Moehle j , seismic design of reinforced concrete buildings, 2014

Jack Moehle is the T.Y. and Margaret Lin Professor of Engineering in the Department of Civil and Environmental Engineering at the University of California, Berkeley. He received his Ph.D. from the University of Illinois and joined the U.C. Berkeley faculty in 1980. His research and teaching activities are mainly in structural engineering, with emphasis on reinforced concrete and earthquake engineering. He was founding director of the Pacific Earthquake Engineering Research Center, a multicampus research center that advanced the concepts and practice of performancebased earthquake engineering. As a licensed Civil Engineer in the State of California, Dr. Moehle works regularly as a consulting engineer, offering advice and expert peer review on highway and mass transit systems, water distribution systems, existing construction, and highrise buildings. He has played a leading role in developing professional guidance and design standards, including Improved Seismic Design Guidelines for California Highway Bridges (ATC 32); Guidelines for Evaluation and Repair of Masonry and Concrete Walls (FEMA 306); Guidelines for Seismic Rehabilitation of Buildings (FEMA 273 and ASCE 356); Development of NextGeneration PerformanceBased Seismic Design Procedures for New and Existing Buildings (FEMA P58); and Guidelines for PerformanceBased Seismic Design of Tall Buildings (Tall Buildings Initiative, PEER). He has served on the Boards of Directors of the Structural Engineers Association of Northern California, the Earthquake Engineering Research Institute, and the American Concrete Institute. His awards include the Lindau Award, the Siess Award, and the Boase Award from the American Concrete Institute; the Huber Research Prize from the American Society of Civil Engineers; the Annual Distinguished Lecturer and Outstanding Paper Award from the Earthquake Engineering Research Institute; and Honorary Member and College of Fellows of the Structural Engineers Association of California. He is an elected member of the U.S. National Academy of Engineering. He has been a member of the ACI 318 Building Code Committee since 1989, chair of ACI 318H (Seismic Provisions) from 1995 to 2014, and is chair of the ACI 318 Building Code Committee for the 2014–2019 code cycle.

About the Author

Jack Moehle is the T.Y and Margaret Lin Professor of Engineering in the Department of Civil and

Environmental Engineering at the University of California, Berkeley He received his Ph.D from theUniversity of Illinois and joined the U.C Berkeley faculty in 1980 His research and teachingactivities are mainly in structural engineering, with emphasis on reinforced concrete and earthquakeengineering He was founding director of the Pacific Earthquake Engineering Research Center, amulticampus research center that advanced the concepts and practice of performance-basedearthquake engineering As a licensed Civil Engineer in the State of California, Dr Moehle worksregularly as a consulting engineer, offering advice and expert peer review on highway and masstransit systems, water distribution systems, existing construction, and high-rise buildings He has

played a leading role in developing professional guidance and design standards, including Improved

Seismic Design Guidelines for California Highway Bridges (ATC 32); Guidelines for Evaluation and Repair of Masonry and Concrete Walls (FEMA 306); Guidelines for Seismic Rehabilitation of Buildings (FEMA 273 and ASCE 356); Development of Next-Generation Performance-Based Seismic Design Procedures for New and Existing Buildings (FEMA P-58); and Guidelines for Performance-Based Seismic Design of Tall Buildings (Tall Buildings Initiative, PEER) He has

served on the Boards of Directors of the Structural Engineers Association of Northern California, theEarthquake Engineering Research Institute, and the American Concrete Institute His awards includethe Lindau Award, the Siess Award, and the Boase Award from the American Concrete Institute; theHuber Research Prize from the American Society of Civil Engineers; the Annual DistinguishedLecturer and Outstanding Paper Award from the Earthquake Engineering Research Institute; andHonorary Member and College of Fellows of the Structural Engineers Association of California He

is an elected member of the U.S National Academy of Engineering He has been a member of the ACI

318 Building Code Committee since 1989, chair of ACI 318H (Seismic Provisions) from 1995 to

2014, and is chair of the ACI 318 Building Code Committee for the 2014–2019 code cycle

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To Melissa, For time, encouragement, diversions.

