Earthquake-Resistant Design Concepts

An Introduction to the NEHRP Recommended Seismic 

Provisions for New Buildings and Other Structures

FEMA P-749 / December 2010

Prepared for the

Federal Emergency Management Agency of the U. S. Department of Homeland Security

By the National Institute of Building Sciences Building Seismic Safety Council

National Institute of Building Sciences 

Building Seismic Safety Council 

Washington, DC

2010
NOTICE: Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of 

the Federal Emergency Management Agency of the Department of Homeland Security. Additionally, neither FEMA nor any of its employees make any warranty, expressed or implied, nor assume any legal liability or responsibility for the accuracy, completeness, or usefulness 

of any information, product, or process included in this publication.

This report was prepared under Contract HSFEHQ-06-C-1139 between the Federal Emergency Management Agency and the National 

Institute of Building Sciences. For further information on Building Seismic Safety Council activities and products, see the Council’s website (www.bssconline.org) or write the Building Seismic Safety Council, National Institute of Building Sciences, 1090 Vermont, Avenue, 

N.W., Suite 700, Washington, D.C. 20005; phone 202-289-7800; fax 202-289-1092; e-mail bssc@nibs.org. Copies of this report may be 

obtained from the FEMA Publication Distribution Facility at 1-800-480-2520. The report can also be downloaded in pdf form from the 

FEMA website or the BSSC website.

About The Building Seismic Safety Council

The Building Seismic Safety Council (BSSC) was established in 1979 under the auspices of the National Institute of Building Sciences as a forum-based mechanism for dealing with the complex regulatory, technical, social, and economic issues involved 

in developing and promulgating building earthquake hazard mitigation regulatory provisions that are national in scope. By 

bringing together in the BSSC all of the needed expertise and all relevant public and private interests, it was believed that 

issues related to the seismic safety of the built environment could be resolved and jurisdictional problems overcome through 

authoritative guidance and assistance backed by a broad consensus.

The BSSC is an independent, voluntary membership body representing a wide variety of building community interests. Its fundamental purpose is to enhance public safety by providing a national forum that fosters improved seismic safety provisions for 

use by the building community in the planning, design, construction, regulation, and utilization of buildings.

2010 BSSC BOARD OF DIRECTION

Chair – William Holmes, Rutherford & Chekene

Vice Chair – James Cagley, Cagley and Associates (representing the Applied Technology Council)

Secretary – Curtis Campbell, J. E. Dunn Construction Company (representing the Associated General Contractors of America)

Ex Officio – David Bonneville, Degenkolb Engineers

Members – Bradford Douglas, American Wood Council; Cynthia J. Duncan, American Institute of Steel Construction; John E. 

Durrant, American Society of Civil Engineers; Melvyn Green, Melvyn Green and Associates (representing the Earthquake Engineering Research Institute); Jay W. Larson, PE, FASCE, American Iron and Steel Institute; Joseph Messersmith, Portland Cement 

Association; Ronald E. Piester, RA, New York State Department, Division of Code Enforcement and Administration (representing the International Code Council); Timothy Reinhold, Institute for Building and Home Safety; R. K. Stewart, FAIA, Hon. FRAIC, 

Hon. JAI, LEED AP, Perkins + Will (representing the National Institute of Building Sciences); Gregory Schindler, KPFF Consulting 

Engineers (representing the National Council of Structural Engineers Associations); Charles Spitz, NCARB, AIA, CSI, Architect/

Planner Code Consultant (representing the American Institute of Architects); S. Shyam Sunder, National Institute of Standards 

and Technology (representing the Interagency Committee for Seismic Safety in Construction); Robert D. Thomas, National 

Concrete Masonry Association

BSSC MEMBER ORGANIZATIONS: AFL-CIO Building and Construction Trades Department, American Concrete Institute, American Consulting Engineers Council, American Wood Council, American Institute of Architects, American Institute of Steel Construction, American Iron and Steel Institute, American Society of Civil Engineers, American Society of Heating, Refrigeration,

and Air-Conditioning Engineers, American Society of Mechanical Engineers, American Welding Society, APA - The Engineered 

Wood Association, Applied Technology Council, Associated General Contractors of America, Association of Engineering 

Geologists, Association of Major City Building Officials, Brick Industry Association, Building Owners and Managers Association 

International, California Seismic Safety Commission, Canadian National Committee on Earthquake Engineering, Concrete 

Masonry Association of California and Nevada, Concrete Reinforcing Steel Institute, Division of the California State Architect, 

Earthquake Engineering Research Institute, General Services Administration Seismic Program, Hawaii State Earthquake Advisory 

Board, Institute for Business and Home Safety, Interagency Committee on Seismic Safety in Construction, International Code 

Council, International Masonry Institute, Masonry Institute of America, Metal Building Manufacturers Association, Mid-America 

Earthquake Center, National Association of Home Builders, National Concrete Masonry Association, National Conference of 

States on Building Codes and Standards, National Council of Structural Engineers Associations, National Elevator Industry, Inc., 

National Fire Sprinkler Association, National Institute of Building Sciences, National Ready Mixed Concrete Association, Portland 

Cement Association, Precast/Prestressed Concrete Institute, Rack Manufacturers Institute, Steel Deck Institute, Inc., Structural 

Engineers Association of California, Structural Engineers Association of Central California, Structural Engineers Association of 

Colorado, Structural Engineers Association of Illinois, Structural Engineers Association of Kansas and Missouri, Structural Engineers 

Association of Kentucky, Structural Engineers Association of Northern California, Structural Engineers Association of Oregon, 

Structural Engineers Association of San Diego, Structural Engineers Association of Southern California, Structural Engineers Association of Texas, Structural Engineers Association of Utah, Structural Engineers Association of Washington, The Masonry Society, 

U.S. Army Corps of Engineers Engineer Research and Development Center–Construction Engineering Research Laboratory, 

Western States Clay Products Association, Wire Reinforcement Institute 

AFFILIATE MEMBERS: Baltimore Aircoil Company, Bay Area Structural, Inc., Building Technology, Incorporated, CH2M Hill, City 

of Hayward, California, Felten Engineering Group, Inc., Flexhead Industries, H&H Group, HLM Design, LaPay Consulting, Inc., 

Sea Hawk Enterprises, Inc., Square D Company, State Farm Insurance Company, Steel Joist Institute, Vibration Mountings and 

Controls, York–a Johnson Controls Company, Inc.
FOREWORD, PREFACE AND ACKNOWLEDGEMENTS iii

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Foreword

One goal of the Federal Emergency Management Agency (FEMA) and the National 

Earthquake Hazards Reduction Program (NEHRP) is to encourage design and 

building practices that address the earthquake hazard and minimize the resulting 

risk of damage and injury. Publication of this document, which is a companion 

guide to the 2009 edition of the NEHRP Recommended Seismic Provisions for 

New Buildings and Other Structures (FEMA P-750), reaffirms FEMA’s ongoing 

support of efforts to achieve this goal. First published in 1985, the 2009 edition 

of the Provisions marks the seventh in a series of updates to the document. 

The Provisions and the building codes and consensus standards based on its 

recommendations are technical documents used primarily by the professionals 

who design and construct buildings and other structures. Understanding the 

basis for the seismic regulations in the nation’s codes and standards is nevertheless 

important to others outside the technical community including elected officials, 

decision-makers in the insurance and financial communities, and individual 

building or business owners and other concerned citizens. This document is 

intended to provide these interested individuals with a readily understandable 

explanation of the intent and requirements of seismic design in general and the 

Provisions in particular.

FEMA wishes to express its deepest gratitude for the significant efforts of the over 

200 volunteer experts as well as the BSSC Board of Direction, member organizations, consultants, and staff who made possible the 2009 NEHRP Recommended 

Seismic Provisions and, by extension, this report. Americans unfortunate enough 

to experience the earthquakes that will inevitably occur in the future will owe 

much, perhaps even their lives, to the contributions and dedication of these individuals. Without the expertise and efforts of these men and women, this document and all it represents with respect to earthquake risk mitigation would not 

have been possible.

Federal Emergency Management Agency of the

U. S. Department of Homeland Security 

FOREWORD, PREFACE AND ACKNOWLEDGEMENTS v

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Preface and Acknowledgements

This document reflects very generous contributions of time and expertise on 

the part of the many individuals who participated in the development of the 

2009 NEHRP Recommended Seismic Provisions for New Building and Other 

Structures. The Building Seismic Safety Council (BSSC) is particularly grateful 

to Ronald O. Hamburger, SE, PE, SECB, Senior Principal, Simpson Gumpertz and 

Heger, San Francisco, California. Not only did Mr. Hamburger serve as chair 

of the Provisions Update Committee responsible for both the 2003 and 2009 

editions of the Provisions, but he also drafted this report. The BSSC also wishes 

to acknowledge the conscientious support and assistance of Michael Mahoney, 

Geophysicist, FEMA, Mitigation Directorate, Building Science Branch, and the 

project officer overseeing development of this introduction to the concepts 

presented in the Provisions.
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

vii

Table of Contents

Forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Preface and Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Chapter 1 | The U.S. Building Regulatory Process and Its Approach to Seismic Risk	 3

1.1 Model Building Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Consensus Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Code Adoption and Enforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 The NEHRP and the NEHRP Recommended Seismic Provisions . . . . . . . . . . . . . . . . . . . . 7

Chapter 2 | Seismic Risk and Performance

2.1 Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 Acceptable Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Geologic Earthquake Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.4 Seismic Hazard Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Chapter 3 | Design and Construction Features Important to Seismic Performance

3.1 Stable Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.2 Continuous Load Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.3 Adequate Stiffness and Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.4 Regularity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.5 Redundancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.6 Ductility and Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.7 Ruggedness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Chapter 4 | Buildings, Structures, and Nonstructural Components

4.1 Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.1.1 Structural Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.1.2 Nonstructural Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.2 Nonbuilding Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.3 Protective Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.4 Existing Buildings and Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Chapter 5 | Design Requirements 

5.1 Seismic Design Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.2 Site Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.3 Design Ground Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

5.4 Structural System Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

5.5 Configuration and Regularity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
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5.6 Required Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.6.1 Seismic Design Category A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.6.2 Seismic Design Category B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.6.3 Seismic Design Category C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

5.6.4 Seismic Design Categories D, E, and F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5.7 Stiffness and Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

5.8 Nonstructural Components and Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

5.9 Construction Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Chapter 6 | Future Directions 

6.1 Rationalization of Design Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

6.2 Manufactured Component Equivalence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

6.3 Nonbuilding Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

6.4 Nonstructural Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

6.5 Performance-based Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

6.5 Damage-tolerant Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Selected References and Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

FIGURES

Chapter 1

Figure 1 Examples of how NEHRP-funded basic research and application activities stimulate 

earthquake risk mitigation (image courtesy of NIST). 9

Chapter 2

Figure 2 Major tectonic plates (courtesy of U.S. Geological Survey). For a more complete 

explanation of plate tectonics, see http://pubs.usgs.gov/gip/dynamic/dynamic.pdf/ 15

Figure 3 Fault movements can break the ground surface, damaging buildings and other 

structures. This fence near Point Reyes was offset 8 feet ( 2.5 m) when the San 

Andreas Fault moved in the 1906 San Francisco (magnitude 7.8) earthquake (photo 

courtesy of USGS ). 16

Figure 4 Vertical fault offset in Nevada resulting from the 1954 Dixie Valley earthquake 

(photo by K. V. Steinbrugge). 16

Figure 5 Earthquakes can trigger landslides that damage roads, buildings, pipelines, and other 

infrastructure. Steeply sloping areas underlain by loose or soft rock are most 

susceptible to earthquake-induced landslides. The photo on the left shows 

Government Hill School in Anchorage, Alaska, destroyed as a result of a landslide 

induced by the 1964 earthquake; the south wing of the building collapsed into a 

graben at the head of the landslide (photo courtesy of USGS). The home shown on 

the right was destroyed when the hillside beneath it gave way following the 1994 

magnitude 6.7 Northridge earthquake (FEMA photo). 17
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

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Figure 6 Top photo shows liquefaction-induced settlement of apartment buildings in the 

1964 earthquake in Nigata, Japan (photo courtesy of the University of Washington). 

The bottom photo shows one of many manholes that floated to the surface as a result 

of soil liquefaction caused by the 2004 Chuetsu earthquake near Nigata, Japan 

(photo courtesy of Wikimedia Commons). 18

Figure 7 Lateral spreading damage to highway pavement near Yellowstone Park resulting from 

the 1959 Hegben Lake earthquake (photo courtesy of the USGS). 19

Figure 8 Locations of earthquakes in the continental United States between 1750 and 1996. 

Although not shown in this map, Alaska, Hawaii, Puerto Rico, and the Marianas also 

experienced earthquakes during this period. 21

Figure 9 Acceleration response spectrum for the 1940 Imperial Valley earthquake, 

north-south component. 24

Figure 10 Generalized shape of smoothed response spectrum. 24

Figure 11 Hazard curve for spectral acceleration at a site in Berkeley, California. 26

Figure 12 1940 Imperial Valley earthquake north-south and east-west spectra. 27

Figure 13 Collapse fragility curve for a hypothetical structure. 28

Figure 14 Distribution of short-period risk-targeted maximum considered earthquake response 

acceleration, SS, for the conterminous United States. 30

Figure 15 Distribution of 1-second period risk-targeted maximum considered earthquake response 

acceleration, S1, for the conterminous United States. 32

Chapter 3

Figure 16 Collapse of a tilt-up building in the 1971 San Fernando earthquake 

(photo by P. Yanev). 36

Figure 17 Houses in Watsonville, California, that fell off their foundations in the 1989 

Loma Prieta earthquake. 37

Figure 18 First story of an apartment building in San Francisco, California, leaning 

to the side after the 1989 Loma Prieta earthquake. 38

Figure 19 Imperial County Services Building, El Centro, California (courtesy of USGS). 

The photo on the right shows the crushed columns at the base of the building. 39

Figure 20 Failure of an unreinforced masonry wall in a building in Santa Cruz, California, 

in the 1989 Loma Prieta earthquake. 41
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Chapter 4

Figure 21 Wood studs and structural panel sheathing of typical wood frame bearing 

wall construction. 44

Figure 22 Typical low-rise concrete bearing wall building. 44

Figure 23 A three-story masonry bearing wall building. 45

Figure 24 A high-rise braced frame building in San Francisco, California. 46

Figure 25 A tall steel moment-frame structure under construction. 47

Figure 26 Structures commonly found in petroleum refineries and chemical plants. 49

Figure 27 Seismic design criteria for steel storage racks of the type used in large 

warehouses and big-box retail stores are included in the Provisions. 50

Figure 28 The San Bernardino County Justice Center in California was one of the first 

base-isolated buildings in the United States. 52

Chapter 5

Figure 29 Seismic Design Categories for low-rise buildings of ordinary occupancy on 

alluvial soils. 62

Figure 30 Generalized design response spectrum. 67

Figure 31 Map of long-period transition period, TL, for the continental United States. 68

Figure 32 Re-entrant corner irregularity. 73

Figure 33 Diaphragm discontinuity irregularity. 73

Figure 34 Out-of-plane offset irregularity. 73

Figure 35 Examples of buildings with a soft first story, a common type of stiffness irregularity. 74

Figure 36 Examples of in-plane discontinuity irregularities. 75

Figure 37 Required seismic design forces for Seismic Design Category A structures. 76

Figure 38 Continuity forces for Seismic Design Category A structures. 77

Figure 39 Distribution of lateral earthquake force in three-story structure. 79

Figure 40 Eccentric application of story forces. 82

Figure 41 Deflection of diaphragm under lateral loading. 83

Figure 42 Interstory drift. 86
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TABLES

Chapter 2

Table 1 Modified Mercalli Intensity Scale 22

Chapter 5

Table 2 Seismic Design Categories, Risk, and Seismic Design Criteria 58

Table 3 Occupancy 59

Table 4 Site Class and Soil Types 61

Table 5 Values of Site Class Coefficient Fa as a Function of Site Class 67

Table 6 Values of Site Class Coefficient Fv as a Function of Site Class 67
EXECUTIVE SUMMARY 1

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Executive Summary

Of the 500,000 or so detectable earthquakes that occur on Planet Earth each year, 

people will “feel” about 100,000 of them and about 100 will cause damage.1 

Although most earthquakes are moderate in size and destructive potential, a severe 

earthquake occasionally strikes a community that is not adequately prepared and 

thousands of lives and billions of dollars in economic investment are lost. 

For example, a great earthquake and the fires it initiated destroyed much of San 

Francisco in 1906 and a significant portion of Anchorage, Alaska, was destroyed 

by a large earthquake in 1964. Within the past 200 years, major destructive 

earthquakes also occurred in Charleston, South Carolina, and Memphis, Tennessee. Within the past 50 years, smaller but damaging earthquakes occurred several 

times in both Los Angeles and Seattle. Overall, more than 20 states have a moderate or high risk of experiencing damaging earthquakes. Earthquakes are truly a 

national problem.

One of the key ways a community protects itself from potential earthquake disasters is by adopting and enforcing a building code with appropriate seismic design 

and construction standards. The seismic requirements in U.S. model building 

codes and standards are updated through the volunteer efforts of design professionals and construction industry representatives under a process sponsored by 

the Federal Emergency Management Agency (FEMA) and administered by the 

Building Seismic Safety Council (BSSC). At regular intervals, the BSSC develops 

and FEMA publishes the NEHRP (National Earthquake Hazards Reduction 

Program) Recommended Seismic Provisions for New Buildings and Other 

Structures (referred to in this publication as the NEHRP Recommended Seismic 

Provisions or simply the Provisions). The Provisions serves as a resource used by 

the codes and standards development organizations as they formulate sound seismic-resistant design and construction requirements. The Provisions also provides 

design professionals, building officials, and educators with in-depth commentary 

on the intent and preferred application of the seismic regulations. 

The 2009 edition of the Provisions (FEMA P-750) and the building codes and 

consensus standards based on its recommendations are, of necessity, highly 

technical documents intended primarily for use by design professionals and 

others who have specialized technical training. Because of this technical focus, 

these documents are not clearly understandable to those not involved in design 

and construction. Nevertheless, understanding the basis for the seismic regula-

_____________________________________

1

For more information, see http://earthquake.usgs.gov/learning/facts.php.
2 EXECUTIVE SUMMARY

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

tions contained in the nation’s building codes and standards is important to many 

people outside this technical community including elected officials, decisionmakers in the insurance and financial communities, and individual business owners and other citizens. This introduction to the NEHRP Recommended Seismic 

Provisions is intended to provide these interested individuals with a readily 

understandable explanation of the intent of the earthquake-resistant design and 

requirements of the Provisions.

Chapter 1 explains the history and purpose of building regulation in the United 

States, including the process used to develop and adopt the nation’s building codes 

and the seismic requirements in these codes. Chapter 2 is an overview of the performance intent of the Provisions. Among the topics addressed are the national 

seismic hazard maps developed by the U.S. Geological Survey (USGS); the seismic 

design maps adopted by the Provisions as a basis for seismic design; and seismic 

risk, which is a function of both the probability that a community will experience 

intense earthquake ground shaking and the probability that building construction 

will suffer significant damage because of this ground motion. Chapter 3 identifies the design and construction features of buildings and other structures that are 

important to good seismic performance. Chapter 4 describes the various types of 

structures and nonstructural components addressed by the Provisions. Chapter 

5 is an overview of the design procedures contained in the Provisions. Chapter 

6 addresses how the practice of earthquake-resistant design is likely to evolve in 

the future. A glossary of key technical terms, lists of notations and acronyms used 

in this report, and a selected bibliography identifying references that may be of 

interest to some readers complete this report.
CHAPTER 1 3

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Building regulation in the United States began in the late 1800s when major cities began to adopt and enforce building codes in response to the large conflagrations that frequently occurred in these densely populated urban areas. The early 

building codes were intended primarily to reduce the 

fire risk but, over time, their scope was broadened to 

address many other issues deemed important to protecting public health, safety, and welfare – including natural 

hazards like earthquakes – and they became known as 

“model” building codes since they could be tailored to 

reflect community concerns before they were adopted. 

