5 bridge engineering handbook seismic design

Published in five books: Fundamentals, Superstructure Design, Substructure Design, Seismic Design, and Construction and Maintenance, this new edition provides numerous worked-out examp

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Over 140 experts, 14 countries, and 89 chapters are represented in the second edition of

the Bridge Engineering Handbook This extensive collection highlights bridge engineering

specimens from around the world, contains detailed information on bridge engineering,

and thoroughly explains the concepts and practical applications surrounding the subject

Published in five books: Fundamentals, Superstructure Design, Substructure Design,

Seismic Design, and Construction and Maintenance, this new edition provides numerous

worked-out examples that give readers step-by-step design procedures, includes

contributions by leading experts from around the world in their respective areas of bridge

engineering, contains 26 completely new chapters, and updates most other chapters

It offers design concepts, specifications, and practice, as well as the various types of

bridges The text includes over 2,500 tables, charts, illustrations, and photos The book

covers new, innovative and traditional methods and practices; explores rehabilitation,

retrofit, and maintenance; and examines seismic design and building materials

The fourth book, Seismic Design contains 18 chapters, and covers seismic bridge analysis

and design

What’s New in the Second Edition:

• Includes seven new chapters: Seismic Random Response Analysis,

Displacement-Based Seismic Design of Bridges, Seismic Design of Thin-Walled Steel and CFT Piers,

Seismic Design of Cable-Supported Bridges, and three chapters covering Seismic

Design Practice in California, China, and Italy

• Combines Seismic Retrofit Practice and Seismic Retrofit Technology into one

chapter called Seismic Retrofit Technology

• Rewrites Earthquake Damage to Bridges and Seismic Design of

Concrete Bridges chapters

• Rewrites Seismic Design Philosophies and Performance-Based Design Criteria

chapter and retitles it as Seismic Bridge Design Specifications for the United States

• Revamps Seismic Isolation and Supplemental Energy Dissipation chapter and retitles

it as Seismic Isolation Design for Bridges

This text is an ideal reference for practicing bridge

engineers and consultants (design, construction,

maintenance), and can also be used as a reference for

students in bridge engineering courses

SECOND EDITION

seismic design

Bridge Engineering Handbook

Bridge Engineering Handbook, Second Edition: Fundamentals Bridge Engineering Handbook, Second Edition: Superstructure Design Bridge Engineering Handbook, Second Edition: Substructure Design Bridge Engineering Handbook, Second Edition: Seismic Design Bridge Engineering Handbook, Second Edition: Construction and Maintenance

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Contents

Foreword vii

Preface to the Second Edition ix

Preface to the First Edition xi

Editors xiii

Contributors xv

1 Geotechnical Earthquake Considerations 1

Charles Scawthorn and Steven L Kramer 2 Earthquake Damage to Bridges 53

Mark Yashinsky, Jack Moehle, and Marc Eberhard 3 Dynamic Analysis 99

Wei Zhang, Murugesu Vinayagamoorth, and Lian Duan 4 Seismic Random Response Analysis 133

Jiahao Lin, Yahui Zhang, and Yan Zhao 5 Nonlinear Analysis 163

Mahamad Akkari and Lian Duan 6 Displacement-Based Seismic Design of Bridges 201

M J Nigel Priestley, Mervyn J Kowalsky, and Gian Michele Calvi 7 Seismic Bridge Design Specifications for the United States. 237

Roy A Imbsen 8 Seismic Design of Concrete Bridges 279

Larry Wu 9 Seismic Design of Steel Bridges 301

Chia-Ming Uang, Michel Bruneau, and Keh-Chyuan Tsai 10 Seismic Design of Thin-Walled Steel and CFT Piers 337

Yoshiaki Goto 11 Seismic Design of Cable-Supported Bridges 381

Jian Ren Tao and Semyon Treyger

12 Seismic Isolation Design for Bridges 449

Roy A Imbsen and Larry Wu

13 Seismic Retrofit Technology 481

Kevin I Keady, Fadel Alameddine, and Thomas E Sardo

14 Soil–Foundation–Structure Interaction 513

Wen-Shou Tseng and Joseph Penzien

15 Seismic Design Practice in California 567

Mark Yashinsky and Lian Duan

16 Seismic Design Practice in China 599

Kehai Wang, Qian Li, Han Wei, and Yue Li

17 Seismic Design Practice in Italy 633

Gian Michele Calvi, Paolo Emilio Pinto, and Paolo Franchin

18 Seismic Design Practice in Japan 661

Shigeki Unjoh

Foreword

Throughout the history of civilization bridges have been the icons of cities, regions, and countries All bridges are useful for transportation, commerce, and war Bridges are necessary for civilization to exist, and many bridges are beautiful A few have become the symbols of the best, noblest, and most beautiful that mankind has achieved The secrets of the design and construction of the ancient bridges have been lost, but how could one not marvel at the magnificence, for example, of the Roman viaducts?

The second edition of the Bridge Engineering Handbook expands and updates the previous edition

by including the new developments of the first decade of the twenty-first century Modern bridge engineering has its roots in the nineteenth century, when wrought iron, steel, and reinforced concrete began to compete with timber, stone, and brick bridges By the beginning of World War II, the transportation infrastructure of Europe and North America was essentially complete, and it served to sustain civilization as we know it The iconic bridge symbols of modern cities were in place: Golden Gate Bridge of San Francisco, Brooklyn Bridge, London Bridge, Eads Bridge of St Louis, and the bridges of Paris, Lisbon, and the bridges on the Rhine and the Danube Budapest, my birthplace, had seven beauti-ful bridges across the Danube Bridge engineering had reached its golden age, and what more and better could be attained than that which was already achieved?

Then came World War II, and most bridges on the European continent were destroyed All seven bridges of Budapest were blown apart by January 1945 Bridge engineers after the war were suddenly forced to start to rebuild with scant resources and with open minds A renaissance of bridge engineering started in Europe, then spreading to America, Japan, China, and advancing to who knows where in the world, maybe Siberia, Africa? It just keeps going! The past 60 years of bridge engineering have brought us many new forms of bridge architecture (plate girder bridges, cable stayed bridges, segmen-tal prestressed concrete bridges, composite bridges), and longer spans Meanwhile enormous knowl-edge and experience have been amassed by the profession, and progress has benefitted greatly by the

availability of the digital computer The purpose of the Bridge Engineering Handbook is to bring much of

this knowledge and experience to the bridge engineering community of the world The contents pass the whole spectrum of the life cycle of the bridge, from conception to demolition

encom-The editors have convinced 146 experts from many parts of the world to contribute their knowledge and to share the secrets of their successful and unsuccessful experiences Despite all that is known, there are still failures: engineers are human, they make errors; nature is capricious, it brings unexpected sur-prises! But bridge engineers learn from failures, and even errors help to foster progress

