Seismic Isolation
for Designers
and Structural Engineers
R. Ivan Skinner Trevor E. Kelly Bill (W.H.) Robinson
Seismic Isolation
for Designers
and Structural Engineers
R. Ivan Skinner
Trevor E. Kelly
Holmes Consulting Group
www.holmesgroup.com
Bill (W.H.) Robinson
Robinson Seismic Ltd
www.rslnz.com
Preface (i)
Acknowledgements (iii)
Author Biographies (iv)
Frequently Used Symbols And Abbreviations (v)
CHAPTER 1: INTRODUCTION... 1
1.1 Seismic Isolation in Context... 1
1.2 Flexibility, Damping and Period Shift ... 3
1.3 Comparison of Conventional & Seismic Isolation Approaches ... 5
1.4 Components in an Isolation System ... 6
1.5 Practical Application of the Seismic Isolation Concept... 7
1.6 Topics Covered in this Book ... 9
CHAPTER 2: GENERAL FEATURES OF STRUCTURES WITH SEISMIC ISOLATION ... 11
2.1 Introduction ... 11
2.2 Role of Earthquake Response Spectra and Vibrational Modes in the Performance of Isolated Structures... 11
2.2.1 Earthquake Response Spectra ... 11
2.2.2 General Effects of Isolation of the Seismic Responses of Structures ... 15
2.2.3 Parameters of Linear and Bilinear Isolation Systems ... 16
2.2.4 Calculation of Seismic Responses ... 20
2.2.5 Contributions of Higher Modes to the Seismic Responses of Isolated Structures... 21
2.3 Natural Periods and Mode Shapes of Linear Structures – Unisolated and Isolated... 22
2.3.1 Introduction... 22
2.3.2 Structural Model and Controlling Equations ... 22
2.3.3 Natural Periods and Mode Shapes ... 24
2.3.4 Example – Modal Periods and Shapes...25
2.3.5 Natural Periods and Mode Shapes with Bilinear Isolation... 26
2.4 Modal and Total Seismic Responses ... 27
2.4.1 Seismic Responses Important for Seismic Design... 27
2.4.2 Modal Seismic Responses ... 28
2.4.3 Structural Responses for Modal Responses... 30
2.4.4 Example – Seismic Displacements and Forces ... 30
2.4.5 Seismic Responses with Bilinear Isolators ... 31
2.5 Comparisons of Seismic Responses of Linear and Bilinear Location Systems ... 34
2.5.1 Comparative Study of Seven Cases... 34
2.6 Guide to Assist the Selection of Isolation Systems... 38
CHAPTER 3: ISOLATOR DEVICES AND SYSTEMS... 43
3.1 Isolator Components and Isolator Parameters... 43
3.1.1 Introduction... 43
3.1.2 Combination of Isolator Components to Form Different Isolation Systems ... 43
3.2 Plasticity of Metals ... 46
3.3 Steel Hysteretic Dampers ... 49
3.3.1 Introduction... 49
3.3.2 Types of Steel Damper ... 51
3.3.3 Approximate Force-Displacement Loops for Steel-Beam Dampers... 52
3.3.4 Bilinear Approximation to Force-Displacement Loops ... 55
3.3.5 Fatigue Life of Steel-Beam Dampers ... 57
3.3.6 Summary of Steel Dampers ... 59
3.4 Lead Extrusion Dampers ... 59
3.4.1 General ... 59
3.4.2 Properties of the Extrusion Damper... 62
3.5.1 Rubber Bearings for Bridges and Isolators... 66
3.5.2 Rubber Bearing, Weight Capacity Wmax... 67
3.5.3 Rubber Bearing Isolation: Stiffness, Period and Damping... 68
3.5.4 Allowable Seismic Displacement Xb... 70
3.5.5 Allowable Maximum Rubber Strains ... 72
3.5.6 Other Factors in Rubber Bearing Design... 74
3.5.7 Summary of Laminated Rubber Bearings ... 74
3.6 Lead Rubber Bearings... 74
3.6.1 Introduction... 74
3.6.2 Properties of the Lead Rubber Bearing... 77
3.7 Further Isolator Components and Systems... 83
3.7.1 Isolator Damping Proportional to Velocity ... 83
3.7.2 PTFE Sliding Bearings ... 84
3.7.3 PTFE Bearings Mounted on Rubber Bearings... 85
3.7.4 Tall Slender Structures Rocking with Uplift ... 85
3.7.5 Further Components for Isolator Flexibility ... 86
3.7.6 Buffers to Reduce the Maximum Isolator Displacement... 87
3.7.7 Active Isolation Systems ... 88
CHAPTER 4: ENGINEERING PROPERTIES OF ISOLATORS ... 89
4.1 Sources of Information... 89
4.2 Engineering Properties of Lead Rubber Bearings... 89
4.2.1 Shear Modulus ... 90
4.2.2 Rubber Damping... 90
4.2.3 Cyclic Change in Properties ... 91
4.2.4 Age Change in Properties ... 93
4.2.5 Design Compressive Stress... 94
4.2.6 Design Tension Stress... 94
4.2.7 Maximum Shear Strain... 95
4.2.8 Bond Strength ... 97
4.2.9 Vertical Deflections... 97
4.3 Engineering Properties of High Damping Rubber Isolators... 100
4.3.1 Shear Modulus ... 100
4.3.2 Damping... 101
4.3.3 Cyclic Change in Properties ... 102
4.3.4 Age Change in Properties ... 103
4.3.5 Design Compressive Stress... 103
4.3.6 Maximum Shear Strain... 103
4.3.7 Bond Strength ... 103
4.3.8 Vertical Deflections... 103
4.3.9 Wind Displacements ... 104
4.4 Engineering Properties of Sliding Type Isolators... 104
4.4.1 Dynamic Friction Coefficient ... 105
4.4.2 Static Friction Coefficient... 106
4.4.3 Effect of Static Friction on Performance ... 108
4.4.4 Check on Restoring Force ... 110
4.4.5 Age Change in Properties ... 110
4.4.6 Cyclic Change in Properties ... 111
4.4.7 Design Compressive Stress... 111
4.4.8 Ultimate Compressive Stress... 111
4.5 Design Life of Isolators ... 111
4.6 Fire Resistance... 111
4.7 Effects of Temperature on Performance... 112
4.8 Temperature Range for Installation... 112
CHAPTER 5: ISOLATION SYSTEM DESIGN ... 113
5.1 Introduction ... 113
5.1.1 Assessing Suitability... 113
5.1.2 Design Development for an Isolation Project ... 115
5.2.1 Codes ... 116
5.2.2 Empirical Data ... 116
5.2.3 Definitions... 116
5.2.4 Range of Rubber Properties ... 117
5.2.5 Vertical Stiffness and Load Capacity ... 118
5.2.6 Vertical Stiffness ... 118
5.2.7 Compressive Rated Load Capacity... 119
5.2.8 AASHTO 1999 Requirements... 120
5.2.9 Tensile Rated Load Capacity... 121
5.2.10 Bucking Load Capacity ... 121
5.2.11 Lateral Stiffness and Hysteresis Parameters for Bearing... 122
5.2.12 Lead Core Confinement... 125
5.3 Basis of an Isolation System Design Procedure... 126
5.3.1 Elastomeric Based Systems... 127
5.3.2 Sliding and Pendulum Systems... 127
5.3.3 Other Systems... 127