Building codes generally are intended to be applied by 

architects and engineers but also are used for various 

purposes by safety inspectors, environmental scientists, 

real estate developers, contractors and subcontractors, 

manufacturers of building products and materials, 

insurance companies, facility managers, tenants, and 

others.

Today, most U.S. communities formally adopt a building code and have a system 

in place for building regulation, but this was and still is not always the case. In 

fact, some rural areas in America still have not adopted a building code and, 

in these areas, it is legal to design and construct structures using any standards 

deemed appropriate by the designers and builders. Further, not all codes enforced at the local level will result in adequate earthquake-resistant design and 

construction. Some communities in the central and eastern United States, for 

example, are at significant risk of experiencing damaging earthquakes but do 

not acknowledge this risk and, consequently, have not adopted adequate seismic 

design and construction requirements into their local building codes. As a result, 

although the cost of incorporating appropriate seismic resistance into new construction is small, many buildings continue to be constructed without adequate 

protection, leaving people in these communities at considerable risk.

One of the primary ways a 

community protects itself and 

its individual citizens from 

potential earthquake disasters 

is by adopting and enforcing a 

building code with appropriate 

seismic design and construction 

requirements.

Chapter 1

THE U.S. BUILDING REGULATORY PROCESS 

AND ITS APPROACH TO SEISMIC RISK
4 CHAPTER 1

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

1.1	 Model Building Codes

By the mid-1900s, three organizations were publishing model building codes for 

adoption by U.S. communities and each represented a major geographic region:

• The Building Officials and Code Administrators International (BOCAI) 

published the National Building Code that served as the basis for most 

building regulation in the northeastern and central states. 

• The Southern Building Code Congress International (SBCCI) published 

the Standard Building Code that was commonly adopted throughout the 

southeastern part of the country. 

• The International Conference of Building Officials (ICBO) published 

the Uniform Building Code that was commonly adopted in the western 

United States. 

Each of the three building codes tended to develop particular strengths in certain 

areas. The National Building Code was heavily influenced by the major cities 

in the northeastern and central states and developed strong provisions on fire 

resistance and urban construction. The Standard Building Code was influenced 

primarily by building interests in the southeastern states where hurricanes were a 

common hazard and consequently developed advanced wind design requirements. 

The Uniform Building Code, reflecting the interest of the western states, became a 

leader in the development and adoption of earthquake design provisions.

The three organizations continued to issue their model codes for more than 50 

years, typically publishing revised and updated editions every three years. All 

three used a similar process that began with a public call for proposals for change. 

Anyone could respond to these public calls and submit a proposal to change the 

code. Typical code changes involved the prohibition of certain types of construction or the introduction of requirements governing the design of other types of 

construction. These proposals generally were made by proponents of building 

products and construction processes as well as by individual building officials and 

design professionals and associations representing these interests. Code change 

proposals often were made in response to observations that some types of construction performed poorly in certain events (e.g., fires or earthquakes) or situations (e.g., in areas of very heavy snow) and that changes in design or construction were needed to improve performance. Once proposals were submitted, the 

model code organization would hold a series of hearings to obtain public input 

on the validity of the proposals and the organization’s membership would then 

vote to either reject or accept the proposals, sometimes modifying the original 

proposal in the process.
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

In the late 1990s, the three original code development organizations (BOCAI, 

ICBO, and SBCCI) agreed to merge into a single organization called the International Code Council (ICC) and, in 2000, published a single series of model 

building codes called the International or I-Codes. The I-Codes are intended to be 

nationally and internationally applicable and include:

• The International Building Code (IBC) that addresses almost all types of 

buildings including residential, commercial, institutional, government, 

and industrial structures;

• The International Residential Code (IRC) that addresses one- and twofamily dwellings; and

• The International Existing Buildings Code (IEBC) that addresses existing 

buildings.

The ICC publishes new editions of these codes every three years (i.e., 2000, 2003, 

2006, 2009, 2012). Currently, all 50 states and most U.S. communities have 

adopted building codes based on the I-Codes. Depending on the state and its 

specific regulations, some adopt the codes verbatim while others modify or adopt 

only portions of the model codes. The development and widespread adoption of 

the I-Codes is beneficial in that it has created a more uniform regulatory environment in which design professionals and contractors need to become familiar with 

only a single set of requirements regardless of where they are practicing.

1.2 Consensus Standards

As the model building codes were evolving, various industries (e.g., concrete, 

masonry, steel, wood) established professional associations to develop technical 

criteria for the design and construction of structures using each industry’s specialized materials and systems. Eventually, the industry associations began issuing 

their guidance documents in the form of industry standards developed following 

rigorous consensus procedures promulgated by the American National Standards 

Institute (ANSI) and the model code organizations began adopting those documents into their codes by reference. The industry consensus standards typically 

are revised and updated every five years.

Among the more important consensus standards presently referenced by the 

building codes are the following:

• Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7, 

published by the Structural Engineering Institute of the American Society 

of Civil Engineers;
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

• Building Code Requirements for Reinforced Concrete, ACI 318, published by the American Concrete Institute;

• National Design Specification, NDS, published by the American Forest 

and Paper Association;

• Specification for Steel Buildings, AISC 360, published by the American 

Institute of Steel Construction;

• North American Specification for the Design of Cold Formed Steel 

Structural Members, AISI S100, published by the American Iron and Steel 

Institute; and

• Building Code Requirements and Specification for Masonry Structures,

TMS 402/ACI 530/ASCE 5 and TMS 602/ACI 530.1/ASCE 6, jointly published by the Masonry Society, the American Concrete Institute, and the 

American Society of Civil Engineers.

1.3 Code Adoption and Enforcement

Building codes are adopted by state and local governments to protect the health, 

safety, and welfare of the public by establishing minimum acceptable design and 

construction requirements intended to provide safe and reliable buildings and 

structures. These codes affect all aspects of building construction including structural stability, fire resistance, means of egress, ventilation, plumbing and electrical 

systems, and even energy efficiency. Once adopted by a state or local government, 

the building code becomes law and is typically enforced by a government official. 

This official generally is identified as the Chief Building Official but he or she may 

have another title such as Fire Marshall or Clerk. Collectively, the people empowered to enforce the requirements of a building code are identified in the codes as 

the Authority Having Jurisdiction (AHJ).

In communities that have adopted a building code, it is illegal to construct a 

structure unless the AHJ issues a building permit. Before issuing the permit, the 

AHJ typically will review the design documents to ensure that they were prepared 

by an appropriately qualified and licensed (generally by the state) professional 

and that they conform, in a general sense, to the technical requirements of the 

building code. Once the AHJ is satisfied that a design conforms to the applicable 

requirements and appropriate fees are paid, the AHJ issues a permit for construction, a document commonly referred to as the “building permit” that generally is 

posted at the construction site. 

During the construction period, the AHJ requires a series of inspections to ensure 

that the design is being properly executed by the builders. These inspections may 
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

be directly performed by the AHJ or the AHJ’s staff, by private individuals or firms 

with the appropriate qualifications, or by a combination of the two. When an 

inspection is performed, the conformance of the construction with the design and 

code requirements is documented by a series of reports and/or by the inspector’s 

signature on the building permit. If an inspector finds that the construction does 

not conform in some way to the code requirements, the builder must correct this 

situation before a sign-off is given. Upon completion of construction and submittal of documentation by the builder of evidence that the building has passed 

all required inspections, the AHJ will issue an “occupancy permit” that allows the 

structure to be open to the public. If a building is occupied without this permit, 

the AHJ can require that other law enforcement officials vacate the premises and 

lock it. Even after an occupancy permit has been issued for a structure, the AHJ 

can revoke the permit if there is reason to believe that the structure has become 

unsafe in some way. It is not uncommon for this to occur after a fire, earthquake, 

hurricane, or other event that causes extreme damage to buildings and structures. 

This also can occur if a building’s occupants allow its various systems to deteriorate to a point at which the structure is no longer safe for use.

1.4 	The NEHRP and the NEHRP

Recommended Seismic Provisions

Even though the largest earthquakes affecting the United States actually occurred 

in the central states, most 20th century U.S. earthquakes struck in the western 

states – primarily Alaska, California, and Washington – and most Americans think 

of earthquakes as a West Coast problem. As a result, the development of seismic 

requirements for building codes occurred primarily in the western states, notably California. These earthquake design requirements initially were developed 

by volunteers from the Structural Engineers Association of California (SEAOC) in 

cooperation with ICBO. These initial requirements appeared as a non-mandatory 

appendix in the 1927 Uniform Building Code. Over the years, as more earthquakes occurred in western states, SEAOC worked with its sister associations in 

other states, most notably Washington, to refine and improve these regulations 

and eventually they were moved into the body of the code and became mandatory.

During the early years of seismic code provision development, the principal basis 

for code changes was observation of the performance of actual buildings in earthquakes. When an earthquake occurred, engineers and building officials would 

survey the damage and, when certain types of construction performed poorly, 

they would develop code changes to address the observed problems. Noteworthy 

code changes resulted after earthquakes that occurred in Long Beach, California, 
8 CHAPTER 1

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

in 1933; Olympia, Washington, in 1949; Kern County, California, in 1952; and 

Prince William Sound, Alaska, in 1964. By 1970, many West Coast engineers and 

building officials believed they had developed a building code capable of providing buildings with superior earthquake performance. However, in 1971, a 

magnitude 6.6 earthquake occurred in Sylmar, California, a community located in 

the San Fernando Valley just north of Los Angeles, and resulted in extensive damage to many modern code-conforming structures and the collapse of some such 

structures. 

This earthquake made it clear that the building code needed significant improvement, but the involved engineers and building officials concluded they did not 

have the resources to address the problem adequately on a volunteer basis. Several 

things occurred in response to this need. First, SEAOC formed a nonprofit entity 

– the Applied Technology Council (ATC) – to seek the funding needed to assemble 

the best available talent to research problems with the building code requirements 

and to develop recommendations for improving those requirements. 

At about the same time, Congress passed the Earthquake Hazards Reduction Act 

of 1977 (Public Law 95-124) that established the National Earthquake Hazards 

Reduction Program (NEHRP). Under the NEHRP, four federal agencies – the Federal Emergency Management Agency (FEMA), the National Institute of Standards 

and Technology (NIST), the National Science Foundation (NSF), and the United 

States Geological Survey (USGS) – were authorized and provided with dedicated 

funding to develop effective ways to mitigate earthquake risks to the national 

economy and the life safety of building occupants. The NEHRP has been reauthorized periodically since that time, and it has funded and continues to support 

many important initiatives involving basic research and the application of this research in ways that will foster broad-scale mitigation of earthquake risks. Figure 1 

identifies some of the many activities conducted under the NEHRP and the agency 

primarily responsible for each.

Under the NEHRP, the USGS focuses on identification of the level of earthquake 

hazard throughout the United States. As part of this effort, USGS operates a 

network of strong-ground-motion instruments that record the effects of earthquakes at sites that range from a few to hundreds of kilometers from the event’s 

geographic origin. These data permit the USGS to identify the likely intensity of 

future earthquakes throughout the United States and to develop the national seismic hazard maps that serve as the basis for the design maps incorporated into the 

NEHRP Recommended Seismic Provisions and building codes and standards.
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NSF fosters technological leadership by sponsoring basic research and the 

development of new generations of scientists and engineers. Over the years it 

has sponsored a broad range of earthquake engineering research including field 

investigations of damage caused by earthquakes and laboratory and analytical research performed by individual students and their professors. NSF also originally 

funded national earthquake engineering research centers to conduct fundamental 

research focused on mitigating U.S. earthquake hazards. Much of this research is 

reflected in requirements contained in today’s building codes. One of the early 

research programs sponsored by NSF under the NEHRP was the development by 

ATC of a guidance document containing recommendations for next-generation 

seismic building code requirements. Published in 1978, this document, Tentative 

Provisions for the Development of Seismic Regulations for Buildings, acknowledged that the new concepts and procedures presented should be evaluated in 

comparative designs to test their workability, practicability, enforceability, and cost 

impact before they were considered for code adoption. Later, FEMA took over this 

initiative and funded the BSSC to conduct this comparative design effort, which 

resulted in consensus-approved modifications to the original document. These 

Figure 1 Examples of 

how NEHRP-funded 

basic research and 

application activities 

stimulate earthquake 

risk mitigation (image 

courtesy of NIST).
10 CHAPTER 1

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

amended seismic design procedures then served as the basis for the initial edition 

of the NEHRP Recommended Seismic Provisions and, hence, the procedures 

reflected in today’s building codes.

NIST conducts research and development work and also supports public/private 

partnerships that perform such work with the goal of improving the technological 

competitiveness of the United States. It has sponsored and participated in research 

that led to development of some of the seismic-resistant technologies reflected 

in the current model building codes. In the 2004 reauthorization of the NEHRP 

program, NIST was identified as the lead NEHRP agency with responsibility for 

coordinating the activities of the four NEHRP agencies and for establishing an 

advisory committee to assess scientific and engineering trends, program effectiveness, and program management.

FEMA provides public and individual assistance after an earthquake disaster occurs, speeding community recovery and minimizing the disaster’s impact on the 

nation as a whole. Under the NEHRP, it sponsors the development of tools and 

practices that will encourage the development of a more earthquake-resistant nation. It is in this role that, in the early 1980s, FEMA funded the development of a 

resource document that would serve as the basis for future seismic regulations in 

building codes. This effort resulted in the 1985 edition of the NEHRP Recommended Provisions. As noted above, the first edition of the Provisions reflected 

the results of a series of trial designs conducted to test the ATC report and was 

presented in a format that could be directly adopted by building codes. FEMA has 

continued to sponsor regular updating of the Provisions since 1985 (initially a 

new edition was published every three years but now every five years). 

The first building code adoption of the Provisions occurred in 1992 when both 

BOCAI and SBCCI adopted seismic provisions in their buildings codes based on 

the 1991 edition of the Provisions. In 1998, the Structural Engineering Institute of the American Society of Civil Engineers adopted the 1997 edition of the 

Provisions almost verbatim into the ASCE/SEI 7 standard. Two years later, the 

2000 International Building Code also adopted seismic provisions based on the 

1997 Provisions and, since that time, both the IBC and ASCE/SEI 7 standard have 

continued to base their seismic design criteria on the recommendations contained 

in the latest edition of the Provisions.

A key step in this process occurred in 1990 when Executive Order 12699, Seismic 

Safety of Federal and Federally Assisted or Regulated New Building Construction, was issued. This executive order required all new federally owned, leased, 

regulated, or funded structures to be constructed using building codes that 

contained suitable seismic standards and charged the Interagency Committee 

on Seismic Safety in Construction (ICSSC) to identify appropriate standards for 
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

seismic safety in building construction. The ICSSC identified the Provisions as 

the appropriate reference standard, thus providing a great incentive for the model 

code development organizations to adopt the Provisions as the basis for their 

seismic requirements so that new construction involving federal money could use 

their model building codes.

 
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

2.1 Basic Concepts

Every year, 100,000 or more earthquakes that can be felt by people occur worldwide. These earthquakes range from very small events felt by only a few individuals to great earthquakes that destroy entire cities.2

 The number of lives lost and 

the amount of economic losses that result from an earthquake depend on the size, 

depth and location of the earthquake, the intensity of the ground shaking and 

related effects on the building inventory, and the vulnerability of that building 

inventory to damage. 

Today’s design professionals know how to design and construct buildings and 

other structures that can resist even the most intense earthquake effects with little 

damage. However, designing structures in this manner can significantly increase 

their construction cost. Even in the areas of highest earthquake risk in the United 

States, severe earthquakes occur infrequently, often with 100 or more years between events capable of causing widespread damage. Given that many structures 

have, on average, useful lives of 50 years, constructing every structure so that it is 

invulnerable to earthquake damage would not be a wise use of society’s resources. 

Instead, the NEHRP Recommended Seismic Provisions, and the building codes 

and industry standards that reflect the Provisions requirements, are based on the 

concept of “acceptable risk,” which involves the establishment of minimum standards that attempt to balance the cost of seismic-resistant construction against the 

chance of incurring unacceptable losses in future earthquakes.

2.2 Acceptable Risk

Defining acceptable risk is difficult because the risk that is acceptable to one 

person may be unacceptable to many others. Often a person’s perception of an 

acceptable level of risk depends on whether or not the person believes he or she 

will be personally affected and how much the person is being asked to personally spend to avoid the risk. The NEHRP Recommended Seismic Provisions has 

adopted the following target risks as the minimum acceptable for buildings and 

structures constructed in the United States:

Chapter 2

SEISMIC RISK AND PERFORMANCE

____________________________

2

The U.S. Geological Survey website at http://earthquake.usgs.gov/learning/facts.php provides an abundance of information on earthquakes 

and their effects on the built environment. Some of the information presented there is somewhat technical but much of it has been prepared 

for the general public and parts of the website are designed for children and their teachers.
14 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

• A small chance (on the order of 10 percent) that any structure will experience partial or total collapse as a result of the most intense earthquake 

ground motion considered by the building codes. These very rare and 

intense earthquake effects are called risk-targeted maximum considered 

earthquake (MCER) ground motions and the probability of their occurrence varies across the nation. This collapse-prevention goal is intended 

as the primary means of ensuring life safety in that most casualties in past 

earthquakes occurred as a result of structural collapse. Although protection at this level does not guarantee no lives will be lost, it should prevent 

the loss of tens of thousands of lives in individual earthquake events such 

as those that occurred in Armenia, China, Haiti, Turkey, and other nations 

in recent years.

• Limit the chance of collapse (to perhaps 6 percent) as a result of MCER

ground shaking for structures intended primarily for public assembly in 

a single room or area (e.g., theaters or convention centers), for structures 

with a very large number of occupants (e.g., high-rise office buildings 

and sports arenas); and for structures housing a moderately large number 

of people with limited mobility (e.g., prisons) or who society generally 

regards as particularly vulnerable and important to protect (e.g., school 

children).

• For structures that contain a large quantity of toxic materials that could 

pose a substantial risk to the public (e.g., some chemical plants), provide 

a small probability that structural damage will result in release of those 

materials.

• Limit the chance of total or partial collapse as a result of MCER ground 

motions (to approximately 3 percent) for structures deemed essential to 

emergency response following a natural disaster (e.g., police and fire stations and hospitals) and further limit the chance that earthquake shaking 

will cause damage to these structures or to their architectural, mechanical, 

electrical, and plumbing systems sufficient to prevent their post-earthquake use.

• For all structures, minimize the risk that, in likely earthquakes, debris generated by damage to cladding, ceilings, or mechanical or electrical systems 

will fall on building occupants or pedestrians.

• To the extent practicable, avoid economic losses associated with damage 

to structural and nonstructural systems as a result of relatively frequent 

moderate earthquake events.

2.3 Geologic Earthquake Effects

The earth’s crust is composed of a series of large plates as shown in Figure 2. 

These “tectonic plates” are constantly being pushed and twisted by forces created 

by the earth’s rotation and the flow of magma within the earth’s molten core. At 

their boundaries, the plates are locked together by friction, which prevents them 

from moving relative to one another. Over a period of hundreds to thousands 
CHAPTER 2 15

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

of years, stress builds up along these boundaries. Occasionally, the stress along 

a plate boundary exceeds the frictional force that locks the plates together or 

the stress at an internal location in a plate exceeds the strength of the rock itself. 

When this occurs, the rock fractures or slips at locations of overstress, releasing 

stored energy and causing an earthquake.

Figure 2 Major tectonic plates (courtesy 

of USGS). For a more 

complete explanation 

of plate tectonics, see 

http://pubs.usgs.gov/

gip/dynamic/dynamic.

pdf/

Most earthquakes occur along plate boundaries or in other areas of the earth’s 

surface that have previously slipped in earthquakes. These locations are collectively known as “faults.” Faults often concentrate near the plate boundaries but 

can also occur within the interior of a plate. Future earthquakes are most likely to 

occur on existing faults; however, stress patterns in the earth shift over time and 

occasionally new faults are created.

