DEVELOPMENT OF NATIONAL GUIDELINES FOR SEISMIC ANALYSIS AND DESIGN OF (ENGINEERED) BUILDINGS IN SRI LANKA Hiththara Gedara Shyamini Rasangika Kularathna (128031D) Degree of Master of Science Department of Civil Engineering University of Moratuwa Sri Lanka July 2013 [i] DECLARATION “I declare that this is my own work and this dissertationdoes 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/dissertation, 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 (such as articles or books). Signature: Date: The above candidate has carried out research for the MastersDissertation under my supervision. Signature of the supervisor: Date [ii] DEVELOPING NATIONAL GUIDELINES FOR SEISMIC ANALYSIS AND DESIGN OF (ENGINEERED) BUILDINGS IN SRI LANKA Abstract Sri Lanka was believed to have no seismic threat compared to other natural disasters such as landslides, floods, droughts which often cause widespread devastations. However, it has now been realized that Sri Lanka can no longer be considered as isolated from seismic threat following recent past events occurred in and around island. Designers of structures in Sri Lanka often used to avoid seismic consideration in the design procedure essentially of buildings as Sri Lanka is located within the Indo-Australian plate and thus, the chances of inter-plate type earthquakes which take place at the plate boundaries causing significant damages are remote. However, it is possible to take place intra-plate type earthquakes at any place within the tectonic plate. A notable example of a damaging intra-plate earthquake is the devastating Gujarat Earthquake in 2001. The only available document for the purpose of seismic design of buildings in Sri Lanka is “Earthquake resistant detailing for buildings in Sri Lanka” published by the Society of Structural Engineers, Sri Lanka. The present study is therefore aimed to provide advice on how all of these factors would affect the need for seismic design in Sri Lanka and provide guidance on suitable analysis and design procedures for buildings when the seismic consideration is explicitly warranted for a structure. The proposed guidelines in this study are based on Euro Code 8 (EN 1998-1: 2004): “Design of Structures for Earthquake Resistance”. Euro Code 8 was selected for this purpose because it allows national choices in defining seismic characteristics such as peak ground accelerations, response spectra etc. in the seismic design procedure. It also allows national choices in selecting analysis and design procedures of buildings to resist seismic events. Therefore, this study mainly focuses on these national choices and suitable values are proposed and discussed depending on the available limited seismic data in Sri Lanka. Whenever there is no enough data, suitable approaches are given comparing similar seismic codes such as IS 1893-1: 2002 and AS 1170.4: 2007. Finally, two case studies are carried out in order to present how the developed guidelines are used in the seismic design procedure of buildings specifically in Sri Lanka. The two buildings selected for this purpose represent buildings with high consequences of failure during an earthquake so that it clearly shows the significance of seismic consideration in the design procedure of buildings. Key words: intra-plate earthquake, seismic design guidelines, Sri Lankan National Annex to EC 8 [iii] ACKNOWLEDGMENTS I would like to express my deepest gratitude to my supervisor Dr. C.S.Lewangamage and co-supervisor Prof. M.T.R.Jayasinghe the former head of the Civil Engineering Department for their support and encouragement during this study. Their research experience and the understanding of the subject were of great importance for results achieved through this study. I also would like to thank Prof. Ranjith Dissanayake, University of Peradeniya for being the chairperson of the progress review committee. His guidance and the support made this work to be nourished by several experts in the field. I should also mention the support given by Dr.L.L.Ekanayake being as a progress review committee member. The support given by Prof. H.S.Thilakasiri being as the research coordinator of Civil Engineering Department is also remarkable during this study. This project would not be in this extent without the financial support given by the Disaster Management Centre (DMC), Sri Lanka through United Nations Development Programme (UNDP). I would like to extend my thanks to Dr. K.K.Wijesundara, South Asia Institute of Technology and Medicine (SAITM) for providing valuable data that made this study to progress. I would also be thankful to Prof. Nimal Senevirathna from University of Peradeniya for his valuable comments on the work carried out in this study. Last but not least, I present my deepest thanks to all other members of academic staff and non-academic staff of Civil Engineering Department, University of Moratuwa who gave me the support in various means to finalize this project successfully. iv TABLE OF CONTENTS Declaration i Acknowledgement ii Abstract iii Table of contents iv List of Figures x List of Tables xii List of Appendices xv 1. INTRODUCTION .............................................................................................. 1 1.1. Background ................................................................................................... 1 1.1.1. Seismic hazard in and around Sri Lanka ................................................... 2 1.2. Earthquake resistant design of buildings ....................................................... 5 1.2.1. Building response to earthquakes .............................................................. 5 1.2.2. Performance based methodologies of earthquake resistant design ............ 7 1.3. Scope and Objectives .................................................................................... 8 1.4. Overview of the report .................................................................................. 9 2. LITERATURE REVIEW ................................................................................ 11 2.1. General ........................................................................................................ 