Risk Management Series
Designing
for Earthquakes
A Manual for Architects
FEMA 454 / December 2006


FEMA

FEMA 454 / December 2006

Risk Management Series
Designing 
for Earthquakes
A Manual for Architects
Providing Protection to People and Buildings


Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of FEMA. Additionally, neither FEMA or any of its employees makes any warrantee, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, product, or process included in this publication. Users of information from this publication assume all liability arising from such use.

FOREWORD AND ACKNOWLEDGMENTS
BACKGROUND AND PURPOSE
In 1978, the National Science Foundation supported the American Institute of 
Architects in the preparation of a document entitled Designing for Earthquakes. 
This document, which has long been out of print, was a compendium of papers 
presented at the 1978 Summer Seismic Institutes for Architectural Faculty, held 
at the University of Illinois and Stanford University.   

FEMA has long fostered a strong relationship with the architectural community. 
It was decided that Designing for Earthquakes, which had remained for many years 
a major reference for architects and related professions, should to be updated 
to reflect advances in technology and understanding that had occurred since the 
original document was published.  

The need for updating this publication was prompted by the fact that literature 
on natural hazard mitigation directed towards the architectural profession is 
scarce, in spite of the fact that architects can make a significant contribution 
to hazard risk reduction. While many textbooks exist on the design of structures 
and the nature of earthquakes, they are of a specialist nature, directed to 
their own disciplines, and written in their own special language.

Currently no single publication exists that provides up-to-date information 
necessary to architects, presented in a form that is attractive, readable, and 
intelligible to a non-specialist audience.  This revised publication will fill 
that gap.

The present publication, under the same title as the original document, is a 
completely new work.  It follows the general approach of the original in that it 
consists of a series of chapters that provide the foundation for an 
understanding of seismic design, each authored by an expert in the field. The 
authors were given freedom to decide the scope of their chapters; and thus this 
publication represents expert opinion rather than consensus.  An outside expert 
review panel has reviewed two drafts of the publication to ensure that the 
selected topics are covered in an accurate, interesting, and useful way. 
Designing for Earthquakes: a Manual for Architects is intended to explain the 
principles of seismic design for those without a technical background in 
engineering and seismology. The primary intended audience is that of architects 
and includes practicing architects, architectural students, and faculty in 
architectural schools who teach structures and seismic design.  For this reason, 
the text and graphics are focused on those aspects of seismic design that are 
important for the architect to know.  

Earthquakes in the United States are regional in their occurrence. While 
California is famous for its earthquake,, other states, such as Texas, have much 
less concern for the threat of temblors. However, architectural practice is 
becoming increasingly national and global, and the architect in Texas may find 
that the next project is in California. Thus it has become necessary for the 
professional architect to have some knowledge of the earthquake problem and how 
design seeks to control it. 

Because of its non technical approach, this publication will also be useful to 
anyone who has an interest and concern for the seismic protection of buildings, 
including facility managers, building owners and tenants, building committee 
participants, emergency service personnel, and building officials. Engineers and 
engineering students will also gain from this discussion of seismic design from 
an architectural viewpoint.

The principles discussed are applicable to a wide range of building types, both 
new and existing. The focus is on buildings that are designed by a team that 
includes architects, engineers and other consultants.

ACKNOWLEDGMENTS 
This publication has been produced by the Earthquake Engineering Research 
Institute (EERI) of Oakland, California, under a grant from FEMA/DHS.

Writing Team

Christopher Arnold, FAIA, RIBA	Building Systems Development, Inc. 		
						Palo Alto, California.

Bruce Bolt (deceased) 		Professor Emeritus, Dept. of Civil and
					Environmental Engineering and Earth and 
					Planetary Science, University of California,
					Berkeley, California.

Douglas Dreger 			Associate Professor, Dept. of Earth and 		
					Planetary Science
					University of California, Berkeley, California.

Eric Elsesser			Structural Engineer
					Forell/Elsesser Engineers, Inc.
					San Francisco, California.

Richard Eisner, FAIA		Regional Administrator
					Governors Office of Emergency Services
					Oakland, California.