The slippage within the rock during an earthquake can occur near the earth’s surface or many kilometers beneath it. When it extends to the surface, it can result 

in abrupt lateral (Figure 3) and vertical (Figure 4) offsets known as “ground fault 

ruptures.” The forces produced by these ground fault ruptures can be very large, 

and it is very difficult to design structures for locations where ruptures occur so 

that they will not be ripped apart. The best defense against damage from ground 
16 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

fault rupture is to avoid building over the known trace of an active fault. A fault 

is considered to be active if there is evidence that it has moved within the past 

10,000 years.

Figure 4 Vertical fault offset in Nevada resulting from the 1954 Dixie Valley earthquake (photo by K. V. 

Steinbrugge).

Figure 3 Fault movements 

can break the ground 

surface, damaging buildings and other structures. 

This fence near Point Reyes 

was offset 8 feet ( 2.5 m) 

when the San Andreas Fault 

moved in the 1906 San 

Francisco (magnitude 7.8) 

earthquake (photo courtesy 

of USGS).

Fault
CHAPTER 2 17

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

The energy released when an earthquake occurs radiates outward in the form 

of random vibrations in all directions from the area of slippage within the rock. 

These vibrations are felt on the surface as “ground shaking.” Ground shaking 

can last from a few seconds in small earthquakes to several minutes in the largest 

earthquakes, and it causes more than 90 percent of earthquake damage and losses.

In addition to causing direct damage to structures, ground shaking can cause 

several types of ground failure that also damage structures. Among the most 

common ground failures caused by earthquakes are landslides. An earthquakeinduced landslide typically will occur on a steeply sloping site with loose soils. 

Earthquake-induced landslides have destroyed buildings and even entire communities in past earthquakes (Figure 5). For example, landslides resulting from the 

1964 Prince William Sound earthquake that affected Anchorage, Alaska, destroyed 

an entire subdivision.

 

Figure 5 Earthquakes can trigger landslides that damage roads, buildings, pipelines, and other infrastructure. Steeply sloping areas underlain by loose or soft rock are most susceptible to earthquakeinduced landslides. The photo on the left shows Government Hill School in Anchorage, Alaska, 

destroyed as a result of a landslide induced by the 1964 earthquake; the south wing of the building 

collapsed into a graben at the head of the landslide (photo courtesy of USGS). The home shown 

on the right was destroyed when the hillside beneath it gave way following the 1994 magnitude 6.7 

Northridge earthquake (FEMA photo).
18 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Another significant earthquake-induced ground failure is soil liquefaction. Soil 

liquefaction can occur when loose saturated sands and silts are strongly shaken. 

The strong shaking compacts or densifies these materials and, in the process, 

forces out a portion of the water that saturates them. As the water is pushed out, 

it flows upward creating a condition in which the soils lose bearing pressure. 

When soil liquefaction occurs, structures supported on the liquefied soils can sink 

and settle dramatically and underground structures can float free (Figure 6). 

Figure 6 Top photo 

shows liquefactioninduced settlement of 

apartment buildings in 

the 1964 earthquake in 

Nigata, Japan (photo 

courtesy of the University of Washington). The 

bottom photo shows one 

of many manholes that 

floated to the surface as 

a result of soil liquefaction caused by the 2004 

Chuetsu earthquake 

near Nigata, Japan 

(photo courtesy of Wikimedia Commons).
CHAPTER 2 19

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

A ground instability related to liquefaction is lateral spreading. When liquefaction 

occurs on sites with even a mild slope, surface soils can move downhill, much like 

a fluid, and carry with them any structures they support. Figure 7 shows damage 

to pavement on a site that experienced liquefaction and lateral spreading. 

Figure 7 Lateral spreading damage to highway pavement near Yellowstone Park resulting from the 

1959 Hegben Lake earthquake (photo courtesy of the USGS).

Whether a building will experience any of these earthquake-induced ground failures depends on where it is located relative to potential causative faults, the local 

geology and types of soil present at the building site, and the site’s topography. 
20 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

2.4 Seismic Hazard Analysis

Earthquakes have occurred in nearly every region of the United States and have 

damaged buildings in all 50 states. Figure 8 is a map of the continental United 

States showing the locations of earthquakes that occurred between 1750 and 

1996. The locations of these earthquakes are shown using symbols that represent 

the maximum intensity of earthquake effects that were reported for each earthquake based on the Modified Mercalli Intensity (MMI) scale. 

The MMI scale ranges from MMI I (earthquakes that are not felt) to MMI XII 

(earthquakes causing total destruction). Table 1 presents one of several common 

versions of the MMI scale. It is a qualitative scale based on how people react to 

the earthquake ground shaking and other effects as well as the damage suffered by 

typical structures. A quick review of Figure 8 reveals that the largest concentration 

of earthquakes in the United States has occurred in California and western Nevada 

but that the most intense earthquakes (represented by red squares) actually occurred elsewhere. Other areas of frequent earthquake activity include the Pacific 

Northwest; the intermountain region of Utah, Idaho, Wyoming, and Montana; a 

band that extends along the Mississippi embayment and into the Saint Lawrence 

Seaway; and a belt that extends along the entire Appalachian Mountain range. Isolated earthquakes have occurred in most other regions of the nation as well.
CHAPTER 2 21

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 8 Locations of earthquakes in the continental United States between 1750 and 1996. Although 

not shown on this map, Alaska, Hawaii, Puerto Rico, and the Marianas also experienced earthquakes 

during this period.
22 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Table 1 Modified Mercalli Intensity Scale

Intensity Description

I Not felt except by a very few under especially favorable 

conditions

II Felt only by a few persons at rest, especially on upper floors 

of buildings

III Felt quite noticeably by persons indoors, especially on upper 

floors of buildings. Many people do not recognize it as an 

earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV Felt indoors by many, outdoors by few during the day. At 

night, some awakened. Dishes, windows, doors disturbed; 

walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rock noticeably.

V Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks 

may stop.

VI Felt by all, many frightened. Some heavy furniture moved; a 

few instances of fallen plaster. Damage slight.

VII Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; 

considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial 

collapse. Damage great in poorly built structures. Fall of 

chimneys, factory stacks, columns, monuments, walls. Heavy 

furniture overturned.

IX Damage considerable in specially designed structures; welldesigned frame structures thrown out of plumb. Damage 

great in substantial buildings, with partial collapse. Buildings 

shifted off foundations.

X Some well-built wooden structures destroyed; most masonry 

and frame structures destroyed with foundations. Rails bent.

XI Few, if any (masonry) structures remain standing. Bridges 

destroyed. Rails bent greatly.

XII Damage total. Lines of sight and level are distorted. Objects 

thrown into the air.
CHAPTER 2 23

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

In a general sense, the risk of high-intensity earthquake ground shaking in a 

region is related to the frequency and intensity of earthquakes that affected the 

region in the past. Most active earthquake faults will produce small earthquakes 

relatively frequently and large earthquakes less often. If a fault produces a small 

magnitude earthquake, the intensity of shaking will be slight. As earthquake magnitude increases, so does the maximum intensity of effects produced and the size 

of the geographic area that experiences these effects. The most intense effects of 

an earthquake generally occur at sites closest to the area on the fault that produced 

the earthquake. Sites with hard rock formations near the surface will experience 

less intense shaking than sites that have loose or soft soils or deep deposits of soils 

over the rock. 

Seismologists, geotechnical engineers, and earth scientists use seismic hazard 

analysis to quantify the probability that a site will experience high-intensity 

ground shaking. Although it is not possible to predict the specific size, location or 

time of future earthquakes with any certainty, these specialists use data on the past 

activity rate for a fault, as well as information on its length and how quickly stress 

builds up in the rock along the fault, to determine the probability that the fault 

will produce future earthquakes of various sizes. The mathematical relationships 

used to express these probabilities are called “recurrence relationships.” 

In addition, earth scientists and geotechnical engineers use data on the intensity 

of motion that was experienced at sites with known soil types and at known 

distances from past earthquakes to develop ground motion prediction equations 

(GMPEs) that indicate the likely intensity of motion at a site if an earthquake of 

a specific size and at a specific distance from a site occurs. Using the recurrence 

equations for individual faults and the GMPEs, these earth scientists and engineers 

develop mathematical relationships that indicate the probability of different intensities of ground shaking occurring at specific sites. 

Because the MMI scale referred to above is a qualitative measure of earthquake 

intensity, it is not directly useful for structural design. Instead structural engineers 

quantify earthquake intensity using a mathematical relationship known as an 

“acceleration response spectrum.” An acceleration response spectrum is a curve 

that shows the peak acceleration that different structures with different dynamic 

properties would experience if subjected to a specific earthquake motion. Figure 

9 is a representative acceleration response spectrum obtained from a recording of 

the 1940 earthquake in Imperial Valley, California. The horizontal axis is structural 

period, a measure of the dynamic properties of structures. If a structure is pushed 

to the side by a lateral force (e.g., a strong gust of wind) and then released, it will 

vibrate back and forth. A structure’s “period” is the amount of time, in seconds, 

that a structure will take to undergo one complete cycle of free vibration. Tall 

structures like high-rise buildings tend to have natural periods on the order of 
24 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

The vertical axis of the acceleration response spectrum is the acceleration that 

a structure will experience depending upon its period. “Spectral acceleration,” 

that is, the acceleration derived from a response spectrum, is designated Sa and is 

usually calculated in units of the acceleration due to gravity, g. This plot indicates 

that tall structures with long natural periods of about 3 seconds or more would 

experience relatively slight accelerations (0.1g or less) when subjected to this 

earthquake while short structures with periods of 1 second or less would experience accelerations of approximately 0.7g.

The acceleration response spectrum will be different at each site and for each 

earthquake. Factors that affect the shape and amplitude of the spectra include the 

earthquake’s magnitude, depth, distance from the site, and the types of soil present. Ground shaking at each site and in each earthquake is unique. To facilitate 

representation of these complex phenomena, building codes specify the use of 

smoothed spectra similar to that shown in Figure 10.

Figure 9 Acceleration response 

spectrum for the 

1940 Imperial Valley 

earthquake, northsouth component.

Figure 10 

Generalized shape 

of smoothed 

response spectrum.

several seconds while short buildings have natural periods of a few tenths of a 

second. Structural engineers use the symbol T to denote the period of a structure.
CHAPTER 2 25

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

The generalized smoothed response spectrum shown in Figure 10 can be derived 

for any site in the United States based on three parameters: 

• The spectral response acceleration at short periods, SS, 

• The spectral response acceleration at 1-second period, S1, and 

• The long-period transition period, TL. 

The U.S. Geologic Survey (USGS) has performed a national seismic hazard analysis 

to determine the values of the SS, and S1 parameters for different recurrence 

intervals (probabilities of exceedance) on a Cartesian grid with 2 kilometer spacing. The results of this analysis are reflected in the 2009 NEHRP Recommended 

Seismic Provisions.

For a given set of geographic coordinates expressed by longitude and latitude, it 

is possible using software developed by the USGS to display a plot of any of these 

three acceleration parameters as a function of annual frequency of exceedance. 

Figure 11 is one such plot showing the annual frequency of exceedance for the 

SS

 parameter for a site located in Berkeley, California. Such plots are known as 

hazard curves.

In Figure 11, the vertical axis is the “annual frequency of exceedance” for SS or, 

in other words, the number of times in any one year that, on average, the specific site can experience ground shaking greater than or equal to that shown on 

the horizontal axis. The average return period is the number of years likely to 

elapse between two events, each producing ground shaking of at least the level 

indicated on the horizontal axis. It is equal to the inverse of the annual frequency 

of exceedance (i.e., 1 divided by the annual frequency). For example, Figure 11 

indicates that for this particular site, ground shaking producing a short-period 

spectral acceleration with a value of 0.7g has an annual frequency of exceedance 

of 10-2 per year, which is equivalent to an average return period of 100 years. At 

an annual frequency of exceedance of 10-3 (return period of 1,000 years), one 

would expect shaking producing a short-period response acceleration (SS) value of 

2.1g and, at an annual frequency of exceedance of 10-4 (return period of 10,000 

years), of 3.7g.

All hazard curves take the approximate form of Figure 11. This form indicates that 

low-intensity earthquakes producing low accelerations occur relatively frequently 

while high-intensity earthquakes producing large highly damaging accelerations 

occur rarely.
26 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

2.5 Maximum Considered Earthquake 

Shaking

If subjected to sufficiently strong ground shaking, any structure will collapse. The 

goal of the Provisions is to provide assurance that the risk of structural collapse is 

acceptably small, while considering that there are costs associated with designing 

and constructing structures to be collapse-resistant. The Provisions defines a reference earthquake shaking level, termed risk-targeted maximum considered earthquake shaking (MCER), and seeks to provide a small probability (on the order of 

10 percent or less) that structures with ordinary occupancies will collapse when 

subjected to such shaking. The acceptable collapse risk for structures that house 

large numbers of persons or that fulfill important societal functions is set lower 

than this and additional objectives associated with maintaining post-earthquake 

occupancy and functionality are added. This section describes how MCER shaking 

is determined under the Provisions.

There is no single unique ground shaking acceleration that will cause the collapse of a particular structure. In part, this is because the ground motion actually 

experienced at each site, in each earthquake, and in each direction is unique and 

unpredictable. Consider the 1940 Imperial Valley earthquake for which a response 

spectrum was shown in Figure 9. Two ground motion recording instruments 

captured the ground shaking from that earthquake at that particular site in El 

Centro, California. These instruments were oriented so that one recorded ground 

accelerations in the north-south direction and the other in the east-west direction. 

Figure 12 plots the acceleration response spectra for the motions recorded by 

these two instruments. 

Figure 11 Hazard 

curve for spectral 

acceleration at a site 

in Berkeley, California.
CHAPTER 2 27

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

As Figure 12 shows, for this earthquake and for this site, structures with a natural 

period of about 1 second or less would be more strongly affected by shaking in 

the north-south direction than by shaking in the east-west direction. Although 

north-south shaking for this earthquake and this site was generally stronger than 

east-west shaking, the north-south direction of shaking was not necessarily the 

most severe direction. The most severe shaking may have occurred at some other 

orientation. The peaks and valleys in the spectra for the two directions of shaking 

also are somewhat different, meaning that each of these two directions of shaking 

would affect structures somewhat differently and shaking in other orientations 

also would affect structures differently. Both spectra are for a single earthquake 

and a single site. The same earthquake produced ground shaking with different spectra at other sites and other earthquakes at this site would likely produce 

spectra different from those shown. Thus, it is impossible to precisely predict 

either the acceleration spectra that will occur at a site in future earthquakes or 

what ground acceleration will cause a structure to collapse. In addition to the randomness of ground motions, other factors that make precise collapse predictions 

impossible include variability in the strength of construction materials and quality 

of workmanship as well as inaccuracies in the models that engineers use to assess 

structural response to earthquakes.

Figure 12 1940 

Imperial Valley 

earthquake northsouth and east-west 

spectra.

It is possible, however, to develop a probabilistic estimate of the acceleration that 

will cause collapse of a structure. These probabilistic estimates are called “fragility 

functions.” Figure 13 presents a plot of a typical structural fragility function. The 

horizontal axis is the spectral response acceleration of earthquake shaking while 

the vertical axis is the probability of collapse if the structure experiences ground 

motion having that spectral response acceleration. For the hypothetical structure 

represented by this fragility curve, there is approximately a 20 percent chance that 

the structure will collapse if subjected to ground shaking with a spectral response 

acceleration at its fundamental period of 0.5g, a 50 percent chance that it will 

collapse if subjected to ground shaking with a spectral response acceleration at 

its fundamental period of 0.8g, and a near certainty that the structure will col-
28 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

lapse if it experiences ground shaking producing spectral response accelerations 

at its fundamental period in excess of about 2.5g. Fragility functions such as this 

are assumed to conform to a mathematical relationship known as a lognormal 

distribution. Such functions are completely defined by two parameters: the probability of collapse at a particular value of the spectral acceleration and the dispersion, which is a measure of the uncertainty (width of the curve) associated with 

collapse vulnerability assessment.

The risk that a structure will collapse is a product of its fragility (Figure 13) and 

the seismic hazard at its site represented in the form of a hazard curve like that in 

Figure 11. By mathematically combining the two functions (fragility and hazard), 

it is possible to calculate the probability that a structure will collapse in any given 

year or number of years (for seismic mapping, the period of time considered is 

usually 50 years). For ordinary structures, the NEHRP Recommended Seismic 

Provisions seeks to provide a probability of 1 percent or less in 50 years that a 

structure will experience earthquake-induced collapse. 

In order to determine the return period for MCER shaking at a particular site that 

will achieve this target risk, the Provisions uses an iterative process in which: 

• A trial return period for MCER shaking is selected; 

• The spectral response acceleration at this return period for the site, assuming reference soil conditions, is determined from the site’s hazard curve; 

• A standard structural fragility function having a 10 percent probability of 

collapse at this spectral response acceleration and a dispersion of 0.6 is 

constructed; and

• The hazard and fragility curves are integrated to produce an annual collapse probability, which is then converted to a 50-year collapse probability. 

If the collapse probability determined in this manner is 1 percent in 50 years, the 

trial return period for MCER shaking was appropriate. If the computed collapse 

probability was more than 1 percent in 50 years, this indicates that the trial return 

Figure 13 Collapse 

fragility curve for 

a hypothetical 

structure.
CHAPTER 2 29

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

period was too short, and a new, longer return period must be selected and the 

process is repeated. If the computed collapse probability is less than 1 percent in 

50 years, the trial return period was too long, and a new, shorter return period is 

selected. This process is repeated until the return period results in a 1 percent in 50 

year collapse probability. This return period defines the MCER shaking probability. 

Once this is known, seismic hazard analysis can be used to define the spectral response acceleration values at various periods, typically 0.3 seconds and 1.0 second.

Using this process, the USGS determined hazard curves and MCER shaking parameters Ss

 and S1 for sites having reference soil conditions on a 2 kilometer by 2 kilometer grid across the United States. At most sites in the United States, the MCER

shaking defined by these parameters generally has a mean recurrence interval of 

approximately 2,500 years. At sites where earthquakes occur relatively frequently 

like some in California, the recurrence interval is somewhat shorter than this; at 

sites that rarely experience earthquakes, the recurrence interval may be somewhat 

longer. 

In some regions of the country with major active faults that are capable of producing large-magnitude earthquakes frequently (on the order of every few hundred 

to perhaps one thousand years), the above process would yield earthquake ground 

motions so severe that it is not practicable to design most structures to withstand 

them. These large motions are driven in part by statistics rather than by physical 

data and, in fact, the mapped shaking parameters at some sites in these regions are 

so large that they exceed the strongest ground shaking that has ever been recorded. 

In these regions, a deterministic estimate of the ground shaking that would occur 

at these sites if the nearby fault produced a maximum magnitude event is used in 

place of the risk-based shaking. By doing this, the NEHRP Recommended Seismic 

Provisions allows for a somewhat higher risk for structures that are constructed 

very close to these major active faults. The USGS has produced a series of composite maps that include either the risk-targeted ground shaking parameters or the 

ground shaking parameters for a maximum magnitude earthquake, whichever controls. These maps are referenced in the building codes and standards as the basis 

for determining design ground shaking for individual buildings. 

Figures 14 and 15 present maps for the continental United States that show the 

values of the Ss

 and S1 coefficients developed by the USGS and recommended 

by the Provisions for use in determining design ground motions in the United 

States. These Ss and S1 coefficients represent, respectively, the lesser of the spectral 

response acceleration for MCER shaking or the deterministic shaking, whichever 

controls as specified in more detail in the Provisions. See Chapter 5 for a more 

simple depiction of the seismic risk in the United States and its territories that 

reflects the maps approved for inclusion in the 2012 edition of the International 

Residential Code. 
30 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 14 Distribution of short-period risk-targeted maximum considered earthquake response 

acceleration, SS

, for the coterminous United States.
CHAPTER 2 31

EARTHQUAKE-RESISTANT DESIGN CONCEPTS
32 CHAPTER 2

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 15 Distribution of 1-second period risk-targeted maximum considered earthquake response 

acceleration, S1

, for the coterminous United States.
CHAPTER 2 33

EARTHQUAKE-RESISTANT DESIGN CONCEPTS
CHAPTER 3 35

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

To satisfy the performance goals of the NEHRP Recommended Seismic Provisions, a number of characteristics are important to the design of buildings and 

structures to ensure that they will behave adequately in strong earthquakes. These 

include:

• Stable foundations,

• Continuous load paths,

• Adequate stiffness and strength,

• Regularity,

• Redundancy,

• Ductility and toughness, and

• Ruggedness.