5.4 Step-By-Step Implementation of a Design Procedure... 127
5.4.1 Example of Illustrate Calculations ... 128
5.4.2 Design Code ... 129
5.4.3 Units... 129
5.4.4 Seismic and Building Definition ... 130
5.4.5 Material Definition ... 131
5.4.6 Isolator Types and Load Data... 133
5.4.7 Isolator Dimensions... 134
5.4.8 Calculate Bearing Properties ... 136
5.4.9 Gravity Load Capacity ... 138
5.4.10 Calculate Seismic Performance... 139
5.4.11 Seismic Load Capacity ... 143
5.4.12 Assess Factors of Safety and Performance ... 144
5.4.13 Properties for Analysis ... 146
5.4.14 Hysteresis Properties ... 147
CHAPTER 6: EFFECT OF ISOLATION ON BUILDINGS ... 149
6.1 Prototype Buildings ... 149
6.1.1 Building Configuration... 149
6.1.2 Design of Isolators... 150
6.1.3 Evaluation Procedure... 156
6.1.4 Comparison with Design Procedure... 158
6.1.5 Isolation System Performance... 164
6.1.6 Building Inertia Loads... 166
6.1.7 Floor Accelerations ... 175
6.1.8 Optimum Isolation Systems... 180
6.2 Example Assessment of Isolator Properties... 182
CHAPTER 7: SEISMIC ISOLATION OF BUILDINGS AND BRIDGES... 185
7.1 Introduction to Isolation of Buildings... 185
7.2 Scope of Building Example ... 185
7.3 Seismic Input... 186
7.4 Design of Isolation System ... 187
7.5 Analysis Models ... 189
7.6 Analysis Results ... 191
7.6.1 Summary of Results ... 195
7.7 Test Conditions ... 195
7.8 Production Test Results... 196
7.9 Summary ... 197
7.10 Implementation in Spreadsheet ... 198
7.10.1 Material Definition ... 198
7.10.2 Project Definition ... 199
7.10.3 Isolator Types and Load Data... 200
7.10.4 Isolator Dimensions... 201
7.10.5 Isolator Performance ... 203
7.10.6 Properties for Analysis ... 205
7.11 Introduction to Isolation for Bridges... 207
7.13.1 The 1991 AASHTO Guide Specifications... 209
7.13.2 The 1999 AASHTO Guide Specifications... 210
7.14 Use of Bridge Specifications for Building Isolator Design ... 210
7.15 Design of Isolation Systems... 212
7.15.1 Non-Seismic Loads ... 212
7.15.2 Effect of Bent Flexibility... 213
7.16 Analysis of Isolated Bridges ... 215
7.17 Design Procedure for Bridge Isolation... 216
7.17.1 Example Bridge... 216
7.17.2 Design of Isolators... 218
7.17.3 Accounting for Bent Flexibility in Design ... 220
7.17.4 Evaluation of Performance ... 224
7.17.5 Effect of Isolation System on Displacements...228
7.17.6 Effect of Isolation on Forces ... 229
7.17.7 Summary ... 231
7.18 Implementation in Spreadsheet ... 231
7.18.1 Material Properties ... 232
7.18.2 Dimensional Properties ... 232
7.18.3 Load and Design Data... 233
7.18.4 Isolation Solution ... 234
CHAPTER 8: APPLICATIONS OF SEISMIC ISOLATION... 237
8.1 Introduction ... 237
8.2 Structures Isolated in New Zealand ... 239
8.2.1 Introduction... 239
8.2.2 Road Bridges ... 242
8.2.3 South Rangitikei Viaduct with Stepping Isolation ... 244
8.2.4 William Clayton Building... 245
8.2.5 Union House ... 247
8.2.6 Wellington Central Police Station... 249
8.3 Structures Isolated in Japan... 251
8.3.1 Introduction... 251
8.3.2 The C-1 Building, Cuchu City, Tokyo ... 255
8.3.3 The High-Tech R&D Centre, Obayashi Corporation ... 255
8.3.4 Comparison of Three Buildings with Different Seismic Isolation Systems ... 256
8.3.5 Oiles Technical Centre Building ... 258
8.3.6 Miyagawa Bridge... 259
8.4 Structures Isolated in the USA ... 261
8.4.1 Introduction... 261
8.4.2 Foothill Communities Law and Justice Centre, San Bernandino, California ... 263
8.4.3 Salt Lake City and Country Building: Retrofit... 264
8.4.4 USC University Hospital, Los Angeles ... 265
8.4.5 Sierra Point Overhead Bridge, San Francisco... 266
8.4.6 Sexton Creek Bridge, Illinois ... 267
8.5 Structures Isolated in Italy... 268
8.5.1 Introduction... 268
8.5.2 Seismically Isolated Bridges ... 268
8.5.3 The Mortaiolo Bridge... 269
8.6 Isolation of Delicate or Potentially Hazardous Structures or Substructures... 274
8.6.1 Introduction... 274
8.6.2 Seismically Isolated Nuclear Power Stations... 275
8.6.3 Protection of Capacity Banks, Haywards, New Zealand... 275
8.6.4 Seismic Isolation of a Printing Press in Wellington, New Zealand ... 277
CHAPTER 9: IMPLEMENTATION ISSUES... 279
9.1 Introduction ... 279
9.2 Isolator Locations and Types... 279
9.2.1 Selection of Isolation Plane ... 279
9.2.2 Selection of Device Type ... 283
9.3.1 Form of Seismic Input... 292
9.3.2 Recorded Earthquake Motions... 293
9.3.3 Near Fault Effects ... 300
9.3.4 Variations in Displacements ... 300
9.3.5 Time History Seismic Input ... 302
9.3.6 Selecting and Scaling Records for Time History Analysis... 302
9.3.7 Selecting Records from a Set... 303
9.3.8 Comparison of Earthquake Scaling Factors... 304
9.4 Detailed System Analysis ... 306
9.4.1 Single Degree-of-Freedom Model ... 307
9.4.2 Two Dimensional Non-Linear Model ... 307
9.4.3 Three Dimensional Equivalent Linear Model ... 307
9.4.4 Three Dimensional Model-Elastic Superstructure, Yielding Isolators ... 308
9.4.5 Fully Non-Linear Three Dimensional Model... 308
9.4.6 Device Modeling... 308
9.4.7 ETABS Analysis for Buildings ... 309
9.4.8 Concurrency Effects ... 313
9.5 Connection Design ... 316
9.5.1 Elastomeric Based Isolators... 316
9.5.2 Sliding Isolators ... 321
9.5.3 Installation Examples... 322
9.6 Structural Design 9.6.1 Design Concepts... 327
9.6.2 UBC Requirements ... 328
9.6.3 MCE Level of Earthquake ... 332
9.6.4 Non-Structural Components ... 332
9.6.5 Bridges... 333
9.7 Specifications ... 333
9.7.1 General ... 333
9.7.2 Testing... 335
CHAPTER 10: FEASIBILITY ASSESSMENT, EVALUATION AND FURTHER DEVELOPMENT OF SEISMIC ISOLATION ... 337
10.1 Decision-Making in a Seismic Isolation Context ... 337
10.1.1 Seismic Isolation Decisions to be Made ... 337
10.1.2 Seismic Isolation Decisions in the Wellington Area... 338
10.2 Construction Projects in New Zealand and India 1992 to 2005... 339
10.2.1 Introduction... 339