William Holmes 			Structural Engineer	
					Rutherford & Chekene, 
					Consulting Engineers, Oakland, California.

Gary McGavin			Gary L. McGavin, AIA,
					Professor, Dept. of Architecture, 
					California State University, Pomona

Christine Theodoropoulos, AIA, PE	Head, Dept. of Architecture
					University of Oregon, Eugene

Project Team 

Milagros Kennett 			Architect/Project Officer, 
					Risk Management Series, Mitigation Division, 
					Building Science and Technology, 
					Department of Homeland Security/ FEMA

Susan Tubbesing, 			Executive Director
Project Director			Earthquake Engineering Research Institute, 	
					Oakland, California.
Christopher Arnold, FAIA, RIBA		President
Co-Project Director and Editor		Building Systems Development, Inc.
					Palo Alto, California.

James Godfrey				Special Projects Manager 
Project Coordinator			Earthquake Engineering Research Institute,
						Oakland, California.

Tony Alexander, AIA, RIBA		Graphics Consultant
Publication Design and Graphics	Palo Alto, California.

Wanda Rizer					design4impact
RMS Publications, format and cover	Abbottstown, Pennsylvania

External Review Panel

Leo E. Argiris				Principal
					Arup , New York, New York

Charles Bloszies, AIA			Charles Bloszies, Architecture/Structures, 
					San Francisco, California.

Gary Chock				President, Martin & Chock Inc. 
					Honolulu, Hawaii.

Charles Davis, FAIA		EHDD Architects, San Francisco, California.

Deane Evans, FAIA			Executive Director, Center for Architecture and 
					Building Science Research, 
					NJ Inst. of Technology, Newark, New Jersey.

Kirk Martini			Professor, Dept. of Architecture
					University of Virginia, 	
					Charlottesville, Virginia.

Jack Paddon, AIA			Williams + Paddon, 
					Architects+ Planners Inc 
					Roseville, California.

Todd Perbix, PE			Perbix Bykonen Inc, Seattle, Washington.


FOREWORD AND ACKNOWLEDGMENTS
Background and Purpose							i
	Acknowledgments								iii

CHAPTER 1 INTRODUCTION
			 Christopher Arnold
1.1	Background								1-1 1.2	The 
Architects Role in Seismic Design					1-4 1.3	Contents
								1-5 1.4	The Bottom Line	
						1-8

CHAPTER 2 NATURE OF EARTHQUAKES AND SEISMIC HAZARDS			   Bruce Bolt 
and Douglas Dreger
2.1	Introduction								2-1 
2.2	Observations of Earthquakes						2-3 	
	2.2.1	Plate Tectonics and Seismicity					2-3 	
	2.2.2	Earthquake Fault Types						2-6 	
	2.2.3	Earthquake Effects						2-10 
2.3	Seismic Waves and Strong Motion					2-15 	
	2.3.1	Seismic Instrumental Recordings and Systems			2-15	
	2.3.2	Types of Earthquake Waves					2-17 
2.4	Seismic Sources and Strong Motion					2-22 	
	2.4.1	Earthquake Magnitude						2-23	
	2.4.2	Elastic Rebound and its Relationship to				 	
		Earthquake Strong Motion					2-25 	
	2.4.3	Source Directivity and its Effect on Strong				
		Ground Motions						2-27 
2.5	Strong Ground Motion							2-28 	
	2.5.1	Duration of Strong Shaking					2-29 	
	2.5.2	Estimating Time Histories					2-29
2.6	Seismic Hazard								2-32 	
	2.6.1	Empiric Attenuation Curves					2-32 	
	2.6.2	Probabilistic Seismic Hazard Analysis (PSHA) and Building Codes	2-
35 		2.6.3	Rapid Response: Shake Maps					2-40

