Seismic Retrofit of Existing Reinforced Concrete Buildings

Understand the complexities and challenges of retrofitting building infrastructure

Across the world, buildings are gradually becoming structurally unsound. Many were constructed before seismic load capacity was a mandatory component of building standards, and were often built with low-quality materials or using unsafe construction practices. Many more are simply aging, with materials degrading, and steel corroding. As a result, efforts are ongoing to retrofit existing structures, and to develop new techniques for assessing and enhancing seismic load capacity in order to create a safer building infrastructure worldwide.

Seismic Retrofit of Existing Reinforced Concrete Buildings provides a thorough book-length discussion of these techniques and their applications. Balancing theory and practice, the book provides engineers with a broad base of knowledge from which to approach real-world seismic assessments and retrofitting projects. It incorporates knowledge and experience frequently omitted from the building design process for a fuller account of this critical engineering subfield.

Seismic Retrofit of Existing Reinforced Concrete Buildings readers will also find:

• Detailed treatment of each available strengthening technique, complete with advantages and disadvantages
• In-depth guidelines to select a specific technique for a given building type and/or engineering scenario
• Step-by-step guidance through the assessment/retrofitting process

Seismic Retrofit of Existing Reinforced Concrete Buildings is an ideal reference for civil and structural engineering professionals and advanced students, particularly those working in seismically active areas.

Stelios Antoniou, Ph.D, is Managing Director of Seismosoft Ltd., a company that develops state-of-the-art software tools for nonlinear analysis, structural assessment, and structural strengthening, as well as CEO and Director of the Repair and Strengthening Section of Alfakat S.A., a construction company specializing in seismic load strengthening and retrofits. He holds degrees in civil engineering and earthquake engineering from the National Technical University of Athens, Greece, as well as both an MSc in Earthquake Engeineering and a Ph.D. in advanced structural analysis from Imperial College, London, UK.

Chapter 1 Introduction  1

1.1 General        1

1.2 Why do old RC Buildings need Strengthening?             3

1.3 Main Differences between Assessment and Design Methodologies    4

1.4 Whom is this Book for?          6

1.5 Main Standards for the Seismic Evaluation of Existing Structures         7

1.6 References 10

Chapter 2 Know Your Building: The importance of accurate knowledge of the structural configuration       12

2.1 Introduction               12

2.2 What Old RC Buildings are like            12

2.2.1 Lack of Stirrups      14

2.2.2 Unconventional Reinforcement in the Members     15

2.2.3 Large, Lightly Reinforced Shear Walls or Lack of Shear Walls               16

2.2.4 Lap Splices               16

2.2.5 Corrosion 17

2.2.6 Geometry: Location of Structural Members              18

2.2.7 Geometry: Bad Alignment of the Columns 18

2.2.8 Geometry: Arbitrary Alterations during Construction or during the Lifetime of the Building  18

2.2.9 Bad Practices with respect to the Mechanical and Electrical Installations      19

2.2.10 Soft Ground Storeys          20

2.2.11 Short Columns     20

2.2.12 Different Construction Methods  21

2.2.13 Foundation Conditions     22

2.2.14 Discussion             22

2.2.15 One Final Example             24

2.3 How come our Predecessors were so Irresponsible? 25

2.4 What the Codes say - Knowledge Level and the Knowledge Factor      26

2.5 Final Remarks            29

2.6 References 30

Chapter 3 Measurement of Existing Buildings, Destructive and Non-destructive Testing   31