The Bridge Engineering Handbook, second edition consists of five books:

Fundamentals, Superstructure Design, and Substructure Design present the many topics necessary

for planning and designing modern bridges of all types, made of many kinds of materials and systems,

and subject to the typical loads and environmental effects Seismic Design and Construction and

Maintenance recognize the importance that bridges in parts of the world where there is a chance of

earthquake occurrences must survive such an event, and that they need inspection, maintenance, and possible repair throughout their intended life span Seismic events require that a bridge sustain repeated dynamic load cycles without functional failure because it must be part of the postearthquake lifeline for

the affected area Construction and Maintenance touches on the many very important aspects of bridge

management that become more and more important as the world’s bridge inventory ages

The editors of the Bridge Engineering Handbook, Second Edition are to be highly commended for

undertaking this effort for the benefit of the world’s bridge engineers The enduring result will be a safer and more cost effective family of bridges and bridge systems I thank them for their effort, and I also thank the 146 contributors

Theodore V Galambos, PE

Emeritus professor of structural engineering

University of Minnesota

Preface to the Second Edition

In the approximately 13 years since the original edition of the Bridge Engineering Handbook was published

in 2000, we have received numerous letters, e-mails, and reviews from readers including educators and practitioners commenting on the handbook and suggesting how it could be improved We have also built up a large file of ideas based on our own experiences With the aid of all this information, we have completely revised and updated the handbook In writing this Preface to the Second Edition, we assume readers have read the original Preface Following its tradition, the second edition handbook stresses professional applications and practical solutions; describes the basic concepts and assumptions omitting the derivations of formulas and theories; emphasizes seismic design, rehabilitation, retrofit and main-tenance; covers traditional and new, innovative practices; provides over 2500 tables, charts, and illus-trations in ready-to-use format and an abundance of worked-out examples giving readers step-by-step design procedures The most significant changes in this second edition are as follows:

• The handbook of 89 chapters is published in five books: Fundamentals, Superstructure Design,

Substructure Design, Seismic Design, and Construction and Maintenance.

• Fundamentals, with 22 chapters, combines Section I, Fundamentals, and Section VI, Special

Topics, of the original edition and covers the basic concepts, theory and special topics of bridge engineering Seven new chapters are Finite Element Method, High-Speed Railway Bridges, Structural Performance Indicators for Bridges, Concrete Design, Steel Design, High Performance Steel, and Design and Damage Evaluation Methods for Reinforced Concrete Beams under Impact Loading Three chapters including Conceptual Design, Bridge Aesthetics: Achieving Structural Art in Bridge Design, and Application of Fiber Reinforced Polymers in Bridges, are completely rewritten Three special topic chapters, Weigh-In-Motion Measurement of Trucks on Bridges, Impact Effect of Moving Vehicles, and Active Control on Bridge Engineering, were deleted

• Superstructure Design, with 19 chapters, provides information on how to design all types of bridges

Two new chapters are Extradosed Bridges and Stress Ribbon Pedestrian Bridges The Prestressed Concrete Girder Bridges chapter is completely rewritten into two chapters: Precast–Pretensioned Concrete Girder Bridges and Cast-In-Place Posttensioned Prestressed Concrete Girder Bridges The Bridge Decks and Approach Slabs chapter is completely rewritten into two chapters: Concrete Decks and Approach Slabs Seven chapters, including Segmental Concrete Bridges, Composite Steel I-Girder Bridges, Composite Steel Box Girder Bridges, Arch Bridges, Cable-Stayed Bridges, Orthotropic Steel Decks, and Railings, are completely rewritten The chapter Reinforced Concrete Girder Bridges was deleted because it is rarely used in modern time

• Substructure Design has 11 chapters and addresses the various substructure components A new

chapter, Landslide Risk Assessment and Mitigation, is added The Geotechnical Consideration chapter is completely rewritten and retitled as Ground Investigation The Abutments and

Retaining Structures chapter is divided in two and updated as two chapters: Abutments and Earth Retaining Structures.

• Seismic Design, with 18 chapters, presents the latest in seismic bridge analysis and design New

chapters include Seismic Random Response Analysis, Displacement-Based Seismic Design of Bridges, Seismic Design of Thin-Walled Steel and CFT Piers, Seismic Design of Cable-Supported Bridges, and three chapters covering Seismic Design Practice in California, China, and Italy Two chapters of Earthquake Damage to Bridges and Seismic Design of Concrete Bridges have been rewritten Two chapters of Seismic Design Philosophies and Performance-Based Design Criteria, and Seismic Isolation and Supplemental Energy Dissipation, have also been completely rewritten and retitled as Seismic Bridge Design Specifications for the United States, and Seismic Isolation Design for Bridges, respectively Two chapters covering Seismic Retrofit Practice and Seismic Retrofit Technology are combined into one chapter called Seismic Retrofit Technology

• Construction and Maintenance has 19 chapters and focuses on the practical issues of bridge

structures Nine new chapters are Steel Bridge Fabrication, Cable-Supported Bridge Construction, Accelerated Bridge Construction, Bridge Management Using Pontis and Improved Concepts, Bridge Maintenance, Bridge Health Monitoring, Nondestructive Evaluation Methods for Bridge Elements, Life-Cycle Performance Analysis and Optimization, and Bridge Construction Methods The Strengthening and Rehabilitation chapter is completely rewritten as two chap-ters: Rehabilitation and Strengthening of Highway Bridge Superstructures, and Rehabilitation and Strengthening of Orthotropic Steel Bridge Decks The Maintenance Inspection and Rating chapter is completely rewritten as three chapters: Bridge Inspection, Steel Bridge Evaluation and Rating, and Concrete Bridge Evaluation and Rating

• The section on Worldwide Practice in the original edition has been deleted, including the chapters

on Design Practice in China, Europe, Japan, Russia, and the United States An international team

of bridge experts from 26 countries and areas in Africa, Asia, Europe, North America, and South

America, has joined forces to produce the Handbook of International Bridge Engineering, Second

Edition, the first comprehensive, and up-to-date resource book covering the state-of-the-practice

in bridge engineering around the world Each of the 26 country chapters presents that country’s historical sketch; design specifications; and various types of bridges including girder, truss, arch, cable-stayed, suspension, and so on, in various types of materials—stone, timber, concrete, steel, advanced composite, and of varying purposes—highway, railway, and pedestrian Ten bench-mark highway composite girder designs, the highest bridges, the top 100 longest bridges, and the top 20 longest bridge spans for various bridge types are presented More than 1650 beautiful bridge photos are provided to illustrate great achievements of engineering professions

