SEISMIC VULNERABILITY ASSESMENT OF REINFORCED CONCRETE FRAMED BUILDINGS IN SRI LANKA: A CASE STUDY Susitha Kumara Abeywickrama Gunaratne 09/8912 Degree of Master of Engineering in Structural Engineering Design Department of Civil Engineering University of Moratuwa Sri Lanka February 2013 SEISMIC VULNERABILITY ASSESMENT OF REINFORCED CONCRETE FRAMED BUILDINGS IN SRI LANKA: A CASE STUDY Susitha Kumara Abeywickrama Gunaratne 09/8912 Thesis submitted in partial fulfillment of the requirements for the degree Master of Engineering in Structural Engineering Design Department of Civil Engineering University of Moratuwa Sri Lanka February 2013 i    DECLARATION I declare that this is my own work and this thesis does not incorporate without acknowledgement any material previously submitted for a Degree or Diploma in any other University or institute of higher learning and to the best of my knowledge and belief it does not contain any material previously published or written by another person except where the acknowledgement is made in the text. Also, I hereby grant to University of Moratuwa the non-exclusive right to reproduce and distribute my thesis, in whole or in part in print, electronic or other medium. I retain the right to use this content in whole or part in future works. Signature: Date: The above candidate has carried out research for the Masters thesis under my supervision. Signature of the supervisor: Date: ii    ACKNOWLEDGEMENTS I would like to convey my sincere gratitude to: The Department of Civil Engineering, University of Moratuwa for selecting me for M.Eng Degree in Structural Engineering Design Course and Central Engineering Consultancy Bureau (CECB) for providing full sponsorship and granting the duty leave for successful completion of the course. Dr. C.S. Lewangamage, Research supervisor for guiding me throughout the research and for his encouragement up to its completion. And Dr.K.Baskaran, Research Coordinator for timely organizing the research presentations and reviewing the progress of the research. Eng.K.L.S.Sahabandu, Additional General Manager (Design, Research and Development), CECB for directing me to some of the short courses and seminars related to the subject of research and helping me for this study. Dr.Yogendra Singh, Asst. Professor, Dept of Earthquake Engineering, Indian Institute of Technology (IIT-Roorke) for giving me the assistance in this regard. Dr. Naveed Anwer, Associate Director, Asian Center for Engineering Computations and Software, AIT(Thailand) for giving me valuable comments on nonlinear analysis through Email and during his short visit to Sri Lanka. Last but not the least, my friends who helped me and worked together up to the completion of the course. S.K.A.Gunaratne iii    ABSTRACT Earthquakes are one of nature’s greatest hazards to life. Sri Lanka is considered to be in an aseismic zone away from major plate boundaries or any active faults. However, the first Earthquake hazard recorded on 14th April 1615 in Colombo with 2000 deaths and destroying 200 houses. Since then, there have been many seismic events in Sri Lanka and neighbouring areas which are small to moderate in magnitude of which the trimmers were felt by the people in some of the regions in Sri Lanka. In addition, geologists suspect that there is a formation of a new plate boundary dividing Indo-Australian plate. Moreover, there is a possibility of occurrence of an intra-plate type earthquake within Indo-Australian plate (Eg. Maharashtra earthquake in 1993). Considering the above facts, there is a risk of occurrence of a small to moderate type earthquake in the vicinity of Sri Lanka. Hence, it is high time to commence not only the design and detailing of the buildings for seismic resistance but also the seismic assessment and retrofitting of the existing public buildings , because almost all of the existing buildings in Sri Lanka have not been designed or detailed for earthquake resistance. However, detailed seismic performance assessments are new to Sri Lanka though most of the earthquakes proven countries have already been reaping the benefits of such assessments considering in-situ conditions of the buildings. Therefore, it is intended to study the seismic performance of a medium rise building which is a reinforced concrete framed building situated in Colombo, Sri