10.2.2 Retrofits... 339
10.2.3 Te Papa Tongarewa ... 339
10.2.4 Other Seismically Isolated Buildings ... 341
10.2.5 Bhuj Hospital ... 341
10.3 A Feasibility Study for Seismic Isolation... 343
10.3.1 Te Papa Tongarewa, the Museum of New Zealand... 343
10.3.2 Description... 343
10.3.3 Seismic Design Criteria ... 344
10.3.4 Feasibility Study... 345
10.3.5 Isolation System Design ... 346
10.3.6 Evaluation of Structural Performance ... 346
10.3.7 Results of ANSR-II Analysis... 348
10.3.8 Conclusions ... 348
10.4 Performance in Real Earthquakes ... 350
10.5 New Approaches to Seismic Isolation... 353
10.5.1 Introduction... 353
10.5.2 The RoBall... 353
10.5.3 The RoGlider... 356
10.6 Project Management Approach... 358
10.7 Future ... 359
Reference List………361
Additional Resources……….367
PREFACE
This is a revised version of the book “An Introduction to Seismic Isolation” published by Wiley and Sons in 1993. There have been many changes in the course of this revision and this is reflected in the changed title - “Seismic Isolation for Designers and Structural Engineers”.
This new book builds on the previous one and uses much of the previous material, but it has different authorship and more focus on practical applications. It acknowledges the pioneering work that has been done over the past 30 to 40 years but aims to present seismic isolation in a different way, as an established technique that could be considered widely, even routinely, as an option by designers and structural engineers. Trevor Kelly’s input as a practising structural engineer has transformed the book into a modern version that makes full use of computer science techniques. (A CD Rom is included).
The original book was authored by R I Skinner, W H Robinson and GH McVerry, who were at the time working at the Department of Scientific and Industrial Research (DSIR) in Wellington, New Zealand. The book recorded the innovative earthquake engineering research that had been carried out at the DSIR over the previous 25 years (see Chapter 3 of both books, and Chapter 8 of this book, which is carried over from the previous Chapter 6). However the book also marked the end of an era, as it was written at a time when change was in the air and the DSIR was about to be disestablished.
The DSIR was closed down mid-1992 as part of a New-Zealand wide drive to making science more commercial. Bill Robinson has risen to this challenge by forming Robinson Seismic Limited (RSL), an engineering company specialising in applications of seismic isolation to protect structures from earthquake damage. Bill has continued as one of the authors of this new book.
The new author is Trevor Kelly, a structural engineer with Holmes Consulting, which has been involved in the design and supply of seismic isolation systems for almost 20 years. Trevor’s interest is in the structural engineering aspects of applying seismic isolation/damping and the new chapters that he has written emphasise the engineering aspects.
The new book therefore retains the mathematical tools in Chapters 1, 2 and 3 of “An Introduction to Seismic Isolation” but replaces the empirical methods of succeeding chapters with detailed design and documentation material of the type that a structural engineer would need to implement isolation. This is followed through with examples of practical designs.
This book provides both theory and design aspects of seismic isolation. This will be useful for structural engineers and teachers of engineering courses. For other structural components (concrete frames, steel braces etc) the engineering student is taught the theory (lateral loads, bending moments) but then also the design (how to select sizes, detail reinforcing, bolts). This book will do the same for seismic engineering.
The book provides practical examples of computer applications as well as device design examples so that the structural engineer is able to do a preliminary design that won’t specify impossible constraints. The book also addresses the steps that need to be taken to ensure the design is code compliant.
The structural engineer is the key to adoption of seismic isolation technology. The book aims to provide enough design information so that the structural engineer can be confident on implementing seismic isolation; otherwise he/she won’t want to take the risk even if the architect or owner is enthusiastic. Firms like RSL and Holmes Consulting will continue to be available to provide expert advice and the benefits of their considerable experience in the field of device design and seismic isolation.
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The engineering credentials and expertise of Bill Robinson and his company, RSL, are evident from Chapters 3 and 8 (which describe the invention and development of the Lead Rubber Bearing). Bill Robinson has been honoured world wide for this work and has received recognition in New Zealand from the scientific community and the business community. There has also been considerable interest in the public domain, in the display of seismic isolation in Te Papa Tongarewa, the Museum of New Zealand on the waterfront in Wellington, which was built on Lead Rubber Bearings (see Chapter 9).
The engineering credentials of Trevor Kelly are evident from his work as Technical Director of Holmes Consulting Group (HCG), part of the Holmes Group, which is New Zealand's largest specialist structural engineering company, with over 90 staff in three main offices in New Zealand plus 25 in the San Francisco office. Trevor heads the seismic isolation division of HCG in the Auckland office. He has over 15 years experience in the design and evaluation of seismic isolation systems in the United States, New Zealand and other countries and is a licensed Structural Engineer in California.