In areas of highest seismic risk (i.e., where the strongest earthquakes may occur) 

and for the most important structures in those areas, the Provisions requires 

inclusion of all of these features in the design and construction of buildings and 

other structures. In areas of lower seismic risk and for less important structures, 

the Provisions permits some of these features to be neglected if the structures 

are designed stronger. This chapter presents a brief overview of these important 

features of seismic design.

3.1	 Stable Foundations

In addition to being able to support a structure’s weight without excessive settlement, the foundation system must be able to resist earthquake-induced overturning forces and be capable of transferring large lateral forces between the structure 

and the ground. Foundation systems also must be capable of resisting both 

transient and permanent ground deformations without inducing excessively large 

displacements in the supported structures. On sites that are subject to liquefaction or lateral spreading, it is important to provide vertical bearing support for 

the foundations beneath the liquefiable layers of soil. This often will require deep 

foundations with drilled shafts or driven piles. Because surface soils can undergo 

large lateral displacements during strong ground shaking, it is important to tie together the individual foundation elements supporting a structure so that the strucChapter 3

DESIGN AND CONSTRUCTION FEATURES 

IMPORTANT TO SEISMIC PERFORMANCE
36 CHAPTER 3

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

ture is not torn apart by the differential ground displacements. A continuous mat 

is an effective foundation system to resist such displacements. When individual 

pier or spread footing foundations are used, it is important to provide reinforced 

concrete grade beams between the individual foundations so that the foundations 

move as an integral unit.

3.2	 Continuous Load Path

It is very important that all parts of a building or structure, including nonstructural components, be tied together to provide a continuous path that will transfer 

the inertial forces resulting from ground shaking from the point of origination to 

the ground. If all the components of a building or structure are not tied together 

in this manner, the individual pieces will move independently and can pull apart, 

allowing partial or total collapse to occur. Figure 16 shows the near total collapse 

of a concrete tilt-up structure near Los Angeles that occurred in the 1971 San 

Fernando earthquake. This collapse occurred because the exterior concrete walls, 

which supported the structure’s wood-framed roof, were not adequately connected to the roof; under the influence of strong shaking, the walls pulled away, allowing both the walls and roof to collapse. Figure 17 shows houses in Watsonville, 

California, that were not connected to their foundations and, as a result, fell off 

their foundations during strong shaking from the 1989 Loma Prieta earthquake. 

If structures are properly tied together to provide a continuous load path, damage 

like this can be avoided.

Figure 16 Collapse of a tilt-up building in the 1971 San Fernando earthquake (photo by P. Yanev).
CHAPTER 3 37

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 17 Houses in Watsonville, California, that fell off their foundations in the 1989 Loma Prieta 

earthquake.

3.3	 Adequate Stiffness and Strength

Strong earthquake shaking will induce both vertical and lateral forces in a structure. The lateral forces that tend to move structures horizontally have proven to 

be particularly damaging. If a structure has inadequate lateral stiffness or strength, 

these lateral forces can produce large horizontal displacements in the structure 

and potentially cause instability. Figure 18 shows large permanent deformation 

in the first story of a four-story apartment building in the Marina District of San 

Francisco, California, damaged by the 1989 Loma Prieta earthquake. Greater 

strength and stiffness at the first story would have prevented this damage.
38 CHAPTER 3

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

3.4	 Regularity

A structure is “regular” if the distribution of its mass, strength, and stiffness is 

such that it will sway in a uniform manner when subjected to ground shaking – 

that is, the lateral movement in each story and on each side of the structure will 

be about the same. Regular structures tend to dissipate the earthquake’s energy 

uniformly throughout the structure, resulting in relatively light but well-distributed damage. In an irregular structure, however, the damage can be concentrated in 

one or a few locations, resulting in extreme local damage and a loss of the structure’s ability to survive the shaking. Figure 19 shows the Imperial County Services 

Building in El Centro, California, an irregular structure that was damaged by the 

1979 Imperial Valley earthquake. 

Figure 18 First story of 

an apartment building 

in San Francisco, California, leaning to the side 

after the 1989 Loma 

Prieta earthquake.
CHAPTER 3 39

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 19 Imperial County 

Services Building, El 

Centro, California (photocourtesy of USGS). The 

photo on the right shows 

the crushed columns at 

the base of the building.

This six-story structure had several types of irregularity including end shear walls 

that stopped below the second floor and a first story with less strength and stiffness than the stories above. As a result, earthquake energy dissipation and damage were concentrated in the first story columns, a condition that could not be 

repaired and required demolition of the building after the earthquake. In a more 

severe earthquake, this type of damage could have caused the building to collapse.

3.5	 Redundancy

As noted above, for economic reasons the NEHRP Recommended Seismic Provisions reflects a design philosophy that anticipates damage to buildings and other 

structures as a result of strong earthquake shaking. If all of a structure’s strength 

and resistance is concentrated in only one or a few elements, the structure will 

not have any residual strength if these elements are seriously damaged and it 

could collapse. If a structure is redundant, a relatively large number of elements 

participate in providing a structure’s strength and, if only a few are badly damaged, the remaining elements may have adequate residual strength to prevent collapse. This can be thought of as not putting all of your earthquake-resistant eggs 

in one basket.
40 CHAPTER 3

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

3.6	 Ductility andToughness

Ductility and toughness are structural properties that relate to the ability of a 

structural element to sustain damage when overloaded while continuing to carry 

load without failure. These are extremely important properties for structures 

designed to sustain damage without collapse. 

Most structural elements are designed to provide sufficient strength to support 

anticipated loads without failure and enough stiffness so that they will not deflect 

excessively under these loads. If such an element is subjected to a load substantially larger than it was designed to carry, it may fail in an abrupt manner, losing 

load-carrying capacity and allowing the structure to collapse. Masonry and concrete, for example, will crush when overloaded in compression and will crack and 

pull apart when placed in tension or shear. Wood will crush when overloaded in 

compression, will split when overloaded in shear, and will break when overloaded 

in tension. Steel will buckle if overloaded in compression and will twist when 

loaded in bending if not properly braced but will yield when overloaded in tension. When steel yields, it stretches a great deal while continuing to carry load, 

and this property allows it to be used in structures of all types to provide them 

with ductility and toughness. Figure 20 shows the failure of an unreinforced 

masonry wall in a building in Santa Cruz, California, in the 1989 Loma Prieta 

earthquake. Such buildings, having no steel reinforcement in the masonry, are not 

very ductile or tough and frequently collapse in earthquakes.

In masonry and concrete structures, steel is used in the form of reinforcing bars 

that are placed integrally with the masonry and concrete. When reinforced masonry and concrete elements are loaded in bending or shear, the steel reinforcing 

bars will yield in tension and continue to carry load, thus protecting the masonry 

and concrete from failure. In wood structures, steel fasteners (typically nails, 

bolts, and straps) bind the pieces of wood together. When the wood is loaded in 

shear or bending, these steel connectors yield and protect the wood from splitting 

and crushing. In steel structures, ductility is achieved by proportioning the structural members with sufficient thickness to prevent local buckling, by bracing the 

members to prevent them from twisting, and by joining the members together 

using connections that are stronger than the members themselves so the structure 

does not pull apart. In structures of all types, ductility and toughness are achieved 

by proportioning the structure so that some members can yield to protect the rest 

of the structure from damage.

The measures used to achieve ductility and toughness in structural elements are 

unique to each construction material and to each type of structural system. The 

building codes specify the measures to use to provide ductility and toughness 
CHAPTER 3 41

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 20 Failure of an unreinforced masonry wall in 

a building in Santa Cruz, California, in the 1989 Loma 

Prieta earthquake.

3.7	 Ruggedness

Ruggedness is a property of some mechanical and electrical equipment and other 

nonstructural building components that permits these items to remain functional 

after experiencing strong shaking. A rugged piece of equipment will have adequate structural strength and will be composed of components that do not lose 

their ability to properly perform their intended functions when shaken. For example, some types of electrical control equipment employ mercury type switches 

to activate certain operations. In strong shaking, the mercury, being liquid, can 

flow and trigger electrical shutdowns. Such equipment would not be considered 

rugged as opposed to equipment with mechanical or solid state switches. Similarly, some computer equipment is intentionally constructed with slide-out boards 

and cards. If these boards or cards can be dislodged by shaking and fall out of 

the equipment, the equipment would not be considered rugged unless the cards 

were provided with locking mechanisms that would prevent them from becoming 

dislodged during shaking. Ruggedness of equipment usually can be demonstrated 

only by subjecting the equipment to shaking, either in a real earthquake or using 

special laboratory devices (called shake tables) that simulate the shaking induced 

by earthquakes.

through reference to the various materials industry design standards (e.g., ACI 

318, AISC 341, TMS 402), each of which contain detailed requirements for obtaining structural ductility. 
CHAPTER 4 43

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Chapter 4

BUILDINGS, STRUCTURES, AND 

NONSTRUCTURAL COMPONENTS

The NEHRP Recommended Seismic Provisions includes seismic design and 

construction requirements for a wide range of buildings and structures and their 

nonstructural components. This chapter presents an overview of those different 

types of buildings, structures, and nonstructural components.

4.1	 Buildings

Generally, a building can be defined as an enclosed structure intended for human 

occupancy. However, a building includes the structure itself and nonstructural 

components (e.g., cladding, roofing, interior walls and ceilings, HVAC systems, 

electrical systems) permanently attached to and supported by the structure. The 

scope of the Provisions provides recommended seismic design criteria for all 

buildings except detached one- and two-family dwellings located in zones of 

relatively low seismic activity and agricultural structures (e.g., barns and storage 

sheds) that are only intended to have incidental human occupancy. The Provisions also specifies seismic design criteria for nonstructural components in buildings that can be subjected to intense levels of ground shaking.

4.1.1 Structural Systems

Over many years, engineers have observed that some structural systems perform 

better in earthquakes than others. Based on these observations, the Provisions

design criteria for building structures are based on the structural system used. 

Structural systems are categorized based on the material of construction (e.g., 

concrete, masonry, steel, or wood), by the way in which lateral forces induced by 

earthquake shaking are resisted by the structure (e.g., by walls or frames), and by 

the relative quality of seismic-resistant design and detailing provided.

The Provisions recognizes six broad categories of structural system:

• Bearing wall systems,

• Building frame systems,

• Moment-resisting frame systems,
44 CHAPTER 4

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

• Dual systems,

• Cantilever column systems, and

• Systems not specifically designed for seismic resistance.

In bearing wall systems, structural walls located throughout the structure provide 

the primary vertical support for the building’s weight and that of its contents as 

well as the building’s lateral resistance. Bearing wall buildings are commonly 

used for residential construction, warehouses, and low-rise commercial buildings 

of concrete, masonry, and wood construction. Figures 21, 22, and 23 show typical bearing wall buildings. 

Figure 21 Wood studs 

and structural panel 

sheathing of typical 

wood frame bearing wall 

construction.

Figure 22 Typical low-rise 

concrete bearing wall 

building.
CHAPTER 4 45

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Building frames are a common structural system for buildings constructed of 

structural steel and concrete. In building frame structures, the building’s weight 

is typically carried by vertical elements called columns and horizontal elements 

called beams. Lateral resistance is provided either by diagonal steel members 

(termed braces) that extend between the beams and columns to provide horizontal rigidity or by concrete, masonry, or timber shear walls that provide lateral 

resistance but do not carry the structure’s weight. In some building frame 

structures, the diagonal braces or walls form an inherent and evident part of the 

building design as is the case for the high-rise building in San Francisco shown in 

Figure 24. In most buildings, the braces or walls may be hidden behind exterior 

cladding or interior partitions.

Figure 23 A three-story 

masonry bearing wall 

building.
46 CHAPTER 4

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Moment-resisting frame systems are commonly used for both structural steel and 

reinforced concrete construction. In this form of construction, the horizontal 

beams and vertical columns provide both support for the structure’s weight and 

the strength and stiffness needed to resist lateral forces. Stiffness and strength are 

achieved through the use of rigid connections between the beams and columns 

that prevent these elements from rotating relative to one other. Although somewhat more expensive to construct than bearing wall and braced frame structural systems, moment-resisting frame systems are popular because they do not 

require braced frames or structural walls, therefore permitting large open spaces 

and facades with many unobstructed window openings. Figure 25 shows a steel 

moment-resisting frame building under construction.

Dual systems, an economical alternative to moment-resisting frames, are commonly used for tall buildings. Dual system structures feature a combination of 

moment-resisting frames and concrete, masonry, or steel walls or steel braced 

Figure 24 A high-rise braced frame 

building in San Francisco, California.
CHAPTER 4 47

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

frames. The moment-resisting frames provide vertical support for the structure’s 

weight and a portion of the structure’s lateral resistance while most of the lateral 

resistance is provided either by concrete, masonry, or steel walls or by steel braced 

frames. Some dual systems are also called frame-shear wall interactive systems.

Cantilever column systems are sometimes used for single-story structures or in 

the top story of multistory structures. In these structures, the columns cantilever 

upward from their base where they are restrained from rotation. The columns 

provide both vertical support of the building’s weight and lateral resistance to 

earthquake forces. Structures using this system have performed poorly in past 

earthquakes and severe restrictions are placed on its use in zones of high seismic 

activity.

In regions of relatively low seismic risk, the NEHRP Recommended Seismic 

Provisions permits the design and construction of structural steel buildings that 

do not specifically conform to any of the above system types. These buildings are 

referred to as “structures not specifically detailed for seismic resistance.”

Figure 25 A tall steel 

moment-frame structure under construction.
48 CHAPTER 4

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

In addition to these basic structural systems and the primary materials of construction, the Provisions also categorizes structural systems based on the quality 

and extent of seismic-resistant detailing used in a structure’s design. Systems that 

employ extensive measures to provide for superior seismic resistance are termed 

“special” systems while systems that do not have such extensive design features 

are typically called “ordinary” systems. The Provisions also includes design rules 

for structural systems intended to provide seismic resistance that is superior to 

that of “ordinary” systems but not as good as that of “special” systems; these systems are called “intermediate” systems.

4.1.2 Nonstructural Components

In addition to the structural framing and the floor and roof systems, buildings 

include many components and systems that are not structural in nature but that 

can be damaged by earthquake effects. The types of nonstructural components 

covered by the NEHRP Recommended Seismic Provisions include:

• Architectural features such as exterior cladding and glazing, ornamentation, ceilings, interior partitions, and stairs;

• Mechanical components and systems including air conditioning equipment, ducts, elevators, escalators, pumps, and emergency generators;

• Electrical components including transformers, switchgear, motor control 

centers, lighting, and raceways;

• Fire protection systems including piping and tanks; and

• Plumbing systems and components including piping, fixtures, and equipment.

The design and construction requirements contained in the Provisions are intended to ensure that most of these components are adequately attached to the supporting structure so that earthquake shaking does not cause them to topple or fall, 

injuring building occupants or obstructing exit paths. For those pieces of equipment and components that must function to provide for the safety of building 

occupants (e.g., emergency lighting and fire suppression systems), the Provisions

provides design criteria intended to ensure that these systems and components 

will function after an earthquake. The Provisions also includes recommendations 

intended to ensure that nonstructural components critical to the operability of essential facilities such as hospitals can operate following strong earthquake shaking.
CHAPTER 4 49

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

4.2	 Nonbuilding Structures

The NEHRP Recommended Seismic Provisions also includes seismic design criteria for many structures that are not considered to be buildings. These structures 

are called nonbuilding structures and include:

• Storage tanks, pressure vessels, and pipe supports such as those commonly 

found in petroleum refineries and chemical plants (Figure 26);

• Water towers;

• Chimneys and smokestacks;

• Steel storage racks (Figure 27);

• Piers and wharves;

• Amusement structures including roller coasters; and

• Electrical transmission towers.

Some nonbuilding structures, however, are not covered by the design recommendations contained in the Provisions because they are of a highly specialized nature 

and industry groups that focus on the design and construction of these structures 

have developed specific criteria for their design. Some such structures are highway and railroad bridges, nuclear power plants, hydroelectric dams, and offshore 

petroleum production platforms.

Figure 26 Structures 

commonly found in 

petroleum refineries 

and chemical plants.
50 CHAPTER 4

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Just as it does for buildings, the NEHRP Recommended Seismic Provisions classifies nonbuilding structures based on the structural system that provides earthquake resistance. Some nonbuilding structures use structural systems commonly 

found in buildings such as braced frames and moment frames. These structures 

are identified as nonbuilding structures with a structural system similar to buildings, and the design requirements for these structures are essentially identical to 

those for building structures. Other nonbuilding structures are called nonbuilding structures with structural systems not similar to buildings, and the Provisions

contains special design requirements that are unique to the particular characteristics of these structures.

Figure 27 Seismic design criteria for 

steel storage racks of the type used in 

large warehouses and big-box retail 

stores are included in the Provisions.
CHAPTER 4 51

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

4.3	 Protective Systems

Most of the seismic-resistant structural systems used in both buildings and 

nonbuilding structures are variations of systems that were traditionally used in 

structures not designed for earthquake resistance. Over the years, engineers and 

researchers improved the earthquake resistance of these traditional systems by 

observing their behavior in laboratory tests and actual earthquakes and incrementally refining the design criteria to achieve better performance. Nevertheless, 

these systems are still designed with the intent that they will sustain damage when 

subjected to design-level or more severe earthquake effects.

Beginning in the 1970s, engineers and researchers began to develop systems 

and technologies capable of responding to earthquake ground shaking without 

sustaining damage and thereby protecting the building or structure. The NEHRP 

Recommended Seismic Provisions presently includes design criteria for two such 

technologies – seismic isolation and energy dissipation systems.

Seismic isolation systems consist of specially designed bearing elements that are 

typically placed between a structure and its foundation (Figure 28). Two types of 

bearing are commonly used – one is composed of layers of natural or synthetic 

rubber material bonded to thin steel plates in a multilevel sandwich form and the 

second consists of specially shaped steel elements coated with a low-friction material. Both types of bearings are capable of accommodating large lateral displacements while transmitting relatively small forces into the structure above. When 

these isolation systems are placed in a structure, they effectively “isolate” the 

building from ground shaking so that, when an earthquake occurs, the building 

experiences only a small fraction of the forces that would affect it if it were rigidly 

attached to its foundations.

Energy dissipation systems are composed of structural elements capable of dissipating large amounts of earthquake energy without experiencing damage, 

much like the shock absorbers placed in the suspensions of automobiles. Energy 

dissipation systems usually are placed in a structure as part of a diagonal bracing 

system. Several types of energy dissipation system are available today including 

hydraulic dampers, friction dampers, wall dampers, tuned mass dampers, and 

hysteretic dampers.

Hydraulic dampers are very similar to automotive shock absorbers. They consist 

of a double acting hydraulic cylinder that dissipates energy by moving a piston 

device through a viscous fluid that is contained within an enclosed cylinder. 

Friction dampers are essentially structural braces that are spliced to the structure 

using slotted holes and high-strength bolts with a tactile material on the mating 

surfaces of the connection. When the braces are subjected to tension or com-
52 CHAPTER 4

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

pression forces, they slip at the splice connection and dissipate energy through 

friction. Wall dampers are a form of viscous damper that consists of vertical 

plates arranged in a sandwich configuration with a highly viscous material. One 

set of plates is attached to one level of a structure and another set to the adjacent 

level. When the structure displaces laterally in response to earthquake shaking, 

the plates shear the viscous material and dissipate energy. Hysteretic dampers 

dissipate energy by yielding specially shaped structural elements that are placed 

in series with conventional wall or brace elements. Tuned mass dampers consist 

of a large mass on a spring-like device. When they are mounted on a structure, 

the lateral displacement of the structure excites the mass, which then begins to 

move and dissipate significant portions of the earthquake’s energy, protecting the 

structure in the process.