 		2.6.3	Rapid Response: Shake Maps					2-43
2.7	Conclusions						2-44     
2.8	Acknowledgments					2-47 
2.9	Cited and Other Recommended References		2-47   2.10	Web Resources
						2-49-
Appendices
I	Quantifying Earthquake Size				2-51   		II
	Earthquake Statistics				2-52 		III	Seismic Wave 
Types					2-53      IV	Measurement of Damping and 
Theoretical Form of 			Wave Amplitude 					2-
54     V	Directivity and Shaking Patterns around a Ruptured 		
	Fault							2-54 VI 	Earthquake 
Probability Estimation			2-55
CHAPTER 3 - SITE EVALUATION AND SELECTION   Richard Eisner
3.1	Introduction						3-1 
3.2	Selecting and Assessing Building Sites in Earthquake 			
	Country							3-1 		3.2.1
	Performance Criteria, Site Selection and Evaluation  		3-2 	
	3.2.2	Building Program and Site Evaluation				3-2

3.3	The Importance of having the Right Team- Using 
Geotechnical Engineering Expertise					3-3
			
3.3.1	The Site Assessment Process					3-4
3.3.2	Geotechnical Report Content					3-4
3.3.3	Additional Investigations to Determine Landslide 
and Liquefaction							3-4
3.3.4	Information Sources for the Assessment Process			3-5

3.4	Local Government Hazard Assessment  DMA 2000			3-6

3.5	Tools for Getting Started							3-6
3.5.1	Understanding Regional Earthquake Risk- the 
Big Picture of Expected Ground Motion				3-6	

3.6`	Earthquake Hazards to Avoid						3-10
3.6.1	Earthquake Fault Zones						3-10	
3.6.2	Ground Failure due to Liquefaction				3-13
3.6.3	Areas of Intensified Ground Motion				3-19
3.6.4	Ground Failure, Debris Flows and Land Slides			3-21

3.7	Off-Site Issues that Effect Site Selection					3-
25
3.7.1	Access and Egress						3-25
3.7.2	Infrastructure							3-25
3.7.3	Adjacency							3-26

3.8	Earthquake and Tsunami Hazards					3-26
3.8.1	Special Considerations for Coastal Area Site 
Assessment  							3-27

3.9	Conclusion								3-31

CHAPTER 4  EARTHQUAKE EFFECTS ON BUILDINGS   Christopher Arnold

4.1	Introduction								4-1

4.2	Inertial Forces and Acceleration						4-1

4.3	Duration , Velocity and Displacement					4-3

4.4	Ground Amplification							4-4

4.5	Period and Resonance							4-5
4.5.1	Natural Periods							4-5
4.5.2	Ground Motion and Building Resonance				4-7
4.5.3	Site Response Spectrum						4-9

4.6	Damping								4-12

4.7	Dynamic Amplification							4-14

4.8	Higher Forces and Uncalculated Resistance				4-14

4.9	Ductility									4-15

4.10	Strength, Stiffness, Force Distribution and Stress 
Concentration								4-16
4.10.1	Strength and Stiffness						4-17
4.10.2	Force Distribution and Stress Concentration			4-18

4.11	Torsional Forces								4-22

4.12	Nonstructural Components						4-23

4.13	Construction Quality							4-25

4.14	Conclusion								4-26

4.15	References								4-27

4.16	To Find Out More							4-27

CHAPTER 5  SEISMIC ISSUES IN ARCHITECTURAL DESIGN   Christopher Arnold

5.1	Introduction								5-1

5.2	The Basic Structural Systems						5-1
5.2.1	The Vertical Lateral Resistance Systems				5-2
5.2.2	Diaphragms-the Horizontal Resistance System		5-4
5.2.3	Optimizing the Structural/Architectural Configuration	5-6

5.3	The Effects of Configuration Irregularity				5-9
5.3.1	Stress Concentrations					5-9
5.3.2	Torsion							5-9

5.4	Configuration irregularity in the Seismic Code			5-10

5.5	Four Serious Configuration Conditions				5-12
5.5.1	Soft and Weak Stories					5-14
5.5.2	Discontinuous Shear Walls				5-18
5.5.3	Variations in Perimeter Strength and Stiffness		5-19
5.5.4	Re-entrant Corners					5-24	

5.6	Other Architectural/Structural Issues				5-28
5.6.1	Overturning: Why Buildings Fall Down, Not Over		5-28
5.6.2	Perforated Shear Walls					5-30
5.6.3	Strong Beam, Weak Column				5-31
5.6.4	Setbacks and Planes of Weakness			5-31