Since 1954 the company has designed a wide range of structures in the commercial and industrial fields. HCG has been progressive in applications of seismic isolation and since its first isolated project, Union House, in 1982, has completed six isolated structures. On these projects HCG provided full structural engineering services. In addition, for over 8 years HCG provided design and analysis services to Skellerup Industries of New Zealand and later Skellerup Oiles Seismic Protection (SOSP), a San Diego based manufacturer of seismic isolation hardware. Isolation hardware used on their projects included Lead Rubber Bearings (LRBs), High Damping Rubber Bearings (HDR), Teflon on stainless steel sliding bearings, sleeved piles and steel cantilever energy dissipators.
The company has developed design and analysis software to ensure effective and economical implementation of seismic isolation for buildings, bridges and industrial equipment. Expertise encompasses the areas of isolation system design, analysis, specifications and evaluation of performance. In writing Chapters 4, 5, 6 and 7 of this book Trevor has drawn on his practical experience in the field and explains methods of calculating seismic responses using state-of-the-art computer software such as ETABS, used for the linear and nonlinear analysis of buildings.
This book is the product of three expert engineers who have, over a long period of years, worked separately and collaboratively to design and develop earthquake isolation solutions and to incorporate them into existing and new structures. Collaboration on this book is a further joint venture and has a two-fold aim — to be used for the benefit of professionals looking to apply earthquake isolation techniques, and to be used in educating a new generation of structural engineers and designers.
ACKNOWLEDGEMENTS
In presenting this new book, which builds on and revises the previous book “An Introduction to Seismic Isolation”, thanks are due to all those who made the previous book possible, especially Graeme McVerry, as well as those who have been involved in its revision. Thanks to Barbara Bibby who again provided editing services, to Heather Naik and the staff and shareholders of Robinson Seismic Ltd and Holmes Consulting Group, to the Book Committees at RSL and Holmes Consulting who have reviewed it, and to FORST for providing funding for its production.
Trevor Kelly
Holmes Consulting Group 34 Waimarei Avenue Paeroa
NEW ZEALAND
www.holmesgroup.com
Bill Robinson
Robinson Seismic Ltd P O Box 33093 Petone
NEW ZEALAND www.rslnz.com
Ivan Skinner
31 Blue Mountains Road Silverstream
Wellington New Zealand
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AUTHOR BIOGRAPHIES
Trevor E Kelly, Technical Director, Holmes Consulting Group 34 Waimarei Avenue, Paeroa, New Zealand
www.holmesgroup.com
Trevor completed a BE at the University of Canterbury in 1973 and an ME in 1974. His research report, related to the nonlinear analysis of concrete structures, initiated an interest in this field which has continued throughout his career. He has worked as a structural engineer in New Zealand and California and is a Chartered Engineer in NZ and licensed Structural Engineer in California. Over the last 20 years, he has specialised in structural engineering fields which utilise nonlinear analysis, such as base isolation, energy dissipation and performance based evaluation of existing buildings. In his current position, Trevor directs the technical developments at Holmes Consulting Group, particularly as they relate to structural analysis and computer software development.
Dr William H Robinson, Founder & Chief Engineer, Robinson Seismic Ltd P O Box 33093, Petone, New Zealand
www.rslnz.com
Bill began his career as a mechanical engineer, graduating ME at the University of Auckland before working for two construction companies. He then changed fields to physical metallurgy, completing a PhD in 1965 at the University of Illinois. Two years as a research fellow followed, working in solid-state physics at the University of Sussex before returning to New Zealand to work as a scientist with the Physics and Engineering Laboratory (PEL) at the Department of Scientific and Industrial Research (DSIR) in December 1967.
Bill’s interest in seismic isolation led to the invention and development of the lead extrusion damper (1970) and the lead-rubber bearing (1974) (See Chapter 3) and has become his major research and engineering interest. Other research during his career as a scientist and later Director of PEL has included Antarctic sea-ice research, attempts to detect gravitational waves, the successful development of an ultrasonic viscometer and ultrasonically modulated ESR. The first version of this book was written with Ivan Skinner and Graeme McVerry in the last days of the DSIR. Ten years ago Bill founded Robinson Seismic Ltd, which is based in Lower Hutt, New Zealand and has contacts and clients all over the world.
Dr R Ivan Skinner
31 Blue Mountains Road, Silverstream, Wellington, New Zealand
Ivan’s early activities prepared him for a contribution towards reducing earthquake impacts on structures, including early applications of seismic isolation such as the Rangitikei stepping bridge and the William Clayton building. He obtained a BE Hons in 1951, and a DSc in 1976, from the University of Canterbury, New Zealand. In 1953 he joined the Physics & Engineering Laboratory, PEL, in Lower Hutt where his wide-ranging activities included designing a vibration isolation system for the lab’s new electron microscope.
During 1959-78 he led the Engineering Seismology Section of PEL where he applied his knowledge of electrodynamics to modelling structures and their dynamic responses to severe earthquakes. Other section priorities included the development of a New Zealand strong-motion earthquake recording network used throughout NZ and overseas; engineering studies of informative earthquake attacks worldwide; contributions as a UNESCO expert in earthquake engineering and developing special components for seismic isolation to give more reliable earthquake resistance at lower cost.
After the completion of the Seismic Isolation book with Robinson and McVerry in 1993, Ivan became Director of the New Zealand Earthquake Commission’s Research Foundation, from
which he retired at the end of 2005.