Preface

Acknowledgments

1 Seismic Design and Performance Verification

1.1 Earthquake Resistance in Concrete Buildings

1.2 Early Developments

1.3 Current Practices

1.3.1 Building Codes

1.3.2 Conceptual Design

1.3.3 Prescriptive Design Approach

1.3.4 Performance-Based Design Approach

1.3.5 Construction Inspection

1.4 Building Performance

1.4.1 Anticipated Response of Buildings to Earthquake Ground Shaking

1.4.2 Performance Concepts

1.4.3 Use, Occupancy, and Risk Classifications

1.4.4 Building Performance Expectations

1.5 Performance Verification

1.5.1 Limit State Design

1.5.2 Serviceability Limit State

1.5.3 Ultimate Limit State (Load and Resistance Factor Design)

1.5.4 Capacity Design

1.5.5 Displacement-Based Design

1.5.6 Performance Evaluation under Earthquake Ground Shaking

1.6 The Purpose and Organization of This Book

References

2 Steel Reinforcement

2.1 Preview

2.2 Steel Reinforcement Used in Buildings

2.2.1 Standard Steel Reinforcement

2.2.2 Reinforcement Grades and Availability

2.2.3 Permitted Reinforcement

2.3 Steel Reinforcement under Monotonic Loading

2.3.1 General Characteristics of the Stress–Strain Relation2.3.2 Tensile Properties of Steel Reinforcement

2.3.3 Compressive Properties of Steel Reinforcing Bars2.3.4 Strain Rate Effect

2.4 Reinforcing Bars under Cyclic Loading

3.3.1 Materials Characteristics and Proportions

3.3.2 Curing Time and Conditions

3.3.3 In-Place Concrete

3.3.4 Test Specimen Parameters

3.3.5 Expected Strength in Structures

3.4 Behavior in Uniaxial Monotonic Loading

3.4.1 Compressive Stress–Strain Response

3.4.2 Tensile Strength

3.4.3 Strain Rate Effects

3.5 Behavior in Uniaxial Cyclic Loading

3.6 Behavior in Multi-axial Stress States

3.6.1 Plain Concrete in Biaxial Stress State

3.6.2 Reinforced Concrete in Biaxial Loading

3.6.3 Plain Concrete in Triaxial Stress State

4.2 Behavior of Confined Concrete Sections

4.3 Mechanism of Concrete Confinement

4.3.1 Passive Confinement of Concrete

4.3.2 Columns with Spiral and Circular Hoop Reinforcement4.3.3 Columns with Rectilinear Hoop Reinforcement

4.3.4 Loading Rate Effect

4.3.5 Aggregate Density Effect

4.3.6 Compressive Strength Effect

4.3.7 Cyclic Loading Effect

4.3.8 Reinforcement Details

4.4 Analytical Modeling of Confined Concrete

4.4.1 Strain at Peak Stress

4.4.2 Maximum Strain Capacity for Confined Concrete

5.4 Service Load Behavior of Compression Members

5.4.1 Linear Elastic Response

5.4.2 Effects of Drying Shrinkage and Creep

5.5 Inelastic Behavior of Compression Members

5.5.1 Cover and Core Concrete

5.5.2 Longitudinal Reinforcement

5.5.3 Load–Displacement Response

5.5.4 Transverse Reinforcement Required for Ductility5.6 Tension Members

5.7 Reversed Cyclic Loading

5.7.1 Stability of Longitudinal Reinforcement

5.7.2 Stability of Axially Loaded Members

5.8 Chapter Review

References

6 Moment and Axial Force

6.1 Preview

6.2 Some Observations about Flexural Behavior

6.3 Internal and External Force Equilibrium

6.6.2 Linear-Elastic Response of Uncracked Sections

6.6.3 Linear-Elastic Response of Cracked Sections

6.6.4 Flexural Stiffness at Service Loads

6.6.5 Response at Ultimate Limit States

6.6.6 Compression Stress Block Parameters

6.6.7 Automation of Moment–Curvature Calculations

6.8.1 General Observations about Axial Force, Moment, and Curvature

6.8.2 Construction of P-M-Ø Relations by Hand Calculations

6.8.3 Axial Force, Moment, and Curvature Response

6.8.4 Nominal, Probable, and Design Strengths

6.8.5 Reinforcement Limits

6.9 Walls

6.9.1 Geometry and Reinforcement

6.9.2 Axial Force, Moment, and Curvature Response

6.9.3 Nominal, Probable, and Design Strengths

6.11.2 Nonlinear Inelastic Range

6.12 Reversed Cyclic Loading

6.12.1 General Aspects of Response to Reversed Cyclic Loading

6.12.2 Laboratory Tests

References

7 Shear in Beams, Columns, and Walls

7.1 Preview

7.2 Some Observations about Shear in Flexural Members

7.3 Relations among Moment, Shear, and Bond

7.4 Beam Action and Arch Action

7.5 Internal Forces in Members with Transverse Reinforcement

7.6 Strut-and-Tie Models

7.6.1 Plastic Truss Analysis for Beams

7.6.2 Example Strut-and-Tie Models

7.7 Proportioning of Strut-and-Tie Models

7.8 Transverse Reinforcement Detailing

7.9 Empirical Approach for Shear Strength of Beams and Columns

7.9.1 Strength of Members without Transverse Reinforcement

7.9.2 Members with Transverse Reinforcement

7.9.3 ACI 318 Design Equations and Requirements for Beams and Columns

7.9.4 Comparison of ACI 318 and Truss Models

7.10 Effects of Inelastic Cyclic Loading

7.11 Diagonally Reinforced Beams

7.12 Shear in Structural Walls

7.12.1 Wall Classification Based on Slenderness

7.12.2 Slender Structural Walls

7.12.3 Squat Structural Walls

7.12.4 Shear in Panel Zones

7.13 Interface Shear Transfer

8.2 Some Observations about Bond and Anchorage

8.3 Relations among Bond Stress and External Forces

Compression8.6 Lap Splices

8.6.1 Tension Lap Splices

8.6.2 Compression Lap Splices

8.11 Effects of Inelastic Cyclic Loading

8.11.1 Straight Bar Anchorages

9.2 Forces in Beam-Column Connections

9.2.1 Connection Forces from Gravity and Lateral Loading9.2.2 Calculation of Joint Shear

9.4.5 ASCE 41 Joint Strength

9.5 Beam-Column Joints with Transverse Reinforcement

9.5.1 Interior Connections

9.5.2 Exterior Connections

9.5.3 Tee (Roof) Connections

9.5.4 Corner Connections9.5.5 Predictive Models for Joint Shear Strength9.6 Recommended Design Procedure for Beam-Column Joints

9.6.1 Classify Joints According to Loading Conditions and Geometry9.6.2 Determine Joint Shear Demands

9.6.3 Size the Connection for Joint Shear Demands9.6.4 Develop Beam and Column Longitudinal Reinforcement9.6.5 Provide Joint Confinement

9.6.6 Provide Adequate Strength and Detailing in Columns9.7 Beam-Column Joint Deformations

10.4 Moments, Shears, and Deformations in Slab-Column Framing

10.5 Flexural Reinforcement in Slab-Column Frames

11.2 Earthquakes and Engineering Representation of Seismic Hazard

11.2.1 Earthquakes and Earthquake Hazards11.2.2 Engineering Characterization of Ground Motion11.2.3 Site-Specific Seismic Hazard Evaluation

11.2.4 Design Response Spectra in U.S Building Codes11.3 Earthquake Demands on Building Structures

11.3.1 Linear-Elastic Response11.3.2 Nonlinear Inelastic Response11.3.3 Drift and Ductility Demands11.4 Earthquake-Resisting Buildings

11.5 Design Approach

11.5.1 Strength-Based Design in Accordance with ASCE 711.5.2 Displacement-Based Design

11.5.3 Performance-Based Design11.6 Chapter Review

12.3.1 Design a Strong-Column/Weak-Beam System12.3.2 Detail Beams and Columns for Ductile Flexural Response12.3.3 Avoid Nonductile Failure Modes