Although seismic isolation and energy dissipation systems have been available for 

more than 20 years, their use in new buildings has been confined primarily to 

very important structures that must remain functional after a strong earthquake 

and to buildings housing valuable contents such as museums or data centers. This 

is because their use adds to the construction cost for a structure and most owners have not viewed the additional protection provided by these technologies as 

worth the additional cost. 

Figure 28 The San Bernardino County Justice Center 

in California was one of the first base-isolated 

buildings in the United States.

4.4	 Existing Buildings and Structures

The NEHRP Recommended Seismic Provisions primarily addresses the design of 

new buildings and structures. However, the most significant seismic risks in the 

United States today are associated with existing buildings and structures designed 

and constructed prior to the adoption and enforcement of current seismic design 

requirements in building codes. It is possible to upgrade these existing hazard-
CHAPTER 4 53

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

ous structures so that they will perform better in future earthquakes and some 

communities in the United States have adopted ordinances that require seismic 

upgrades of the most hazardous types of existing building. 

Chapter 34 of the International Building Code and Appendix 11B of the ASCE/

SEI 7 standard include requirements aimed at improving the seismic resistance of 

existing structures, typically as part of a significant expansion, repair, or alteration 

of the building. These requirements are intended to prevent existing buildings 

from being made more hazardous than they already are (by either reducing their 

current strength or adding mass to them) and to trigger a seismic upgrade of 

these buildings when their expected useful life is extended by a major renovation 

project.

When a structurally dependent addition to an existing building is proposed, 

Appendix 11B of ASCE/SEI 7 requires that the entire structure, including the 

original building and the addition, be brought into compliance with the seismic 

requirements for new construction. The upgrade requirement is waived if it can 

be demonstrated that the addition does not increase the seismic forces on any 

existing element by more than 10 percent unless these elements have the capacity 

to resist the additional forces and that the addition in no way reduces the seismic 

resistance of the structure below that required for a new structure.

The ASCE/SEI 7 standard contains similar requirements for building alterations 

such as cutting new door openings into walls, cutting new stairway openings in 

floors, or relocating braces within a structure. Such alterations trigger a requirement to bring the entire structure into conformance with the seismic requirements for new buildings unless the alteration does not increase the seismic force 

on any element by more than 10 percent, the seismic resistance of the structure is 

not reduced, the forces imposed on existing elements do not exceed their capacity, 

new elements are detailed and connected to the structure in accordance with the 

requirements for new structures, and a structural irregularity is not created or 

made more severe.

Both ASCE/SEI 7 and the International Building Code require a seismic upgrade 

of an existing structure when an occupancy change will result in a higher risk to 

the public. An example of such an occupancy change would be the conversion of 

a normally unoccupied warehouse building into condominiums or an emergency 

shelter intended to provide living space for the public after a disaster. Further 

discussion of occupancies and the design requirements associated with them is 

contained in the next chapter.

Although both ASCE/SEI 7 and the International Building Code require that existing buildings and structures be upgraded to comply with the requirements for 
54 CHAPTER 4

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

new structures under some circumstances, it often is impractical and technically 

impossible to do this for many structures because they are constructed of systems 

and materials that are on longer permitted by the building codes and for which 

suitable design criteria are no longer available. In order to obtain literal compliance with the requirements to upgrade such structures, it would be necessary to 

demolish the nonconforming elements and replace them with new conforming 

construction, which is seldom economically practical. Recognizing this, FEMA 

has developed a series of publications specifically intended to help engineers identify the likely performance of existing nonconforming buildings and design effective means of upgrading these structures. Several of these publications have since 

evolved into national consensus standards issued by the American Society of Civil 

Engineers and they are widely accepted by building officials as suitable alternatives 

to the requirements of the building code for existing structures.

One such standard, ASCE/SEI 31-02, Seismic Evaluation of Existing Buildings, is 

based on FEMA 310 and employs a tiered methodology that enables engineers to 

determine whether buildings are capable of meeting either life safety or immediate occupancy performance objectives. The lowest tier of evaluation provides a 

simple checklist to assist the engineer in identifying deficiencies that are known to 

have caused poor performance in buildings in past earthquakes. Higher tier evaluations utilize progressively more complex analytical procedures to quantitatively 

evaluate an existing building’s probable performance.

ASCE/SEI 41-06, Seismic Rehabilitation of Existing Buildings, is based on several FEMA publications (notably, FEMA 273/274 and FEMA 356) and provides 

design criteria for the seismic upgrading of existing buildings to meet alternative 

performance criteria ranging from a reduction of collapse risk to the capability to 

survive design-level earthquakes and remain functional. FEMA 547, Techniques 

for the Seismic Rehabilitation of Existing Buildings, is an important companion 

document to ASCE/SEI 41-06; it provides engineers with alternative structural 

techniques that can be used to effectively upgrade existing buildings.

Many jurisdictions have adopted ordinances that require owners of some types 

of buildings known to be particularly hazardous to perform seismic upgrades of 

these structures. The targets of such ordinances include unreinforced masonry 

buildings, older precast concrete tiltup buildings, and wood frame buildings with 

weak first stories or inadequately attached to their foundations. Some of these 

ordinances adopt technical provisions contained in the International Existing 

Buildings Code produced by the International Code Council as a companion publication to the IBC. Other ordinances permit the use of the ASCE 41 procedures or 

specify other acceptable procedures developed for that particular community.
CHAPTER 4 55

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Owners often elect to undertake upgrades of buildings independent of requirements contained in the building codes or locally adopted ordinances. These 

upgrades may range from incremental projects that address specific building 

deficiencies to complete upgrades intended to provide performance equivalent 

or superior to that anticipated by the building code for new construction. The 

International Building Code includes permissive language that enables such 

upgrades so long as the engineer designing the upgrade can demonstrate that the 

proposed changes do not create new seismic deficiencies or exacerbate existing 

seismic deficiencies. FEMA 390 through 400 suggest some ways to incrementally 

improve a building’s seismic performance and FEMA 420 is an engineering guide 

for use with those publications.
CHAPTER 5 57

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Chapter 5

DESIGN REQUIREMENTS

5.1	 Seismic Design Categories

The NEHRP Recommended Seismic Provisions recognizes that, independent of 

the quality of their design and construction, not all buildings pose the same seismic risk. Factors that affect a structure’s seismic risk include:

• The intensity of ground shaking and other earthquake effects the structure 

is likely to experience and

• The structure’s use including consideration of the number of people who 

would be affected by the structure’s failure and the need to use the structure for its intended purpose after an earthquake.

The Provisions uses the Seismic Design Category (SDC) concept to categorize 

structures according to the seismic risk they could pose. There are six SDCs ranging from A to F with structures posing minimal seismic risk assigned to SDC A 

and structures posing the highest seismic risk assigned to SDC F. As a structure’s 

potential seismic risk as represented by the Seismic Design Category increases, the 

Provisions requires progressively more rigorous seismic design and construction 

as a means of attempting to ensure that all buildings provide an acceptable risk 

to the public. Thus, as the SDC for a structure increases, so do the strength and 

detailing requirements and the cost of providing seismic resistance. Table 2 summarizes the potential seismic risk associated with buildings in the various Seismic 

Design Categories and the primary protective measures required for structures in 

each of the categories. 

As noted in Table 2, structures are assigned to a Seismic Design Category based 

on the severity of ground shaking and other earthquake effects the structure may 

experience and the nature of the structure’s occupancy and use. The nature of the 

structure’s occupancy and use used in determining a Seismic Design Category is 

broken into four categories of occupancy as summarized in Table 3. 
58 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

SDC Building Type and Expected MMI Seismic Criteria

A Buildings located in regions having a very small probability of 

experiencing damaging earthquake effects

No specific seismic design requirements but 

structures are required to have complete lateralforce-resisting systems and to meet basic structural 

integrity criteria.

B Structures of ordinary occupancy 

that could experience moderate 

(MMI VI) intensity shaking

Structures must be designed to resist seismic forces.

C Structures of ordinary occupancy 

that could experience strong 

(MMI VII) and important structures 

that could experience moderate 

(MMI VI) shaking

Structures must be designed to resist seismic forces.

Critical nonstructural components must be 

provided with seismic restraint.

D Structures of ordinary occupancy 

that could experience very strong 

shaking (MMI VIII) and important 

structures that could experience 

MMI VII shaking

Structures must be designed to resist seismic forces.

Only structural systems capable of providing good 

performance are permitted.

Nonstructural components that could cause injury 

must be provided with seismic restraint.

Nonstructural systems required for life safety 

protection must be demonstrated to be capable 

of post-earthquake functionality.

Special construction quality assurance measures 

are required.

E Structures of ordinary occupancy 

located within a few kilometers 

of major active faults capable of 

producing MMI IX or more intense 

shaking

Structures must be designed to resist seismic forces.

Only structural systems that are capable of 

providing superior performance permitted.

Many types of irregularities are prohibited.

Nonstructural components that could cause injury 

must be provided with seismic restraint.

Nonstructural systems required for life safety 

protection must be demonstrated to be capable 

of post-earthquake functionality.

Special construction quality assurance measures 

are required.

F Critically important structures 

located within a few kilometers 

of major active faults capable of 

producing MMI IX or more intense 

shaking

Structures must be designed to resist seismic forces.

Only structural systems capable of providing 

superior performance permitted are permitted.

Many types of irregularities are prohibited.

Nonstructural components that could cause injury 

must be provided with seismic restraint.

Nonstructural systems required for facility function 

must be demonstrated to be capable of postearthquake functionality.

Special construction quality assurance measures 

are required.

Table 2 Seismic Design Categories, Risk, and Seismic Design Criteria
CHAPTER 5 59

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Table 3 Occupancy 

Category Representative Buildings Acceptable Risk

I Buildings and structures that 

normally are not subject to human 

occupancy (e.g., equipment 

storage sheds, barns, and other 

agricultural buildings) and that do 

not contain equipment or systems 

necessary for disaster response or 

hazardous materials.

Low probability of earthquake-induced 

collapse.

II Most buildings and structures of 

ordinary occupancy (e.g., residential, commercial, and industrial 

buildings) except those buildings 

contained in other categories.

Low probability of earthquake-induced 

collapse.

Limited probability that shaking-imposed 

damage to nonstructural components will 

pose a significant risk to building occupants.

III Buildings and structures that:

• Have large numbers of occupants (e.g., high-rise office 

buildings, sports arenas, and 

large theaters),

• Shelter persons with limited 

mobility (e.g., jails, schools, and 

some healthcare facilities); 

• Support lifelines and utilities 

important to a community’s 

welfare; or

• Contain materials that pose 

some risk to the public if released.

Reduced risk of earthquake-induced 

collapse relative to Occupancy Category II 

structures. 

Reduced risk of shaking-imposed damage 

to nonstructural components relative to 

Occupancy Category II structures.

Low risk of release of hazardous materials or 

loss of function of critical lifelines and utilities.

IV Buildings and structures that:

• Are essential to post-earthquake 

response (e.g., hospitals, police 

stations, fire stations, and emergency communications centers) 

or

• House very large quantities of 

hazardous materials.

Very low risk of earthquake inducedcollapse.

Low risk that the building or structure will 

be damaged sufficiently to impair use in 

post-earthquake response and recovery 

efforts. Very low risk of release of hazardous 

materials.

The intensity of earthquake shaking and other effects used to assign structures to a 

Seismic Design Category is determined using the national seismic maps previously 

presented in Figures 14 and 15. Figure 14 is used to determine a short-period 

shaking parameter, SS. This acceleration parameter is the maximum shaking 

considered for the design of low-rise buildings located on sites conforming to 

a reference soil condition. Figure 15 is used to determine the 1-second period 

shaking parameter, S1. This shaking parameter, also derived for sites conforming 

to a reference soil condition, is important to the design of taller buildings.
60 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

In order to determine a structure’s Seismic Design Category, it is necessary to 

determine the value of the Ss and S1 parameters at the building site, adjust those 

values to account for the soil conditions actually present at the building site, and 

then reduce the values by two-thirds to represent design-level ground shaking. 

The resulting design acceleration parameters are labeled SDS and SD1, respectively. 

In general, sites that have deep deposits of soft soils will have larger values of the 

design acceleration parameters than sites with shallow deposits of firm soils or 

near-surface rock. More discussion of these parameters appears below. 

In communities where soil conditions vary, similar buildings constructed on different sites may be assigned to different Seismic Design Categories and this can 

result in very different seismic design requirements for similar buildings in the 

same city. Figure 29 provides a series of maps3 of the United States and its territories showing the Seismic Design Category for low-rise Occupancy Category I and 

II structures located on sites with average alluvial soil conditions. This map is used 

in the International Residential Code (IRC). Structures of a higher Occupancy 

Category would be assigned to a higher SDC. Tall structures and structures on 

sites with other than average alluvial soils also may be assigned to different SDCs.

5.2	 Site Class

Site soil conditions are important in determining Seismic Design Category. Hard, 

competent rock materials efficiently transmit shaking with high-frequency 

(short-period) energy content but tend to attenuate (filter out) shaking with 

low-frequency (long-period) energy content. Deep deposits of soft soil transmit 

high-frequency motion less efficiently but tend to amplify the low-frequency 

energy content. If the nature and depth of the various soil deposits at a site are 

known, geotechnical engineers can perform a site response analysis to determine 

the importance of these effects. For most sites, however, these effects can be approximated if the nature of soil at the site is known. The NEHRP Recommended 

Seismic Provisions uses the concept of Site Class to categorize common soil 

conditions into broad classes to which typical ground motion attenuation and amplification effects are assigned.

Site Class is determined based on the average properties of the soil within 100 feet 

(30 meters) of the ground surface. Geotechnical engineers use a variety of parameters to characterize the engineering properties of these soils, including general soil classifications as to the type of soil, (e.g. hard rock, soft clay), the number 

________________________

3

The Seismic Design Category maps that follow are those approved for inclusion in the 2012 edition of the International Residential Code. In the International Building Code and the Provisions, Categories D0

, D1 and D2 are 

combined into a single Category D.
CHAPTER 5 61

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

of blows (N) needed to drive a standard penetration tool 1 foot into the soil using 

a standard hammer, the velocity (vs) at which shear waves travel through the 

material as measured by on-site sonic and other tests, and the shear resistance of 

the soil (su) as measured using standard laboratory test procedures. Table 4 lists 

the six Site Classes recognized by the NEHRP Recommended Seismic Provisions

and the engineering parameters used to define them. On many sites, the nature of 

soils will vary with depth below the surface. 

Table 4 Site Class and Soil Types

Site Class General Description Shear Wave 

Velocity, vs 

(ft/sec)

Blows/foot (N) Shear strength, 

su (psf)

A Hard rock >5,000

B Rock 2,500-5,000

C Very dense soil and 

soft rock

1,200-2,500 >50 >2,000

D Stiff soil 600-1,200 15-50 1,000 – 2,000

E Soft clay soil <600 <15 <1,000

F Unstable soils

 

Rock associated with Site Class A is typically found only in the eastern United 

States. The types of rock typically found in the western states include various volcanic deposits, sandstones, shales, and granites that commonly have the characteristics appropriate to either Site Class B or C. Sites with very dense sands and gravels or very stiff clay deposits also may qualify as Site Class C. Sites with relatively 

stiff soils including mixtures of dense clays, silts, and sands are categorized as Site 

Class D, and this is the most common site class throughout the United States. Sites 

along rivers or other waterways underlain by deep soft clay deposits are categorized as Site Class E. Sites where soils are subject to liquefaction or other ground 

instabilities are categorized as Site Class F and site-specific analyses are required.

As indicated above, the properties of the soils in the 100 feet below ground 

surface must be known to determine the Site Class, and this requires an investigation that includes drilling borings into the soil and removing samples of the 

soil at various depths in order to classify it. The NEHRP Recommended Seismic 

Provisions permits any site to be categorized as Site Class D unless there is reason 

to believe that it would be more properly classified as Site Class E or F. However, 

classification of a site as conforming to either Site Class A, B, or C generally will 

lead to a more economical structural design than an assumption that a site conforms to Class D because Site Classes A, B, and C produce less intense shaking than 

does Site Class D.
62 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 29 Seismic Design Categories for low-rise buildings of ordinary occupancy on alluvial soils.
CHAPTER 5 63

EARTHQUAKE-RESISTANT DESIGN CONCEPTS
64 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 29 continued
CHAPTER 5 65

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 29 continued
66 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

5.3	 Design Ground Motion

In order to determine the Seismic Design Category for a structure, it is first necessary to determine the design ground motion, which is one of the primary factors 

used to determine the required seismic resistance (strength) of structures and 

supported nonstructural components.

Design ground motion is defined by an acceleration response spectrum having a 

shape similar to that shown previously in Figure 10 and characterized by the following parameters:

• SDS – short-period design response acceleration, in units of percent g

• SD1 – one-second period design response acceleration, in units of percent g

• Ts – transition period from constant response acceleration to constant response velocity, in units of seconds

• TL - transition period from constant response velocity to constant response 

displacement, in units of seconds

Figure 30 is the generalized form of the design acceleration response spectrum 

showing each of these parameters. The values of SDS and SD1, respectively, are 

determined as follows:

 (1)

 (2)

In these equations, Fa and Fv are coefficients related to the Site Class that indicate, respectively, the relative amplification or attenuation effects of site soils on 

short-period (high-frequency) and long-period (low-frequency) ground shaking 

energy. Tables 5 and 6 present the values of these coefficients for the Site Classes 

defined above.

Ss and S1 are the mapped values of MCER spectral accelerations for reference soil 

conditions. The USGS maintains a web-based application accessible at http://

earthquake.usgs.gov/research/hazmaps/ that will calculate values of Ss, S1, SDS, 

and SD1 based on input consisting of either geographic coordinates (latitude and 

longitude) or postal zip code and Site Class. It should be noted that the use of zip 

code to determine these acceleration parameters is not recommended in regions 

of the nation where structures are assigned to Seismic Design Category D or 

higher because there can be great variation in the value of these parameters across 

the area encompassed by a postal zip code. A number of internet sites include 

SDS = 2

3 Fa

SS

SD1 = 2

3 Fv

S1
CHAPTER 5 67

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

look-up features for longitude and latitude of a site based on address; one such 

site is http://www.zipinfo.com/search/zipcode.htm.

Table 5 Values of Site Class Coefficient Fa as a Function of Site Class

Value of Short-Period MCE Response Acceleration, SS

Site Class Ss < 0.25 Ss = 0.5 Ss = 0.75 Ss = 1.0 Ss ≥ 1.25

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.2 1.2 1.1 1.0 1.0

D 1.6 1.4 1.2 1.1 1.0

E 2.5 1.7 1.2 0.9 0.9

F Site specific study required.

Table 6 Values of Site Class Coefficient Fv as a Function of Site Class

Value of 1-Second MCE Response Acceleration, S1

Site Class S1 ≤ 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 ≥ 0.5

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.7 1.6 1.5 1.4 1.3

D 2.4 2.0 1.8 1.6 1.5

E 3.5 3.2 2.8 2.4 2.4

F Site specific study required.

Figure 30 Generalized design response 

spectrum.

The value of TL is obtained from a map prepared by the USGS based on the maximum magnitude earthquake anticipated to produce strong shaking in a region. 

Figure 31 presents this map of TL

 values (in units of seconds) for the continental 

United States.
68 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 31 Map of long-period transition period, TL, for the continental United States.

TS = SD1SDS

The value of TS (in units of seconds) is calculated by the following equation:

(3)
CHAPTER 5 69

EARTHQUAKE-RESISTANT DESIGN CONCEPTS
70 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

5.4	 Structural System Selection

The next step in the design process consists of selecting an appropriate seismicforce-resisting system (SFRS). As explained in Chapter 3, the seismic-force-resisting systems for building structures and nonbuilding structures with structural 

systems like buildings are categorized by construction material (e.g., concrete, 

masonry, steel, or wood), type of system (bearing wall, braced frame, moment 

frame, dual, or cantilever column), and level of seismic detailing (special, intermediate, ordinary, or not detailed for seismic resistance). Structures assigned to 

Seismic Design Category A can use any type of SFRS as long as the system is complete and provides minimum specified strength. Structures assigned to Seismic 

Design Categories B or higher must utilize one of the specific SFRSs or combinations of these systems listed in Table 12.2-1 of the ASCE/SEI 7 standard. This table 

lists more than 90 different structural systems providing designers with a wide 

range of choices. 