The 146 bridge experts contributing to these books have written chapters to cover the latest bridge engineering practices, as well as research and development from North America, Europe, and Pacific Rim countries More than 80% of the contributors are practicing bridge engineers In general, the handbook is aimed toward the needs of practicing engineers, but materials may be re-organized to accommodate several bridge courses at the undergraduate and graduate levels

The authors acknowledge with thanks the comments, suggestions, and recommendations made during the development of the second edition of the handbook by Dr Erik Yding Andersen, COWI A/S, Denmark; Michael J Abrahams, Parsons Brinckerhoff, Inc.; Dr Xiaohua Cheng, New Jersey Department of Transportation; Joyce E Copelan, California Department of Transportation; Prof Dan

M Frangopol, Lehigh University; Dr John M Kulicki, Modjeski and Masters; Dr Amir M Malek, California Department of Transportation; Teddy S Theryo, Parsons Brinckerhoff, Inc.; Prof Shouji Toma, Horrai-Gakuen University, Japan; Dr Larry Wu, California Department of Transportation; Prof Eiki Yamaguchi, Kyushu Institute of Technology, Japan; and Dr Yi Edward Zhou, URS Corp

We thank all the contributors for their contributions and also acknowledge Joseph Clements, acquiring editor; Jennifer Ahringer, project coordinator; and Joette Lynch, project editor, at Taylor & Francis/CRC Press

Preface to the First Edition

The Bridge Engineering Handbook is a unique, comprehensive, and state-of-the-art reference work and

resource book covering the major areas of bridge engineering with the theme “bridge to the twenty-first century.” It has been written with practicing bridge and structural engineers in mind The ideal readers will be MS-level structural and bridge engineers with a need for a single reference source to keep abreast

of new developments and the state-of-the-practice, as well as to review standard practices

The areas of bridge engineering include planning, analysis and design, construction, maintenance, and rehabilitation To provide engineers a well-organized, user-friendly, and easy-to-follow resource, the handbook is divided into seven sections Section I, Fundamentals, presents conceptual design, aesthetics, planning, design philosophies, bridge loads, structural analysis, and modeling Section II, Superstructure Design, reviews how to design various bridges made of concrete, steel, steel-concrete composites, and timbers; horizontally curved, truss, arch, cable-stayed, suspension, floating, movable, and railroad bridges; and expansion joints, deck systems, and approach slabs Section III, Substructure Design, addresses the various substructure components: bearings, piers and columns, towers, abut-ments and retaining structures, geotechnical considerations, footings, and foundations Section IV, Seismic Design, provides earthquake geotechnical and damage considerations, seismic analysis and design, seismic isolation and energy dissipation, soil–structure–foundation interactions, and seismic retrofit technology and practice Section V, Construction and Maintenance, includes construction of steel and concrete bridges, substructures of major overwater bridges, construction inspections, main-tenance inspection and rating, strengthening, and rehabilitation Section VI, Special Topics, addresses in-depth treatments of some important topics and their recent developments in bridge engineering Section VII, Worldwide Practice, provides the global picture of bridge engineering history and practice from China, Europe, Japan, and Russia to the U.S

The handbook stresses professional applications and practical solutions Emphasis has been placed

on ready-to-use materials, and special attention is given to rehabilitation, retrofit, and maintenance The handbook contains many formulas and tables that give immediate answers to questions arising from practical works It describes the basic concepts and assumptions, omitting the derivations of formulas and theories, and covers both traditional and new, innovative practices An overview of the structure, organization, and contents of the book can be seen by examining the table of contents pre-sented at the beginning, while the individual table of contents preceding each chapter provides an in-depth view of a particular subject References at the end of each chapter can be consulted for more detailed studies

Many internationally known authors have written the chapters from different countries covering bridge engineering practices, research, and development in North America, Europe, and the Pacific Rim This handbook may provide a glimpse of a rapidly growing trend in global economy in recent years toward international outsourcing of practice and competition in all dimensions of engineering

In general, the handbook is aimed toward the needs of practicing engineers, but materials may be reorganized to accommodate undergraduate and graduate level bridge courses The book may also be used as a survey of the practice of bridge engineering around the world.

The authors acknowledge with thanks the comments, suggestions, and recommendations during the development of the handbook by Fritz Leonhardt, Professor Emeritus, Stuttgart University, Germany; Shouji Toma, Professor, Horrai-Gakuen University, Japan; Gerard F Fox, Consulting Engineer; Jackson

L Durkee, Consulting Engineer; Michael J Abrahams, Senior Vice President, Parsons, Brinckerhoff, Quade & Douglas, Inc.; Ben C Gerwick, Jr., Professor Emeritus, University of California at Berkeley; Gregory F Fenves, Professor, University of California at Berkeley; John M Kulicki, President and Chief Engineer, Modjeski and Masters; James Chai, Senior Materials and Research Engineer, California Department of Transportation; Jinrong Wang, Senior Bridge Engineer, URS Greiner; and David W Liu, Principal, Imbsen & Associates, Inc

We thank all the authors for their contributions and also acknowledge at CRC Press Nora Konopka, acquiring editor, and Carol Whitehead and Sylvia Wood, project editors

Editors

Dr Wai-Fah Chen is a research professor of civil engineering at the

University of Hawaii He was dean of the College of Engineering at the University of Hawaii from 1999 to 2007, and a George E Goodwin Distinguished Professor of Civil Engineering and head of the Department

of Structural Engineering at Purdue University from 1976 to 1999

He earned his BS in civil engineering from the National Cheng-Kung University, Taiwan, in 1959, MS in structural engineering from Lehigh University in 1963, and PhD in solid mechanics from Brown University

in 1966 He received the Distinguished Alumnus Award from the National Cheng-Kung University in 1988 and the Distinguished Engineering Alumnus Medal from Brown University in 1999

Dr Chen’s research interests cover several areas, including tutive  modeling of engineering materials, soil and concrete plasticity, structural connections, and structural stability He is the recipient of several national engineering awards, including the Raymond Reese Research Prize and the Shortridge Hardesty Award, both from the American Society of Civil Engineers, and the T R Higgins Lectureship Award in 1985 and the Lifetime Achievement Award, both from the American Institute of Steel Construction In 1995, he was elected to the U.S National Academy of Engineering In 1997, he was awarded Honorary Membership by the American Society of Civil Engineers, and in 1998, he was elected to the Academia Sinica (National Academy of Science) in Taiwan

consti-A widely respected author, Dr Chen has authored and coauthored more than 20 engineering books

and 500 technical papers His books include several classical works such as Limit Analysis and Soil

Plasticity (Elsevier, 1975), the two-volume Theory of Beam-Columns (McGraw-Hill, 1976 and 1977), Plasticity in Reinforced Concrete (McGraw-Hill, 1982), and the two-volume Constitutive Equations for Engineering Materials (Elsevier, 1994) He currently serves on the editorial boards of more than 15

on tall steel buildings, and for the World Bank on the Chinese University Development Projects, among many others Dr Chen has taught at Lehigh University, Purdue University, and the University of Hawaii