12.3.4 Avoid Interaction with Nonstructural Components12.4 Seismic Response of Special Moment Frames

12.4.1 Observations on Dynamic Response12.4.2 Frame Yielding Mechanisms

12.4.3 Member Forces12.4.4 Member Deformation Demands and Capacities12.5 Modeling and Analysis

12.5.1 Analysis Procedure12.5.2 Stiffness Recommendations12.5.3 Foundation Modeling

12.6 Proportioning and Detailing Guidance

12.6.1 Beam Flexure and Longitudinal Reinforcement12.6.2 Joint Shear and Anchorage

12.6.3 Beam Shear and Transverse Reinforcement12.6.4 Column Design and Reinforcement

12.7 Additional Requirements

12.7.1 Special Inspection12.7.2 Material Properties

12.7.3 Additional System Design Requirements12.8 Detailing and Constructability Issues

12.8.1 Longitudinal Bar Compatibility12.8.2 Beam and Column Confinement12.8.3 Bar Splices

12.8.4 Concrete PlacementReferences

13 Special Structural Walls

13.1 Preview

13.2 The Use of Special Structural Walls

13.2.1 Structural Walls in Buildings13.2.2 When to Use Structural Walls13.2.3 Wall Layout

13.2.4 Wall Foundations13.2.5 Wall Configurations13.2.6 Wall Reinforcement13.2.7 Wall Proportioning13.3 Principles for Design of Special Structural Walls

13.3.1 Slender Walls13.3.2 Squat Walls13.3.3 Diaphragms and Foundations13.4 Observations on the Behavior of Special Structural Walls13.4.1 Slender versus Squat Walls

13.4.2 Flexural Response of Walls13.4.3 Stability of Flexural Compression Zone13.4.4 Dynamic Response

13.4.5 Backstay Effects13.4.6 Walls with Cap Beams and Outriggers13.4.7 Frame–Wall Interaction

13.5 Analysis Guidance

13.5.1 Analysis Procedures13.5.2 Stiffness Recommendations13.5.3 Effective Flange Width13.5.4 Foundation Modeling13.5.5 Limit Analysis and Redistribution of Coupled Walls13.6 Load and Resistance Factors for Wall Design

13.7 Preliminary Proportioning

13.7.1 Proportioning for Base Shear

13.7.2 Proportioning for Drift13.8 Design of Slender Walls with Single Critical Section

13.8.1 Moment and Axial Force Design of Intended Plastic Hinge13.8.2 Shear Design of the Intended Plastic Hinge

13.8.3 Shear-Friction Design of the Intended Plastic Hinge13.8.4 Requirements above the Intended Plastic Hinge13.9 Design of Walls without an Identified Critical Section

13.10 Squat Walls

13.10.1 Conventionally Reinforced Squat Walls13.10.2 Diagonally Reinforced Squat Walls13.11 Wall Piers

13.12 Coupled Wall Systems

13.12.1 Coupling Beams13.12.2 Coupled Walls13.13 Wall Panel Zones

13.14 Wall Transfer at Podium and Subterranean Levels

13.15 Outriggers

13.16 Geometric Discontinuities

13.16.1 Walls with Openings13.16.2 Columns Supporting Discontinuous Walls13.16.3 Thickness Transitions

13.16.4 Foundation Steps13.17 Additional Requirements

13.17.1 Special Inspection13.17.2 Material Properties13.17.3 Additional System Design Requirements13.18 Detailing and Constructability Issues

13.18.1 Reinforcement Cage Fabrication13.18.2 Boundary Element Confinement13.18.3 Bar Compatibility

14.2.2 Example Applications14.2.3 Performance of Gravity Framing in Past Earthquakes14.3 Principles for Design of Gravity Framing

14.3.1 Control Deformation Demands14.3.2 Confine Column Sections Where Yielding Is Expected14.3.3 Avoid Shear and Axial Failures

14.4 Analysis Guidance

14.4.1 Analysis Procedure14.4.2 Stiffness Recommendations14.5 Design Guidance

14.5.1 Design Actions14.5.2 Columns

14.5.3 Beams14.5.4 Beam-Column Joints14.5.5 Slab-Column Framing14.5.6 Slab-Wall Framing14.5.7 Wall Piers

14.6 Additional Requirements

14.6.1 Special Inspection14.6.2 Material Properties14.7 Detailing and Constructability Issues

15.4 Diaphragm Behavior and Design Principles

15.4.1 Dynamic Response of Buildings and Diaphragms15.4.2 Intended and Observed Behavior

15.5 Analysis Guidance

15.5.1 Design Lateral Forces15.5.2 Diaphragm Modeling and Analysis Approaches15.5.3 Idealized Load Paths within the Diaphragm15.5.4 Displacement Compatibility for Flexible Diaphragms15.5.5 Ramps

15.5.6 Diaphragm Slabs-on-Ground15.6 Design Guidance

15.6.1 Load and Resistance Factors

15.6.2 Chord Longitudinal and Confinement Reinforcement15.6.3 Diaphragm Shear Strength

15.6.4 Force Transfer (Including Collector Forces) to Vertical Elements15.6.5 Reinforcement Development

15.6.6 Special Cases15.7 Additional Requirements

15.7.1 Material Properties15.7.2 Inspection Requirements15.7.3 Bracing Columns to Diaphragms15.7.4 Interaction of Diaphragm Reinforcement with Vertical Elements15.8 Detailing and Constructability Issues

15.8.1 Diaphragm Reinforcement15.8.2 Collector and Chord Detailing15.8.3 Confinement

15.8.4 Shear Transfer15.8.5 Mechanical Splices15.8.6 Conduits and Embedded Services15.8.7 Location of Construction JointsReferences

16 Foundations

16.1 Preview

16.2 Foundation Elements in Earthquake-Resisting Buildings

16.2.1 Shallow Foundations16.2.2 Deep Foundations16.2.3 Grade Beams and Structural Slabs-on-Ground16.3 Soil–Structure Interaction

16.4 Geotechnical Investigation Report

16.5 Foundation Performance Objectives and Design Values

16.7.1 Behavior and Analysis Considerations16.7.2 Geotechnical Considerations

16.7.3 Mat Foundation Design and Reinforcement Details

16.8 Deep Foundations

16.8.1 Behavior and Analysis Considerations

16.8.2 Geotechnical Considerations

16.8.3 Pile Design and Reinforcement Details

16.8.4 Pile Cap Design and Reinforcement Details

Preface

his book emphasizes the behavior and design requirements for earthquake-resistant reinforcedconcrete buildings Design of a building for earthquake effects requires a different perspectivethan is required for other load effects Earthquake loads are mainly absent during the life of abuilding, but suddenly may be applied with an intensity that drives the structure beyond thelinear range of response in multiple loading cycles Earthquake response of a structure isdynamic, with distributed inertial forces that act in all directions simultaneously To meet establishedperformance objectives under earthquake loading, a building requires a structural system that isappropriately configured, proportioned, and detailed These complicated design conditions arebeyond the scope of traditional reinforced concrete or earthquake engineering textbooks This bookaims to provide the focused and in-depth treatment necessary to fully understand the designrequirements for earthquake-resistant concrete buildings