11 2.2. Performance based earthquake engineering ................................................ 12 2.3. Ground motion and Building response ........................................................ 17 2.3.1. Ground motion ......................................................................................... 18 2.3.2. Building reaction to ground motion ......................................................... 19 2.4. Evolution of Earthquake Resistant Design of Structures ............................ 19 2.5. Different Code Approaches to Earthquake Resistant Design of Buildings . 22 2.5.1. Target performance level ......................................................................... 23 v 2.5.2. Specification of hazard and defining seismic action ............................... 25 2.5.2.1.FEMA 450 ....................................................................................... 25 2.5.2.2.Euro Code 8 ..................................................................................... 27 2.5.2.3.AS 1170.4: 2007 .............................................................................. 29 2.5.2.4.IS 1893-1: 2002 ............................................................................... 30 2.5.3. Structural analysis and design criteria ..................................................... 31 2.5.3.1.Screening process ............................................................................ 32 2.5.3.2.Seismic analysis methods ................................................................ 34 3. DEVELOPING NATIONAL GUIDELINES FOR SEISMIC ANALYSIS AND DESIGN OF BUILDINGS TO EURO CODE 8 .................................. 36 3.1. General ........................................................................................................ 36 3.2. Implementation of Euro Code 8 .................................................................. 37 3.2.1. Background to Euro Codes ...................................................................... 37 3.2.2. Overview and implementation of the Euro Code 8 ................................. 38 3.3. Seismic Analysis and Design Procedure in Euro Code 8 ............................ 40 3.3.1. Performance requirements (EN 1998-1: 2004/2.1 (1) P) ......................... 40 3.3.2. Compliance .............................................................................................. 42 3.3.3. Reliability Differentiation (EN 1998-1: 2004/2.1 (2) P, (3) P) ............... 43 3.3.4. Earthquake ground motion (EN 1998-1: 2004/3.2) ................................. 44 3.3.4.1.Peak ground acceleration ................................................................. 44 3.3.4.2.Elastic response spectrum (EN 1998-1: 2004/3.2.2) ....................... 45 3.3.4.2.1.Horizontal elastic response spectrum ....................................... 45 3.3.4.2.2.Vertical elastic response spectrum ............................................ 49 3.3.4.3.Design response spectrum (EN 1998-1/3.2.2.5) .............................. 49 3.3.5. Behavior factor (q) and ductility class ..................................................... 50 vi 3.3.6. Seismic analysis of buildings ................................................................... 52 3.3.6.1.Screening process (EN 1998-1/3.2.1 (4) & (5)) .............................. 52 3.3.6.2.Structural Regularity (EN 1998-1/4.2.3) ......................................... 54 3.3.6.2.1.Criteria for regularity in plan .................................................... 55 3.3.6.2.2.Criteria for regularity in elevation ............................................ 55 3.3.6.3.Modelling of structures ................................................................... 56 3.3.6.4.Methods of analysis (EN 1998-1/4.3.3) .......................................... 57 3.3.6.4.1.Lateral force method of analysis (EN 1998-1/4.3.3.2) ............. 57 3.3.6.4.2.Modal response spectrum analysis (EN 1998-1/4.3.3.3) .......... 59 3.3.6.4.3.Non-linear methods of analysis (EN 1998-1/4.3.3.4) ............... 60 3.3.6.5.Other important issues in seismic analysis to EC 8 ......................... 60 3.3.6.5.1.Seismic mass (EN 1998-1/3.2.4) .............................................. 60 3.3.6.5.2.Accidental torsional effects (EN 1998-1/4.3.3.3.3) .................. 61 3.3.6.5.3.Seismic load combination ......................................................... 61 3.3.7. Design verification ................................................................................... 64 3.3.7.1.Ultimate limit state (EN 1998-1/4.4.2) ............................................ 64 3.3.7.2.Serviceability limit state (EN 1998-1/4.4.3) ................................... 65 3.4. Implementation of Euro Code 8 in Sri Lanka ............................................. 65 3.4.1. Proposed seismic design approach for Sri Lanka .................................... 66 3.4.1.1.Seismic action .................................................................................. 66 3.4.1.2.Behavior factor and ductility class .................................................. 69 3.4.1.3.Structural model and the method of analysis .................................. 70 3.4.1.4.Design verification .......................................................................... 71 3.4.2. Peak ground acceleration and elastic response spectra ............................ 73 3.4.2.1.Peak ground acceleration ................................................................. 73 3.4.2.2.Elastic ground acceleration response spectrum ............................... 76 vii 3.5. Nationally Determined Parameters (NDPs) for Sri Lanka .......................... 86 4. CASE STUDY 01 .............................................................................................. 87 4.1. General ........................................................................................................ 87 4.2. Case Study 1: Three storey school building ................................................ 87 4.3. Performance requirements ........................................................................... 89 4.4. Structural regularity ..................................................................................... 89 4.4.1. Criteria for regularity in plan ................................................................... 