Dr Lian Duan is a senior bridge engineer and structural steel

commit-tee chair with the California Department of Transportation (Caltrans)

He  worked at the North China Power Design Institute from 1975 to

1978 and taught at Taiyuan University of Technology, China, from 1981

to 1985

He earned his diploma in civil engineering in 1975, MS in structural engineering in 1981 from Taiyuan University of Technology, China, and PhD in structural engineering from Purdue University in 1990

Dr Duan’s research interests cover areas including inelastic behavior

of reinforced concrete and steel structures, structural stability, seismic bridge analysis, and design With more than 70 authored and coauthored papers, chapters, and reports, his research focuses on the development of unified interaction equations for steel beam-columns, flexural stiffness

of reinforced concrete members, effective length factors of compression members, and design of bridge structures

Dr Duan has over 35 years experience in structural and bridge engineering He was lead engineer for

the development of Caltrans Guide Specifications for Seismic Design of Steel Bridges He is a registered

professional engineer in California He served as a member for several National Highway Cooperative Research Program panels and was a Transportation Research Board Steel Committee member from

2000 to 2006

He is the coeditor of the Handbook of International Bridge Engineering, (CRC Press, 2014) He received

the prestigious 2001 Arthur M Wellington Prize from the American Society of Civil Engineers for the

paper, “Section Properties for Latticed Members of San Francisco-Oakland Bay Bridge,” in the Journal

of Bridge Engineering, May 2000 He received the Professional Achievement Award from Professional

Engineers in California Government in 2007 and the Distinguished Engineering Achievement Award from the Engineers’ Council in 2010

Buffalo, New York

Gian Michele Calvi

IUSS Pavia and Eucentre

Qian Li

Research Institute of HighwayThe Ministry of TransportBeijing, China

Yue Li

Research Institute of HighwayThe Ministry of TransportBeijing, China

Jiahao Lin

Dalian University of TechnologyDalian, China

Jack Moehle

University of CaliforniaBerkeley, California

Joseph Penzien

International Civil Engineering Consultants, Inc

Berkeley, California

Paolo Emilio Pinto

Sapienza University of RomeRome, Italy

Charles Scawthorn

University of CaliforniaBerkeley, Californiaand

SPA RiskSan Francisco, California

Jian Ren Tao

HNTB CorporationSan Jose, California

Semyon Treyger

HNTB CorporationBellevue, Washington

Shigeki Unjoh

Public Works Research InstituteIbaraki, Japan

Research Institute of Highway

The Ministry of Transport

Beijing, China

Han Wei

Research Institute of Highway

The Ministry of Transport

Beijing, China

Larry Wu

California Department of TransportationSacramento, California

Mark Yashinsky

California Department of TransportationSacramento, California

Wei Zhang

Taiyuan University of TechnologyTaiyuan, China

Yahui Zhang

Dalian University of TechnologyDalian, China

Yan Zhao

Dalian University of TechnologyDalian, ChinaContents

1.1 Introduction

Earthquakes are naturally occurring broad-banded vibratory ground motions, due to a number of causes, including tectonic ground motions, volcanism, landslides, rockbursts, and man-made explosions, the most important of which are caused by the fracture and sliding of rock along tectonic faults within the earth’s crust For most earthquakes, shaking and/or ground failure are the dominant and most wide-spread agents of damage Shaking near the actual earthquake rupture lasts only during the time when the fault ruptures, a process that takes seconds or at most a few minutes The seismic waves generated

by the rupture propagate long after the movement on the fault has stopped; however, spanning the globe

in about 20 minutes Typically earthquake ground motions are powerful enough to cause damage only

in the near field (i.e., within a few tens of kilometers from the causative fault)—in a few instances, long-period motions have caused significant damage at great distances, to selected lightly damped structures, such as in the 1985 Mexico City earthquake, where numerous collapses of mid- and high-rise buildings were due to a Magnitude 8.1 earthquake occurring at a distance of approximately 400 kilometers from Mexico City

1

Geotechnical Earthquake

Considerations

Charles Scawthorn

University of California

Steven L Kramer

University of Washington

1.1 Introduction 1

1.2 Seismology 2

1.3 Measurement of Earthquakes 3

1.4 Strong Motion Attenuation and Duration 11

OpenSHA Attenuation Calculator 1.5 Probabilistic Seismic Hazard Analysis 16

Open Source PSHA Tools 1.6 Site Response 18

Evidence for Local Site Effects • Methods of Evaluation • Site Effects for Different Soil Conditions 1.7 Earthquake-Induced Settlement 29

Settlement of Dry Sands • Settlement of Saturated Sands 1.8 Ground Failure 31

Liquefaction • Liquefaction Susceptibility • Initiation of Liquefaction • Lateral Spreading • Global Instability • Retaining Structures 1.9 Soil Improvement 42

Densification Techniques • Drainage Techniques • Reinforcement Techniques • Grouting/Mixing Techniques Defining Terms 44

References 46

1.2 Seismology

Plate Tectonics: In a global sense, tectonic earthquakes result from motion between a number of large

plates comprising the earth′s crust or lithosphere (approximately 15 large plates in total, with many smaller “platelets”) These plates are driven by the convective motion of the material in the earth′s mantle, which in turn is driven by heat generated at the earth′s core Relative plate motion at the fault interface is constrained by friction and/or asperities (areas of interlocking due to protrusions in the fault surfaces) However, strain energy accumulates in the plates, eventually overcomes any resis-

tance, and causes slip between the two sides of the fault This sudden slip, termed elastic rebound by

Reid (1910) based on his studies of regional deformation following the 1906 San Francisco earthquake, releases large amounts of energy, which constitute the earthquake The location of initial radiation of

seismic waves (i.e., the first location of dynamic rupture) is termed the hypocenter, whereas the tion on the surface of the earth directly above the hypocenter is termed the epicenter Other terminol- ogy includes near-field (within one source dimension of the epicenter, where source dimension refers

projec-to the length of faulting), far-field (beyond near-field) and meizoseismal (the area of strong shaking and

damage) Energy is radiated over a broad spectrum of frequencies through the earth, in body waves and surface waves (Bolt, 1993) Body waves are of two types: P waves (transmitting energy via push–pull motion), and slower S waves ( transmitting energy via shear action at right angles to the direction

of motion) Surface waves are also of two types: horizontally oscillating Love waves (analogous to S body waves) and horizontally and vertically oscillating Rayleigh waves

Faults are typically classified according to their sense of motion (Figure 1.1) Basic terms include

transform or strike slip (relative fault motion occurs in the horizontal plane, parallel to the strike of the

fault), and dip-slip (motion at right angles to the strike, up- or down-slip), which includes normal

(dip-slip motion, two sides in tension, move away from each other), reverse (dip-(dip-slip, two sides in sion, move toward each other) and thrust (low-angle reverse) faulting

compres-Generally, earthquakes will be concentrated in the vicinity of faults, faults that are moving more idly than others will tend to have higher rates of seismicity, and larger faults are more likely than others

rap-to produce a large event Many faults are identified on regional geological maps, and useful information

on fault location and displacement history is available from local and national geological surveys in areas of high seismicity An important development has been the growing recognition of blind thrust faults, which emerged as a result of the several earthquakes in the 1980s, none of which were accompa-nied by surface faulting (Stein and Yeats, 1989)

Reverse fault Normal fault

Strike-slip fault

FIGURE 1.1 Fault types.

