for 31% Damping Performance point 0.8 36% Damping 0.6 FRP-confined pier 0.4 31% Damping 0.2 Seismic demand curve for 35% Damping As-built pier 0 0.05 0.1 0.15 0.2 0.25 0.3 Spectral displacement (m) Figure 5.22 Evaluation of seismic adequacy of Northridge bride pier of section aspect ratio of 190 Chapter Case studies 4920mm 1230mm 5388mm Longitudinal steel =70nos. #18 (diameter 57.33mm) bars Transverse Steel = #5 (diameter 16mm) links @ 300c/c Figure 5.23 Cross-section details of modified bridge pier 191 Chapter Case studies 20000 16000 FRP-confined pier (numerical) Shear (kN) 12000 As-built pier (numerical) FRP-confined pier (theoretical) As-built pier (theoretical) 8000 By ACI 318 (2008) 4000 By Priestley et al. (1996) Shear failure points 0 10 15 20 Curvature ductility factor 25 30 Figure 5.24 Identification of failure points of Northridge bridge pier with section aspect ratio 4.4 192 Chapter Case studies 1.2 Seismic demand curve (37% Damping) Spectral acceleration(g) 0.8 37% Damping Seismic demand curve (40% Damping) 0.6 As-built pier 43% Damping 0.4 Performance point 0.2 FRP-confined pier 0 0.1 0.2 0.3 0.4 Spectral displacement (m) 0.5 0.6 Figure 5.25 Evaluation of seismic adequacy of Northridge bridge pier with aspect ratio of 4.4 193 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 6.1 Conclusions The study focused on the effectiveness of the externally bonded FRP systems to increase the axial load capacity, shear capacity and ductility of FRP-confined capsule-shaped columns under axial and lateral loading. As reviewed in Chapter 2, the effectiveness of FRP confinement is maximum in circular columns. On the contrary, for square and rectangular columns, the confining pressure provided by the FRP systems varies over the cross-sections resulting in partial confinement of the concrete sections. Moreover, with the increase in section aspect ratios of rectangular columns, the confinement effectiveness decreases significantly. To increase the FRP confinement effect in case of rectangular columns, reprofiling of the column was proposed in this study. The shorter ends of the rectangular section could be rounded off by adding circular concrete segments and thus forming a capsuleshaped section before applying the transverse FRP reinforcement. The principal conclusions from this study are as follows: 1. According to the prediction by the proposed confinement models, the axial load capacities of capsule-shaped sections could be enhanced significantly by FRP confinement. Both confinement models predicted the axial load capacities of tested columns with reasonable accuracy. 2. Test results showed maximum enhancement in axial load capacity of 68% for capsule-shaped columns with section aspect ratio of 2. While for 194 Chapter Conclusions and recommendations capsule-shaped columns with section aspect ratio of 4, the average increase in axial load capacities was around 30%. 3. A unified equation to calculate the effective confinement coefficient (ke) of circular, rectangular and capsule-shaped columns was proposed. Also, a parametric study has been done to calculate ke by varying the section aspect ratio (h/b) and corner radius to width ratio (r/b) of columns. For capsule-shaped columns with aspect ratio 2, the effective confined concrete area was 81% of the total area of column section. While for aspect ratio 4, the effective concrete area was calculated as only 36% of the total area which clearly explain the less enhancements in axial load capacities of columns with higher section aspect ratios. 4. Theoretical and numerical models were proposed to predict the lateral load-displacement relationship of FRP-confined capsule-shaped columns. In these models, the confined compressive strength (f’cc) as obtained using the proposed confinement models as described in Chapter was used to calculate the ultimate confined compressive strain (εcu) of the FRP-confined columns, 5. As shown in Chapter 4, the predicted load-displacement profiles by theoretical and numerical models matched satisfactorily with the observed profiles. Thus, these models can be effectively used to predict the performance of laterally loaded FRP-confined capsule-shaped columns with high aspect ratios. 6. It was found that the unconfined columns subjected to lateral loading were failed in shear irrespective of shape parameters. The CFRP confinement with one ply of FRP sheet in full height of the column prevented the shear 195 Chapter Conclusions and recommendations failure of columns; and the additional ply in the plastic hinge zones significantly enhanced the ductility of columns for all three shape parameters. 7. The maximum displacement ductility of FRP-confined columns with shape parameter 0.19 and 0.5 were found as 5.6 and 5.0. On the other hand, for shape parameter 0, that is, for rectangular column, the displacement ductility was only 2.6. This indicates the efficiency of proposed reprofiling methods to enhance the FRP confinement effect for rectangular columns with high aspect ratios and thereby to increase the inelastic displacement. 8. For FRP-confined column with shape parameter 0.19 (LC-0.19-2), the CFRP confinement efficiently prevented the stiffness and strength degradation. As a result, the displacement ductility was found to be as high as 4.1. 9. To examine the proposed reprofiling and retrofitting technique in case of full-scale structure, pushover analysis has been carried out numerically on a building frame and a bridge pier. The FE analysis of FRP-retrofitted frame showed an increase in overall drift ratio of 3.9 times than the as-built frame. 10. By FRP-retrofitting of Northridge bridge pier with section aspect ratio 3, the shear capacity was enhanced by 11% with an increase of 31% in curvature ductility. Thus, the seismic capacity of the pier was found adequate as per Northridge earthquake demand. The lateral load capacity of FRP-confined bridge pier with aspect ratio 4.4 was governed by the flexural failure and the seismic capacity of the pier exceeded the seismic demand. 