90 4.4.2. Criteria for regularity in elevation ........................................................... 99 4.5. Determining the structural eccentricities, torsional radii and radii of gyration ................................................................................................. 92 4.5.1. Structural eccentricity .............................................................................. 92 4.5.2. Torsional radius ....................................................................................... 93 4.5.3. Radius of gyration of the floor mass in plan (lx and ly) ............................ 93 4.6. Seismic action .............................................................................................. 94 4.6.1. Design response spectra ........................................................................... 95 4.6.1.1.Behavior factor (q) .......................................................................... 96 4.6.1.2.Design peak ground acceleration..................................................... 98 4.7. Structural Model .......................................................................................... 99 4.8. Design characteristics of the model ............................................................. 99 4.9. Fundamental period of vibration of the building ....................................... 102 4.9.1. Fundamental period of vibration using empirical expressions .............. 103 4.9.2. Fundamental period of vibration using Rayleigh method ..................... 104 4.9.3. Fundamental period of vibration using modal analysis ......................... 105 4.10. Methods of analysis ................................................................................... 105 4.10.1.Lateral force method of analysis .......................................................... 106 viii 4.10.1.1.Estimation of self-weight and seismic mass ................................ 107 4.10.1.2.Calculating seismic base shear .................................................... 109 4.10.1.3.Distribution of lateral forces ........................................................ 110 4.10.2.Modal response spectrum analysis ....................................................... 111 4.10.2.1.General ........................................................................................ 111 4.10.2.2.Periods, effective masses and modal shapes ............................... 111 4.11. Torsional effects ........................................................................................ 114 4.12. Displacements and drift ............................................................................. 114 4.12.1.Displacement ........................................................................................ 114 4.12.2.Inter-storey drift ................................................................................... 115 4.13. P-Δ effects ................................................................................................. 117 4.14. Results ....................................................................................................... 119 4.14.1.Shears ................................................................................................... 119 4.14.2.Member internal forces ........................................................................ 120 5. CASE STUDY 2 .............................................................................................. 123 5.1. General ...................................................................................................... 123 5.2. Description of the Building ....................................................................... 123 5.3. Seismic Action .......................................................................................... 124 5.3.1. Structural type of the building ............................................................... 127 5.3.2. Behavior factor ...................................................................................... 127 5.3.3. Design peak ground acceleration ........................................................... 127 5.4. Structural Regularity ................................................................................. 128 5.5. Structural Model ........................................................................................ 128 5.6. Modal Response Spectrum Analysis ......................................................... 129 5.6.1. General ................................................................................................... 129 ix 5.6.2. Seismic mass of the building ................................................................. 131 5.6.3. Periods, effective masses and modal shapes .......................................... 131 5.7. Accidental Torsional Effects ..................................................................... 133 5.8. Static Forces by Lateral Force Method ..................................................... 134 5.9. Storey Shear and Base Shear ..................................................................... 136 5.10. Displacements and drifts ........................................................................... 137 5.10.1.Displacements ...................................................................................... 137 5.10.2.Inter-storey drift ................................................................................... 137 5.11. Criterion of the Second Order Effect (P-∆ effects) ................................... 138 5.12. Results ....................................................................................................... 139 6. CONCLUSIONS AND RECOMMENDATIONS ....................................... 142 6.1 General ............................................................................................................ 142 6.2 Implementation of the proposed guidelines .................................................... 