5.7	Irregular Configurations: A Twentieth Century Problem		5-32
5.7.1	A New Vernacular: the International Style and		5-33
      its Seismic Implications

5.8	Designing for Problem Avoidance				5-36
5.8.1	Use of Regular Configurations				5-36
5.8.2	Designs for Irregular Configurations			5-36

5.9	Beyond the international Style: Towards a  Seismic
Architecture?							5-37	
5.9.1	The Architects Search for Forms  Symbolic and 
Metaphorical						5-38
5.9.2	New Architectural Prototypes Today			5-40
5.9.3	Towards an Earthquake Architecture			5-45
5.9.4	Expressing the Lateral Force Systems			5-46
5.9.5	The Earthquake as a Metaphor				5-50

5.10	Conclusion							5-52

5.11	References							5-53

5.12	To Find Out More						5-53

CHAPTER 6  THE REGULATION OF SEISMIC DESIGN   Christine Theodoropoulos

6.1	Introduction							6-1

6.2	Earthquakes and Code Action					6-2
6.2.1	Early Twentieth Century					6-2
6.2.2	The 1920s and the First Seismic Code			6-3
6.2.3	Mid century Codes and the Introduction  of
Statewide Regulations					6-5
6.2.4	Late Twentieth Century: the Move toward New
 Model Building Codes					6-8
6.2.5	Current State of Seismic Code Development		6-13

6.3	Code Intent							6-13
6.3.1	The Purpose of Code Provisions				6-13
6.3.2	Conflicts between Intent, Expectations and 
Performance						6-14

6.4	Performance Based Seismic Design				6-15
6.4.1	Prescriptive Design, Performance Design and 
the Code						6-16
6.4.2	Definitions of Performance Based Seismic Design		6-17
6.4.3	Implementing Performance Based Seismic Design		6-18

6.5	Seismic Design Provisions					6-20
6.5.1	Code-defined Parameters				6-20
6.5.2	Performance Levels					6-20
6.5.3	Performance Based Seismic Engineering			6-22
6.5.4	Engineering Analysis Methods				6-24

6.6	Nonstructural Codes 						6-28

6.7	Conclusion							6-31

6.8	References							6-31

CHAPTER 7	SEISMIC DESIGN  PAST, PRESENT AND FUTURE   Eric Elsesser

7.1	Introduction							7-1

7.2	A Brief Summary of 100 Years of Structural Seismic Design	7-1

7.3	Historic and Current Structural-Seismic Systems
7.3.1	Early Structural Systems- Pre-1906 San Francisco		7-2
	Earthquake
7.3.2	The Early Years (1906-1940)				7-3
7.3.3	The Middle years (1945-1960)				7-3
7.3.4	The Mature Years (1960-1985)				7-4
7.3.5	The Creative years (1985-2000)				7-4

7.4	Background and Progression of Structural-Seismic Concepts	7-6
7.4.1	Development of Seismic Resisting Systems		7-6
7.4.2	Pictorial History of Seismic Systems			7-7

7.5	Commentary on Structural Frameworks				7-30
7.5.1	Steel Building Frameworks				7-30
7.5.2	Concrete Building Frameworks				7-32

7.6	System Characteristics						7-34
7.6.1	Elastic Design-Linear Systems				7-35
7.6.2	Post Elastic Design-Nonlinear Drift			7-35
7.6.3	Cyclic Behavior						7-35
7.6.4	Performance Based Seismic Design			7-36
7.6.5	Nonlinear Performance Comparisons			7-36
7.6.6	Energy Dissipation					7-38
7.6.7	The Design Approach-Force vs. Energy			7-38

7.7	The Search for the Perfect Seismic System				7-39
7.7.1	Structural Mechanisms					7-39
7.7.2	Semi-Active and Active Dampers				7-40
7.7.3	Cost Effective Systems					7-41
7.7.4	Avoiding the Same mistakes				7-44
7.7.5	Configurations are Critical				7-45
7.7.6	Common Sense Structural Design-Lessons Learned	7-46
	