The content emphasizes the mechanics of reinforced concrete behavior and the designrequirements applicable to buildings located in “highly seismic” regions The content will also be ofvalue to engineers interested in the seismic evaluation of existing structures, design in regions oflower seismicity, and the general design of concrete structures for routine and extreme loadingconditions

Although the mechanics of reinforced concrete is universal, the performance expectations andassociated design requirements may vary by region This book mainly follows the requirements of the

2014 edition of the American Concrete Institute’s Building Code Requirements for Structural

Concrete (ACI 318-14) and Commentary Those requirements are augmented by additional

recommendations derived from other codes, guidelines, and the general literature, as deemedappropriate by the author Dual units [U.S customary units and International System of Units (SI)] areused throughout

The target audience is twofold: (1) graduate students with structural engineering emphasis and (2)practicing structural engineers For graduate students, this book provides a logical progression ofcontent that builds knowledge of reinforced concrete construction, including design methods,behavior of structural materials and members, and the assembly of structural members into completebuildings capable of resisting strong earthquake shaking This content has been developed and honedthrough years of graduate student instruction For the practitioner, this book can build knowledge andserve as a reference resource to help solve challenging design problems The book draws extensivelyfrom research literature and the experience of the author working with practicing structural engineers.The presentation emphasizes practical aspects, with numerous illustrations of concepts andrequirements

Topics are organized in four main parts The first part (Chapter 1) reviews design methodsapplicable to the earthquake-resistant design of reinforced concrete buildings The second part(Chapters 2 to 4) discusses material properties of steel, concrete, and confined concrete that areimportant for seismic performance and design The third part (Chapters 5 to 10) covers the behavior

of structural concrete components, including tension and compression members, beams, columns,walls, beam-column connections, and slab-column and slab-wall framing subjected to axial force,

moment, shear, and imposed inelastic deformations The last part of the book (Chapters 11 to 16)addresses seismic design of moment-resisting frames, structural walls, gravity frames, diaphragms,and foundations Taken together, these four parts provide comprehensive coverage of the mechanicsand design of earthquake-resistant concrete buildings.

The book is suitable for advanced undergraduate or graduate courses in structural engineering Atthe University of California, Berkeley, it serves as a resource for a first-semester graduate course onseismic design of reinforced concrete buildings, touching on selected subjects in most of the chapters,but leaving the remaining chapter content for individual study The book could also be used in a two-semester sequence, the first semester covering design methods, materials, and structural components(Chapters 1 to 10) and the second semester covering the design of earthquake-resistant structuralsystems (Chapters 11 to 16)

Numerous graduate students have read early drafts of this book in graduate classes, and individualexperts have reviewed individual chapters Readers are encouraged to send further suggestions forimprovements, clarifications, and corrections to my attention at moehleRCSeismic@gmail.com

Jack MoehleAugust 2014

Acknowledgments

n early text on seismic design of concrete buildings1 begins with the line: “Considerableknowledge has been gained in the last three decades about the phenomena of ground motion,the characteristics of structures, and their behavior in earthquakes.” In the intervening fivedecades, knowledge and methods for earthquake-resistant concrete buildings have grown at anincreasing rate The key contributions to this book are acknowledged in an extensive list ofreferences at the end of each chapter

I am grateful for the contributions of several individuals and organizations as noted below

Instructors at the University of Illinois at Urbana–Champaign introduced me to reinforcedconcrete and inspired a lifelong study of the subject Especially, Professors William Gamble andMete Sozen emphasized the mechanics of reinforced concrete and instilled an appreciation of the roleand the limits of mechanics in engineering practice and the building code Professor Sozen hascontinued as a lifelong mentor

The University of California, Berkeley, has extended to me the privilege of teaching courses andconducting research on the subject of this book over three decades An extraordinary group of facultymembers and graduate students provided me with challenges, ideas, solutions, and a testing groundfor much of the content of this book

Many structural and geotechnical engineers have collaborated with me on research, code andguideline development efforts, and structural/earthquake engineering design and assessment projects.These interactions have revealed engineering problems and solutions that served as the basis formany practical recommendations presented in the book

Several individuals contributed directly to this book Nicholas Moehle processed the data insupport of the confined concrete models of Chapter 4 Santiago Pujol of Purdue University, while onleave at UC Berkeley in 2014, led developments on panel zone shear that are presented in Chapters 7and 13 Ian McFarlane, Michael Valley, and John Hooper of Magnusson Klemencic Associates; JayLove and Wayne Low of Degenkolb Engineers; and Dom Campi of Rutherford & Chekene discussedand provided examples of foundation design Steve Kramer of the University of Washington providedextensive references on geotechnical earthquake engineering and foundation design, and MarshallLew of AMEC, Los Angeles, provided references on retaining wall design

The National Institute of Standards and Technology, under the auspices of the U.S NationalEarthquake Hazard Reduction Program and the leadership of John (Jack) Hayes, supported thedevelopment of three technical briefs that were the starting point of Chapters 12, 13, and 15 Co-authors of these technical briefs were John Hooper, Dave Fields, and Chris Lubke of MagnussonKlemencic Associates; Dominic Kelly of Simpson Gumpertz & Heger; and Tony Ghodsi andRajnikanth Gedhada of Englekirk Structural Engineers

Many individuals and organizations permitted the use of copyrighted images and tables that addedconsiderably to the presentation The American Concrete Institute was especially generous infacilitating the use of numerous images and other content

Several individuals reviewed various chapters and example problems, including Ron Hamburger

of Simpson Gumpertz & Heger; David Gustafson of the Concrete Reinforcing Steel Institute; JulioRamirez and Santiago Pujol of Purdue University; Gustavo Parra of the University of Wisconsin,Madison; Ian McFarlane and Michael Valley of Magnusson Klemencic Associates; Dom Campi ofRutherford & Chekene; and Professors Paulo Monteiro and Yousef Bozorgnia, and Graduate StudentResearchers Carlos Arteta, John N Hardisty, and Ahmet Tanyeri of the University of California,Berkeley.