FREQUENTLY USED SYMBOLS AND ABBREVIATIONS
(Chapters 1, 2, 3 and 8)
β
: tuning parameter for combined primary-secondary system, namely(ω
p-ω
s)/ ω
aβ
ij : analogue to β, for multimode primary-secondary systemsҐ
e,rn : elastic-phase participation factor at position r in mode nҐ
n(z
) : mode-n participation factor at position zҐ
Nn : mode-n participation factor at top floor of structure (position N)Ґ
n : weighting factor for the nth mode of vibrationҐ
n(I)
: isolated mode weight factorҐ
n(U)
: unisolated mode weight factorҐ
rn : participation factor for response to ground excitation for a mass at level r of a structure vibrating in the nth modeҐ
y,rn : yielding-phase participation factor at position r in mode nγ
xz : shear strain of rubber discγ
: interaction parameter of combined primary-secondary system, given by ms/mpγ
: ‘engineering’ shear strainγ
ij : interaction parameter, analogue toγ
, for multimode primary-secondary systemsγ
n : wave number of mode n, possibly complexγ
y : shear-strain coordinate of yield point∆
n : difference between nth root of equation (4.17) and (n-1)π
δ
d : nonclassical damping parameter in combined primary-secondary systemδ
ij : analogue toδ
d, for multimode primary-secondary systemsε
: ωb/ωFB1= ratio of frequencies of rigid-mass isolated structure and first-mode unisolated structure, used for expressing orders of perturbationε
: strain = (increment in length)/(original length)ε
m : maximum amplitude of cyclic strainε
y : strain coordinate of yield pointΘ
n : variation of spatial phase of mode-n displacement down shear beamT
T
ζ
p : damping of primary structureζ
a : average damping of combined primary-secondary system, given byζ
a= (ζ
p+ζ
s)/2
ζ
d : damping difference of combined primary-secondary system, given byζ
d= ζ
p -ζ
sζ
FBn : fraction of critical viscous damping of (unisolated) fixed-base mode nζ
: velocity- (viscous-) ‘damping factor’ or ‘fraction of critical damping’ for single-mass oscillatorζ
b : velocity-damping factor for isolatorζ
B : ‘effective’ damping factor of bilinear isolator, given by sum of velocity- and hysteretic-damping factorsζ
b1 : velocity-damping factor in ‘elastic’ region of bilinear isolatorζ
b2 : velocity-damping factor in ‘plastic’ or ‘yielded-phase’ region of bilinear isolatorζ
h : hysteretic damping factor of bilinear isolatorζ
n : fraction of critical viscous damping of mode n; also called mode-n damping factorμ
j0 : modal mass of free-free mode j :u
j0[M] u
j0μ
sj : jth modal mass of secondary system =Φ
sj[M
s] Φ
sjξ
n(t)
: modal (relative displacement) coordinate for mode n at timet ρ
: uniform density of shear beam representing a uniform shear structureσ
: nominal stress, as used in ‘scaled’ (σ-ε
) curves for steel dampers in Chapter 3σ
: stress = force/area (Pascals)σ
y : stress coordinate of yield pointτ
: nominal shear stress, as used in ‘scaled’ (σ-ε
) curves for steel dampers in Chapter 3τ
: shear stress = (shear force)/area (Pascals)τ
y : shear-stress coordinate of yield pointΦ
: [Φ1, … Φ2, … Φ3], the mode shape matrix, a function of space, not timeΦ
n,Φ
m : mode shape in the nth or mth mode of vibrationΦ
e,rn : elastic-phase modal shape at position r in mode nΦ
y,rn : yielding-phase modal shape at position r in mode nΦ
n(z,t) : shape of mode n, used interchangeably with un(z,t); normalized to unity at the toplevel
Ψ
n : phase angle of participation factor vectorΓ
nω
s : (circular) frequency of secondary structureω
p : (circular) frequency of primary structureω
a : average frequency of combined primary-secondary system, given byω
a= (ω
p+ ω
s) /2
ω
pi : analogue toω
p, for multimode primary-secondary systemω
sj : analogue toω
s, for multimode primary-secondary systemω
1(U)
: unisolated undamped first-mode natural (circular) frequency, the same asω
FBiω
FFn : mode-n natural (circular) frequency with ‘free-free’ boundary conditionsω
b : isolator frequency =√
(Kb/M) for a rigid mass Mω
FB1 : natural (circular) frequency of (unisolated) fixed-base made 1, equivalent toω
1(U) ω
FBn : natural (circular) frequency of (unisolated) fixed base mode nω
n : undamped natural (circular) frequency of mode n, related to frequencyf
n byω
n= 2πf
nω
d : damped natural (circular) frequency of single-mass oscillatorω
n : undamped natural (circular) frequency of single-mass oscillator, or nth-mode natural frequency of multi-degree-of-freedom linear oscillatorA : area of rubber bearing in Chapter 3
A : cross-sectional area of shear beam representing a uniform shear structure Ah : area of bilinear hysteresis loop
an(t) : absolute acceleration of mode n
A′ : overlap area of rubber bearing in Chapter 3
b : subscript denoting base isolator
ůb, ůb(t) : relative velocity of base mass with respect to ground
B : subscript denoting bilinear isolator
BF : ‘bulge factor’ describing the ratio Sr/Sr,1 of total shear to first-mode shear at level r in a structure, particularly at mid-height
c(r,s) : interlevel velocity-damping coefficient, defined only for r ≥ s
Cb : coefficient of velocity-damping for a base isolator, with units such as Nm-1s = kgs s-1 CF : correction factor linking displacement of bilinear isolator to equivalent spectral
displacement
ck : stiffness-proportional damping coefficient of shear beam representing a uniform shear structure
CK : overall stiffness-proportional damping coefficient CkA/L of uniform shear structure cm : mass-proportional damping coefficient of shear beam representing a uniform shear
structure
CM : overall mass-proportional damping coefficient CmAL of uniform shear structure crs : element of damping coefficient matrix
[C] : damping coefficient matrix, with elements crs related to c(r,s) e : subscript used to denote ‘elastic-phase’
e : subscript used to denote ‘experimental model’ in ‘scaled’ (σ-ε) or ( τ- γ) curves for steel dampers in Chapter 3
E : Young’s modulus = σ/ε in elastic region
f : force-scaling factor, as used in ‘scaled’ (σ-ε) or ( τ- γ) curves for steel dampers in Chapter 3
F : force or shear-force as obtained from ‘scaled’ (σ-ε) or ( τ- γ) curves for steel dampers in Chapter 3
FAr(T,ξ) : floor acceleration spectrum at rth level of a structure
Fb : isolator force arising from bilinear resistance to displacement Fe′ : residual force in elastic phase of bilinear isolator
FF : subscript denoting ‘free-free’ boundary condition corresponding to perfect isolation
FFn : subscript denoting mode-n ‘free-free’ vibration
Fn(z) : maximum seismic force per unit height, at height z of mode n Fr : maximum inertia load on the masss mr at level r
Frn : maximum seismic force of mode n at the rth point of a structure Fy′ : residual force in yielding phase of bilinear isolator
G : constant shear modulus of shear beam representing a uniform shear structure G0 : white noise power spectrum
hr : height of rth level of a structure
I : ‘degree of isolation’ or ‘isolation ratio’ given by
ω
FB1/ω
b=Tb/TFB1=Tb/T1(U) k : stiffness of single-mass oscillatorK : overall stiffness GA/L of uniform shear structure
k(r,s) : interlevel stiffness, such that k(r,r-1) = KN for a N-mass uniform structure and k(1,o) = Kb it if is isolated
Kb : stiffness of linear isolator
KB : ‘effective’ or ‘secant’ stiffness of bilinear isolator Kb(r) : stiffness of rubber component of lead rubber bearing Kb1 : ‘initial’ or ‘elastic’ stiffness of bilinear isolator
Kb2 : ‘post-yield’ or ‘plastic’ stiffness of bilinear isolator
Kc : stiffness of spring introduced to isolator to reduce higher-mode responses (Figure 2.2c)
Kn : stiffness of nth ‘spring’ in discrete linear chain system krs : element of stiffness matrix
[K] : stiffness matrix, with elements krs related to k(r,s)
ℓ
: length-scaling factor, as used in ‘scaled’(σ-ε)
or( τ- γ)
curves for steel dampers in Chapter 3L : length of shear beam representing a uniform shear structure m : mass of single-mass oscillator
M : mass pAL of uniform shear structure
M : total mass of structure; together with the mass of the isolator this gives MT
Mb : isolator (base) mass mp : mass of primary structure mr : mass at rth level
: M/N for a uniform structure with N levels ms : mass of secondary structure
MT : total mass of structure plus isolator
N : number of masses in discrete linear system ..