Some types of SFRS have proven to exhibit undesirable behavior when subjected 

to very intense ground shaking; therefore, the use of these SFRSs in higher SDCs 

is restricted. Some structural systems are prohibited from use in these design 

categories and other structural systems are permitted only for buildings and structures meeting specific height and weight limitations. Some notable restrictions on 

structural systems include the following:

• Plain concrete and plain masonry bearing wall systems are not permitted 

in Seismic Design Categories C or higher.

• Ordinary concrete and ordinary masonry bearing wall systems are not 

permitted in Seismic Design Categories D or higher.

• Ordinary concentric braced steel frames are not permitted in Seismic 

Design Categories D and E for buildings in excess of 35 feet in height or 

in Seismic Design Category F for buildings of any height.

• Braced frames and walls of any material cannot be used as the only SFRS 

in structures exceeding 160 feet in height in Seismic Design Categories D, 

E, or F unless certain configuration limitations are met.

• Braced frames and walls of any material cannot be used as the only SFRS 

in structures exceeding 240 feet in height in Seismic Design Categories D, 

E, or F regardless of building configuration.

Many other limitations apply to the individual SFRSs listed in the ASCE/SEI 7 table.
CHAPTER 5 71

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

In order to qualify as a particular SFRS, the structure’s seismic-force-resisting 

elements must be designed and detailed to conform to the specific requirements 

contained in industry specifications. For example, special concentric braced 

steel frames must comply with the design requirements contained in Chapter 13 

of AISC 341, Seismic Provisions for Structural Steel Buildings. Intermediate 

concrete moment resisting frames must be designed and detailed to conform to 

the requirements contained in Section 21.12 of ACI 318, Building Code Requirements for Structural Concrete. Table 12.2.1 of ASCE/SEI 7 references the mandatory specification requirements for each structural system. Part 1 of the NEHRP 

Recommended Seismic Provisions adds additional design and detailing requirements for some structural systems. 

For nonbuilding structures with structural systems similar to buildings, ASCE/SEI 

7 Table 15.4-1 provides an alternative set of limitations on system use that considers the reduced human occupancies and different characteristics of nonbuilding 

structures. ASCE/SEI 7 Table 15.4-2 provides similar information for nonbuilding 

structures that do not have structural systems similar to buildings.

All three of the ASCE/SEI 7 tables specify the values of the three design coefficients 

used to determine the required strength and stiffness of a structure’s seismicforce-resisting system:

• R is a response modification factor that accounts for the ability of some 

seismic-force-resisting systems to respond to earthquake shaking in a 

ductile manner without loss of load-carrying capacity. R values generally 

range from 1 for systems that have no ability to provide ductile response 

to 8 for systems that are capable of highly ductile response. The R factor 

is used to reduce the required design strength for a structure.

• Cd is a deflection amplification coefficient that is used to adjust lateral 

displacements for the structure determined under the influence of design 

seismic forces to the actual anticipated lateral displacement in response 

to design earthquake shaking. The Cd factors assigned to the various 

structural systems are typically similar to, but a little less than, the R

coefficients, which accounts in an approximate manner for the effective 

damping and energy dissipation that can be mobilized during inelastic 

response of highly ductile systems. Generally, the more ductile a system 

is, the greater will be the difference between the value of R and Cd.

• Ωo is an overstrength coefficient used to account for the fact that the actual 

seismic forces on some elements of a structure can significantly exceed 

those indicated by analysis using the design seismic forces. For most 

structural systems, the Ωo coefficient will have a value between 2 and 3.
72 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

5.5	 Configuration and Regularity

The design procedures contained in the NEHRP Recommended Seismic Provisions were developed based on the dynamic response characteristics of structures 

that have regular configurations with a relatively uniform distribution of mass 

and stiffness and continuous seismic-force-resisting elements. To the extent that 

structures have nonuniform distribution of strength or stiffness and discontinuous structural systems, the assumptions that underlie the design procedures can 

become invalid. These conditions are known as irregularities, and structures that 

have one or more of these irregularities are termed “irregular structures.” 

Some irregularities trigger requirements for the use of more exact methods of 

analysis that better account for the effects of these irregularities on the distribution of forces and deformations in the structure during response to earthquake 

shaking. Other irregularities trigger requirements for portions of the structure to 

be provided with greater strength to counteract the effects of the irregularity. Still 

other irregularities have led to very poor performance in past earthquakes and are 

prohibited from use in structures assigned to Seismic Design Categories E or F.

The Provisions identifies two basic categories of irregularity: horizontal or plan 

irregularity and vertical irregularity. Horizontal irregularities include:

• Torsional irregularity – This condition exists when the distribution of 

vertical elements of the seismic-force-resisting system within a story, 

including braced frames, moment frames and walls, is such that when the 

building is pushed to the side by wind or earthquake forces, it will tend 

to twist as well as deflect horizontally. Torsional irregularity is determined 

by evaluating the difference in lateral displacement that is calculated at opposite ends of the structure when it is subjected to a lateral force.

• Extreme torsional irregularity – This is a special case of torsional irregularity in which the amount of twisting that occurs as the structure is 

displaced laterally becomes very large. Structures with extreme torsional 

irregularities are prohibited in Seismic Design Categories E and F.

• Re-entrant corner irregularity – This is a geometric condition that occurs 

when a building with an approximately rectangular plan shape has a missing corner or when a building is formed by multiple connecting wings. 

Figure 32 illustrates this irregularity.

• Diaphragm discontinuity irregularity – This occurs when a structure’s 

floor or roof has a large open area as can occur in buildings with large 

atriums. Figure 33 illustrates this irregularity.
CHAPTER 5 73

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

• Out-of-plane offset irregularity – This occurs when the vertical elements 

of the seismic-force-resisting system, such as braced frames or shear 

walls, are not aligned vertically from story to story. Figure 34 illustrates 

this irregularity.

• Nonparallel systems irregularity – This occurs when the structure’s 

seismic-force-resisting does not include a series of frames or walls that are 

oriented at approximately 90-degree angles with each other.

Figure 32 Re-entrant corner irregularity.

Figure 33 Diaphragm discontinuity irregularity.

Figure 34 Out-of-plane offset irregularity.
74 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Vertical irregularities include the following:

• Stiffness soft-story irregularity – This occurs when the stiffness of one 

story is substantially less than that of the stories above. This commonly 

occurs at the first story of multistory moment frame buildings when the 

architectural design calls for a tall lobby area. It also can occur in multistory bearing wall buildings when the first story walls are punched with a 

number of large openings relative to the stories above. Figure 35 illustrates these two conditions.

Figure 35 Examples of buildings with a soft first story, 

a common type of stiffness irregularity.

• Extreme stiffness soft-story irregularity – As its name implies, this is an 

extreme version of the first soft-story irregularity. This irregularity is prohibited in Seismic Design Categories E and F structures.

• Weight/mass irregularity – This exists when the weight of the structure at 

one level is substantially in excess of that at the levels immediately above 

or below it. This condition commonly occurs in industrial structures 

where heavy pieces of equipment are located at some levels. It also can 

occur in buildings that have levels with large mechanical rooms or storage 

areas.

• In-plane discontinuity irregularity – This occurs when the vertical elements of a structure’s seismic-force-resisting system such as its walls or 

braced frames do not align vertically within a given line of framing or the 

frame or wall has a significant setback. Figure 36 provides examples of 

this irregularity. 
CHAPTER 5 75

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 36 Examples of in-plane discontinuity irregularities.

• Weak-story irregularity – This occurs when the strength of the walls or 

frames that provide lateral resistance in one story is substantially less than 

that of the walls or frames in the adjacent stories. This irregularity often 

accompanies a soft-story irregularity but does not always do so.

• Extreme weak-story irregularity – As its name implies, this is a special case 

of the weak-story irregularity. Structures with this irregularity are prohibited in Seismic Design Categories E and F. 

5.6	 Required Strength

Earthquake shaking induces both vertical and horizontal forces in structures. 

These forces vary during an earthquake and, for brief periods ranging from a 

few tenths of a second to perhaps a few seconds, they can become very large. In 

structures assigned to Seismic Design Categories D, E or F, these forces easily can 

exceed the forces associated with supporting the structure’s weight and contents. 

In keeping with the basic design philosophy of accepting damage but attempting 

to avoid collapse, the NEHRP Recommended Seismic Provisions requires that 

structures be provided with sufficient strength to resist specified earthquake forces 

in combination with other loads. Typically, engineers design a structure so that 

only some of the structure’s elements (e.g., beams, columns, walls, braces) and 

their connections provide the required seismic resistance. As previously noted, the 

system created by these elements and their connections is called the seismic-forceresisting system (SFRS). The specific combinations of seismic load with other 

loads, including dead and live loads, that members of the SFRS must be proportioned to resist are specified in the ASCE/SEI 7 standard.
76 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

The specified earthquake forces are typically a fraction of the forces that design 

level earthquake shaking will actually produce in these structures. The magnitude 

of the specified earthquake forces and how they are calculated depends on the 

structure’s Seismic Design Category, the type of structural system that is used, the 

structure’s configuration, and the type of element or connection being designed. 

These are described briefly below.

5.6.1 Seismic Design Category A

Structures assigned to Seismic Design Category A are required to have adequate 

strength to resist three different types of specified forces:

• Global system lateral forces,

• Continuity forces, and

• Wall anchorage forces.

The global system lateral forces on elements of the SFRS are determined by applying a total static lateral force, equal to 1 percent (0.01) of the structure’s weight 

and that of its supported nonstructural components and contents at each level, 

in each of two perpendicular directions. The forces in each direction are applied 

independently, but when the forces are applied in a given direction, they must be 

applied simultaneously at all levels. Figure 37 illustrates this concept.

Figure 37 Required seismic design forces for 

Seismic Design Category A structures.

The design professional must use methods of elastic structural analysis to determine the individual forces in each of the SFRS elements and their connections 

under the influence of these global applied loads.

Roof – Weight WR

2nd Floor – Weight W2

FR2=0.01WR

F22=0.01W2

FR1=0.01WR

F21=0.01W2
CHAPTER 5 77

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Continuity forces apply to those elements that “tie” or interconnect a small piece 

of a structure (e.g., a cantilevered deck to the main structure). The NEHRP 

Recommended Seismic Provisions specifies that such forces be equal to 5 percent 

(0.05) of the weight of the smaller portion of the structure as illustrated in Figure 

38.

Figure 38 Continuity forces for Seismic Design Category A structures.

In addition to the forces illustrated in Figure 37, the Provisions also requires that 

each beam, girder, truss, or other framing member that provides vertical support 

for a floor or roof be connected to its supporting member with sufficient strength 

to resist a force applied along the axis of the member equal to 5 percent of the 

weight supported by the member.

Wall anchorage forces are intended to prevent the type of failure illustrated previously in Figure 16. The Provisions requires that all concrete and masonry walls 

in Seismic Design Category A structures be connected to the floors and roofs 

that provide out-of-plane support for the wall and that these connections have a 

strength not less than 280 pounds per linear foot of wall.

5.6.2 Seismic Design Category B

The forces illustrated above are sometimes called lateral forces because they result 

from actions that attempt to move the structure, or a portion of the structure, 

laterally to the side. Elements of structures in Seismic Design Category B must be 

designed for both lateral earthquake forces and vertical earthquake forces. Every 

structural element in these structures must be designed for stresses that result 

from vertical earthquake forces whether the element is part of the SFRS or not. 

These vertical forces are a result of vertical ground shaking. To account for these 

forces, the NEHRP Recommended Seismic Provisions requires that the stresses 

due to vertical earthquake shaking be taken as a fraction of the stresses in the 

members due to the weight of the structure itself and its permanent attachments 

(i.e., the dead load, D). The fraction is given by the formula:

2nd Floor Deck – Weight WD

F1

=0.05WD
78 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

(4)

In this equation, Ev is the magnitude of forces due to vertical earthquake shaking, D is the magnitude of force due to the weight of the structure itself and its 

permanent attachments, and SDS is the design spectral response acceleration at 

0.2-second period determined in accordance with Equation 1.

The lateral earthquake forces are determined using procedures that approximate 

calculation of the structure’s dynamic inelastic response to horizontal earthquake 

shaking. Several methods are available for calculating these lateral forces:

• Nonlinear response history analysis is a complex technique that calculates the forces and deformations induced in a structure in response to 

a particular earthquake record and accounts explicitly for the structure’s 

dynamic and hysteretic properties. This is an elegant technique but it is 

computationally complex and, except for some structures incorporating 

seismic isolation or energy dissipation systems, it is not required so it is 

almost never used for the design of structures assigned to Seismic Design 

Category B. 

• Linear dynamic analysis, commonly called response spectrum analysis 

(RSA), is substantially less complex than nonlinear response history analysis. It accounts for a structure’s dynamic properties but only approximates 

the effects of nonlinear behavior. Its use is not required for the design of 

Seismic Design Category B structures but it is occasionally employed to 

design highly irregular or tall structures.

• The so-called equivalent lateral force (ELF) method is a simplification of 

the response spectrum analysis method, and it produces similar estimates 

of the earthquake forces and displacements for structures that are relatively regular and have primary response to earthquake shaking in their first 

mode. The first mode is the deformed shape associated with the lowest 

period at which a structure will freely vibrate. All structures in Seismic 

Design Category B can be designed using the ELF technique and it is the 

method most commonly used in this design category. 

EV =0.2SDSD
CHAPTER 5 79

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

The actual magnitude of forces that act on a structure during earthquake shaking depends on the deflected shape of the structure as it responds to earthquake 

shaking and on the weight of the structure at each level. Figure 39 illustrates this 

concept for a three-story structure.4

Figure 39 Distribution of lateral earthquake force in three-story structure.

In Figure 39, V is the total lateral earthquake force, which is sometimes called the 

“base shear.” F1 is the force applied at level 1, F2 is the force applied at the second 

level, and F3 is the applied force at the third level. According to the Provisions,

the total lateral force or base shear, V, has a value given by the formula:

(5)

In this equation, Cs is the seismic base shear coefficient and W is the structure’s 

seismic weight. The seismic weight is equal to the weight of the structure and all 

permanently attached nonstructural components and systems including cladding, 

roofing, partitions, ceilings, mechanical and electrical equipment, etc. In storage and warehouse occupancies, W also includes 25 percent of the design storage 

load. For buildings with a flat roof in areas susceptible to a snow load of 30 psf 

or more, the seismic weight also includes 20 percent of the uniform design snow 

load. 

________________________

4

The Provisions also contains a simplified version of the equivalent lateral force (ELF) procedure that can be used for 

some low-rise structures. This simplified design procedure is almost identical to the ELF procedure described above 

except that the equations used to determine the base shear forces (V) and story forces (Fi) are simplified, and it is 

not necessary to determine the deflections of the structural system. For buildings that do not have the irregularities described in Section 5.5, the simplified procedure and the full ELF procedure will produce very similar results; 

however, these results are sometimes relatively conservative. The simplified procedure cannot be used for buildings 

that have torsional irregularities because it does not provide for distribution of forces considering eccentric (torsional) 

effects. Therefore, before the simplified procedure can be used for a building with diaphragms that are not flexible, the 

building must be evaluated to determine if it is torsionally sensitive. In addition, since the simplified procedure does 

not include an evaluation of lateral deflection, it can be used only for buildings with relatively stiff structural framing 

systems including bearing wall systems and some types of building frame systems.

V = CSW
80 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

The base shear coefficient (Cs) depends on a number of factors including the 

structure’s fundamental period of vibration (T), the structure’s Occupancy Category (discussed in Section 5.1), and the type of seismic-force-resisting system 

used (discussed in Section 5.4). The fundamental period of vibration (T) is the 

amount of time, in seconds, the structure will take to undergo one complete 

cycle of motion if it is laterally displaced and released (similar to what is shown 

in Figure 39). For structures with fundamental periods of vibration less than 

the mapped value of TL at their site, the base shear coefficient (Cs) is taken as the 

lesser of the value given by:

(6)

(7)

where SDS and SD1 are the spectral response acceleration parameters obtained from 

Equations 1 and 2 as indicated previously, R is the response modification coefficient discussed in Section 5.4; I is an occupancy importance factor, the value 

of which depends on the Occupancy Category previously described in Section 

5.1, and T is the structure’s fundamental period of vibration. The quantity R/I in 

Equations 6 and 7 is an expression of the permissible amount of inelastic structural response. The value of R is determined from the ASCE/SEI 7 standard based 

on the selected structural system. For buildings in Occupancy Category I or II, the 

importance factor (I) has a value of 1.0. For structures in Occupancy Categories 

III and IV, the importance factors are 1.25 and 1.5, respectively. Thus, for structures in higher occupancy categories, less inelastic behavior is permitted, which is 

consistent with the desired reduced risk of damage.

For structures with a fundamental period of vibration greater than TL

, the value of 

Cs can be determined using the formula:

(8)

The value of the base shear coefficient for any structure, however, cannot be taken 

as less than the value obtained from the following formula:

(9)

CS = 

SDS

(R/I)

CS = 

SD1

(R/I)T

CS = 

SD1TL

(R/I)T2

CS = 0.44SDSI
CHAPTER 5 81

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

The lateral earthquake force (Fi) applied at each story “i” is obtained from the 

following formula:

(10)

In Equation 10, the superscript k has a value of unity for structures with a fundamental period (T) less than or equal to 0.5 second, has a value of 2 for structures 

with a fundamental period greater than or equal to 2.5 seconds, and has a value 

that is linearly interpolated from these values for structures with a fundamental 

period that falls between these values. The value of the period can be determined 

using either a series of approximate formula that depend on the type of seismicforce-resisting system used or methods of structural dynamics that consider the 

distribution of the structure’s mass and stiffness.

The fundamental period (T), seismic base shear force (V), and individual story 

forces (Fi

) must be computed and applied independently in each of the structure’s 

two primary orthogonal directions of response. The major vertical elements of 

the seismic-force-resisting system (frames or walls) will be aligned in these two 

orthogonal directions in most structures but, when this is not the case, any two 

orthogonal axes may be used. The story forces (Fi) are applied as static loads, and 

an elastic analysis is performed to determine the distribution of seismic forces in 

the various beams, columns, braces, and walls that form the vertical elements of 

the seismic-force-resisting system. These forces then are combined with the forces associated with dead, live, vertical seismic, and other forces using load combinations contained in the ASCE/SEI 7 standard and evaluated against permissible 

strengths contained in the various materials design standards referenced by ASCE/

SEI 7. The design seismic forces on some elements in irregular structures must be 

amplified by the Ω0 coefficient described previously. The purpose of design using 

these amplified forces is to avoid damage to elements whose failure could result in 

widespread damage and collapse of the structure.

The lateral forces (Fi) at each level are applied at a location that is displaced from 

the center of mass of the level by a distance equal to 5 percent of the width of the 

level perpendicular to the direction of application of the force. Figure 40 illustrates this concept. If the structure is not symmetrical, the 5 percent displacement 

of the point of application of the forces must be taken to both sides of the center 

of mass, and the design seismic forces on the elements must be taken as the highest forces obtained from either point of application. The purpose of this eccentric 

application of the forces is to account for any potential torsional loading that may 

Fi =

wi

hk

 V i

n

wj

hk

j j=i
82 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

occur if, for example, one side of a building is occupied during earthquake shaking while the other side is vacant. This requirement also is intended to ensure that 

all structures have a minimum amount of resistance to torsional effects.

Figure 40 Eccentric application of story forces.