This book would not have been possible without the support of my wife, Melissa, who gave ideas

on content and organization, proofread the chapters, and provided continual encouragement throughmany lost evenings and weekends

_

1Blume, J.A., N.M Newmark, and L.H Corning (1961) Design of Multistory Reinforced Concrete Buildings for Earthquake

Motions, Portland Cement Association, Evanston, IL, 318 pp.

CHAPTER 1

Seismic Design and Performance Verification

1.1 Earthquake Resistance in Concrete Buildings

Cast-in-place reinforced concrete construction is naturally well suited to earthquake resistance As amonolithic construction form, it can readily provide a continuous load path to resist forces andmaintain structural integrity during earthquake shaking Concrete structures can also have high rigidity

to protect nonstructural elements and contents from the deformations that are caused by earthquakeshaking To be earthquake-resistant, however, a reinforced concrete building should have anappropriate and clearly defined lateral-force-resisting system that is proportioned and detailed toresist the expected earthquake demands Without these features, reinforced concrete buildings can besusceptible to localized and relatively brittle failures The aim of modern design procedures is toproduce a structural system having the stiffness, strength, and deformation capacity necessary to resistearthquake shaking with acceptable performance The aim of this book is to describe the requirements

of earthquake-resistant concrete buildings, starting from a fundamental materials level and endingwith conceptual considerations and the detailed requirements for design and construction of completestructural systems

This book is organized into a series of chapters that sequentially build the knowledge required forseismic design of concrete buildings Chapter 1 introduces basic concepts of building performanceand the methods commonly used to verify performance Chapters 2 to 4 present common structuralmaterials used in reinforced concrete buildings Chapters 5 to 10 present elements and connections ofreinforced concrete construction, including methods for modeling, design, and verification Finally,Chapters 11 to 16 present requirements for seismic design and construction of complete structuralsystems Together, these chapters provide a strong foundation for conceiving, designing, and verifyingreinforced concrete buildings for seismic resistance

This book emphasizes conventionally reinforced, cast-in-place construction Structural systemsthat use precast or prestressed concrete, or that use specialized “self-centering” systems, are notemphasized However, a thorough understanding of the subjects covered in this book will serve as aneffective basis for the design of such systems

1.2 Early Developments

Reinforced concrete was introduced around the middle of the 19th century The earliest forms ofreinforced concrete construction included many patented systems that are unfamiliar today By thebeginning of the 20th century, publication of papers, books, and codes introduced the mechanics andconstruction requirements of reinforced concrete to an increasing number of engineers, leading toacceleration in the use of reinforced concrete Many engineers of this period understood theimportance of detailing for reinforcement continuity, but general requirements for earthquakeresistance were little understood, and construction in seismically active regions did not differsignificantly from construction in apparently non-seismic regions

Concepts of seismic design for concrete buildings, including proportioning and detailing for

ductile response, were introduced by Blume et al (1961) Borrowing from technologies developedfor blast resistance and with a nascent understanding of earthquake design requirements, that bookintroduced flexural ductility concepts, capacity design for shear, requirements for reinforcementcontinuity, and the use of transverse reinforcement to confine heavily strained sections Concurrentactivities by the Structural Engineers Association of California (SEAOC, 1963 and later editions)further contributed to knowledge about seismic design requirements for buildings in California Bythe mid-1960s, a wide audience of practicing structural engineers was learning about therequirements for earthquake-resistant concrete buildings (Figure 1.1) Salient requirements for suchbuildings included use of transverse reinforcement to make the strength in shear greater than the shearoccurring at flexural strength; hoops with 135° and 180° hooks to keep hoops closed after spalling ofcover concrete, and with tight spacing to confine concrete in potential yielding regions; andeffectively continuous longitudinal reinforcement developed within the beam-column joints, withsplices located away from yielding regions.

FIGURE 1.1 Page from 1967 Structural Engineers Association of California seminar notes (SEAOC, 1967) Used with permission

from A Tarics and SEAONC.

The recommendations of Blume et al (1961) and the Structural Engineers Association ofCalifornia (SEAOC, 1963 and later) were not immediately adopted as requirements by U.S buildingcodes It took the 1971 San Fernando earthquake (NOAA, 1973), and its demonstration of the

vulnerabilities of some concrete buildings, to instigate code changes By 1976, the Uniform BuildingCode (UBC, 1976) introduced many of the recommendations of Blume et al and SEAOC as buildingcode requirements Early developments in other countries have been reported by Park (1986), Otani(1995), and Fardis (2013).

1.3 Current Practices

Experience, research, computational capabilities, and developments in conceptual thinking have led

to important advances in the practice of earthquake engineering since the 1970s Today, an engineerhas available a wide variety of procedures that can be used for seismic assessment and design ofbuildings These are contained in building codes, standards, guidelines, and the general literature onstructural and earthquake engineering

Earthquake-resistant reinforced concrete construction practices have by now become widelyadopted Figure 1.2 shows photographs of three recently constructed buildings designed according tocurrent seismic design principles Figure 1.2a shows a mid-rise building on the University ofCalifornia, Berkeley campus Located less than one kilometer from the active trace of the Haywardearthquake fault, the building features reinforced concrete structural walls for lateral force resistance

Figure 1.2b illustrates a high-rise frame-wall building under construction in the Pacific Northwest of

the United States Figure 1.2c illustrates a 300-m-tall core-wall building that was under construction

at the time of the 2010 Chile earthquake Each of these structural systems relies on resistant structural systems designed using methods that will be featured in this book

earthquake-FIGURE 1.2 Modern buildings designed for earthquake resistance in regions of high seismicity.