Xn(z) : maximum absolute seismic acceleration of mode n at position z n* : complex conjugate associated with mode n
NL : nonlinearity factor
OMn(z) : overturning moment at height z of mode n
OMrn : maximum overturning moment at point r, and height hr, of mode-n of a structure
p : subscript used to denote ‘primary’ in primary-secondary systems P : peak factor, namely ratio of peak response to RMS response
p : subscript used to denote ‘prototype’ in ‘scaled’
(σ-ε)
or( τ- γ)
curves for steel dampers in Chapter 3P : peak factor, namely ratio of peak response to RMS response Pa : amplitude-scaling factor such that
ü
g(t) = P
aü
ElCentro(t/P
p)
Pn : complex frequency of mode n, see equation (4.7)
Pn0 : zeroth-order term in the perturbation expression for the complex frequency Pni : i-th term in perturbation expression for the nth-mode complex frequency Pp : frequency-scaling factor such that
ü
g(t) = P
aü
ElCentro(t/P
p)
Pps : peak factor for secondary structure when mounted on primary structure Ps : peak factor for secondary structure when mounted on the ground Q : force across Coulomb slider at which it yields
Qy : yield force at which changeover from elastic to plastic behaviour occurs, at yield displacement Xy
Qy : shear-force coordinate of yield point
Qy/W : yield force-to-weight ratio of bilinear isolator
S : shape factor of elastomeric bearing = (loaded area)/(force-free area)
SA(T,
ζ
) : spectral absolute acceleration for period T and dampingζ
, as seen on response spectrum, Figure 2.1Sb : maximum base-level shear Sbn : maximum base shear in mode n
SD(T,
ζ
) : spectral relative displacement for period T and dampingζ
, as seen on response spectrum, Figure 2.1Sn (z) : maximum seismic shear at height z of mode n
Srn : maximum shear force at the rth point of a structure oscillating in mode n Sv(T,
ζ
) : spectral relative velocity for period T and dampingζ
t : time
T : superscript indicating ‘transpose’
T : natural period
T1(U) : unisolated undamped first-mode period, the same as TFB1
Tb : natural period of linear base isolator = 2
π/ω
bTB : ‘effective’ period for bilinear isolator
Tb1 : period associated with Kb1, in ‘elastic’ region of bilinear isolator Tb2 : period associated with Kb2, in ‘plastic’ region of bilinear isolator Tn(1) : isolated nth period
Tn(U) : unisolated nth period
u
: vector containing the displacements uru
(z,t) : relative displacement, at position z in the structure, in the horizontal x direction, with respect to the ground at time t; often written as u, without arguments, in thedifferential form of the equation of motion
ü
(z,t) : relative acceleration with respect to ground of position z at time tu
1 : displacement of bilinear isolatoru
b, ub(t) : relative displacement of base mass with respect to groundü
b,ü
b(t) : acceleration of base mass with respect to groundu
bj0 : base displacement in free-free mode ju
bn(t) : nth-mode relative displacement, with respect to ground, at base of structure at time tu
e,rn : elastic-phase displacement at position r in mode nü
e,rn : elastic-phase relative acceleration at position r in mode nu
FBn(z,t) : fixed-base mode-n relative displacement with respect to ground at position z at time tu
FFN(z,t) : ‘free-free’ mode-n relative displacement with respect to ground, at position z and time tU
Ln,U
Nn : amplitude of nth-mode displacement at position z=L (top of shear beam) (possibly complex); amplitude at top of discrete N-component structureu
n(z) : nth mode shape, used interchangeably with Φn(z); usually normalisation is not definedu
n(z,t) : mode-n relative displacement, with respect to ground, of position z at time tu
n0 : zeroth-order term in the perturbation expression for the mode shapeu
ps : displacement of secondary structure mounted on the primary structureü
ps : acceleration of secondary structure mounted on the primary structureu
rn(t) : Φrnξ
n(t)
= displacement of mode-n at rth level of structure, where Φrn is the spatial variation andξ
n is the time variationu
s : displacement of secondary structure mounted on the groundü
s : acceleration of secondary structure mounted on the groundu
y,rn : yielding-phase displacement at position r in mode nü
y,rn : yielding-phase relative acceleration at position r in mode nu
n : displacement vector for discrete linear system in nth modev : vector comprising the relative velocity and relative displacement vectors vn : vector v for mode n
W : total weight of structure
X : displacement, as obtained from ‘scaled’
(σ-ε)
or( τ- γ)
curves for steel dampers in Chapter 3Xb : maximum relative displacement of isolator or of base of isolated structure XNn : maximum mode-n relative displacement at top floor of structure (position N) Xp : peak response of primary structure when mounted on the ground
Xp(RMS) : RMS response of primary structure when mounted on the ground
Xps : peak response of secondary structure when mounted on primary structure Xr : maximum relative displacement with respect to ground at any level r Xrn : peak value of mode-n relative displacement at the rth point of a structure Xs : peak response of secondary structure when mounted on the ground Xs(RMS) : RMS response of secondary structure when mounted on the ground
Xy : displacement coordinate of yield point ..
Xrn : peak value of mode-n absolute acceleration at the rth point of a structure .
Xrn : peak value of mode-n relative velocity at the rth point of a structure z : vertical coordinate; height of a point of a structure
Zn(t) : relative displacement response, of one-degree-of-freedom oscillator of undamped natural frequency
ω
n and dampingζ
n, to ground accelerationü
g(t)LIST OF COMMONLY USED ABBREVIATIONS
CQC : abbreviation for ‘Complete Quadratic Combination’, a method of adding responses of several modes
DSIR : Department of Scientific and Industrial Research, New Zealand LRB : Lead rubber bearing
MDOF : abbreviation for multiple-degree-of-freedom MWD : Ministry of Works and Development, New Zealand
PEL : Physics and Engineering Laboratory of the DSIR, later DSIR Physical Sciences PTFE : polytetrafluoroethylene
SRSS : abbreviation for ‘Square Root of the Sum of the Squares’, a method of adding responses of several modes
1DOF : abbreviation for one-degrees-of-freedom 2DOF : abbreviation for two-degrees-of-freedom
1.1 SEISMIC ISOLATION IN CONTEXT
A large proportion of the world's population lives in regions of seismic hazard, at risk from earthquakes of varying severity and varying frequency of occurrence. Earthquakes cause significant loss of life and damage to property every year.