7.8	Conclusions							7-51

7.9	References							7-51

CHAPTER 8  EXISTING BUILDINGS EVALUATION AND RETROFIT   William Holmes

8.1	Introduction							8-1
8.1.1	Contents of Chapter					8-1
8.1.2	Reference to other Relevant Chapters			8-1



8.2	Background							8-1
8.2.1	Changes in Building Practice and Seismic 
Design Requirements Resulting in Buildings that are 
Currently Considered Inadequate				8-3
8.2.2	Philosophy Developed for Treatment of Existing
Buildings						8-6
8.2.3	Code Requirements Covering Existing Buildings		8-9

8.3	The FEMA Program to Reduce the Seismic Risk from 
Existing Buildings						8-16
8.3.1	FEMASponsored Activity for Existing Buildings		8-16
8.3.2	The FEMA Model Building Types				8-21

8.4	Seismic Evaluation of Existing Buildings				8-22
8.4.1	Expected Performance by Building Types			8-32
8.4.2	Evaluation of Individual Buildings				8-34

	8.4.3	Other Evaluation Issues						8-38

8.5	Seismic retrofit of Existing Buildings					8-45
8.5.1	Categories of Retrofit Activity					8.5.1
8.5.2	Conceptual Design of a Retrofit Scheme for an 
Individual Building						8-48
8.5.3	Other Retrofit Issues						8-51
8.5.4	Examples							8-55

8.6	Special Issues with Historic Buildings					8-59
8.6.1	Special Seismic Considerations					8-59
8.6.2	Common Issues of Tradeoffs					8-60
8.6.3	Examples							8-60

8.7	Conclusion								8-64

8.8	References								8-70
8.8.1	References from Text						9-70
8.8.2	To Learn More							9-71

CHAPTER 9  NONSTRUCTURAL DESIGN PHILOSOPHY   Gary McGavin

9.1 	Introduction								9-1

9.2	What is Meant by the Term Nonstructural?				9-2
9.2.1	Architectural Components					9-4
9.2.2	Mechanical and Electrical Components				9-5

9.3	Nonstructural Seismic Design and Normal Seismic Design		9-6

9.4	Effects of Improper Nonstructural Design					9-6

9.5	Damage to Nonstructural Systems and Components 			9-8

9.6	Design Details for Nonstructural Damage Reduction			9-16
9.6.1	Precast Concrete Cladding Panels				9-16
9.6.2	Suspended Ceilings						9-17
9.6.3	Lighting Fixtures							9-17
9.6.4	`Heavy (masonry) Full-height Non-Load Bearing Walls		9-17
9.6.5	Partial-height Masonry Walls					9-18
9.6.6	Partial-height metal Stud Walls					9-18
9.6.7	Parapet Bracing							9-18
9.6.8	Sheet Metal Ductwork						9-19
9.6.9	Piping								9-19
9.6.10	Vibration-isolated Equipment					9-20
9.6.11	Emergency Power Equipment					9-20
9.6.12	Tall Shelving							9-20
9.6.13	Gas Water Heaters						9-20

9.7 	The Need for Systems Design						9-21

9-8	Who is Responsible for Design?						9-22

9-9	Nonstructural Codes							9-25

9-10	Methods of Seismic Qualification						9-25
9.10.1	Design Team Judgment						9-25
9.10.2	Prior Qualification						9-26
9.10.3	Mathematical Analysis and other Qualification 
	Methods							9-26

9.11	Some Myths regarding Nonstructural Design				9-27

9.12	What can the Architect do to Decrease Nonstructural
Damage?								9-29

9.13	The Complexity of Retrofitting Existing Buildings				9-
30

9.14	Conclusions								9-30

9.15	References										9-31

CHAPTER 10  DESIGN FOR EXTREME HAZARDS

10.1    Introduction 

10.2	Multi-hazard Design System Interactions




[End of Front Matter]

INTRODUCTION BY CHRISTOPHER ARNOLD
1.1	THE BACKGROUND
Earthquakes have long been feared as one of nature's most terrifying phenomena.  
Early in human history, the sudden shaking of the earth and the death and 
destruction that resulted were seen as mysterious and uncontrollable.  
We now understand the origin of earthquakes and know that they must be accepted 
as a natural environmental process, one of the periodic adjustments that the 
earth makes in its evolution. Arriving without warning, the earthquake can, in a 
few seconds, create a level of death and destruction that can only be equalled 
by the most extreme weapons of war.  This uncertainty, combined with the 
terrifying sensation of earth movement, creates our fundamental fear of 
earthquakes.  

