An Introduction to Seismic Isolation,R. Ivan Skinnerl , William H. Robinson,And Graeme H. McVerry

R.I. Skinner W. H. Robinson G. H. McVerry

An Introduction to Seismic Isolation R. Ivan Skinner l , William H. Robinson 2 and Graeme H. McVerry) DSIR Physical Sciences, Welling/on. New Zealand

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Contents

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i\rkllowledgments

Jocar.mda Wiley LId, G.P.O. Box 859. Brisbane, Qucellsland 4001. Auslralia

xiii·

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"n'qllcnlly used Symbols and Abbreviations

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INTRODUCTION

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Seismic Isolation in COnlcxt

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Acxibility, Damping and Period Shift

4

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Cortlp,lrison of Conventional and Seismic Isolation Approaches

7

1,1

Components in an lsolalion System

8

",I>

Practical Application of the Seismic Isolation Concepc

I.

lOpics Covered in this Book

II

(:ENERAL FEATURES OF STRUCTURES WITH SEISMIC ISOLATION

Skinoer. R. IV1Il (Robert Ivan) An inlroduction 10 seismic i$oIation I R. Ivan Skinoer, William H. Robinson. and Grneme H. MeVelT)'.

p,

,,

Introduction

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Role of E.1nhquake Response Spectra and Vibrntional Modes in the PerfQnIl,lilce of Isot:llcalancc

Sc"nlIC rc'I"Hl"C.~ uf .'lrtlclurcs willi bilinear isolalioll and torsional IIllh.lll"lI.'C SllIllm.uy ~.4

233

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CONTENTS

5 A BASIS FOR THE DESIGN OF SEISMICALLY ISOLATED STRUCTURES 5.1

General Approach to the Design of Structures with Seismic Isolation

5.1.1

Introduction

6.4.2

Foothill Communities Law aoo Justice Centre. San Bernardino. California

308 308

240

6.4.3

Salt Lake City and County Building: retrofit

Design eanhquakes

242

6.4-4

USC University Hospital. Los Angeles

313

5.1.4

'Trade-off' between reducing base shear and increasing disploccmcnt Higher.mode effeels

246

6.4.5 6.4.6

Sierra Point Ovefficad Bridge. San Francisco Sexton Creek Bridge. Illinois

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249

5.1.6 1be locus of yield-points for a given NL and Ks. for a bilinear isolator

251

Design Procedures 5.2.1 Selcction of linear or non·linear isolation system

'" '" '"

5.2.2

Design cqUatKlns for linear isolu)on systems

5.2.3

Design

pr'(lttdu~

for bilinear isolation systems

'" 261

Isolation of capacitor banks Design of seismic isolation for a hypothetical eight-storey shear building Aseismic Design of Bridges with Superstructure Isolation

26\

5.4.1 Seismic features with superwucture isolation 5.4.2 Seismic responses modified by supersuucture isolation

270

5.4.3

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Discussion

Guidelines and Codes for the Design of Seismically Isolated Buildings and Bridges

6 APPLICATIONS OF SEISMIC ISOLATION

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271

276

281

Inlroduetion

281

6.2

Structures Isolated in New Zealand

283

6.2.1

Introduction

283

6.2.2 6.2.3

Road bridges Sooth Rangitikei Viaduct with stepping isolation

284 287

'" '" 297

6.2.4

William Clayton Building

6.2.5

Union 1·louse

6.2.6

Wellington Central Police Station

Structures Isolated in Japan

299

6.3.1

Inlroduction The C_I Building. Fuchu City. Tokyo

299

The High-Tech R&I) CCll1rc. Obayashi Corpomtion

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6.3.2 6.3.3 6.3.4

Com[lC wOlking relationships which haye deYeloped over the years between "III II'W.IU.:hcI-' :llId de~ign engineers in this field worldwide, We should like 10 III 11I1. Ilww l:ollc:lgllc, for their contribution to lhis book, roth indireclly. Ihrough • ,,11.,tkll,111011 (lYe" the ye"f'., :md directly. by supplying us wilh infomlalion and 1,llllhlJ'I"I'II';, p..,nMllly lor Chapter 6, I!llll-!l'

xii

PREFACE

We dedicate this book to the memory of the late OUo Glogau, Chief Structural Engineer of the Ministry of Works and Development, whose active support led to the early application of seismic isolation in New Zealand. We should also like to thank the support staff at DSIR Physical Sciences for the devoted effort which has made this book possible.