11. In simulating the Northridge bridge pier, the column was assumed as uniform along its height. But, in reality, many building columns and piers are tapered along the height. For retrofitting of tapered columns by external FRP 196 Chapter Conclusions and recommendations confinement, it is recommended to reprofile the tapered plastic hinge region as a uniform section before the installation of FRP systems. 6.2 Recommendations for further research Some recommendations for further research are suggested below: 1. The current study described in this thesis is confined to relatively small scale FRP-confined capsule-shaped columns under axial and lateral loads. To examine the effectiveness of FRP confinement for real applications in buildings and bridges, experimental investigation is necessary on large scale and full scale FRP-confined capsule-shaped columns. 2. In the current study, the enhancements in lateral load capacity and ductility of FRP-confined capsule-shaped columns have been evaluated for a high section aspect ratio to examine the effectiveness of reprofiling. 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Mater., 7, 125138. 204 [...]... on FRP- confined columns under axial and lateral loading have been reviewed and discussed in the following sections 2.2 FRP- confined columns under axial loading In recent years, the effectiveness of FRP confinement has been studied for circular, square and rectangular sections (Mirmiran and Shahawy 1997; Miyauchi et al 1999; Rochette and Labossiere 2000; Pessiki et al 2001; Chaallal et al 2003; Lam and. .. out on FRP- confined columns under pushover and cyclic load The effectiveness of the two proposed reprofiling methods has been examined by comparing the shear capacity and ductility of FRP- confined reprofiled columns with the unconfined columns The effectiveness of FRP confinement has been evaluated by comparing the performance of unconfined and FRP- confined columns 3 Practical applications of FRP retrofitting...The axial load tests showed significant enhancement in axial load capacities of capsule- shaped columns The proposed confinement models are valid for circular, square, rectangular and capsule- shaped sections and they showed reasonable accuracy to predict the axial load capacity of tested columns The pushover load tests on FRP- confined rectangular and capsuleshaped columns showed the effectiveness... experimental investigation on axially loaded FRP- confined capsule- shaped concrete columns FRP- confined capsule- shaped columns with aspect ratios ranging from 1 to 4 were tested and the test results have been analyzed and compared considering the effect of reprofiling and FRP confinement 5 Chapter 1 Introduction 2 Evaluation of lateral load-displacement response Analytical and numerical models have been... retrofitting In Chapter 4, theoretical and numerical predictions of lateral loaddisplacement characteristics of FRP confined capsule- shaped concrete columns are presented This is followed by a detailed description of lateral load tests study on FRP- confined capsule- shaped columns In Chapter 5, numerical case studies on the performance of a retrofitted four storied building structure and a retrofitted bridge pier... findings from the axial and lateral load studies on FRPconfined columns, numerical case studies have been carried out to evaluate the performance of as-built and FRP- retrofitted residential building and bridge pier under lateral loading 1.5 Organization of the thesis This thesis consists of six chapters Chapter 1 discusses the background of the present research work, and the research objective and scope Chapter... capacity and ductility Also, retrofitting of concrete columns by external FRP systems are an easy and fast repairing method compared to the steel jacketing and concrete jacketing The FRP systems are highly non-corrosive and hence, external jacketing of damaged columns or structures by FRP systems in corrosive environment could be very effective 1.3 FRP- confined concrete columns Externally bonded FRP systems... Strengthening ratios for FRP- confined rectangular columns (Chaallal et al 2003) Figure 2.3 Effect of corner radius on FRP- confined square columns (AlSalloum 2007) Figure 2.4 Stress-strain model of steel - confined concrete (Mander et al 1988) Figure 2.5 Rectangular column confined by steel links Figure 2.6 FRP- confined rectangular column section Figure 2.7 Confinement models of FRP- confined circular column... patterns of unconfined columns Figure 4.17 Failure patterns of CFRP confined columns Figure 4.18 Lateral load vs strain from longitudinal reinforcement for LC-0.19-2 Figure 4.19 Lateral load vs longitudinal strain from FRP surface for LC-0.19-2 Figure 4.20 Failure patterns of column LC-0.19-2 Figure 4.21 Lateral load-displacement characteristics of columns under pushover loading Figure 4.22 Lateral load... effectiveness of FRP confinement was better in circular columns than in rectangular or square columns To predict the confined compressive strength of columns under axial loading, various confinement models for FRP confined concrete column have been proposed (Restrepol and DeVino 1996; Karbhari and Gao 1997; Samaan et al 1998; Miyauchi et al 1999; Saafi et al 1999; Toutanji et al 1999; Lam and Teng 2003; . FRP-CONFINED CAPSULE-SHAPED COLUMNS UNDER AXIAL AND LATERAL LOADINGS TAMALI BHOWMIK NATIONAL UNIVERSITY OF SINGAPORE 2011 FRP-CONFINED CAPSULE-SHAPED COLUMNS UNDER AXIAL AND LATERAL LOADINGS. LITERATURE REVIEW 2.1 General 10 2.2 FRP-confined columns under axial loading 11 2.2.1 Effect of FRP amount and concrete strength 12 2.2.2 Effect of aspect ratio and shape of column 13 2.2.3 FRP-confinement. details and results of a study on the FRP- confined capsule-shaped columns subjected to axial and lateral loads. The main parameters of the study were the effect of section geometry and the