Various a seismic construction designs and technologies have been developed over the years in attempts to mitigate the effects of earthquakes on buildings, bridges and potentially vulnerable contents. Seismic isolation is a relatively recent, and evolving, technology of this kind.
Seismic isolation consists essentially of the installation of mechanisms which decouple the structure, or its contents, from potentially damaging earthquake-induced ground, or support, motions. This decoupling is achieved by increasing the flexibility of the system, together with providing appropriate damping. In many, but not all, applications the seismic isolation system is mounted beneath the structure and is referred to as 'base isolation'.
Although it is a relatively recent technology, seismic isolation has been well evaluated and reviewed (e.g. Lee & Medland, 1978; Kelly, 1986; May 1990 issue of "Earthquake Spectra" ); and has been the subject of international workshops (e.g., NZ-Japan Workshop, 1987; US-Japan Workshop, 1990; Assisi Workshop, 1989; Tokyo Workshop, 1992); is included in the programmes of international, regional and national conferences on Earthquake Engineering (e.g., 9th WCEE World Conference on Earthquake Engineering, Tokyo, 1988; Pacific Conferences, 1987, 1991;
Fourth US Conference, 1990); and has been proposed for specialised applications (e.g., SMIRT 11, Tokyo, 1991).
Seismic isolation may be used to provide effective solutions for a wide range of seismic design problems. For example, when a large multi-storey structure has a critical Civil Defence role which calls for it to be operational immediately after a very severe earthquake, as in the case of the Wellington Central Police Station (see Chapter 8), the required low levels of structural and non-structural damage may be achieved by using an isolating system which limits structural deformations and ductility demands to low values. Again, when a structure or sub-structure is inherently non-ductile and has only moderate strength, as in the case of the newspaper printing press at Petone (see Chapter 8), isolation may provide a required level of earthquake resistance which cannot be provided practically by earlier seismic techniques. Careful studies have been made of classes of structure for which seismic isolation may find widespread application. This has been found to include common forms of highway bridges.
The increasing acceptance of seismic isolation as a technique is shown by the number of retrofitted seismic isolation systems which have been installed. Examples in New Zealand are the retrofitting of seismic isolation to existing bridges and to the electrical capacitor banks at Haywards (see Chapter 8), while the retrofit of isolators under the old New Zealand Parliamentary Buildings was completed in 1993. Many old monumental structures of high cultural value have little earthquake resistance.
The completed isolation retrofit of the Salt Lake City and County Building in Utah is described in some detail in Chapter 8.
Isolation may often reduce the cost of providing a given level of earthquake resistance. The New Zealand approach has been to design for some increase in earthquake resistance, together with some cost reduction, a typical target being a reduction by 5% of the structural cost.
Reduced costs arise largely from reduced seismic loads, from reduced ductility demand and the consequent simplified load-resisting members, and from lower structural deformations which can be accommodated with lower-cost detailing of the external cladding and glazing.
Seismic isolation thus has a number of distinctive beneficial features not provided by other aseismic techniques. We believe that seismic isolation will increasingly become one of the many options routinely considered and utilised by engineers, architects and their clients. The increasing role of seismic isolation will be reflected, for example, in widespread further inclusion of the technique in the seismic provisions of structural design codes.
When seismic isolation is used, the overall structure is considerably more flexible and provision must be made for substantial horizontal displacement. It is of interest that, despite the widely varying methods of computation used by different designers, a consensus is beginning to emerge that a reasonable design displacement should be of the order of 50 to 400 mm, and possibly up to twice this amount if 'extreme' earthquake motions are considered. A 'seismic gap' must be provided for all seismically isolated structures, to allow this displacement during earthquakes.
It is imperative that present and future owners and occupiers of seismically isolated structures are aware of the functional importance of the seismic gap and the need for this space to be left clear. For example, when a road or approach to a bridge is resealed or re-surfaced, extreme care must be taken to ensure that sealing material, stones etc, do not fall into the seismic gap.
In a similar way, the seismic gap around buildings must be kept secure from rubbish, and never used as a convenient storage space.
All the systems presented in this book are passive, requiring no energy input or interaction with an outside source. Active seismic isolation is a different field, which confers different aseismic features in the face of a different set of problems. As it develops, it will occupy a niche among aseismic structures which is different from that occupied by structures with passive isolation. In a typical case, a mass which is a fraction of a percent of the structural mass is driven with large accelerations so that the reaction to its inertia forces tend to cancel the effects of inertia forces arising in the structure as a result of earthquake accelerations. Such a system may be a practical, but expensive, means of reducing the effective seismic loads during moderate, and in some locations frequent, earthquakes. Practical limitations on the size and displacements of the active mass would normally render the system much less effective during major earthquakes.
Moreover, it is difficult to ensure the provision of the increasing driving power required during earthquakes of increased severity. In principle, such an active isolation system might be used to complement a passive isolation system in certain special cases. For example, a structure with passive seismic isolation may be satisfactory in all respects, except that it may contain components which are particularly vulnerable to high-frequency floor-acceleration spectra.
The active-mass power and displacement requirements for the substantial cancellation of these short-period low-acceleration floor spectra may be moderate, even when the earthquake is very severe. Moreover, such moderate power might be supplied by an in-house source, with its dependability increased by the reduced seismic attack resulting from isolation.
A number of factors need to be considered by an engineer, architect or client wishing to decide whether a proposed structure should incorporate seismic isolation.
The first of these is the seismic hazard, which depends on local geology (proximity to faults, soil substructure), recorded history of earthquakes in the region, and any known factors about the probable characteristics of an earthquake (severity, period, etc). Various proposed solutions to the design problem can then be put forward, with a variety of possible structural forms and materials, and with some designs incorporating seismic isolation, some not. The probable level of seismic damage can then be evaluated for each design, where the degree of seismic damage can be broadly categorised as:
(1) minor
(2) repairable (up to about 30% of the construction cost) (3) not repairable, resulting in the building being condemned.
The whole thrust of seismic isolation is to shift the probable damage level from (3) or (2) towards (1) above, and thereby to reduce the damage costs, and probably also the insurance costs.