Geotechnical Earthquake Considerations

1.3 Measurement of Earthquakes

Magnitude: An individual earthquake is a unique release of strain energy—quantification of this energy

has formed the basis for measuring the earthquake event C.F Richter (1935) was the first to define earthquake magnitude, as

trace amplitude in microns recorded on a standard Wood–Anderson short-period torsion seismometer,

located at distances other than 100 km and < 600 km A number of other magnitudes have since been

gen-erated by an earthquake, and thus to the total energy release—an empirical relation by Richter is

=2.5 0.63+

Body wave magnitudes are more commonly used in stable continental regions, because of the deeper

Kanamori, 1979; also denoted as bold face M), which is now the most widely used measure of

numerically almost identical up to magnitude 7.5 Figure 1.2 indicates the relationship between moment magnitude and various magnitude scales

From the foregoing discussion, it can be seen that magnitude and energy are related to fault rupture length and slip Slemmons (1977), Bonilla et al (1984) and (Wells and Coppersmith, 1994) have deter-mined statistical relations between these parameters, for worldwide and regional data sets, aggregated and segregated by type of faulting (normal, reverse, strike-slip) (Wells and Coppersmith, 1994) provide regressions for rupture area, length, width, displacement, and surface rupture length and displacement

as a function of faulting type, the latter of which are given in Table 1.1, which indicates, for example, that

the average displacement is approximately 1m)

Intensity: In general, seismic intensity is a metric of the effect, or the strength, of an earthquake

haz-ard at a specific location Although the term can be generically applied to engineering measures such

as peak ground acceleration (PGA), it is often employed for qualitative measures of location-specific earthquake effects, based on observed human behavior and structural damage Numerous intensity scales developed in preinstrumental times—the most common in use today are the Modified Mercalli

(MMI) (Wood and Neumann, 1931), Table 1.2, Rossi-Forel (R-F), Medvedev–Sponheur–Karnik

(MSK-64, 1981), and Japan Meteorological Agency (JMA) scales

Time History: Strong motion seismometers have been available since the 1930s, and record actual

ground motions specific to their location (Figure 1.3) Typically, the ground motion records, termed

“seismograms” or “time histories,” have recorded acceleration (these records are termed grams”), for many years in analog form on photographic film and, recently, digitally

“accelero-Time histories theoretically contain complete information about the motion at the instrumental

loca-tion, recording three traces or orthogonal records (two horizontal and one vertical) Time histories

(i.e., the earthquake motion at the site) can differ dramatically in duration, frequency content, and amplitude The maximum amplitude of recorded acceleration is termed PGA (also termed the zero period acceleration [ZPA])—peak ground velocity (PGV) and peak ground displacement (PGD) are the maximum respective amplitudes of velocity and displacement Acceleration is normally recorded, with velocity and displacement being determined by integration; however, velocity and displacement

but is often also expressed in terms of the fraction or percent of the acceleration of gravity (980.66 gals,

approxi-mately 100 kine, whereas almost 2 g was recorded in the 1992 Cape Mendocino earthquake

Elastic Response Spectra: If a single degree of freedom mass is subjected to a time history of ground

(i.e., base) motion similar to that shown in Figure 1.3, the mass or elastic structural response can

be readily calculated as a function of time, generating a structural response time history, as shown

in Figure 1.4 for several oscillators with differing natural periods The response time history can be calculated direct integration in the time domain, or by solution of the Duhamel integral However,

2 2

FIGURE 1.2 Relationship between moment magnitude and various magnitude scales (From Campbell, K W.,

Earthquake Spectra, 1(4), 759–804, 1985 With permission; after Heaton, T.H., Tajima F., and Mori, A.W., Surveys

in Geophysics, 8(1), 25–83, 1986.)

this is time-consuming, and the elastic response is more typically calculated in the frequency domain (Clough and Penzien, 1975).

For design purposes, it is often sufficient to know only the maximum amplitude of the response time history If the natural period of the single degree of freedom oscillator (SDOF) is varied across a spectrum of engineering interest (typically, for natural periods from 0.03 to 3 or more sec., or frequencies

TABLE 1.2 Modified Mercalli Intensity Scale of 1931

earthquake Standing motor cars may rock slightly Vibration like passing truck Duration estimated.

disturbed; walls make creaking sound Sensation like heavy truck striking building Standing motorcars rock noticeably.

unstable objects overturned Disturbance of trees, poles, and other tall objects sometimes noticed Pendulum clocks may stop.

damaged chimneys Damage slight.

well-built ordinary structures; considerable in poorly built or badly designed structures Some chimneys broken Noticed by persons driving motor cars.

great in poorly built structures Panel walls thrown out of frame structures Fall of chimneys, factory stacks, columns, monuments, and walls Heavy furniture overturned Sand and mud ejected in small amounts Changes

in well water Persons driving motor cars disturbed.

in substantial buildings, with partial collapse Buildings shifted off foundations Ground cracked conspicuously Underground pipes broken.

ground badly cracked Rails bent Landslides considerable from river banks and steep slopes Shifted sand and mud Water splashed over banks.

pipelines completely out of service Earth slumps and land slips in soft ground Rails bent greatly.

FIGURE 1.3 Typical earthquake accelerograms (From Darragh et al Proceedings of Fifth U.S National

Conference Earthquake Engineering, vol III, 99–108, Earthquake Engineering Research Institute, Oakland CA,

1994 With permission.)

Geotechnical Earthquake Considerations

of 0.3 to 30 + Hz), then the plot of these maximum amplitudes is termed a response spectrum Figure 1.4

pseudo-spectral velocity, pseudo to emphasize that this spectrum is not exactly the same as the relative

Natural vibration period, Time (s)

Time (s) Time (s)

ξ = 2%

El Centro sooe component, May 18, 1940

0 0 0 0

–10 –10 –10

10 10 10

0 –0.4 g –0.4 g

FIGURE 1.4 Computation of deformation (or displacement) response spectrum (From Chopra, A.K., Dynamics

of Structures: A Primer Earthquake Engineering Research Institute, Oakland, CA, 1981.)