Design of any building begins with a conceptual design, in which the structural systems and

materials are identified and configured Once the structural system has been identified andapproximately proportioned, structural analysis and design are used to confirm that the buildingdesign is capable of meeting intended performance objectives Generally this is done following the

requirements of the building code, using either prescriptive or performance-related provisions In a

prescriptive design, the structural analysis and design are implemented in strict accordance with the

prescriptive requirements of the building code, with the implicit assumption that a code-conforming

building will automatically meet the performance objectives In contrast, a performance-based

design can deviate from the prescriptive provisions and use structural analysis and design to

demonstrate that the building nonetheless meets or exceeds the performance objectives of the buildingcode

Regardless of the design approach, competent construction inspection is required to ensure that

the project is constructed in accordance with the design intent The following subsections discussaspects of building codes, conceptual design, prescriptive design, performance-based design, andconstruction inspection Performance objectives are discussed in Section 1.4

1.3.1 Building Codes

A building code is a set of minimum regulations intended to safeguard public health, safety, andgeneral welfare of the occupants The development, adoption, and enforcement of building codes varywidely from one country to another In some countries, building codes are developed by governmentalagencies and are enforced nationwide In other countries, including the United States, the authority toregulate building construction is delegated to local jurisdictions Ideally, the local jurisdictions insuch countries adopt model building codes by reference, making these model codes part of the lawgoverning construction in that jurisdiction

In the United States, most jurisdictions adopt and use the International Building Code (IBC,

2012), which establishes minimum regulations for building systems using prescriptive and

performance-related provisions The IBC, in turn, adopts by reference the standard Minimum Design

Loads for Buildings and Other Structures (ASCE 7-10, 2010), which establishes minimum

requirements for design loads including those associated with earthquakes The IBC also adopts by

reference the standard Building Code Requirements for Structural Concrete (ACI 318-11) and

Commentary (ACI 318-11, 2011),1 which establishes minimum requirements for structural concretedesign

It should be emphasized that the purpose of a building code is to establish the minimumrequirements to safeguard the public health, safety, and general welfare These are generallyestablished through strength, serviceability, durability, and other requirements of the building code It

is also permitted to design a building to exceed the minimum requirements of the code, includingdesign for enhanced performance or sustainability Various design guidelines and standards havebeen developed by professional organizations to present recommended practices that may exceedminimum building code requirements Performance-based design can also be used to targetperformance exceeding the building code performance intent

1.3.2 Conceptual Design

Conceptual design refers to the early design phase in which the structural systems are selected,configured, and approximately proportioned The structural system must fit within the space andfunction of the building, while at the same time providing a suitable load path for anticipated loads,including gravity, wind, and earthquake loads Selection of the structural concept is a keyresponsibility of the structural engineer By selecting a good structural concept, the structural engineerusually can simplify the structural analysis, design, and review process, while providing a highdegree of confidence that the performance objectives will be achieved

Figure 1.3 illustrates typical elements of a reinforced concrete structural system The gravityload-carrying system comprises the roof and floor system, columns and bearing walls, and thefoundation The lateral-force-resisting system comprises diaphragms, vertical elements, and thefoundation The diaphragms tie the building system together into a rigid, three-dimensional unit.Diaphragms also transmit lateral forces to the vertical elements of the lateral-force-resisting system.Chapter 15 describes diaphragm design in detail The building codes limit the seismic-force-resistingvertical elements to moment-resisting frames (Chapter 12), structural walls (Chapter 13), orcombinations of these elements Various foundation elements (Chapter 16) are sized to transmitvertical, horizontal, and overturning forces to the supporting soils

FIGURE 1.3 Typical elements of a reinforced concrete structural system.

The ideal structural system for an earthquake-resistant building is compact and symmetric, withstiffness and strength that are uniformly distributed over the height and across the plan, and withoutirregularities caused by discontinuous or offset structural elements The structural system illustrated

in Figure 1.3 has the desired attributes Figure 1.4 illustrates a range of building configurations, some

of which create design challenges that are better avoided through good conceptual design

FIGURE 1.4 Considerations in building configurations: (a) vertically regular system; (b) discontinuous shear wall; (c) vertical

irregularities in stiffness and mass; (d) horizontal offsets in lateral-force-resisting system; (e) torsionally unbalanced system; (f) isometric view of column transfers and offsets; (g) plan view showing limited connectivity between vertical elements and diaphragm In (d) and (e), ⊠ is used to indicate location of a structural wall; a concrete braced frame is not a recognized seismic-force-resisting system in

contemporary U.S building codes.

1 Figure 1.4a illustrates a regular structural system with continuous lateral resistance provided

by a wall extending full height, and represents a good conceptual design

2 A discontinuous structural wall, especially one that leaves an open first story as shown in

Figure 1.4b, can create a weak and soft story that can be difficult to protect from excessive

damage, and should be avoided

3 Buildings with large changes in stiffness and strength over height (Figure 1.4c) may developconcentrations of inelastic response and damage, especially near the discontinuity

4 Horizontal offsets in lateral systems (Figure 1.4d) create large force transfers across floordiaphragms, while also creating overturning problems for columns supporting discontinuouswalls

5 Eccentricity between center of resistance and center of mass (Figure 1.4e) results in torsionthat creates design challenges Such eccentricities should be minimized

6 Column transfers and offsets (including inclined columns) (Figure 1.4f) disturb the load pathand create large forces in transfer elements and diaphragms

7 Diaphragm openings adjacent to structural walls (Figure 1.4g) limit the ability to transfer

forces between the two elements, and are especially problematic near the base of a buildingwhere forces commonly must be transferred out of the structural walls

Each of these conceptual design problems will be addressed in greater detail at appropriatelocations throughout this book

As will be discussed subsequently, most buildings are designed such that some inelastic response

is anticipated during a design-basis earthquake Hence, conceptual design also involves selection of atarget yielding mechanism For the structural system depicted in Figure 1.3, the intended mechanismmight include flexural yielding of the walls for north-south loading and flexural yielding of the beamsover the height and the columns at the base of the frames for east-west loading The capacity designmethod is commonly used to proportion the structure for the intended mechanism (Section 1.5.4).