Lanka. Pushover procedure given in ATC 40 and the hinge parameters given in FEMA 356 guidelines were used to carry out the performance assessment. One of the outcomes expected from this study is to check the applicability and importance of pushover analysis for seismic assessments of a medium rise building having a large floor area incorporating all As- built details and found that it is applicable and realistic results can be obtained. As per UBC world seismic zoning, Sri Lanka is situated in seismic zone 0. However, considering the future seismic risk, the building was assessed for seismic Zone 1 and 2A. It was found from the study that the building performs at Immediate Occupancy performance level for serviceability earthquake in seismic zone 1(CA=0.06, CV=0.09).But not perform well for serviceability earthquake in seismic zone 2A(CA=0.12, CV=0.18).In addition found that the building can safely withstand a maximum ground motion having acceleration coefficients of CA=0.11 and CV=0.17 performing at Immediate Occupancy performance level. Furthermore, it was observed that seismic performance of the building considered, can be improved significantly in shorter direction than in longer direction by strengthening some of the critical elements. The building performs well in shorter direction than in longer. It was also found that pushover analysis helps to enhance the seismic resistance of a structure significantly by identifying and strengthening the critical elements with a small amount of additional cost. Finally, this analysis gives an indication of the integral seismic resistance of a reinforced concrete framed structure although specifically not designed for seismic loading. The recommendations were also made for proper seismic assessment and resistance verified from the study. This study is to be continued to find out the significance of a 3D analysis compared to a 2D analysis with respect to the accuracy of analysis results and time taken for the evaluation. Furthermore, the seismic performance assessment of this building can be carried out with other software such as PERFORM 3D to verify the analysis results obtained from SAP 2000. Key words: Seismic assessment, Seismic evaluation, Performance based design, Non linear static analysis and Pushover analysis. iv    TABLE OF CONTENTS Declaration of the Candidate & Supervisor i Acknowledgements ii Abstract iii Table of Content iv-vii List of Figures viii-ix List of Tables x List of Abbreviations x List of Appendices x 1. INTRODUCTION 1-4 1.1 Background 1 1.2 Significance of the Study 2 1.3 Objective 2 1.4 Outcome 2 1.5 Building Selected for the Study 3 1.6 The Arrangement of the Report 3 2. LITERATURE SURVEY 5-27 2.1 Background 5 2.2 Guidelines Available for Seismic Evaluation/ Rehabilitation of Buildings 5 2.3 Past Researches on Seismic Assessments of Buildings 6 2.4 Why Seismic Evaluation is needed 7 2.5 Seismic Evaluation of Buildings 7 2.6 Reliability of Assessment of Existing Buildings 9 2.7 Collection of As-Built Data and Documents 10 2.8 Detailed In-Situ Investigation 11 2.8.1 Material Properties 11 2.8.2 Component Properties 11 2.8.3 Condition Assessment 12 2.9 As-Built Data for the Building Modeling 13 2.10 Analytical Methods Available 13 2.11 Non-linear Static Analysis (Push-over Analysis) 14 2.12 Limitations of Non-Linear Static Analysis 15 2.13 Capacity Spectrum Method 16 2.14 Performance Based Design 17 v    2.15 Performance Level 17 2.15.1 Structural Performance Levels 19 2.15.2 Non Structural Performance Levels 20 2.16 Seismic Hazard Level 21 2.17 Performance Objectives 22 2.18 Selection of Performance Objectives 23 2.19 Primary Ground Shaking Criteria 23 2.19.1 Site Geology and Soil Characteristics 24 2.19.2 Site Seismicity Characteristics 24 2.19.3 Site Response Spectra 24 2.20 Outline of Capacity Spectrum Method 25 3. METHODOLOGY 28-61 3.1 Structural form of the Building 28 3.2 Static Pushover Analysis Procedure 28 3.3 Computer Modeling of the Building 31 3.3.1 Material Property Data 31 3.3.2 Defining the Frame sections 32 3.3.3 Modeling of Frame Elements 34 3.3.4 Assigning of As-Built Section Properties to FrameElements 34 3.3.5 Assigning Diaphragm Constraints 34 3.3.6 Assigning Plastic Hinges to Frame Elements 35 3.3.6.1 Frame Hinge Properties 35 3.3.6.2 Hinge Location and Generated Hinge