Maintenance costs should be low for passive systems, though they may be higher for active seismic isolation. As discussed above, the construction costs including seismic isolation usually vary by + 5 to 10% from unisolated options.
The total 'costs' and 'benefits' of the various solutions can then be evaluated, where the analysis has to include the 'value' of having the structure or its contents in as good as possible a condition after an earthquake, and the reduced risk of casualties with reduced damage. In many cases such additional benefits may well follow the adoption of the seismic isolation option.
1.2 FLEXIBILITY, DAMPING AND PERIOD SHIFT
The 'design earthquake' is specified on the basis of the seismicity of a region, the site conditions, and the level of hazard accepted (for example, a '400-year return period' earthquake for a given location would be expected to be less severe than one which occurred on average once every 1000 years).
Design earthquake motions for more seismic areas of the world are often similar to that experienced and recorded at El Centro, California, in 1940, or scalings of this motion, such as '1.5 El Centro'. The spectrum of the El Centro accelerogram has large accelerations at periods of 0.1 to 1 second. Other earthquake records, such as that at Pacoima Dam in 1971 or 'artificial' earthquakes A1 or A2 are also used in specifying the design level.
It must also be recognised that occasionally earthquakes give their strongest excitation at long periods. The likelihood of these types of motions occurring at a particular site can sometimes be foreseen, such as with deep deposits of soft soil which may amplify low-frequency earthquake motions, the old lake-bed zone of Mexico City being the best-known example. With this type of motion, flexible mountings with moderate damping may increase rather than decrease the structural response. The provision of high damping as part of the isolation system gives an important defence against the unexpected occurrence of such motions.
Typical earthquake accelerations have dominant periods of about 0.1 to 1 seconds as shown in Figure 2.1 in the next Chapter, with maximum severity often in the range 0.2 to 0.6 s. Structures whose natural periods of vibration lie within the range 0.1 to 1 seconds are, therefore, particularly vulnerable to seismic attack because they may resonate. The most important feature of seismic isolation is that its increased flexibility increases the natural period of the structure. Because the period is increased beyond that of the earthquake, resonance and near-resonance are avoided and the seismic acceleration response is reduced.
This period shift is shown schematically in Figure 1.1(a) and in more detail in Figure 2.1 in Chapter 2.
Figure 1.1: Effect of increasing the flexibility of a structure:
(a) The increased period and damping lower the seismic acceleration response;
(b) The increased period increases the total displacement of the isolated system, but this is offset to a large extent by the damping. (After Buckle & Mayes, 1990.)
The increased period, and consequent increased flexibility, also affects the horizontal seismic displacement of the structure, as shown in Figure 1.1(b) for the simplest case of a single-mass rigid structure and as shown in more detail in Figure 2.1 in Chapter 2. Figure 1.1(b) shows how excessive displacements are counteracted by the introduction of increased damping. Real values of the maximum undamped displacement for isolated structures could be as large as 1 m in typical strong earthquakes; damping typically reduces this to 50 to 400 mm, and this is the displacement which has to be accommodated by the 'seismic gap.' The actual motion of parts of the structure depends on the mass distribution, the parameters of the isolating system, and the 'participation' of various modes of vibration. This is discussed in detail in Chapters 2 and 6.
Seismic isolation is thus an innovative aseismic design approach aimed at protecting structures against damage from earthquakes by limiting the earthquake attack rather than resisting it.
Conventional approaches to aseismic design provide a structure with sufficient strength, deformability and energy-dissipating capacity to withstand the forces generated by an earthquake, and the peak acceleration response of the structure is often greater than the peak acceleration of the driving ground motion. On the other hand, seismic isolation limits the effects of the earthquake attack, since a flexible base largely decouples the structure from the horizontal motion of the ground, and the structural response accelerations are usually less than the ground accelerations. The forces transmitted to the isolated structure are further reduced by damping devices which dissipate the energy of the earthquake-induced motions.
Figure 1.2(a) illustrates the seismic isolation concept schematically. The building on the left is conventionally protected against seismic attack and that on the right has been mounted on a seismic isolation system. The performance of a pair of real test buildings of this kind, at Tohoku University, Sendai, Japan, is described in Chapter 8. Similar schematic diagrams can be drawn to illustrate the seismic isolation of bridges and of parts of buildings which contain delicate or potentially hazardous contents.
Figure 1.2: (a) Schematic seismic response of two buildings; that on the left is conventionally protected against earthquake, and that on the right has been mounted on a seismic isolation system.
(b) Maximum base shear for a single-mass structure, represented as a linear resonator, with and without seismic isolation. The structure is subjected to Pa times the El Centro NS 1940 accelerogram. (From Skinner & McVerry, 1975).
In Figure 1.2(a) it can be seen that large seismic forces act on the unisolated, conventional structure on the left, causing considerable deformation and cracking in the structure. In the isolated structure on the right, the forces are much reduced, and most of the displacement occurs across the isolation system, with little deformation of the structure itself, which moves almost as a rigid unit. Energy dissipation in the isolated system is provided by hysteretic or viscous damping. For the unisolated system, energy dissipation results mainly from structural damage.
Figure 1.2(b) illustrates the reduction of earthquake induced shear forces which can be achieved by seismic isolation. The maximum responses of seismically isolated structures, as a function of unisolated fundamental period are shown by a solid line and those of the unisolated structures as a dotted line, with results shown for three scalings of the El Centro NS 1940 earthquake motion. It is seen that seismic isolation markedly reduces the base shear in all cases.
1.3 COMPARISON OF CONVENTIONAL & SEISMIC ISOLATION APPROACHES
Many of the concepts of seismic isolation using hysteretic isolators are similar to the conventional failure-mode-control approach ('capacity design') which is used in New Zealand for providing earthquake resistance in reinforced concrete and steel structures.
In both the seismic isolation and failure-mode-control approaches, specially selected ductile components are designed to withstand several cycles well beyond yield under reversed loading, the yield levels being chosen so that the forces transmitted to other components of the structure are limited to their elastic or low ductility, range. The yielding lengthens the fundamental period of the structure, detuning the response away from the energetic period range of most of the earthquake ground motion. The hysteretic behaviour of the ductile components provides energy dissipation to damp the response motions. The ductile behaviour of the selected components ensures sufficient deformation capacity, over a number of cycles of motion, for the structure as a whole to ride out the earthquake attack.
However, seismic isolation differs fundamentally from conventional seismic design approaches in the method by which the period lengthening (detuning) and hysteretic energy dissipating mechanisms are provided, as well as in the