3.1 Introduction               31

3.2 Information needed for the measured drawings         31

3.3 Geometry    34

3.4 Details - Reinforcement         36

3.5 Material Strengths   39

3.6 Concrete Tests – Destructive Methods            40

3.7 Concrete Tests – Non-destructive Methods, NDT        41

3.7.1 Rebound Hammer Test       41

3.7.2 Penetration Resistance Test             42

3.7.3 Pull-off Test             42

3.7.4 Ultrasonic Pulse Velocity Test, UPV               43

3.8 Steel Tests   43

3.9 Infill Panel Tests        44

3.10 What is the Typical Procedure for Monitoring an Existing Building?  44

3.11 Final Remarks          47

3.12 References               47

Chapter 4 Methods for Strengthening Reinforced Concrete Buildings        49

4.1 Introduction               49

4.2 Literature Review     50

4.3 Reinforced Concrete Jackets 53

4.3.1 Application              53

4.3.2 Advantages and Disadvantages       55

4.3.3 Design Issues: Modelling, Analysis and Checks          56

4.4 Shotcrete     57

4.4.1 Introduction            57

4.4.2 Dry mix vs. Wet mix Shotcrete        58

4.4.3 Advantages and Disadvantages of Shotcrete             59

4.4.4 What is it actually called, Shotcrete or Gunite?        60

4.4.5 Materials, Proportioning, and Properties    61

4.4.6 Mix Proportions for the Dry-mix Process     64

4.4.7 Equipment and Crew           65

4.4.8 Curing and Protection         67

4.4.9 Testing and Evaluation        67

4.5 New Reinforced Concrete Shear Walls             67

4.5.1 Application              67

4.5.2 Foundation Systems of new Shear Walls      70

4.5.3 Advantages and Disadvantages       71

4.5.4 Design Issues: Modelling and Analysis          72

4.6 RC Infilling    73

4.6.1 Application              73

4.6.2 Advantages and Disadvantages       73

4.7 Steel Bracing               74

4.7.1 Application              74

4.7.2 Advantages and Disadvantages       77

4.7.3 Design Issues: Modelling Analysis and Checks           78

4.8 Fibre-Reinforced Polymers (FRPs)     78

4.8.1 FRP Composite Materials   78

4.8.2 FRP composites in Civil Engineering and Retrofit     79

4.8.3 FRP Composite Materials   81

4.8.4 FRP Wrapping         82

4.8.5 FRP Laminates        85

4.8.6 Near Surface Mounted FRP Reinforcement 86

4.8.7 FRP Strings               87

4.8.8 Sprayed-FRP           88

4.8.9 Anchoring Issues   89

4.8.10 Advantages and Disadvantages of FRP systems     90

4.8.11 Design Issues       91

4.9 Steel Plates and Steel Jackets              92

4.9.1 Advantages and Disadvantages       94

4.9.2 Design Issues          95

4.10 Damping Devices    95

4.11 Seismic Isolation     96

4.11.1 Type of Base Isolation Systems     99

4.11.2 Advantages and Disadvantages    100

4.11.3 Design Issues       101

4.12 Selective Strengthening and Weakening through Infills          101

4.13 Strengthening of Infills         103

4.13.1 Glass or Carbon FRPs        103

4.13.2 Textile Reinforced Mortars TRM   104

4.13.3 Shotcrete              105

4.14 Connecting New and Existing Members       106

4.14.1 Design issues        107

4.15 Strengthening of individual members            108

4.15.1 Strengthening of RC Columns or Walls       108

4.15.2 Strengthening of RC Beams            109

4.15.3 Strengthening of RC Slabs               111

4.15.4 Strengthening of RC Ground Slabs               111

4.16 Crack Repair - epoxy injections         112

4.17 Protection against corrosion, repair mortars and cathodic protection              113

4.18 Foundation Strengthening 114

4.19 Concluding Remarks Regarding Strengthening Techniques  116

4.20 Evaluation of Different Seismic Retrofitting Solutions: A Case Study 116

4.20.1 Building Configuration      116

4.20.2 Effects of the Infills on the Structural Behaviour    119

4.20.3 Strengthening with Jacketing        121

4.20.4 Strengthening with new RC Walls (entire building height) 122

4.20.5 Strengthening with new RC Walls (ground level only)         124

4.20.6 Strengthening with Braces              126

4.20.7 Strengthening with FRP Wrapping               127

4.20.8 Strengthening with Seismic Isolation          128

4.20.9 Comparison of the Methods          129

4.21 References               130

Chapter 5 Criteria for Selecting Strengthening Methods – Case Studies    156

5.1 Things are Rarely Simple....  156

5.2 Criteria for Selecting Strengthening Method 157

5.3 Basic Principles of Conceptual Design               159