The Tangshan, China, earthquake of 1976 is officially reported to have caused 
255,000 deaths: foreign observers say the total may be much more.  The city of 
Tangshan was essentially leveled as if struck by an atomic bomb (Figure 1-1). 

Figure 1-1 
The city of Tangshan, China, after the 1976 earthquake. The city was leveled and 
over 250,000 of the city's 750,000 inhabitants were killed. 
SOURCE: CHINA ACADEMIC PUBLISHERS, THE MAMMOTH  TANGSHAN EARTHQUAKE OF 1976,  
BEIJING, 1986

However, Tangshan was a city of largely nonengineered, unreinforced masonry 
buildings: this level of destruction is not expected in a city built in 
accordance with recent seismic codes.

As described in this publication, many characteristics of the site, the 
earthquake and the structure influence seismic performance.  It is common for a 
group of engineered buildings to demonstrate extremely varied damage patterns 
within a small area that receives essentially the same ground motion.  The 
effect of poor soils, clearly shown in the San Francisco earthquake of 1906, was 
demonstrated again in the Loma Prieta earthquake of 1989.  The Marina district, 
which was built partially on fill recovered from the debris of the 1906 
earthquake, suffered substantial damage, and some buildings collapsed because of 
the amplified ground motion at the site (Figure 1-2). The United States does not 
rate very high in deadly earthquakes compared to other countries.  In the entire 
history of the United States, the estimated number of earthquake-related deaths 
is only about 3,100, of which some 2,000 are accounted for by the 1906 San 
Francisco quake. 

Figure 1-2 
Collapsed apartment house in the Marina District of San Francisco, caused  by a 
combination of amplified ground motion and a soft story.
SOURCE: NIST

The Northridge (Los Angeles) earthquake of 1994 is the most recent large 
earthquake in the United States. It was responsible for only 57 deaths (of which 
19 were heart attacks deemed earthquake-related).  This was the result of the 
excellence of California design and construction, the time the earthquake 
occurred (4:31am), and because most of the earthquake's energy was directed 
north into a sparsely populated mountain area. However, the economic losses were 
estimated at $46 billion, and the earthquake was the most costly disaster in the 
nation's history until the recent Gulf area hurricane and floods. 
However, the Kobe earthquake of 1995 showed what an earthquake centered on the 
downtown region of a modern city could do, even though Japan vies with the 
United States in the excellence of its seismic design and research. Over 5,000 
deaths occurred, the majority of which happened in old timber frame buildings 
that had not been engineered. The earthquake sought out a weakness in the 
building inventory that had been overlooked. The regional economy, centered on 
the port of Kobe, was crippled, and large sections of the city's freeways 
collapsed (Figure 1-3).  

Figure 1-3
Damage in Kobe after the 1995 earthquake. Extensively damaged by air raids in 
World War II, Kobe was a relatively new city. Major development took place 
during the boom years of the 1970s and 1980s. Over 5,000 people were killed.

At the regional and national levels, economic losses can be very high in 
industrialized countries for earthquakes that kill relatively few people. Even 
moderate earthquakes cause huge economic losses, largely due to the fragility of 
modern buildings' interiors, systems and enclosures. 
While the low loss of life in United States earthquakes has been a cause of 
cautious optimism - now tempered by the experience of Kobe - increasing economic 
losses as a result of earthquakes are becoming a major concern.  For example, in 
the past 30 years, earthquake losses in California, by far the most earthquake-
prone state, have increased dramatically.

Table 1-1 shows a tabulation by the Federal Emergency Management Agency of 
earthquake losses in California between 1964 and 1994 (FEMA 1997).