Acknowledgments

R. Ivan Skinner l William H. Robinson 2 Graeme H. McVcrryJ DStR Physical Sciences, Wellington, New Zealand, 30 June 1992

Ihe datc of submission of this manuscript, 30 June 1992, is auspICIous to us il is the last day of existence of the DSIR, the New Zealand Department III Scientific and Industrial Research (1926-1992) which will now be restructurcd, III~clhcr with other government-funded research organisations, into ten new Crown Hl'\carch Institutes. This means that the three authors will be going in different dill'etions from now on, with R1S finally moving to work in a private capacity, \VIm going to the Institute for Industrial Research and Development and GHMcV, With othcr mcmbers of the engineering seismology section, moving into the Institute lit (icological and Nuclear Sciences. It ~CClllS vcry appropriate at this time to thank the DSIR, as an organisation, for hllVlIlg l)l"Ovided an environment in which our scientific endeavours could flourish. WI' 1l1.\0 acknowledge the strong support of the various Directors of the DSIR I'hy\lcs and Engincering Laboratory (PEL), later known as DSIR Physical Sciences, 1ll1llll'ly Dr M C Probine, Mr M A Collins, Dr W H Robinson and Dr G P Beneridge. Wc Ill.~o wish to thank the members of staff who have contributed 10, and ~t1pponcd, our research and developments over the last 25 years. Many of our 11~~lll.:illtcs ,IPIx:ar as co-authors of publications cited, except for Jiri Babor whom WI' ~hnllid likc to thank for the many calculations which underlie results given III the I..:X1. Excellent suppol'l has been provided over the years by the Mechani\ nl [)evclopmcll1 Workshop of the Physics and Engineering Laboratory, with the tllllllllf:lctUfC of prototypes and testing equipment and the production of full-scale I~lllntlll' componcnts for installation in structures. Wt' ~hould also like to thank the many support staff who have contributed to tht' [1locrfomled in New Zealand :lIld Japan. Somc seismically isolated structure, have now (1992) lx:rl(ITlllCd ..ucce,sfully during real, but so far minor, earth(lu:lke Illotion..,

Ii, TOPICS COVERED IN

nus

BOOK

II

A number of organisations around the world have developed isolation systems different from those at DSLR Physical Sciences. Most have used means other than lhe hysteretic action of ductile metal components to obtain energy dissipation, force limitation and base flexibility, Various systems have used elastomeric bearings without lead plugs, damping being provided either by the use of high-loss n1bber or neoprene malerials in the constnlction of the bearings or by auxiliary viscous dampers. There have been a number of applications of frictional sliding systems, both with and without provision of elastic centring action. There has been substantial work recently on devices providing energy dissipation alone, without isolation, in systems not requiring period shifting. eithcr because of the substantial force reduction from large damping or because the devices were applied in inhercntly long-period struclures, such as suspension bridgcs or tall buildings, where isolation itself produces little benefit. Thcre has also been work on very expensive mechanical linkage systems for obtaining three-dimensional isolation. Seismic isolation has often been considered as a technique only for 'problem' structures or for equipment which requires a special seismic design approach. This may arise because of their function (sensitive or high-risk industrial or commercial facilities such as computer systems. semiconductor manufacturing plant, biotechnology facilities and nuclear power plants); their special imponance after an earthquake (e.g. hospitals. disaster control centres such as police stations, bridges providing vital communication links); poor ground conditions; proximity to a major fault; or other special problems (e.g. increasing the seismic resistance of existing structures). Seismic isolation docs indeed have particular advantages over other approaches in these special circumstanccs, usually being able to provide much bettcr protection under extreme eanhquake motions. Howevcr, its economic use is by no means limited to such cases. In New Zealand, the most common use of seismic isolation has been in ordinary two-lane road bridges of only moderate span. which are by no means special structures. although adminedly lhe implementation of seismic isolation required little modification of the standard design which already used vulcanised laminated-rubber bearings to aecommodate thermal and other movements.

1.6 TOPICS COVERED IN THIS BOOK [n this book we seck to present a parallel development of theoretical and practical aspects of seismic isolation. Thus in Chapter 2 the main concepts are defincd, in Chapter 3 details of various devices arc given, Chapter 4 explores the thcoretical concepts in more (letail, Chapter 5 presents guidelines for design and Chapter 6 gives some details of seismically isolated structures worldwide. In Chapter 2 the principal seismic response features conferred by isolation are outlined, with descriptions and brief explanations, which often anticipate the more extcnsivc studic.. :lIld discussions which appear in Chaplcr 4. Seismic response Spcctr:.1 are intflKluced :... the m:lximum seismic displacemenls and accc1crnlions of lincar I·ma.... dnmped vihr:.l\or-.. II i\ later shown lhat thecctrum "ppro:K:h :11-.0 a"sists in the seismic design of isolated structures, as described in Chapter ~, "ince it allows separate consideration of the character of dc"ign earthqtlltl..c" llud ill c,lrlhquake·re"istant structures. A technique

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