5.4 Some rules of thumb               161

5.5 Case Studies               164

5.5.1 Case Study1: Seismic upgrade of a 5-storey hotel    164

5.5.2 Case Study2: Seismic upgrade of a 4-storey hotel    166

5.5.3 Case Study 3: Seismic upgrade of a 4-storey hotel   167

5.5.4 Case Study 4: Seismic upgrade of a 3-storey residential building       168

5.5.5 Case Study 5: Seismic upgrade of a 3-storey residential building for the addition of two new floors  168

5.5.6 Case Study 6: Seismic strengthening of an 11-storey building             170

5.5.7 Case Study 7: Seismic strengthening of a 5-storey building  170

5.5.8 Case Study 8: Seismic strengthening of a 3-storey building  171

5.5.9 Case Study 9: Strengthening a building damaged by a severe earthquake    171

5.5.10 Case Study 10: Strengthening of an 11-storey building        172

5.5.11 Case Study 11: Strengthening of a two storey building with basement        173

5.5.12 Case Study 12: Strengthening of a weak ground storey with FRP wraps       174

5.5.13 Case Study 13 (several examples): Strengthening of RC slabs           175

5.5.14 Case Study 14: Strengthening of a ground slab       176

5.5.15 Case Study 15: Strengthening of beam that has failed in shear        176

5.5.16 Case Study 16: Demolition and reconstruction of a RC beam           177

5.5.17 Bonus Case Study 1: Strengthening of an industrial building             177

5.5.18 Bonus Case Study 2: Strengthening of an industrial building             177

5.5.19 Bonus Case Study 3: Strengthening of a residential building             178

5.6 References 178

Chapter 6 Performance Levels & Performance Objectives              179

6.1 Introduction               179

6.1.1 Selection of Performance Objectives in the Design of New Buildings               179

6.1.2 Selection of Performance Objectives in the Assessment of Existing Buildings               180

6.2 Seismic Assessment and Retrofit Procedures 180

6.2.1 Seismic Assessment Procedures     180

6.2.2 Seismic Retrofit Procedures             181

6.3 Understanding Performance Objectives         182

6.3.1 Target Building Performance Levels              182

6.3.2 Seismic Hazard Levels         187

6.3.3 Performance Objectives    188

6.3.4 Eurocode 8, Part-3 and other Standards      189

6.3.5 The Rationale for Accepting a Lower Performance Level for Existing Buildings            191

6.4 Choosing the correct Performance Objective 192

6.5 References 194

Chapter 7 Linear and Nonlinear Methods of Analysis        195

7.1 Introduction               195

7.2 General Requirements           197

7.2.1 Loading Combinations        197

7.2.2 Multidirectional Seismic Effects      198

7.2.3 Accidental torsional effects              198

7.3 Linear Static Procedure          199

7.4 Linear Dynamic Procedure    199

7.5 Nonlinear Structural Analysis               201

7.5.1 Nonlinear Structural Analysis in Engineering Practice             201

7.5.2 Challenges Associated with Nonlinear Analysis         204

7.5.3 Some Theoretical Background         204

7.5.4 Implications from the Basic Assumptions of Nonlinear Analysis         210

7.5.5 How Reliable are Numerical Predictions from Nonlinear Analysis Methods? 212

7.5.6 Closing Remarks    213

7.6 Nonlinear Static Procedure  213

7.6.1 Pushover Analysis 213

7.6.2 Information Obtained with Pushover Analysis          214

7.6.3 Theoretical Background on Pushover Analysis          215

7.6.4 Target Displacement           216

7.6.5 Applying Forces vs. Applying Displacements              217

7.6.6 Controlling the Forces or the Displacements             218

7.6.7 Control Node          219

7.6.8 Lateral Load Patterns          220

7.6.9 Pushover Analysis Limitations          220

7.7 Nonlinear Dynamic Procedure            221

7.7.1 Information Obtained with Nonlinear Dynamic Analysis       223

7.7.2 Selecting and Scaling Accelerograms            223

7.7.3 Advantages and Disadvantages of Nonlinear Dynamic Analysis          226

7.8 Comparative Assessment of Analytical Methods         227

7.8.1 Advantages and Disadvantages of the Analytical Methods  227

7.8.2 Selection of the Best Analysis Procedure for Structural Assessment 228

7.9 References 230

Chapter 8 Structural Modelling in Linear and Nonlinear Analysis  233

8.1 Introduction               233

8.2 Mathematical Modelling       233

8.3 Modelling of Beams and Columns      234

8.3.1 Material Inelasticity             234