142 6.3 Significance of seismic consideration in design of buildings in Sri Lank3 ………………………………………………………………………..............143 Reference List 145 Annex A: Summary of nationally determined parameters 148 Annex B: Calculation seismic base shear according to AS 1170: 2007 154 Annex C: Calculation seismic base shear according to IS 1893: 2002 155 [x] LIST OF FIGURES Page Figure 1.1: Tectonic plate boundaries .......................................................................... 3 Figure 1.2: Earthquakes in and around Sri Lanka ........................................................ 5 Figure 1.3: Response spectra for a given ground motion [2] ....................................... 6 Figure 2.1: Inter-relationship between seismic demand and structural capacity as applied to EQRD [3] ............................................................................... 12 Figure 2.2: A global framework for performance-based earthquake engineering [4] 14 Figure 2.3: Performance objectives as defined in Vision 2000 report [7] ................. 15 Figure 2.4: Key steps in performance based design process [9] ................................ 17 Figure 2.5: Evolution of seismic design procedure (adopted from ATC-40) [3] ...... 21 Figure 2-6: General design procedure common to seismic design codes .................. 23 Figure 2.7: Response spectrum in FEMA 450 [14] ................................................... 27 Figure 2.8: Euro code Type 1 elastic response spectra .............................................. 28 Figure 2.9: Euro code Type 2 elastic response spectra .............................................. 29 Figure 2.10: Response spectra defined in AS 1170.4: 2007 ...................................... 30 Figure 2.11: Response spectra defined in IS 1893-1: 2002 ....................................... 31 Figure 3.1: EC 8 frame work for seismic analysis and design of buildings............... 41 Figure 3.2: Basic shape of the horizontal elastic response spectrum in EC 8 ............ 46 Figure 3.3: Steps involved in Seismic analysis and design procedure to EC 8 ......... 53 Figure 3.4: Proposed seismic design approach for Sri Lanka .................................... 67 Figure 3.5: Proposed methodology for modal response spectrum analysis ............... 72 Figure 3.6: Hazard curves for Colombo [22] ............................................................. 74 Figure 3.7: Representation of the relationship between the importance factor and the return period for different value of the seismic exponent ...................... 75 Figure 3.8: Response spectrum for Colombo at rock site and the corresponding response spectrum in IS 1893-1: 2002 [21] ............................................ 78 Figure 3.9: Comparison of soil amplification at the constant acceleration branch .... 80 Figure 3.10: Comparison of EC 8 Type 1 response spectra with IS 1893-1 and AS 1170.4 response spectra for the approximate equivalent soil types ...... 82 [xi] Figure 3.11: Comparison of EC 8 Type 2 response spectra with IS 1893-1 and AS 1170.4 response spectra for the approximate equivalent soil types ...... 83 Figure 4.1: Cross section of the building showing the elevation in Y-direction........ 88 Figure 4.2: (a) & (b) Typical floor plans showing plan dimensions .......................... 89 Figure 4.3: Elastic response spectrum and design response spectra for the three ductility classes ....................................................................................... 96 Figure 4.4: Three dimensional (spatial) model of the building ................................ 101 Figure 4.5: Three fundamental modes of vibration obtained from modal response spectrum analysis .................................................................................. 113 Figure 4.6: Shear force diagrams for perimeter frame 1 (a) lateral force method of analysis (b) Modal response spectrum analysis .................................... 121 Figure 4.7: Bending moment diagrams for perimeter frame 1 (a) Lateral force method of analysis (b) Modal response spectrum analysis .................. 122 Figure 5.1: Floor plan (a) Basement floors (b) Floors above ground level ............. 125 Figure 5.2: Elevation of the building ....................................................................... 126 Figure 5.3: Elastic response spectrum and design response spectrum ..................... 126 Figure 5.4: Structural model of the test building ..................................................... 130 Figure 5.5: Three fundamental modes of vibration .................................................. 133 Figure 5.6: Bending moments (a) and shear force (b) for internal frame 3 for the combined (SRSS) seismic action in both horizontal directions ........... 140 Figure 5.7: Bending moments (a) and shear forces (b) for internal frame 3 for the seismic design combination in EN 1990/6.4.3.4 .................................. 141 [xii] LIST OF TABLES Page Table 2-1: Four discrete target performance levels as defined in FEMA documents [8] ............................................................................................................. 16 Table 2-2: Seismic design category based on SDS [13] .............................................. 32 Table 2-3: Seismic design category based on SD1 [13] .............................................. 33 Table 2-4: Threshold values for screening process in EC 8 [15] ............................... 33 Table 2-5: Analysis methods recommended for different building categories in FEMA 450 [13] ...................................................................................... 35 Table 3-1: Importance classes of buildings and associated importance factors ......... 44 Table 3-2: Values of the parameters describing the recommended Type-1 elastic response spectrum .................................................................................... 47 Table 3-3: Values of the parameters describing the recommended Type-2 elastic response spectrum .................................................................................... 47 Table 3-4: Ground types............................................................................................. 48 Table 3-5: Recommended values of parameters describing the vertical elastic response spectra ....................................................................................... 49 Table 3-6: Consequences of structural regularity on seismic analysis and design .... 