Response spectra form the basis for much modern earthquake engineering structural analysis and design They are readily calculated if the ground motion is known For design purposes however, response spectra must be estimated, either by methods of probabilistic seismic hazard analysis using period-specific attenuation relations, by adjusting an idealized response spectral shape such as shown

in Figure 1.7 combined with estimates of peak ground acceleration and peak ground velocity, or by following code procedures as shown in Figure 1.8 Response spectra may be plotted in any of several ways, as shown in Figure 1.5 with arithmetic axes, and in Figure 1.6, where the velocity response spec-trum is plotted on tripartite logarithmic axes, which equally enables reading of displacement and accel-eration response Response spectra are most normally presented for 5% of critical damping

Inelastic Response Spectra: Although the foregoing discussion has been for elastic response spectra,

most structures are not expected, or even designed, to remain elastic under strong ground motions

Rather, structures are expected to enter the inelastic region—the extent to which they behave

inelasti-cally can be defined by the ductility factor, μ:

u u

10 20 30 40 50

0

(a)

(b)

(c) Natural vibration period, Time (s)

0.5 1.0 1.5

FIGURE 1.5 Response spectra spectrum (From Chopra, A.K., Dynamics of Structures: A Primer Earthquake

Engineering Research Institute, Oakland, CA, 1981.)

Geotechnical Earthquake Considerations

at yield (i.e., that displacement that defines the extreme of elastic behavior) Inelastic response spectra can be calculated in the time domain by direct integration, analogous to elastic response spectra but

with the structural stiffness as a nonlinear function of displacement, k = k(u) If elastoplastic behavior

is assumed, then elastic response spectra can be readily modified to reflect inelastic behavior (Newmark and Hall, 1982), on the basis that (1) at low frequencies (0.3 Hz<) displacements are the same, (2) at high frequencies (>33 Hz), accelerations are equal, and (3) at intermediate frequencies, the absorbed energy

is preserved Actual construction of inelastic response spectra on this basis is shown in Figure 1.9, where

A  specific example, for ZPA = 0.16 g, damping = 5% of critical and μ = 3 is shown in Figure 1.10

Imperial valley earthquake

40 20

800 400 200 100 80 40 20 10 8 4 2 1 6 4 2 1 08

.08 04 02 01.008 006 004 002 001.0008.0006.0004

1

.06 04 02 01 008 006 004 002

10864

4 2

2 1

FIGURE 1.6 Response spectra, tri-partite plot (El Centro S 0º E component) (From Chopra, A.K., Dynamics of

Structures: A Primer Earthquake Engineering Research Institute, Oakland, CA, 1981.)

500 200 100 50

0.002 0.0050.010.02 0.05 0.1 0.2

0.5 12 5 10 20 50 100

Maximum ground motion

A

50 20 10 5 2 1 0.5 0.2 0.1 0.05 0.02 0.01

Acceleration, g

Displacemen

t, cm

FIGURE 1.7 Idealized elastic design spectrum, horizontal motion (ZPA = 0.5 g, 5% damping, one sigma

cumula-tive probability (From Newmark, N M and Hall, W J., Earthquake Spectra and Design Earthquake Engineering

Research Institute, Oakland, CA, 1982.)

FIGURE 1.8 Deterministic lower limit on MCER response spectrum (From ASCE 7-10, Minimum Design Loads

for Buildings and Other Structures, Sei/Asce 7-10 American Society of Civil Engineers, Reston, VA, 2010.)

Geotechnical Earthquake Considerations

1.4 Strong Motion Attenuation and Duration

The rate at which earthquake ground motion decreases with distance, termed attenuation, is a function

of the regional geology and inherent characteristics of the earthquake and its source Equations for

estimation of attenuation are termed ground-motion prediction equations (GMPEs), and until a decade

or two ago were thought to vary significantly among many different regions Douglas (2011) offers an

Elastic spectrum Inelastic spectrum

Elastic spectrum for acceleration and displacement

Aʺ

Inelastic displacement spectrum

Inelastic acceleration spectrum Dʹ

FIGURE 1.9 Inelastic response spectra for earthquakes (After Newmark, N M and Hall, W J., Earthquake

Spectra and Design Earthquake Engineering Research Institute, Oakland, CA, 1982.)

Frequency, Hz

0.1 0.1 0.2 0.5 1 2 5 10 20

50

Elastic response Displacement For µ = 3

Acceleration for µ = 3

Maximum ground motion

Acceleration, g

Displacement, in.

0.0005 0.001

0.01 0.02 0.05

0.1 0.2 0.5 1 2 5 10 20

0.00 2

0.00 5

0.05 0.1

1 2 5

10 20

0.02 0.0 1

0.5

FIGURE 1.10 Example inelastic response spectra (From Newmark, N M and Hall, W J., Earthquake Spectra

and Design Earthquake Engineering Research Institute, Oakland, CA, 1982.).

excellent review of more than 450 such relations developed during the period 1964–2010, many of which are now mainly of historical or only local interest With the availability of more data, the consensus

is emerging (see for example Stafford et al [2008]) that attenuation may be relatively similar within

a broader classification of regions, three of which are currently favored: (1) active tectonic regions, (2)  stable continental regions, and (3) subduction zones

Starting about 2005, a series of projects has developed a strong ground motion database (http://peer berkeley.edu/products/strong_ground_motion_db.html) that has formed the basis for devel-opment of empirical GMPEs for active tectonic regions Five GMPEs for active tectonic regions, termed the Next Generation Attenuation (NGA) equations for PGA, PGV and response spectral

ordinates, have been developed and are presented in the February 2008 Special Issue of Earthquake

Spectra (Stewart et al., 2008) Comparable NGA relations for stable continental regions and

subduc-tion zones are currently under development The NGA models mark a significant advancement in the state-of-the-art in empirical ground-motion modeling and include many effects not generally previously accounted for The remainder of this section discusses the five NGA equations in general, and presents selected details for one of the NGA equations—the reader is referred to the Special Issue for details

The five NGA models are (Abrahamson and Silva, 2008; Boore and Atkinson, 2008; Campbell and Bozorgnia, 2008; Chiou and Youngs, 2008; Idriss, 2008) referred to as AS08, BA08, CB08, CY08 and I08, respectively As noted above, they are meant for estimation of strong ground motion because of shal-low crustal earthquake in active tectonic regions The approach for all five relations, while informed by theoretical considerations, was largely empirical and consisted of regressing data from (depending on the equation) 942 to 2754 recordings from 58 to 135 different earthquakes Function forms of the NGA models are indicated in Table 1.3, where it can be seen that style-of-faulting, depth of rupture, nonlinear site amplification and hanging wall (HW) effects are considered by all or most of the models All models

the soil column) as the soil column stiffness metric, but vary as to the way in which the primary distance

the rupture plane, Figure 1.11 (Abrahamson et al., 2008) present further details on comparisons of the five models

TABLE 1.3 Functional Forms of NGA Models

Source: Data from Abrahamson, N et al., Earthquake Spectra, 24, 45–66, 2008.