Although earthquakes cannot be prevented, modern science and engineering provide 
tools that, if properly used, can greatly reduce their impacts. Science can now 
identify, with considerable accuracy, where earthquakes are likely to occur and 
what forces they will generate. Good seismic engineering can provide structures 
that can survive to a useful degree of predictability.

1.2	THE ARCHITECT'S ROLE IN SEISMIC DESIGN
The key figures in ensuring safe seismic design are the seismologist and the 
structural engineer. However, the architect initiates the building design and 
determines a number of issues relating to its configuration that have a major 
influence on the building's seismic performance. Configuration is defined as the 
building's size and three-dimensional shape, the form and location of the 
structural elements, and the nature and location of nonstructural components 
that may affect seismic performance. Many experienced earthquake engineers say 
that the architect plays the key role in ensuring the satisfactory seismic 
performance of a building.

To develop an effective seismic design, the architect and engineer must work 
together from the inception of the project so that seismic issues and 
architectural requirements can be considered and matched at every stage of the 
design process. For this process to be successful, the architect and engineer 
must have mutual understanding of the basic principles of their disciplines. 
Hence, the architect should have a basic understanding of the principles of 
seismic design so that they will influence the initial design concepts, enabling 
the engineer and architect to work together in a meaningful way, using a 
language that both understand. In turn, the engineer must understand and respect 
the functional and aesthetic context within which the architect works. The 
purpose of this publication is to provide the foundation for these 
understandings and to make the engineering and seismological language of seismic 
design clear to the architect and others who form the design team.
It is not intended that the study of this publication can turn the architect 
into a seismic engineer, capable of performing seismic analysis and creating the 
engineering design for the building. The intent is to help architects and 
engineers become better partners, not to further their separation, and to 
encourage a new level of architect and engineer collaboration.

Inspection and analysis of earthquake-damaged buildings play important roles in 
understanding the effectiveness of seismic design and construction. Although 
earthquake damage often appears random (one building may survive while its 
immediate neighbor will collapse), there are, in fact, patterns of damage that 
relate to the characteristics of the site discussed in Chapter 2 and Chapter 3  
and to the building characteristics discussed on Chapters 4, 5, and 7. 

1.3	THE CONTENTS OF THIS PUBLICATION
Chapter 1 provides an introduction to some of the key issues involved in seismic 
design, including a summary of the effects of earthquakes world-wide and in the 
United States.

The nature of earthquake damage is shown graphically, to provide a context for 
the chapters that follow.

Chapter 2 outlines the characteristics of earthquakes that are important for 
building design and discusses the nature of seismic hazard and how it is 
expressed. The chapter includes up-to-date information on new topics such as 
near-field activity and directivity. 

Chapter 3 discusses the selection and assessment of sites in earthquake hazard 
areas. Important collateral issues such as earthquake-induced landslide and 
liquefaction are covered with special attention to tsunamis. 

Chapter 4 explains the basic ways in which earthquake-induced ground motion 
affects buildings. This includes the ways in which buildings respond to ground 
motion and the characteristics of buildings that may amplify or reduce the 
ground motion that they experience. 

Chapter 5 explains the ways in which fundamental architectural design decisions 
influence building seismic performance, and shows how the building becomes more 
prone to failure and less predictable as the building becomes more complex in 
its overall configuration and detailed execution. A discussion of the ways in 
which architectural configurations are created leads to some speculation on the 
future of architectural design in relation to the seismic problem.
Chapter 6 provides a sketch of the recent history of seismic codes as a means of 
ensuring a minimum level of building safety against earthquakes, and discusses 
some of the key concepts in seismic codes, using the International Building Code 
as a basis. The concept of performance-based design is outlined as a means of 
redressing some of the flaws of current prescriptive methods of building that 
have been revealed in recent earthquakes. 

The principles behind failures caused by architectural decisions are discussed 
in Chapter 4, and specific types of failure are categorized in Chapter 5. These 
two chapters present the core concepts with which the architect should be 
familiar, and make the central argument for the importance of architectural 
design decisions in determining a building's seismic performance.