In addition to determining the seismic forces (E) on the vertical elements of the 

lateral-force-resisting system, the NEHRP Recommended Seismic Provisions

requires determination of the seismic forces on the horizontal elements, typically called diaphragms. In most structures, the diaphragms consist of the floors 

and roofs acting as large horizontal beams that distribute the seismic forces to 

the various vertical elements. Diaphragms are categorized as being rigid, flexible, 

or of intermediate stiffness depending on the relative amounts of deflection that 

occur in the structure when it is subjected to lateral loading. Figure 41 shows the 

deflected shape of a simple single-story rectangular building under the influence 

of lateral forces in one direction. The roof diaphragm has deflection δL

 at the left 

side, δR at the right side and δC at its center. If the deflection at the center of the 

diaphragm (δC) exceeds twice the average of deflections δL and δR at the ends, 

the diaphragm can be considered flexible. The Provisions permits diaphragms 

of untopped wood sheathing or steel deck to be considered flexible regardless of 

the computed deflection. Diaphragms consisting of reinforced concrete slabs or 

concrete-filled metal deck that meet certain length-to-width limitations can be 

considered perfectly rigid. Other diaphragms must be considered to be of intermediate stiffness.
CHAPTER 5 83

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Figure 41 Deflection of diaphragm under lateral loading.

A flexible diaphragm is considered to distribute forces to the supporting vertical 

elements of the seismic-force-resisting system in the same way as a simple beam 

spanning between the vertical elements. For other diaphragms, the distribution 

of forces to the vertical elements must be considered on the basis of the relative 

rigidity of the vertical elements and the diaphragms using methods of structural 

analysis. Regardless, the diaphragm shears and moments at each level (i) of the 

structure must be determined for lateral forces using the following formula:

 (11)

In this formula, Fpxi is the total force to be applied to the diaphragm at level i, Fj is 

the seismic design force at each level j determined from Equation 10, wpxi is the 

seismic weight of the structure tributary to the diaphragm at level i, and wj is the 

seismic weight at each level j of the structure. 

5.6.3 Seismic Design Category C

The design requirements for structures assigned to SDC C are almost identical to 

those for SDC B but there are a few important differences. First, some structural 

systems that can be used for SDC B are not permitted for SDC C because it is believed they will not perform adequately under the more intense ground motions 

associated with SDC C. In addition, SDC C structures with vertical seismic-forceresisting elements (shear walls, braced frames, moment frames, or combinations 

of these systems) located in plan such that they can experience significant seismic 

forces as a result of shaking in either of the major orthogonal building axes must 

be designed considering this behavior. An example of such a structure is one with 

columns common to intersecting braced frames or moment frames aligned in 

different directions. Another example is a structure with vertical elements aligned 

F

pxi

=

 w

pxi n

wj j=i

n



j=i

Fj

δL δC

δR
84 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

in two or more directions that are not orthogonal to each other. The NEHRP 

Recommended Seismic Provisions requires this type of structure to be designed 

considering that forces can be incident in any direction. The Provisions permits 

satisfaction of this requirement by considering 100 percent of the specified design 

forces applied along one primary axis simultaneously with 30 percent of the 

specified design forces in an orthogonal direction. When this approach is used, at 

least two load cases must be considered consisting of 100 percent of the specified 

forces in direction A taken with 30 percent of the specified forces in direction B 

and 30 percent of the specified forces in direction A taken with 100 percent of 

the forces in direction B where directions A and B are, respectively, orthogonally 

oriented to each other. 

For SDC C structures that are torsionally irregular, the 5 percent accidental torsion 

(discussed in the previous section) is amplified by an additional factor related to 

the amount of twisting that occurs when the design seismic forces are applied.

The Provisions also includes anchorage and bracing requirements for nonstructural components in SDC C structures and requires a site-specific geotechnical 

investigation to evaluate the potential for earthquake-induced ground instability 

including liquefaction, landsliding, differential settlement, and permanent ground 

deformation. If the geotechnical investigation report indicates that the site has 

significant potential to experience any of these instabilities, it also must include 

a discussion of potential mitigation strategies that can be used in the foundation 

design.

5.6.4 Seismic Design Categories D, E, and F

The requirements for determination of lateral seismic forces in SDCs D, E, and F 

are very similar to those for SDC C. The ELF method of analysis can be used for 

all structures of wood or cold-formed steel light frame construction and for all 

regular structures having a fundamental period (T) less than or equal to 3.5Ts as 

determined by Equation 3. The simplified analysis procedure also can be used 

for regular structures having three or fewer above-grade stories. Regardless of 

whether structures are regular or not, the design of SDC D, E, and F structures 

must include consideration of seismic forces acting concurrently in two orthogonal directions as discussed above.
CHAPTER 5 85

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

For structures assigned to SDCs E and F, an additional lower bound is placed on 

the base shear coefficient (Cs) determined as follows:

 (12)

This additional limit on base shear is intended to ensure that structures located 

close to major active faults have sufficient strength to resist the large impulsive 

forces that can occur on such sites.

The lateral seismic forces for structures that cannot be determined using either the 

complete ELF or the simplified procedures must be determined using either the 

response spectrum analysis (RSA) or the nonlinear response history procedures. A 

complete discussion of these procedures is beyond the scope of this document but 

can be found in the references at the conclusion of this report.

Finally, the strength design of structures assigned to SDC D, E, or F is subject to 

consideration of the structure’s redundancy. A structure is considered to be sufficiently redundant if the notional removal of any single element in the structure’s 

seismic-force-resisting system (e.g., a shear wall or brace) does not reduce the 

structure’s lateral strength by more than one third and does not create an extreme 

torsional irregularity. If the configuration of a structure’s seismic-force-resisting 

system meets certain prescriptive requirements, a rigorous check of the structure’s 

redundancy is not required. If a structure does not meet these prescriptive requirements or the minimum strength and irregularity criteria described above, the 

required strength of all elements and their connections comprising the seismicforce-resisting system, other than diaphragms, must be increased by 30 percent.

5.7	 Stiffness and Stability

If the simplified analysis procedure (see footnote in Section 5.6.2) is not used, 

Seismic Design Categories B through F structures must be evaluated to ensure 

that their anticipated lateral deflection in response to earthquake shaking does not 

exceed acceptable levels or result in instability. Two evaluations are required – the 

first is an evaluation of the adequacy of the structure’s interstory drift at each level 

and the second is an evaluation of stability.

Interstory drift is a measure of how much one floor or roof level displaces under 

load relative to the floor level immediately below. It is typically expressed as a 

ratio of the difference in deflection between two adjacent floors divided by the 

CSmin= 0.5 S1

 (R/I)
86 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

height of the story that separates the floors. Figure 42 illustrates the concept of 

interstory drift, showing this as the quantity δi, the drift that occurs under the application of the design seismic forces. 

Figure 42 Interstory drift.

The NEHRP Recommended Seismic Provisions sets maximum permissible interstory drift limits based on a structure’s Occupancy Category and construction 

type. The adequacy of a structure in this respect is determined by calculating the 

design story drift, Δ, as follows:

 (13)

In this equation, δi is the computed interstory drift under the influence of the 

design seismic forces, Cd is the deflection amplification coefficient described in 

Section 5.4, and I is the occupancy importance factor. The acceptable drift ratio, 

Δa, varies from 0.007 to 0.025 depending on the structure’s Occupancy Category 

and construction type.

Drift is also an important consideration for structures constructed in close proximity to one another. In response to strong ground shaking, structures located 

close together can hit one another, an effect known as pounding. Pounding can 

induce very high forces in a structure at the area of impact and has been known to 

cause the collapse of some structures. Therefore, the NEHRP Recommended Seismic Provisions requires that structures be set far enough away from one another 

and from property lines so that pounding will not occur if they experience the 

design drifts determined using Equation 13.

Δ = Cd

δi

 Δahi

 I

δi
CHAPTER 5 87

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

In addition, the Provisions requires an evaluation of a structure’s stability under 

the anticipated lateral deflection by calculating the quantity Ө for each story:

 (14)

In this formula, Px is the weight of the structure above the story being evaluated, 

Δ is the design story drift determined using Equation 13, Vx is the sum of the 

lateral seismic design forces above the story, hx is the story height, and Cd is the 

deflection amplification coefficient described earlier. If the calculated value of Ө

at each story is less than or equal to 0.1, the structure is considered to have adequate stiffness and strength to provide stability. If the value of Ө exceeds 0.1, the 

lateral force analysis must include explicit consideration of P-delta effects. These 

effects are an amplification of forces that occurs in structures when they undergo 

large lateral deflection. The limiting value for Ө (Өmax) is calculated as:

 (15)

If the structure exceeds this limiting value, it is considered potentially unstable 

and must be redesigned unless nonlinear response history analysis is used to 

demonstrate that the structure is adequate. In the equation for Өmax, β is calculated as the ratio of the story shear demand under the design seismic forces to the 

story shear strength. It can conservatively be assumed to have a value of 1.0. This 

requirement can become a controlling factor in areas of moderate seismicity for 

relatively flexible structures like steel moment-resisting frames.

5.8	 Nonstructural Components and

	 Systems

In Seismic Design Categories C and higher, nonstructural components and systems 

also must be designed for seismic resistance. The first step in the process is determining the component importance factor, I

p

. Nonstructural components and 

systems that satisfy any of the following criteria are assigned an I

p

 of 1.5:

• The component is required for life-safety purposes following an earthquake. Fire sprinkler systems and emergency egress lighting and similar 

components are included in this category.

Ө = Px

Δ

 Vx

hxCd

Өmax = 

0.5  0.25

 βCd
88 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

• The component contains hazardous material that, if released, could pose 

a threat to life safety. This would include piping carrying potentially toxic 

gases, tanks containing corrosive materials, laboratory equipment containing potentially harmful bacteria, and similar components.

• The component is attached to an Occupancy Category IV structure and is 

required for continued operation of the structure.

Some nonstructural components with a component importance factor of 1.5 

can be further classified as “designated seismic systems.” Designated seismic 

systems are those active mechanical and electrical components that must remain 

operable following an earthquake and those components containing hazardous 

components. In addition to meeting all of the other requirements for nonstructural components, the suppliers of designated seismic system components must 

provide certification that the components have either been subjected to shaketable testing or that earthquake experience data are available to demonstrate that 

the components will be capable of fulfilling their intended purpose following a 

design level earthquake. 

Some nonstructural components including the following are exempt from seismic 

requirements:

• Mechanical and electrical components in Seismic Design Category C structures except those assigned an I

p

 of 1.5.

• Mechanical and electrical components in Seismic Design Category D, E, or 

F structures that are mounted at floor level, have an I

p

 of 1.0, weigh less 

than 400 pounds, and are connected to any piping or ductwork with flexible connections.

• Mechanical and electrical components in Seismic Design Category D, E, 

or F structures that have an I

p

 of 1.0, are mounted more than 4 feet above 

the floor, weigh less than 20 pounds, and are connected to any piping or 

ductwork with flexible connections.

Components that are not exempt must be installed in structures using anchorage and bracing that have adequate strength to resist specified seismic forces. In 

addition, components attached at multiple points in a structure that can move 

differentially with respect to one another must be able to withstand anticipated 

earthquake displacements without failing in a manner that would endanger life 

safety.

The required strength of component attachments is determined as follows:

 (16) F

p

= 0.3SDSI

p

W

p



0.4a

p

SDSW

p



1+2 z



 1.6SDSI

p

W

p

 (R

p

/I

p

) h
CHAPTER 5 89

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

In this formula, Fp is the required attachment force, I

p

 is the component importance factor, Wp

 is the weight of the component, SDS is the design short-period 

response acceleration calculated in accordance with Equation 1, h is the height 

above grade that the component is mounted in the structure, z is the height above 

grade of the component’s point of attachment, h is the total height of the structure; and ap

 and Rp are component-specific coefficients obtained from the ASCE/

SEI 7 standard that are intended to reflect the dynamic amplification of floor accelerations that some types of component can experience and the ability of some 

components to experience overstress without failure. 

In addition to these general strength and deformation requirements, the NEHRP 

Recommended Seismic Provisions identifies design requirements for some architectural components including exterior glazing and ceiling systems. The requirements for exterior glazing are relatively new in the construction industry and are 

not familiar to many cladding system suppliers. They are intended to ensure that 

large quantities of exterior glazing do not break during earthquakes and fall onto 

occupied street and sidewalk areas.

5.9	 Construction Quality Assurance

Post-earthquake investigations have shown that a considerable amount of the serious earthquake damage to modern structures has occurred, not because of design 

deficiencies, but rather because contractors did not construct structural elements 

and nonstructural components as required in the design drawings and specifications. In order to minimize this problem, the NEHRP Recommended Seismic 

Provisions requires formalized construction quality assurance measures as part of 

the design and construction process. Among the key points of these construction 

quality assurance measures are the following:

• The design professional of record is required to designate on the drawings those structural elements that are part of the seismic-force-resistingsystem,

• The design professional of record is required to indicate designated seismic system nonstructural components on the drawings,

• The design professional of record or another qualified design professional 

must observe the construction of some critical elements to ensure that the 

design is properly interpreted and executed,

• The design professional of record is required to develop a formal Quality 

Assurance Plan that identifies the number and types of inspections and 

tests that must be performed during construction, and
90 CHAPTER 5

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

• Qualified independent inspectors must perform special inspections of key 

elements to ensure that the construction is performed in accordance with 

the design intent.
CHAPTER 6 91

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Chapter 6

FUTURE DIRECTIONS

Earthquake engineering has been one of the most rapidly evolving areas of 

structural engineering practice during the past 40 years. Extensive research and 

development has occurred at major universities and new technologies have been 

rapidly adopted into engineering practice. The NEHRP Recommended Seismic 

Provisions plays an essential role in this process by serving as the effective bridge 

between academic research and practical criteria that can be adopted into the 

model building codes and standards. This chapter identifies important areas of 

future development, some of which are introduced in Part 3 of the Provisions and 

are likely to become requirements in future editions of the Provisions.

6.1	 Rationalization of Design Parameters

As described in Chapter 5, the determination of required strength and acceptable 

drift for structures in all but Seismic Design Category A is dependent on a number 

of coefficients (R, Cd, and Ωo) based on the selection of a structural system. The 

values of these coefficients, which are specified in the ASCE/SEI 7 standard, are 

based on historical precedent and engineering judgment rather than on quantitative analytical study. FEMA recently sponsored the development of a rational 

procedure for determining appropriate values for these coefficients, which is 

described in Quantification of Building Seismic Performance Factors, FEMA 

P-695. The National Institute of Standards and Technology is funding pilot studies 

using this methodology to evaluate the adequacy of the design coefficient values 

presently specified in the ASCE/SEI 7 standard. It is expected that additional studies of this type will be performed in the future and that some adjustment of the 

present design coefficients will be made.

6.2	 Manufactured Component

	 	 Equivalence

In recent years, a number of manufacturers have developed proprietary products 

that are intended to be used as replacements for structural elements designed in 

conformance with requirements for various seismic-force-resisting systems specified by the Provisions. As an example, a number of manufacturers have developed 

and market proprietary shear panels for use as alternatives to structural sheathed 

light frame walls and proprietary moment connections for use in structural steel 
92 CHAPTER 6

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

frames. Building officials require guidance as to when such products can safely 

be accepted as equivalents to elements that are designed and constructed in accordance with the requirements of the Provisions. To satisfy this need, FEMA 

recently funded the development of a simplified component-based comparative 

procedure that is described in Quantification of Building Seismic Performance 

Factors: Component Equivalency Methodology, FEMA P-795.

 

6.3	 Nonbuilding Structures

The design requirements for buildings contained in the building codes have been 

developed over many years and are quite mature. However, the building codes 

also are used to regulate the design and construction of a wide range of nonbuilding structures such as industrial plants, tanks, piers, and wharves. These structures 

have long been designed using the criteria for buildings even though the earthquake response characteristics of many of these structures are not similar to those 

of buildings. In recent years, the NEHRP Recommended Seismic Provisions has 

included specific design criteria appropriate to the various types of these structures. Additional development in this area can be expected in the future.

6.4	 Nonstructural Components

Building code requirements for earthquake resistance were developed principally 

to result in structures capable of resisting strong earthquake ground motions 

without collapse. Nevertheless, a significant amount of earthquake economic 

loss results from the failure of nonstructural components such as walls, ceilings, 

glazing, and elevators and injuries and even life loss also can occur. The NEHRP 

Recommended Seismic Provisions now includes extensive criteria for the design 

and installation of these nonstructural components. However, earthquake damage to these components remains a significant factor and installation problems 

continue to be observed. It is likely that substantial additional development will 

occur in this area.

6.5	 Performance-based Design

The NEHRP Recommended Seismic Provisions presents design criteria that are 

intended to result in buildings and other structures capable of withstanding strong 

earthquake effects with acceptable levels of damage and attendant consequences in 

terms of life, economic, and functionality losses. Although the design procedures 

are intended to provide acceptable performance, they are not directly tied to this 
CHAPTER 6 93

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

performance and it is not clear to designers how these criteria should be changed 

in order to provide buildings and structures with different performance capabilities. Further, because these design procedures do not include actual evaluations 

of a building’s performance capability, many buildings designed in conformance 

with the Provisions may not actually be capable of attaining the desired performance. Performance-based design procedures are an alternative to the prescriptive approaches contained in the Provisions that enable engineers to directly 

consider a building’s probable performance as they perform the design and to 

tailor the design to attain specific desired performance. These procedures can 

allow more reliable, and sometimes more economical, attainment of the performance intended by the Provisions and also can allow buildings to be designed for 

superior performance.

Over the past 20 years, the earthquake engineering community has been engaged 

in the development of performance-based procedures directly focused on providing existing buildings with the capability to deliver specific levels or types of desired performance. The ASCE/SEI 31 and ASCE/SEI 41 standards that evolved from 

earlier FEMA-funded studies and products (FEMA 310 and FEMA 356, respectively) provide criteria for the evaluation and upgrading of existing buildings and 

both represent a first generation of performance-based design criteria. FEMA now 

is engaged in a major project with the Applied Technology Council to develop 

the next-generation of performance-based design criteria. These next generation 

criteria will enable engineers to more reliably design and upgrade buildings to 

achieve specific levels of performance as measured by the probable casualties and 

economic and occupancy losses that may result from future earthquakes. 

Engineers currently use performance-based design procedures under a clause in 

the building codes that allows the application of alternative procedures subject to 

the approval of the authority having jurisdiction. Future editions of the NEHRP 

Recommended Seismic Provisions are likely to include procedures for performance-based design that can be adopted by the building codes.

6.6	 Damage-tolerant Systems

The basic premise underlying the design procedures in the NEHRP Recommended Seismic Provisions is that buildings and other structures will be damaged 

when subjected to the effects of rare intense earthquakes. This damage, when 

it occurs, can result in great economic loss. These losses are experienced by the 

owners of the damaged structures, the people who must live or work in them, 

and the nation as a whole because the economic resources needed for damage repair are diverted from other productive uses. In recent years, a significant amount 
94 CHAPTER 6

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

of academic research has been devoted to the development of structural systems 

capable of surviving intense earthquake effects without damage. Should these 

systems (e.g., rocking frame systems, post-tensioned frame systems, and structures with sacrificial links that can be replaced easily following an earthquake) 

become economically competitive with more traditional systems, the technology 

of seismic-resistant construction will change considerably. Future editions of the 

NEHRP Recommended Seismic Provisions are likely to introduce new damagetolerant systems.
GLOSSARY 95

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

GLOSSARY

Acceleration – Rate of change of velocity with time.

Acceleration Response Spectrum – A graphical plot of the maximum acceleration that 

structures having different characteristics will experience when subjected to a specific 

earthquake ground motion.

Addition – An increase in the aggregate floor area, height, or number of stories of a structure.

Alteration – Any construction or renovation to an existing structure other than an addition.

Appendage – An architectural component such as a canopy, marquee, ornamental balcony, 

or statuary.

Amplification – A relative increase in the magnitude of a quantity, such as ground motion 

or building shaking. 

Amplitude – The maximum value of a time-varying quantity.

Architectural Components – Components such as exterior cladding, ceilings, partitions, 

and finishes.

Base – The level at which the horizontal seismic ground motions are considered to be 

imparted to a structure.

Base Shear Force – A term used in linear structural analysis techniques to describe the vector sum of the lateral forces that are applied to the structure to represent the effects of 

earthquake shaking

Beam – A horizontal structural element.

Bearing Wall System – A structural system in which vertical structural walls serve the dual 

purpose of providing vertical support for a significant portion of the structure’s weight 

as well as resistance to lateral forces.

Building – An enclosed structure generally used for human occupancy.