Geotechnical Earthquake Considerations

CB08 is typical of the five models and the following discussion excerpted from Campbell and Bozorgnia (2008) CB08′s basic equation is

the magnitude parameter:

Rupture surface

FIGURE 1.11 Common GMPE distance measures.

the shallow site response term is given by the expression

and Y is the median estimate of the geometric mean horizontal component (referred to as GMRotI50)

as basin or sediment depth (km) The empirical coefficients ci and the theoretical coefficients c, n and ki

are listed in Table 1.4 When PSA < PGA and T ≤ 0.25 s, PSA should be set equal to PGA to be consistent with the definition of pseudo-absolute acceleration This condition occurs only at large distances and small magnitudes

1.4.1 OpenSHA Attenuation Calculator

OpenSHA (discussed further in Section 1.5) has implemented an open-source Java applet termed the “Attenuation Calculator,” available at http://www.opensha.org/glossary-attenuationRelation-USGS_COMBO_2004 The calculator implements the NGA and a number of other GMPEs and permits calculation or comparison of various GMPEs, for specified values Figure 1.12 shows the calculator plotting CB08 for moment magnitude 5.5 and 7.5 events, as a function of distance, with other parameters fixed as indicated The calculator is easy to use, and also provides the data in tabular form

1.5 Probabilistic Seismic Hazard Analysis

The Probabilistic Seismic Hazard Analysis (PSHA) approach entered general practice with Cornell′s (1968) seminal paper, and basically employs the theorem of total probability to formulate

where

Y is a measure of intensity, such as PGA, response spectral parameters PSV, and so on

p(Y|M,R) is the probability of Y given earthquake magnitude M and distance R (i.e., attenuation) p(M) is the probability of a given earthquake magnitude M,

P(R) is the probability of a given distance, R, and

F indicates seismic sources, whether discrete such as faults, or distributed

This process is illustrated in Figure 1.13, where various seismic sources (faults modeled as line sources and dipping planes, and various distributed or area sources, including a background source to account for miscellaneous seismicity) are identified, and their seismicity characterized on the basis of historic seismicity and/or geologic data The effects at a specific site are quantified on the basis of strong ground motion modeling, also termed “attenuation.” These elements collectively are the seismotectonic model—their integration results in the seismic hazard

There is an extensive literature on this subject (National Academy, 1988; Reiter, 1990) so that only key points will be discussed here Summation is indicated, as integration requires closed form solu-

tions, which are usually precluded by the empirical form of the attenuation relations The p(Y|M,R) term

represents the full probabilistic distribution of the attenuation relation—summation must occur over

the full distribution, because of the significant uncertainty in attenuation The p(M) term is referred to

as the magnitude-frequency relation, which was first characterized by Gutenberg and Richter (1954) as

FIGURE 1.12 OpenSHA Attenuation Calculator showing example plot of PGA versus distance for Mw 5.5 (lower

Geotechnical Earthquake Considerations

where N(m) = the number of earthquake events equal to or greater than magnitude m occurring on a

Richter relation can be normalized to

distribution to model large earthquake occurrence (Esteva, 1976) leads to the CDF of earthquake nitude per unit time:

can be evaluated by a least squares regression on historical seismicity data, although the probability of very large earthquakes tends to be overestimated Several attempts have been made to account for this (e.g., Cornell and Merz, 1973) Yegulalp and Kuo (1974) have used Gumbel′s Type III (largest value,

Source 4 (Fault)

-Site

Source 2 (Area)

Source 3 (Area)

Source 1 (Area)

PGA

PGA

Seismotectonic model

FIGURE 1.13 Elements of seismic hazard analysis—seismotectonic model is composed of seismic sources, whose

seismicity is characterized on the basis of historic seismicity and geologic data, and whose effects are quantified at the site via strong motion attenuation models.

has the advantage that w is the largest possible value of the variate (i.e., earthquake magnitude), thus permitting (when w, u, and k are estimated by regression on historical data) an estimate of the source’s largest possible magnitude It can be shown (Yegulalp and Kuo, 1974) that estimators of w, u, and k can be

seismicity data, which is highly complex and something of an art (Donovan and Bornstein, 1978), and the merging of the resulting statistics with estimates of maximum magnitude and seismicity made on the basis of geological evidence (i.e., as discussed in Section 1.3, maximum magnitude can be estimated from fault length, fault displacement data, time since last event, and other evidence, and seismicity can be esti-mated from fault slippage rates combined with time since last event; see Schwartz [1988] for an excellent discussion of these aspects) In a full PSHA, many of these aspects are treated fully or partially probabi-listically, including the attenuation, magnitude–frequency relation, upper and lower bound magnitudes for each source zone, geographical bounds of source zones, fault rupture length, and many other aspects The full treatment requires complex specialized computer codes, which incorporate uncertainty via use

of multiple alternative source zonations, attenuation relations, and other parameters (Electric Power Research Institute [EPRI], 1986; Bernreuter et al., 1989) often using a logic tree format A number of codes have been developed using the public domain FRISK (Fault Risk) code first developed by McGuire (1978)

1.5.1 Open Source PSHA Tools

There are a number of open source PSHA tools now available on the web—two of these stand out and are worthy of mention (Figures 1.14 and 1.15):

OpenSHA is a suite of open source software developed by the U.S Geological Survey, available at http://www.opensha.org/ “As an object-oriented framework, OpenSHA can accommodate arbitrarily complex (e.g., physics based) earthquake rupture forecasts (ERFs), ground-motion models, and engi-neering-response models, which narrows the gap between cutting-edge geophysics and state-of-the-art hazard and risk evaluations.” OpenSHA is now used to develop the U.S National Seismic Hazard Maps, and significantly lowers the barriers to entry for performing PSHA The site not only offers calculational tools, but also tutorials and in-depth reference materials

OpenQuake is the open source implementation of a calculational engine developed by the Global Earthquake Model (www.globalquakemodel.org) and is available at http://beta.globalquakemodel.org/openquake/about/ “OpenQuake refers to all tools, apps and IT-infrastructure being developed to sup-port stakeholders in assessing risk The core of OpenQuake is the web-based risk assessment platform, which will offer an integrated environment for modeling, viewing, exploring, and managing earth-quake risk But there is more: a variety of tools and even databases can be used independently, so users can use GEM′s resources and tools in a flexible way, adjusted to their needs.” The goal of OpenQuake, which incorporates significant elements from OpenSHA, is to go beyond PSHA to PSRA—Probabilistic Seismic Risk Analysis—that is, the probabilistic estimation of earthquake damage and loss