54 Table 3-7: Categories of action as defined in EN 1991-1-1 ...................................... 62 Table 3-8: Recommended values for in EN 1990/Table A1.1 ............................... 63 Table 3-9: Recommended values of φ in EN 1998-1/Table 4.2 ................................ 64 Table 3-10: Building classification in to importance classes ..................................... 68 Table 3-11: Importance factors for different return periods for the k value of 3.0 .... 76 Table 3-12: Proposed importance factors and corresponding return period values ... 76 Table 3-13: Soil classification in EC 8 and IS 1893-1 based on NSPT value ............. 79 Table 3-14: Soil classification in EC 8 and AS 1170.4 based on average shear velocity ................................................................................................... 79 Table 3-15: Comparison of corner period values of EC 8 and IS .............................. 80 Table 3-16: Soil types and corresponding parameters defining response spectra (IS 1893) modified to the format in EC 8 ................................................... 85 [xiii] Table 4-1: Structural eccentricity, torsional radius and radii of gyration in each horizontal direction .................................................................................. 91 Table 4-2: Structural eccentricity in each horizontal direction .................................. 93 Table 4-3: Torsional radii in each horizontal direction .............................................. 93 Table 4-4: Radius of gyration .................................................................................... 94 Table 4-5: Parameters defining elastic response spectrum for soil class ‘hard’ ........ 95 Table 4-6: Design loads used in the analysis ........................................................... 101 Table 4-7: Material properties used in the analysis ................................................. 102 Table 4-8: Fundamental period of vibration using alternative empirical expression given in EC 8 .......................................................................................... 104 Table 4-9: Parameters for calculation of fundamental period of vibration using Rayleigh method .................................................................................... 105 Table 4-10: Fundamental period of vibration obtained from modal analysis .......... 105 Table 4-11: Criteria for the use of lateral force method of analysis for the test building ................................................................................................. 107 Table 4-12: Approximate calculation of dead load of the test building ................... 107 Table 4-13: Approximate calculation of imposed load on the test building ............ 108 Table 4-14: Total seismic mass of the test buildings ............................................... 109 Table 4-15: Seismic base shear for each horizontal direction .................................. 110 Table 4-16: Distribution of seismic base shear at each storey level ........................ 110 Table 4-17: Periods, effective modal mass participation for the test buildings obtained from the modal response spectrum analysis ......................... 112 Table 4-18: Calculation of torsional moments at each horizontal direction ............ 114 Table 4-19: Design displacement (ds) of the test building at each storey level (Lateral force method of analysis) ..................................................................... 115 Table 4-20: Design displacement (ds) of the test building at each storey level (Modal response spectrum analysis method) .................................................... 115 Table 4-21: Parameters defining the criteria for damage limitation requirement (Lateral force method of analysis) ........................................................ 116 Table 4-22: Parameters defining the criteria for damage limitation requirement (modal response spectrum analysis method)....................................... 117 [xiv] Table 4-23: Calculation of inter-storey drift coefficient at each level of the test building (Lateral force method of analysis) ........................................ 118 Table 4-24: Calculation of inter-storey drift coefficient at each level of the test building (Modal response spectrum analysis) ..................................... 118 Table 4-25: Storey shear and base shear for the test building (Lateral force method of analysis)............................................................................................... 119 Table 4-26: Storey shear and base shear for the test building (Modal response spectrum analysis) ............................................................................... 119 Table 5-1: Parameters defining the elastic response spectrum ................................ 124 Table 5-2: Loads applied in the analysis .................................................................. 129 Table 5-3: Period of vibration (T) and effective masses .......................................... 131 Table 5-4: Torsional Moments ................................................................................. 134 Table 5-5: Seismic Mass at each floor level ............................................................ 135 Table 5-6: Seismic base shear .................................................................................. 135 Table 5-7: Seismic horizontal force at each floor level ........................................... 136 Table 5-8: Storey shear forces.................................................................................. 136 Table 5-9: Displacement in centre of mass at each floor level ................................ 137 Table 5-10: Storey drift control for both directions ................................................. 138 Table 5-11: Determination of the inter-storey drift coefficient ............................... 139 [xv] LIST OF APENDICES Appendix Description Page Appendix A Summary of Nationally determined 148 Parameters Appendix B Calculation of seismic base shear 154 According to AS 1170: 2007 Appendix C Calculation of seismic base shear 155 According to IS 1893: 2002