Property 36 3.3.6.3 Concrete Beams in Flexure 39 3.3.6.4 Concrete Columns in Flexure 39 3.3.6.5 Coupled P-M2-M3 Hinge 40 3.3.6.6 Plastic Deformation Curve 41 3.3.6.7 Moment-Rotation Curves 42 3.3.6.8 Performance Levels on Plastic Deformation Curve 42 3.3.6.9 Scaling the Curve 42 3.3.6.10 Strength Loss 43 3.3.7 Assigning the Loads to Beam Elements (Load Patterns) 44 3.3.8 Defining Load Patterns 44 3.3.9 Defining Non Linear Load Cases 46 3.3.9.1 Initial conditions 47 3.3.9.2 Structural Response and Superposition 47 3.3.9.3 Nonlinearity 48 3.3.9.4 Displacement Control 49 3.3.9.4.1 Conjugate Displacement Control 50 3.3.9.5 Output Steps 50 3.3.9.5.1 Saving Multiple Steps 51 3.3.9.5.2 Minimum and Maximum Saved Steps 51 vi    3.3.9.5.3 Save Positive Increments Only 52 3.3.9.6 Nonlinear Solution Control 53 3.3.9.6.1 Maximum Total Steps 54 3.3.9.6.2 Maximum Null (Zero) Steps 54 3.3.9.6.3 Maximum Iterations per Step 55 3.3.9.6.4 Iteration Convergence Tolerance 55 3.3.9.6.5 Event-to-Event Iteration Control 55 3.3.9.7 Hinge Unloading Method 56 3.3.9.7.1 Unload Entire Structure 57 3.3.9.7.2 Apply Local Redistribution 57 3.3.9.7.3 Restart Using Secant Stiffness 58 3.3.10 Defining Load Combinations 59 3.4 Calculation of Seismic Coefficients 59 3.4.1 Soil Type 59 3.4.2 ZEN Value 60 3.4.3 Seismic Zone Factor(Z) 60 3.4.4 E Value 60 3.4.5 Near Source Factor (N) 60 3.4.6 Seismic Coefficients (CA , CV ) 61 4. ANALYSIS RESULTS 62-91 4.1 Modal Participating Mass Ratio 62 4.2 Procedure for Reviewing Analysis Results 63 4.3 Review of Hinge Analysis Results 63 4.4 Review of Pushover Analysis Results of the Building 64 4.5 Pushover Results for the Building (with As-Built Details) 65 4.5.1 Base shear vs Monitored Displacement 65 4.5.2 Pushover Results (ATC 40 Capacity Spectrum Method) for Serviceability Earthquake for Seismic Zone 1 67 4.5.2.1 Pushover Results for X direction 67 4.5.2.2 Pushover Results for Y direction 70 4.5.3 Pushover Results (ATC 40 Capacity Spectrum Method) for Serviceability Earthquake for Seismic Zone 2A 73 4.5.3.1 Pushover Results for X direction 73 4.5.3.2 Pushover Results for Y direction 74 4.5.4 Maximum Acceleration which the Building Safely Withstands 75 4.5.4.1 Maximum Acceleration in X direction 75 4.5.4.2 Maximum Acceleration in Y direction 76 4.6 Pushover Results after Structural Improvements to Some of the Critical Elements 77 vii    4.6.1 Base shear vs Monitored Displacement 79 4.6.2 Pushover Results (ATC 40 Capacity Spectrum Method) for Serviceability Earthquake for Seismic Zone 2A 81 4.6.2.1 Pushover Results in X direction 81 4.6.2.2 Pushover Results in Y direction 84 4.6.3 Maximum Acceleration which the Building safely withstands after Structural Improvements. 87 4.6.3.1 Pushover Results in X direction 87 4.6.3.2 Pushover Results in Y direction 88 4.7 Summary of Analysis Results 89 4.7.1 For the Building with As-Built Details 89 4.7.2 For the Building after Structural Improvements 90 4.7.3 Comparison of Seismic Performance 91 5. CONCLUSION & RECOMMENDATIONS 92-93 5.1 Conclusion 92 5.2 Recommendations 93 5.3 Future work 93 References 94-95 Appendix A - References from Chapter 3, ATC 40(1996) Vol. 1 Appendix B - References from UBC 97 & Chapter 4 ATC 40(1996) Vol. 1 Appendix C - References from Chapter 8, ATC 40(1996) Vol. 1 Appendix D - References from FEMA 356 viii    LIST OF FIGURES Figure 1-1 Isometric view of the building model 4 Figure 2-1 Construction of a 5 percent-Damped Elastic Response spectrum 25 Figure 2-2 Elastic Response spectrum in ADRS Format 25 Figure 2-3 Plot of Demand and Capacity spectrum in ADRS Format 26 Figure 2-4 Selection of a Trial Performance Point 26 Figure 2-5 Bilinear representation of capacity curve 26 Figure 2-6 Plotting of reduced demand spectrum 26 Figure 2-7 Locating the performance point 27 Figure 3-1a SAP 2000 Model (Isometric View) 29 Figure 3-1b SAP 2000 Model (Plan View) 29 Figure 3-1c Typical key plan 30 Figure 3-2a Defining Column Sections 32 Figure 3-2b Defining Column Reinforcements 32 Figure 3-3a Defining Beam Sections 33 Figure 3-3b Defining Beam Reinforcements 33 Figure 3-4 Concept of Modeling Frame Elements 34 Figure 3-5 Assigning Diaphragm Constraints 35 Figure 3-6a Assigning Auto Hinges to Beams 37 Figure 3-6b Assigning Auto Hinges to Columns 38 Figure 3-7a Moment Rotation Data for a M3 Hinge 38 Figure 3-7b Moment