Building Frame System – A structural system in which vertical forces associated with the 

structure’s weight and that of its supported contents are carried by beams and columns 

while lateral forces associated with wind or earthquake loading are carried by either diagonal braces or vertical walls that do not support significant portions of the structure’s 

weight.

Braced Frame – A structural system in which diagonally inclined members provide the 

structure’s primary resistance to lateral forces.

Cantilever Column System – A structural system in which resistance to lateral forces is 

provided by the bending strength of the vertical column elements, which are fixed 

against rotation at their bases and free to translate and rotate at their tops.

Center of Mass – Point on the building plan about which, the building’s weight is evenly 

distributed.

Coefficient of Variation – A measure of the amount of scatter between the average value in 

a normally distributed group or population and the value that is exceeded by only 84 

percent of the members of the population divided by the average value.
96 GLOSSARY

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Column – A slender vertical structural element.

Component (also Element) – Part of an architectural, structural, electrical, or mechanical 

system.

Concrete – A mixture of Portland cement, sand, rock, water, and other materials that is 

placed into forms, and allowed to harden into a structural element.

Concrete Tilt-up Building – A type of reinforced concrete structure in which the exterior 

concrete walls are constructed laying flat against the ground and then tilted vertically 

into position. 

Configuration – The size, shape, and geometrical proportions of a building.

Connection – A method by which different components are joined to one another.

Cycle of Motion – For a shaking object, the motion that occurs as the object moves from 

an initial position to a maximum displacement in one direction, back through the 

initial position to a maximum displacement in the opposite direction, and then back to 

the initial position.

Damping – The natural dissipation of energy that occurs in a vibrating structure as a result 

of friction, cracking, and other behaviors and that eventually brings a vibrating structure to rest.

Damping Device – A structural element that dissipates energy due to relative motion of 

each end of the device. 

Dead Load – The weight of a structure and all of its permanently attached appurtenances 

including cladding and mechanical, plumbing, and electrical equipment

Deflection – The state of being displaced from an initial at-rest position; see also “Drift.”

Deformation – Load-induced distortion of structural or nonstructural elements or components .

Design Earthquake Shaking – In the Provisions, the earthquake shaking that is two/thirds 

of maximum considered earthquake shaking

Design Seismic Map – A map contained in building codes and referenced standards that 

specifies the geographic distribution of the value of ground shaking parameters that are 

specified as minimum values to be used in design.

Designated Seismic System – A nonstructural component that must remain functional to 

protect life safety or to support the operation of an essential facility.

Diaphragm – A horizontal or nearly horizontal assembly of structural elements used to tie 

a structure together, typically at a floor or roof level

Diaphragm Discontinuity Irregularity – A type of horizontal irregularity.

Displacement – Movement of a structure due to applied forces.

Distribution, Force – Portion of the total forces applied to a structure that is resisted by 

each structural element.

Drift – Vertical deflection of a building or structure caused by lateral forces; see also “Story 

Drift.”

Dual System – A structural system in which a combination of moment-resisting frames 

and braced frames or walls are provided to resist lateral forces. 
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Ductility – The ability of some structural systems to experience extensive deformation and 

damage without loss of load-carrying capability

Earthquake – A sudden motion or vibration in the earth caused by the abrupt release of 

energy in the earth’s lithosphere. 

Eccentricity – Non coincidence between the center of mass and center of resistance of 

a building or characteristic of a braced frame design in which the center lines of the 

braces and the structural members to which they are connected do not meet at a point.

ELF - See “Equivalent Lateral Force Procedure.”

Elastic – Capable of recovering size and shape after deformation.

Elastic Analysis – See “Linear Analysis.”

Essential Facility – A building or structure intended for use during post-earthquake recovery operations including police and fire stations, hospitals, and emergency communications centers

Equivalent Lateral Force Procedure – An approximate method of structural analysis used 

to predict the forces and deformations induced in a structure by earthquake ground 

shaking that represents the effects of such shaking as a series of lateral static forces applied to the structure.

Exceedance Probability – The probability that a specified level of ground motion will be 

exceeded at a site or in a region during a specified exposure time.

Extreme Stiffness Irregularity – A type of vertical structural irregularity sometimes also 

referred to as extreme soft story irregularity.

Extreme Torsional Irregularity – A type of horizontal irregularity.

Extreme Weak Story Irregularity – A type of vertical structural irregularity.

Fault – A fracture in the earth’s crust along which displacement of one side of the fracture 

with respect to the other in a direction parallel to the fracture can occur.

Fault, Active – A fault that has moved one or more times in the past 10,000 years.

Fault Trace – The path along the earth’s surface that overlies a zone of fracture in the earth’s 

crust along which past earthquake movement has occurred

Flexible Diaphragm – A floor, roof, or horizontal bracing system that experiences lateral 

deformations equal to or greater than those experienced by the vertical frames or walls 

it connects.

Force – In physics, the influence that causes a free body to undergo an acceleration. Force 

also can be described by intuitive concepts such as a push or pull that can cause an 

object with mass to change its velocity (which includes to begin moving from a state 

of rest) or that can cause a flexible object to deform.

Frame, Braced – A structural framework which derives it resistance to lateral displacement 

through the action of diagonal members.

Frame System, Building – A structural system with an essentially complete space frame 

providing support for vertical loads; seismic forces are resisted by shear walls or braced 

frames. 

Frame System, Moment Resisting – A structural frame that derives resistance to lateral 

displacement through the rigid or nearly rigid interconnection of beams and columns.
98 GLOSSARY

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Frame, Space – A structural system composed of interconnected members, other than 

bearing walls, that is capable of supporting vertical loads and that also may provide 

resistance to seismic forces.

Frame-Shear Wall Interactive System – A type of structural system in which a structure’s 

resistance to lateral forces is provided by a combination of moment-resisting frames 

and shear walls without limitation on the relative strength of each.

Frequency – For a vibrating structure, the number of times per second that the structure 

will undergo one complete cycle of motion. 

g – The acceleration due to gravity or 32 feet per second per second.

Ground Failure – Physical changes to the ground surface produced by an earthquake; these 

include landslides, lateral spreading, and liquefaction.

Grout – A mixture of sand, Portland cement, water, and other elements used to fill voids in 

masonry construction, bond the masonry units together, and bond reinforcing steel.

Hysteretic Properties – For a structural element or member, the variation of stress in the 

element as a function of imposed deformation considering the prior loading history.

Inelastic Structural Response – The force and deformation behavior of a structure after 

the onset of damage.

Intensity – The apparent effect that an earthquake produces at a given location; in the 

United States, intensity generally is measured by the modified Mercalli intensity (MMI) 

scale.

Intermediate System – A structural system that has been designed to provide more ductility and toughness than that required for an “ordinary” system but less than that for a 

“special” system.”

Interstory Drift – The difference in peak lateral displacement from the at-rest position of 

the center of mass of the diaphragm levels immediately above and below a story.

Interstory Dirt Ratio – The ratio of interstory drift in a story to the story height.

In-plane Discontinuity Irregularity – A type of vertical structural irregularity.

Irregularity – A condition relating to a structure’s shape or the distribution of its weight, 

stiffness, or strength that could lead to atypical behavior when subjected to earthquake 

shaking.

Irregular Structure – A structure that has one or more specified irregularities.

Landslide – Disturbance in hillside ground, sometimes caused by earthquake ground motion, in which one land mass slides down and over another,

Lateral Force – A force that affects an element or portion of a structure as a result of the 

building’s horizontal acceleration in an earthquake

Linear Analysis – Any method of structural analysis that ignores the effects of both structural damage and large displacements on internal forces and displacements

Linear Dynamic Analysis – An approximate method of structural analysis that predicts the 

forces and deformations induced in a structure by ground shaking without consideration of the effects of structural damage that may occur.

Liquefaction – The conversion of a solid into a liquid by heat, pressure, or violent motion; 

sometimes occurs to the ground in earthquakes.
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EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Live Load – The weight of objects supported by a structure but not permanently attached 

to it; the live load changes frequently with time and includes the weight of occupants, 

furniture, and similar items.

Loss – Any adverse economic or social consequences caused by earthquakes.

Masonry – A form of structural construction in which individual blocks of fired clay 

(bricks) or concrete are stacked together and joined with mortar to form an integral 

element.

Mass – A constant quantity or aggregate of matter; the inertia or sluggishness that an object, when frictionlessly mounted, exhibits in response to any effort made to start it or 

stop it or to change in any way its state of motion.

Mat Foundation – A form of foundation in which a monolithic reinforced concrete slab 

underlying a large portion of a structure or perhaps the entire structure is used to transfer the structure’s weight to the underlying soil.

Mercalli Scale (or Index) – A measure of earthquake intensity named after Giuseppe Mercalli, an Italian priest and geologist.

Moment – The force effect associated with the application of a force at a distance from the 

point under consideration.

Moment Resisting Frame – A structural system in which the rigid or nearly rigid interconnection of the horizontal beams and vertical columns provides the primary resistance 

to lateral forces.

Monolithic – In reinforced concrete construction, a term used to describe elements that 

are cast in one continuous placement of concrete without joints.

Mortar – A mixture of sand, cement, lime, and water used to bond bricks or concrete 

blocks together to form an integral structural element.

Natural Period – The time, in seconds or fractions of a section, that a structure in free 

vibration will take to undergo one complete cycle of motion

Nonbuilding Structure – Generally, a self-supporting structure, other than a building, that 

carries gravity loads and that may be required to resist the effects of earthquakes.

Nonstructural Components – Components of a building that are not designed to contribute to its structural resistance.

Nonlinear Analysis – Any of several types of structural analysis that consider the effects of 

structural damage and large displacement on forces and displacements.

Nonlinear Response History Analysis – A method of structural analysis that uses numerical integration of the equation of motion to simulate the forces and deformations that 

occur in a structure in response to earthquake shaking considering the effects of structural damage that may occur.

Nonparallel Systems Irregularity – A type of horizontal irregularity.

Nonstructural Component – A portion of a building or structure that is provided for purposes other than acting as a structural element including doors, windows, some types 

of wall, and mechanical and electrical equipment.

Occupancy Category – A categorization of buildings and other structures based on their 

intended use and the risk that structural failure would pose to the public.
100 GLOSSARY

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Ordinary System – A structural system that has been designed with only limited ductility 

and toughness.

Out-of-Plane Offset Irregularity – A type of horizontal irregularity.

P-delta Effects – A tendency of vertical loads placed on a laterally displaced structure to 

increase the lateral displacements, potentially capable of causing instability. 

Permanent Deformation – A change in the permanent shape and geometry of the ground 

or of a structure that occurs as a result of damage sustained during an earthquake.

Period – The elapsed time (generally in fractions of a second or seconds) of a single cycle 

of a vibratory motion or oscillation; the inverse of frequency.

Pier Foundation – A type of cast-in-place concrete pile that has a large diameter, usually 

greater than 18 inches and sometimes as large as 5 or 6 feet.

Pile Foundation – A type of foundation in which a vertical or nearly vertical element (the 

pile) is embedded directly into the ground to transfer the weight of a structure into the 

ground either through friction between the sides of the pile and the surrounding soil 

or end bearing of the pile against stiff soils and rock beneath it. 

Plain Concrete – A structural element of concrete construction that does not include sufficient steel reinforcement or prestressing to be classified as reinforced or prestressed 

concrete.

Plain Masonry – A structural element of masonry construction that does not include sufficient steel reinforcement to be classified as reinforced masonry. Also termed “unreinforced masonry” or “URM.”

Prestressed Concrete – A form of concrete construction in which reinforcement is provided by steel cables or rods that have been embedded in the concrete and then stressed 

in tension to place the concrete in compression.

Recurrence Interval – see “Return Period.”

Redundancy – A property of some structures in which multiple elements are used to 

provide support for the structure so that if one or some of these elements are damaged, 

other elements are available to continue to support the structure.

Re-entrant Corner Irregularity – A type of horizontal irregularity.

Regular Structure – A structure that does not have any specified irregularities.

Reinforced Concrete – A type of structural element formed of concrete with embedded 

steel rod reinforcement.

Reinforced Masonry – A type of structural element formed of masonry units with embedded steel rod reinforcement.

Reinforcing Steel – Round steel bars that have been deformed to provide bond with concrete and/or grout.

Response Spectrum Analysis – An approximate method of linear dynamic analysis that 

computes the forces and deformations induced in a structure by earthquake shaking 

using a response spectrum as the representation of the ground motion.

Resonance – The amplification of a vibratory motion occurring when the period of an 

impulse or periodic stimulus coincides with the period of the oscillating body.
GLOSSARY 101

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Response, Building – The way in which a building reacts to earthquake ground motion; 

includes, for example, rocking, sliding, distorting, and collapsing.

Response Modification Factor – The factor in the equivalent lateral force equation that 

accounts for damping and ductility inherent in the structure; often referred to as the “R 

factor.”

Return Period – The average time interval, in years, that can be expected between repeat 

occurrences of similar extreme events such as earthquakes, floods, snow and ice accumulations.

Rigid Diaphragm – A floor, roof, or horizontal bracing system that deflects substantially 

less than the vertical frames or walls it connects when subjected to lateral forces.

Risk-Targeted Maximum Considered Earthquake Shaking – The most severe earthquake 

effects considered by the 2009 NEHRP Recommended Seismic Provisions.

Seismic Design Category – A categorization of buildings and other structures based on 

consideration of each structure’s seismic risk.

Seismic-Force-Resisting System – The part of a structural system designed to provide 

required resistance to prescribed seismic forces. 

Seismic Hazard Map – A map showing contours of the maximum ground motion intensity 

or acceleration expected across a geographic region within a defined return period or 

probability of exceedance; in the United States, these maps are produced by the U.S. 

Geological Survey.

Seismic-Load-Resisting System – The assembly of columns, beams, braces, walls, and 

other structural elements that provide a structure’s resistance to seismic loads.

Seismic Risk – A measure of the severity of the possible losses associated with the behavior 

of a building or structure in likely earthquakes

Shear – A force that acts by attempting to cause the fibers or planes of an object to slide 

over one another.

Site Class – A system used to categorize site soil conditions in general terms based on 

the stiffness and depth of soil deposits and the likely effect of these characteristics on 

ground shaking strength and frequency content.

Static Load – A force that remains constant with time.

Stiffness – A quantitative measure of the amount of force required to produce a unit 

amount of deflection or displacement in a structure.

Stiffness Irregularity – A type of vertical structural irregularity.

Story Drift – Vertical deflection of a single story of a building caused by lateral forces.

Strain – Deformation of a material per unit of the original dimension.

Strength – The capability of a material or structural member to resist or withstand applied 

forces.

Stress – Applied load per unit area or the internal resistance of a material to deformation 

forces.

Soft Story Irregularity – See “Stiffness Irregularity.”
102 GLOSSARY

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Special System – A structural system that is designed to provide high levels of ductility and 

toughness.

Structural Element – A piece of a structure that is used to both support the structure’s 

weight and that of its supported contents and attachments and resist various types of 

environmental loads including earthquakes and wind.

Structural Steel – An alloy of iron, carbon, and other elements that has been formed by a 

hot rolling process into either flat plates or shaped elements for use in construction.

Spectral acceleration – The maximum acceleration that a structure having a specific natural 

period of vibration would experience when subjected to a particular earthquake.

Spread Footing Foundation – A type of foundation in which individual reinforced 

concrete slabs are placed beneath individual building columns (or sometimes closely 

spaced groups of columns) to transfer the weight supported by the column(s) to the 

underlying soil.

System – An assembly of components or elements designed to perform a specific function 

(e.g., a structural system or a force-resisting system).

Torsion – Structural behavior associated with twisting about a vertical axis for structures or 

a longitudinal axis for individual structural elements.

Torsional Irregularity – A type of horizontal irregularity.

Transient Deformation – Deformation (movement) of the ground or a structure supported on the ground that occurs during an earthquake event; all or a part of this deformation may be disappear after the earthquake is over.

Unreinforced Masonry – Masonry construction that does not include sufficient steel reinforcement to be classified as reinforced masonry; also referred to as “plain masonry.”

Vertical Bearing Support – The mechanism by which the weight of a structure and its supported contents is transferred to and resisted by the ground.

Vertical Force – A force that acts vertically; vertical earthquake forces represent the effects 

of vertical accelerations experienced in an earthquake.

Weak Story Irregularity – A type of vertical structural irregularity.

Weight/Mass Irregularity – A type of vertical structural irregularity.
SELECTED REFERENCES AND BIBLIOGRAPHY 103

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

SELECTED REFERENCES AND BIBLIOGRAPHY

Algermissen, S. T. 1983. An Introduction to the Seismicity of the United States. 

Earthquake Engineering Research Institute, Oakland, California.

American Concrete Institute. 2008. Building Code Requirements for Structural Concrete 

and Commentary, ACI 318-08. American Concrete Institute, Framington Hills, 

Michigan.

American Institute of Steel Construction. 2005. Seismic Provisions for Steel Buildings,

ANSI/AISC 341-05. American Institute of Steel Construction, Chicago, Illinois

American Iron and Steel Institute. 2008. S100 North American Specification for the 

Design of Cold Formed Steel Structural Members. American Iron and Steel Institute, 

Washington, D.C.

American Society of Civil Engineers/Structural Engineering Institute. 2005. Minimum 

Design Loads for Buildings and Other Structures, ASCE/SEI 7-05. American Society of 

Civil Engineers, Reston, Virginia.

American Society of Civil Engineers/Structural Engineering Institute. 2003. Seismic 

Evaluation of Existing Building, ASCE/SEI 31-03. American Society of Civil Engineers, 

Reston, Virginia.

American Society of Civil Engineers/Structural Engineering Institute. 2006. Seismic 

Rehabilitation of Existing Buildings, ASCE/SEI 41-06. American Society of Civil 

Engineers, Reston, Virginia.

Chen, W. F., and C. R. Scawthorn. 2003. Earthquake Engineering Handbook. CRC Press, 

New York, New York.

Chopra, A. K. 2005. Earthquake Dynamics of Structures, A Primer, Earthquake 

Engineering Research Institute, Oakland, California.

Federal Emergency Management Agency. 2009. NEHRP Recommended Seismic 

Provisions for Seismic Regulation of Buildings and Other Structures, FEMA P-750, 

prepared for FEMA by the Building Seismic Safety Council. Federal Emergency 

Management Agency, Washington, D.C.

Federal Emergency Management Agency. 2006. Homebuilders’ Guide to EarthquakeResistant Design and Construction, FEMA 232, prepared for FEMA by the Building 

Seismic Safety Council. Federal Emergency Management Agency, Washington, D.C.

Federal Emergency Management Agency. 2008. Techniques for the Seismic Rehabilitation 

of Existing Building, FEMA P-547. Federal Emergency Management Agency, 

Washington, D.C.

Federal Emergency Management Agency. 2009. A Quantification of Building Seismic 

Performance Factors, FEMA P-695, prepared for FEMA by the Applied Technology 

Council. Federal Emergency Management Agency, Washington, D.C.

Federal Emergency Management Agency. 2009. Reducing the Risks of Nonstructural 

Earthquake Damage – A Practical Guide, FEMA P-74, prepared for FEMA by the 

Applied Technology Council. Federal Emergency Management Agency, Washington, D.C.
104 SELECTED REFERENCES AND BIBLIOGRAPHY

EARTHQUAKE-RESISTANT DESIGN CONCEPTS

Federal Emergency Management Agency. 2007. Risk Management Series – Designing for 

Earthquakes: A Manual for Architects, FEMA P-454. Federal Emergency Management 

Agency, Washington, D.C.

Lee, W. H. K., H. Kanamori, P. C. Jennings, and C. Kisslinger. 2003. Earthquake and 

Engineering Seismology. Academic Press, San Diego, California.

The Masonry Society. 2008. Building Code Requirements for Masonry Structures,

ACI-530-02/ASCE 5-02/TMS402-02. American Concrete Institute, Framington Hills, 

Michigan.

Naeim, F., Ed. 2001. The Seismic Design Handbook. Springer (formerly Kluver Academic 

Publishers), New York/Heidelberg.

Newmark, N. M., and W. J. Hall. 1982. Earthquake Spectra and Design. Earthquake 

Engineering Research Institute, Oakland, California.