8.3.2 Geometric Nonlinearities  235

8.3.3 Modelling of Structural Frame Elements     236

8.4 Modelling of Shear Walls       244

8.5 Modelling of Slabs    244

8.6 Modelling of Stairs   246

8.7 Modelling of Infills    247

8.7.1 A simple Example: infilled frame vs. bare frame       248

8.7.2 Another Example: partially infilled frame (soft storey) vs. bare frame             249

8.7.3 Problems in the Modelling of Infills               250

8.8 Modelling of Beam-Column Joints     251

8.9 Modelling of Bar Slippage     252

8.10 Shear Deformations              252

8.11 Foundation Modelling          253

8.12 How Significant are our Modelling Decisions?            253

8.13 References               254

Chapter 9 Checks and Acceptance Criteria             256

9.1 Introduction               256

9.2 Primary and Secondary Members     257

9.3 Deformation-Controlled & Force-Controlled Actions 258

9.4 Expected vs. Lower-Bound Material Strengths             260

9.5 Knowledge Level & Knowledge Factor             261

9.6 Capacity Checks        261

9.6.1 Capacity Checks for Linear Methods – ASCE 41         262

9.6.2 Capacity Checks for Nonlinear Methods – ASCE 41  264

9.6.3 Capacity Checks for Linear Methods – Eurocode 8, Part-3    265

9.6.4 Capacity Checks for Nonlinear Methods – Eurocode 8, Part-3             266

9.7 Main Checks to be Carried out in an Assessment Procedure  266

9.7.1 Bending Checks     266

9.7.2 Shear Checks          268

9.7.3 Beam-Column Joints            269

9.8 References 270

Chapter 10 Practical Example: Assessment and Strengthening of a 6-storey RC Building    271

10.1 Introduction             271

10.2 Building Description              271

10.3 Knowledge of the Building and Confidence Factor    272

10.3.1 Geometry             272

10.3.2 Reinforcement    272

10.3.3 Material Strengths             273

10.4 Seismic Action and Load Combinations         274

10.5 Structural Modelling             276

10.6 Eigenvalue analysis                277

10.7 Nonlinear Static Procedure 278

10.7.1 Lateral Load Patterns        278

10.7.2 Selection of the Control Node       279

10.7.3 Capacity Curve and Target Displacement Calculation           279

10.7.4 Safety Verifications           282

10.7.5 Chord rotation checks      282

10.7.6 Example of the calculation of Chord rotation capacity         283

10.7.7 Shear Checks        284

10.7.8 Example of the Calculation of Shear Capacity          285

10.7.9 Beam-column Joint Checks             286

10.7.10 Example of the Checks for beam-column Joints   286

10.8 Strengthening of the Building            288

10.8.1 Strengthening with Jackets            288

10.8.2 Designing the Interventions          289

10.8.3 Deliverables         290

10.8.4 Strengthening with Shear Walls    291

10.9 References               292

Appendix A Standards and Guidelines     294

A.1 EUROCODES               294

A.1.1 Performance Requirements             294

A.1.2 Information for Structural Assessment        295

A.1.3 Safety Factors        297

A.1.4 Capacity Models for Assessment and Checks            297

A.1.5 Target Displacement Calculation in Pushover Analysis          301

A.2 ASCE 41-17  304

A.2.1 Performance Requirements             304

A.2.2 Information for Structural Assessment        305

A.2.3 Safety Factors        306

A.2.4 Capacity Models for Assessment and Checks            307

A.2.5 Target Displacement Calculation in the Nonlinear Static Procedure 309

A.3 References 311

Appendix B Poor Construction & Design Practices in Older Buildings          313

B.1 Stirrup Spacing          313

B.2 Lap Splices   313

B.3 Member Alignment 313

B.4 Pipes inside RC Members      313

B.5 Bad Casting of Concrete        313

B.6 Footings       314

Appendix C Methods of Strengthening   315

C.1 Reinforced Concrete Jackets 315

C.2 New Shear Walls       315

C.3 Fibre-reinforced Polymers    315

C.3.1 FRP Wrapping of Columns 315

C.3.2 FRP Wraps in Slabs               315

C.3.3 FRP Wraps for Shear Strengthening              315

C.3.4 FRP Laminates        315

C.3.5 FRP Strings               315

C.4 Steel Braces 315

C.5 Steel Jackets               315

C.6 Steel Plates 316

C.7 Infills              316

C.8 Foundations               316

C.9 Dowels and Anchorages        316

C.10 Demolition with Concrete Cutting   316

C.11 Reinforcement Couplers     316

C.12 Epoxy Injections          316

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