Once the structural system has been configured and approximately proportioned, preferably using

a regular and symmetric layout, it can be analyzed and designed using either the prescriptive or theperformance-based design approach

1.3.3 Prescriptive Design Approach

A prescriptive design is one that adheres strictly to the prescriptive provisions of the building code,such as those contained in the IBC Most building designs follow this approach The provisions areprescriptive in the sense that they prescribe required analysis procedures, strengths, stiffnesses, andcomponent and system details, with little leeway for deviating from the prescription A typicalprescriptive design includes the following steps:

• The building code specifies the intensity of the design loads for dead, live, wind, earthquake,and other effects, and spells out how the loads are to be combined for determining worst effects

In reference to Figure 1.5, dead load (D) is calculated from the weight of the building

components and live load (L) is prescribed based on the building occupancy2 Earthquake load

(E) is determined through a set of prescribed calculations set forth in the building code Unlike other loads, E as specified in the codes is not intended to be a realistic estimation of actual

earthquake loads, but instead is used to set a minimum strength, such that excessive ductility isnot required under anticipated ground shaking The code also prescribes several different loadcombinations, that is, ways in which to apply the specified loads, only one of which is shown inthe figure See Section 1.5.3 for load combinations

• The building code specifies how structural analysis is to be done, including the required

stiffness and strength models In most cases, the structural analysis is strictly linear, althoughstiffness may be reduced to approximately account for nonlinear effects, and some allowance forredistribution of internal actions may be permitted In Figure 1.5, member stiffnesses are

modified to approximate the effects of cracking and axial force, and then linear analysis is

conducted to obtain the required strengths in shear (V u ), moment (M u ), and axial force (P u)

• The building code specifies how to calculate member design strengths, and requires that these

be at least equal to required strengths as determined from the structural analysis See Section1.5.3

• The code also specifies requirements for member dimensions and reinforcement detailing

Figure 1.6 illustrates the types of details that will be prescribed, including requirements forcontinuity of reinforcement, locations and types of splices, and spacing and configuration oftransverse reinforcement Some of the reinforcement quantities and details will be dictated bycalculated member actions, and others will be specified as requirements that are independent ofthe magnitude of the calculated member actions

• The building code specifies displacement limits for members and for the building as a whole

FIGURE 1.5 Determination of design shears, moments, and axial forces in a prescriptive design.

FIGURE 1.6 Examples of types of prescriptive dimensional limits and reinforcement details.

The prescriptive approach has the advantage that it uses well-established and familiar analysisand design techniques, with all of the requirements spelled out in the building code in such a way that

an engineer can reliably implement them and a building code official can check the final design forcompliance with the building code requirements A disadvantage of the prescriptive approach is thatthere is no calculation of expected building performance for future events

In the United States, prescriptive requirements for proportioning and detailing of concretebuildings are contained in the seismic provisions of ACI 318 Building Code Requirements forStructural Concrete (ACI 318) Provisions relevant to other countries include Eurocode 8 (2004),

NBC (2005), and NZS3101 (2006) This book describes member behavior and design in depth so thatthe reader can gain a full understanding of these building code requirements and the performance theyare likely to deliver.

The traditional approach for proportioning and detailing of cast-in-place reinforced concretestructures intends that flexural yielding of members will be the primary source of inelastic response.That approach is emphasized in this book Alternative approaches have introduced prestressed andprecast concrete with nonlinear force-displacement mechanisms that may differ from those of cast-in-place concrete See, for example, PRESSS (various) and PCI (2013)

1.3.4 Performance-Based Design Approach

In addition to the prescriptive design approach, most building codes also contain provisions thatpermit alternative designs The IBC, for example, contains the following language:

104.11 Alternative materials, design and methods of construction and equipment The provisions of this code are not intended

to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.

Such alternative design approaches are often referred to as performance-based approaches, because the main basis for accepting

the alternative method is a demonstration (through testing, analysis, or both) that the resulting building meets or exceeds the

performance intent of the building code Several conditions can trigger the use of performance-based design, including:

• A proposed building uses structural materials, elements, or systems that are not covered by theprescriptive building code provisions The performance-based design may require physical tests

to verify equivalent performance of the new materials Structural analysis may also be required

to demonstrate that a building using the new material will have equivalent performance

• A proposed building height exceeds the prescriptive limits of the building code In this case,structural analysis is used to demonstrate that the taller building can be safe and serviceableeven though it exceeds the prescribed height limit

• An owner or other responsible entity desires building performance that exceeds the minimumperformance objectives of the building code As a first step in this process, the structural

engineer works with the owner or responsible entity to define the enhanced performance

objective Innovative structural materials may be introduced, or structural analysis may be used

to demonstrate enhanced performance, or both

Most performance-based designs rely on the prescriptive building code provisions, with specificexceptions to those provisions that emphasize the unique aspects of the proposed design Theperformance evaluation can then focus mainly on those aspects of the design that are exceptions,greatly simplifying the process

For buildings located in seismically active regions, performance-based seismic design generallyinvolves a seismic hazard analysis to determine site-specific shaking levels, and usually includes theselection of representative earthquake ground motions by which to “test” the structure A nonlinearcomputer model of the building structure is then subjected to these ground motions to determine thebuilding response Key response quantities are analyzed to establish whether the design meets the

performance criteria that have been adopted for the building TBI (2010) and LATBSDC (2011)contain detailed guidelines for performance-based seismic design of tall buildings These sameguidelines can also serve as a basis for performance-based design of other building types.

Performance-based design gives the engineer much greater flexibility in the choice of thestructural system and its design method Such designs, however, typically require additional designeffort and time, advanced engineering capabilities, and a building official who is willing to acceptdesigns not conforming strictly to the prescriptive provisions of the building code Most buildingofficials will not have the expertise necessary to judge the adequacy of a design falling under thealternative methods clause of the building code Therefore, independent peer review is usuallyrequired to advise the building official as to whether a design is satisfactory

to support the structure despite the ground failure Other secondary effects such as tsunami, fire, andlifeline disruption can also be considered in exceptional cases

Effects of ground shaking can be represented through a linear response spectrum, which plots theacceleration of a linear-elastic oscillator as a function of its vibration period Figure 1.7 plots thedesign earthquake response spectrum for a site on the University of California, Berkeley campusbased on the provisions of ASCE 7, for the Design Earthquake (DE) shaking level This responsespectrum is a smoothed representation of the ground motion, having spectral ordinates equal to two-thirds of the Maximum Considered Earthquake (MCE), and serves as a basis for building design (Foradditional details on the design approach, see Chapter 11.) Using the approximation that the

fundamental vibration period of a building is approximately T = 0.1N, we would estimate the period

for a 5-story building to be approximately 0.5 s, while that for a 10-story building would be

approximately 1 s Corresponding spectral accelerations are approximately 1.6g and 0.9g,

respectively Thus, crude estimates for the design base shears, assuming linear-elastic response, are

approximately 1.6W and 0.9W, respectively, where W is the building weight.