Rotation Data for a P-M2-M3 Hinge 39 Figure 3-8 Interaction Surface Definition for a P-M2-M3 Hinge 40 Figure 3-9 Plastic Force/Moment vs Deformation Curve 41 Figure 3-10 Assignment of Frame Loads 44 Figure 3-11 Defining Load Patterns 45 Figure 3-12 Defining Load Cases 45 Figure 3-13 Defining Non Linear Gravity Load Cases 46 Figure 3-14 Defining Non Linear Pushover Load Cases 46 Figure 3-15 Load Application Control 50 Figure 3-16 Assigning saved steps 53 Figure 3-17 Non Linear Parameters 54 Figure 4-1 Modal Participating Mass Ratios 62 Figure 4-2a Base shear vs monitored displacement curve for X-direction 65 Figure4-2b Base shear vs monitored displacement curve for Y-direction 66 Figure 4-3 ADRS Graph (ATC 40 Capacity Spectrum Method) for X-direction 67 Figure 4-4a Details of pushover steps for X-direction 68 Figure 4-4b Hinge state at pushover steps for X-direction 68 Figure 4-5a Hinge state at step 4 for X direction 69 Figure 4-5b Hinge state at step 5 for X direction 69 Figure 4-6 ADRS Graph (ATC 40 Capacity Spectrum Method) for Y-direction 70 Figure 4-7a Details of pushover steps for Y-direction 71 ix    Figure 4-7b Hinge state at pushover steps for Y-direction 71 Figure 4-8a Hinge state at step 8 for Y direction 72 Figure 4-8b Hinge state at step 10 for Y direction 72 Figure 4-9 ADRS Graph (ATC 40 Capacity Spectrum Method) for X-direction 73 Figure 4-10 ADRS Graph (ATC 40 Capacity Spectrum Method) for Y-direction 74 Figure 4-11a Parameters for ATC40 Spectrum for X-direction 75 Figure 4-11b ADRS Graph (ATC40 Spectrum) for X-direction 75 Figure 4-12a Parameters for ATC40 Spectrum for Y-direction 76 Figure 4-12b ADRS Graph (ATC40 Spectrum) for Y-direction 76 Figure 4-13 Key plan showing the locations of structural improvements 78 Figure 4-14a Base shear vs monitored displacement curve for X-direction 79 Figure 4-14b Base shear vs monitored displacement curve for Y-direction 80 Figure 4-15 ADRS Graph for X-direction 81 Figure 4-16a Details of pushover steps for X-direction 82 Figure 4-16b Hinge state at pushover steps for X-direction 82 Figure 4-17a Hinge state at step 6 for X direction 83 Figure 4-17b Hinge state at step 7 for X direction 83 Figure 4-18 ADRS Graph for Y-direction 84 Figure 4-19a Details of pushover steps for Y-direction 85 Figure 4-19b Hinge state at pushover steps for Y-direction 85 Figure 4-20a Hinge state at step 5 forY direction 86 Figure 4-20b Hinge state at step 6 for Y direction 86 Figure 4-21a Maximum Acceleration parameters for X-direction 87 Figure 4-21b ADRS Graph for X-direction 87 Figure 4-22a Maximum Acceleration parameters for Y-direction 88 Figure 4-22b ADRS Graph for Y-direction 88 x    LIST OF TABLES Table 2-1 Combinations of Structural and Nonstructural Performance Levels to Form Building Performance Levels 18 Table 2-2 The Basic Safety Objective in ATC 40 23 Table 3-1 Summary of ZEN Values 61 Table 3-2 Summary of Seismic Coefficient CA ,CV 61 Table 4-1 Amount of R\F at the locations of structural improvements 77 Table 4-2a Performance of the Building (with As-Built Details) for SE 89 Table 4-2b Performance of the Building (with As-Built Details) for DE 89 Table 4-2c Maximum Seismic performance of the Building (with As-Built Details) 89 Table 4-3a Performance of the Building(after structural improvement) for SE 90 Table 4-3b Performance of the Building (after structural improvement) for DE 90 Table 4-3c Maximum Seismic performance of the Building (after structural improvement) 90 Table 4-4 Comparison of Maximum Seismic Coefficients (g) 91 Table 4-5 Percentage of improvement in each direction 91 (after structural improvement) LIST OF APPENDICES Appendix A References from Chapter 3, ATC 40(1996) Vol. 1 Appendix B References from UBC 97 & Chapter 4 ATC 40(1996) Vol. 1 Appendix C References from Chapter 8, ATC 40(1996) Vol. 1 Appendix D References from FEMA 356 LIST OF ABBREVIATIONS Beff Effective damping ratio CA , CV Acceleration coefficients Teff Effective period of vibration Sa Spectral acceleration Sd Spectral displacement ATC Applied Technology Council FEMA Federal Emergency Management Agency V Base shear D Displacement at the top of the building I/O Immediate occupancy performance level L/S Life safety performance level C/P Collapse prevention performance level SP Structural performance level NP Non structural performance level