1.6 Site Response

When seismic waves reach a site, the ground motions they produce are affected by the geometry and properties of the geologic materials at that site At most bridge sites, rock will be covered by some thick-ness of soil that can markedly influence the nature of the motions transmitted to the bridge structure as well as the loading on the bridge foundation The influence of local site conditions on ground response has been observed in many past earthquakes, but specific provisions for site effects were not incorpo-rated in codes until 1976

The manner in which a site responds during an earthquake depends on the near-surface stiffness gradient and on how the incoming waves are reflected and refracted by surface topography, near- surface material boundaries, and deeper basin geometries The interaction between seismic waves and

Geotechnical Earthquake Considerations

near-surface materials can be complex, particularly when surface topography and/or subsurface tigraphy are complex Quantification of site response has generally been accomplished by empirical or analytical methods

stra-1.6.1 Evidence for Local Site Effects

Theoretical evidence for the existence of local site effects has been supplemented by instrumental and observational evidence in numerous earthquakes Nearly 200 years ago (MacMurdo, 1824), variations

in damage patterns were correlated to variations in subsurface conditions; such observations have been repeated on a regular basis since that time With the advent of modern seismographs and strong motion

FIGURE 1.14 OpenSHA home page (http://www.opensha.org/).

FIGURE 1.15 OpenQuake home page (http://beta.globalquakemodel.org/openquake/about/).

Geotechnical Earthquake Considerations

instruments, quantitative evidence for local site effects is now available In the Loma Prieta earthquake, for example, strong motion instruments at Yerba Buena Island and Treasure Island were at virtually identical distances and azimuths from the hypocenter However, the Yerba Buena Island instrument was located on a rock outcrop and the Treasure Island instrument on approximately 14 m of loose hydrauli-cally placed sandy fill underlain by nearly 17 m of soft San Francisco Bay Mud The measured motions, which differed significantly (Figure 1.16), illustrate the effects of local site effects

Site effects are taken here to include local ground response, topographic effects, and basin effects Local ground response refers to the amplification or de-amplification of ground motions associated with (nearly) vertically propagating body waves by near-surface geologic materials Topographic effects are those associated with amplification or de-amplification by two- and three-dimensional topographic irregularities (i.e., features that deviate from level-ground-surface conditions) Basin effects are associ-ated with reflection and refraction of waves at inclined subsurface boundaries and the generation of surface waves near basin edges

1.6.2.1 Empirical Methods

In the absence of site-specific information, local site effects can be estimated on the basis of empirical lation to measured site response from past earthquakes The database of strong ground motion records has increased tremendously over the past 30 years Division of the records within this database according to general site conditions has allowed the development of empirical correlations for different site conditions.Since that time, a number of approaches have been taken to the empirical estimation of site effects Most express site effects in terms of period-dependent amplification factors defined as the ratio of ground surface response spectral acceleration (usually 5% damping) to reference motion spectral accel-eration for a given site condition The available methods use different characteristics to describe site conditions, and some use different definitions of reference motions

0.2 0.4 0.6 0.8

200

–200

0

FIGURE 1.16 Ground surface motions at Yerba Buena Island and Treasure Island in the Loma Prieta earthquake

(From Kramer, S L 1996 Geotechnical Earthquake Engineering, Prentice-Hall, Upper Saddle River, NJ, Figure 8.9,

1996 With permission.)

Reference motions are generally taken as recorded rock motions (Boatwright et al., 1991), most monly “soft rock” such as that commonly associated with the western United States, having a typical shear wave velocity of 760 m/sec In other cases (e.g., Field and Jacob, 1995; Sokolov, 1997), median rock spectra from GMPEs are used for the reference motion values Site conditions have been character-ized in terms of surficial geology, geotechnical classification (Seed and Idriss, 1982; Dickenson, 1994; Rodriguez-Marek et al., 2001), and near-surface shear wave velocity (Borcherdt, 1994; Choi and Stewart, 2005; BSSC, 2009) The latter approach, in which site conditions are characterized by the average shear wave velocity of the upper 30 m of a profile, that is,

i s,i 1

and n = number of sublayers comprising upper 30 m of profile, has become common Choi and Stewart

(2005) proposed that a median amplification factor could be estimated as

rock

(1.23)

factors obtained by regression The second term in the brackets in Equation 1.23 accounts for nonlinear response, in which the degree of amplification depends on the amplitude of the input motion The value

factor for periods between 0.3 and 1.0 seconds are shown in Figure 1.17 amplification at 1.0 seconds is

2 1.5

1.5 0.5

0.5 1.5

0.5

1 2

1 2

1

1.5

0.5 0

PHAr (g)

0.5 1

Geotechnical Earthquake Considerations

and decreasing structural period

Recent GMPEs have included site terms with function forms similar to those of recent tion functions The manner in which the site term coefficients are determined vary from one GMPE to

Bridge codes have historically used relatively simple site classification schemes The AASHTO code

(SPT) resistances or average undrained strengths to be used when shear wave velocity data is not able The average SPT resistances and undrained strengths are computed as

u

i u,i 1

and the criteria indicated in Table 1.5 used to classify the soil profile into one of the six types

The reasonableness of empirically based methods for estimation of site response effects depends on the extent to which site conditions match the site conditions in the databases from which the empirical relationships were derived Whether in the form of amplification factors or GMPEs, empirical expres-sions of site effects are based on regression analyses, and therefore correspond best to sites with charac-teristics, such as shear wave velocity profiles, that are similar to the average characteristics of the profiles

in the databases upon which the expressions are based It is important to recognize the empirical nature

of such methods and the significant uncertainty inherent in the results they produce

1.6.2.2 Analytical Methods

When sufficient information to characterize the geometry and dynamic properties of subsurface soil layers is available, local site effects may be computed by site-specific ground response analyses Site-specific analyses are most useful for sites whose characteristics are significantly different than those

TABLE 1.5 Estimation of Site Class

1 Plasticity index PI > 20

2 Moisture content w ≥ 40%

1 Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive days, collapsible weakly cemented soils

2 Peats and/or highly organic clays (H > 10 ft of peat and/or highly organic clay where H = thickness of soil)

3 Very high plasticity clays (H > 25 ft with plasticity index PI > 75)

4 Very thick soft/medium stiff clays (H > 120 ft.)

Source: AASHTO Guide Specifications for LRFD Seismic Bridge Design, 1st Edition American Association of State

Highway and Transportation Officials, Washington, DC, 2011.