FIGURE 1.7 Design response spectrum for Design Earthquake (DE) for 5% damped linear response in accordance with ASCE 7.

Although it is possible to design buildings to have strengths corresponding to these design baseshears, to do so would require very robust lateral-force-resisting systems Economic and functionalconstraints would make such designs impractical except in unusual cases Thus, most buildings aredesigned with base-shear strength lower than the strength required for linear-elastic response Aconsequence is that inelastic response and corresponding damage must be anticipated for buildingssubjected to DE-level ground motions Expected building performance capability can be determined,

in part, by the degree of inelastic response anticipated and by how that inelastic response is manifest

in damage to the structural system

1.4.2 Performance Concepts

Building performance can be expressed in multiple ways In building design practice today, the most

common approach is to define a series of performance objectives A performance objective is a

statement of the expected building performance conditioned on it having been subjected to aparticular loading For example, TBI (2010) recommends that a tall building be designed to satisfythe following two performance objectives:

1 The building shall have a small probability of life-threatening collapse given that it has beensubjected to rare earthquake ground shaking defined as the Maximum Considered Earthquake(MCE) shaking level

2 The building shall have a small probability of damage requiring repair given that it has beensubjected to more frequent ground shaking defined as the Service Level Earthquake (SLE)

introduction of the Vision 2000 Committee report on performance-based seismic design of buildings(SEAOC, 1995) and the development of performance-based assessment procedures for existingbuildings (ATC 40, 1996; FEMA 273, 1997) Figure 1.8 illustrates the performance objectivessuggested in SEAOC (1995), but using performance level designations of ASCE 41 (which

supersedes FEMA 273) For the Basic Objective, which would apply to the vast majority of buildings, the performance objectives would be Operational for Frequent shaking, Immediate

Occupancy for Occasional shaking, Life Safety for Rare shaking, and Collapse Prevention for Very Rare shaking Proposed return periods for these different shaking levels are shown in parentheses.

For more critical structures, higher performance objectives were suggested (Figure 1.8)

FIGURE 1.8 Performance objectives suggested by SEAOC (1995).

An early concept was to relate performance levels to the physical condition of the building as itwas subjected to increasing lateral deformation (SEAOC, 1995) Figure 1.9 illustrates threeperformance levels introduced in FEMA 273 (1997) and continued in ASCE 41 (2013) The

performance level Immediate Occupancy corresponds to a state in which some damage may have occurred, but after cosmetic repairs the structure can be occupied and functional Collapse

Prevention is a point in the response just prior to onset of collapse Life Safety is a term used to

define a performance state with a “comfortable” margin below the collapse state In ASCE 41, themargin is set at about three-quarters of the displacement corresponding to the collapse performancestate, but in ASCE 7 and the building code, this margin is two-thirds Figure 1.9 implies thatperformance states are a function of the deformations imposed on the structural and nonstructuralsystems Performance of contents and other items that are not rigidly fixed to the structural system caninstead be a function of floor acceleration or velocity

FIGURE 1.9 Visualization of performance levels (Personal communication with R Hamburger.)

Building performance should be defined by the performance of the building system as a whole Itcan be difficult, however, to quantify performance metrics for building systems Therefore, as apractical matter, a common practice is to define system performance based on the performance ofindividual structural (or nonstructural) components that compose the building system In effect, thebuilding performance is defined as being equal to the worst performance of any of the components ofthe building This approach, which is adopted in ASCE 41, tends to be a very conservative approach

The preceding discussion emphasizes the current approach of gauging performance usingstructural engineering metrics, such as displacement, story drift, floor acceleration, inelasticdeformation, and component forces, all compared with values that are considered acceptable It isalso feasible to translate these engineering metrics into damage states, and from there intoconsequences such as casualties, repair costs, and downtime This approach is not commonly appliedtoday, but the capabilities exist and are occasionally applied for special buildings The interestedreader is referred to Yang et al (2009) and FEMA P-58 (2012)

1.4.3 Use, Occupancy, and Risk Classifications

Figure 1.8 introduced the idea that building performance objectives should depend on the risk that thebuilding poses to its occupants and the surrounding community This idea is incorporated in current

U.S building codes In ASCE 7, four different Risk Categories are defined, as summarized in Table

1.1 The vast majority of buildings correspond to Risk Category II ASCE 7 imposes more stringentrequirements for buildings with higher Risk Category, consistent with the larger population of peopleput at risk if the building should fail to perform Higher design forces are also imposed through

application of an importance factor, I e, listed in Table 1.1

TABLE 1.1 Risk Category* of Buildings and Other Structures (adapted from ASCE 7)

1.4.4 Building Performance Expectations

The commentary to ASCE 7 quantifies the intended performance for buildings in different RiskCategories Table 1.2 summarizes the anticipated reliability values These values have not beenvalidated with experience or in-depth analysis, but instead are notional values that represent the intent

of the building code committee

FEMA P-695 (2009) also suggests that the probability of collapse due to MCE ground motionsshould be limited to 10% for Risk Category II buildings FEMA P-695 presents a detailedmethodology for determining collapse probability of classes of buildings Several case studies haveused the methodology to benchmark performance of modern buildings, with results reported in FEMAP-695 (2009) and NIST (2010)

The collapse probabilities in Table 1.2 indicate the probabilities for an individual building giventhat it has been subjected to ground shaking at the MCE level It should be noted that the term MCE,

or Maximum Considered Earthquake, actually does not refer to an earthquake, but instead refers to theshaking that occurs at a site given the occurrence of an earthquake MCE-level shaking generallyindicates both a rare earthquake and unusually high ground shaking given the occurrence of thatearthquake Thus, one should not expect that all buildings in a region will be subjected to MCE-levelshaking in any given earthquake Instead, only a subset of buildings might experience MCE-levelshaking, with the rest experiencing lower shaking intensity Thus, over a region, the collapseprobability for a population of new buildings is lower than the values indicated in Table 1.2