Chapter 7 uses a largely historical approach to show the development of 
earthquake resistant-design in the twentieth century. By tracing the evolution 
of design in the San Francisco Bay region the chapter shows the great 
inventiveness of earthquake engineers throughout the first half of the century, 
the gradual introduction of advanced methods during the latter half of the 
century, and the application of advanced research in base isolation methods and 
energy dissipation devices that has marked the last two decades.

Chapter 8 tackles perhaps the most difficult problem facing the seismic design 
community, that of improving the safety of our existing seismically hazardous 
buildings. The chapter sketches the main issues of the existing building problem 
and outlines current methods of dealing with them. A common typography of 
building types is illustrated together with their seismic deficiencies and 
common retrofit techniques.

This chapter stresses that the structural systems that are in common use have 
different performance characteristics, and the system selection must be properly 
matched to the site conditions, the architectural configuration, and the nature 
of the nonstructural components and systems in order to achieve the desired 
performance. The performance characteristics of commonly used structural 
systems, both those that are obsolete but still present in older buildings and 
those currently defined in the seismic codes, are outlined in Chapter 7, Figures 
7.11A and 7.11b, and also in Chapter 8, Table 8.3. 

Chapter 9 outlines the scope of the nonstructural design problem: the protection 
of the components and systems that transform a bare structure into a functioning 
building. The chapter suggests that the nonstructural problem demands a systems 
approach to its solution in which the critical linkages between systems are 
protected in addition to the components and systems themselves. 

Chapter 10 recognizes that seismic design does not exist in a vacuum but the 
building must also be protected against other hazards, natural and man-made. In 
this regard, one issue is the extent to which protection from one hazard 
reinforces or conflicts with protection from another.  This chapter uses a 
matrix to compare seismic protection methods to those of the key natural 
hazards: flood and high winds, the traditional hazard of fire, and the new 
hazard of physical attack.  

1.4	THE BOTTOM LINE
This publication is an introduction to its subject, and deals more with 
principles than with the many detailed tasks that go into ensuring the seismic 
safety of a building.  These tasks require a team approach in which all the 
participants in the building design and construction process must participate in 
a timely manner. Understanding the principles discussed in this publication will 
assist the design team as they search for affordable solutions that will provide 
building safety without compromising building function, amenity and delight.  
In the confines of a document that contains a huge scope, the authors must 
necessarily be very selective. Seismic hazard is now clearly recognized as a 
national problem, and analytical and experimental research is being pursued in a 
number of regional centers and universities. However, there are great regional 
variations in seismic hazard levels. California, in particular, has had 
extensive experience with damaging earthquakes that have significantly 
influenced building design. Seismic codes, design practices and related land use 
and rehabilitation provisions originated in California and have been refined 
there for decades. Most of the material in this publication, developed by 
authors with first-hand experience, draws on that readily available wealth of 
knowledge and lessons learned. 

Each chapter includes references to other readily available publications and 
other sources that will enable the interested reader to dig deeper into the 
subject matter.

[End of Chapter 1]

NATURE OF EARTHQUAKES AND SEISMIC HAZARDS by Bruce A. Bolt and Douglas Dreger
2.1 Introduction
Seismology has long contributed to engineering and architecture. The founders of 
seismology, defined as the scientific study of earthquakes, were Robert Mallet 
[1810-1881], a civil engineer, and John Milne [1850-1913], a mining engineer. 
They were first stimulated by their field studies of great earthquakes, and then 
posed some basic questions, such as What is the mechanical explanation for the 
damage (or lack of it) when structures are subject to seismic strong ground 
motion? and What are the essential characteristics of seismic waves that 
affect different structures?

Robert Mallet, after the great Neapolitan earthquake of 1857 in southern Italy, 
endeavored to explain the masses of dislocated stone and mortar that he 
observed in terms of mechanical principles and the building type and design. In 
doing so, he established much basic vocabulary, such as seismology, hypocenter 
(often called the earthquake focus), and isoseismal (contours of equal seismic 
intensity). These nineteenth century links between seismology, engineering, and 
architecture have continued ever since.

A later well-known architectural example is Frank Lloyd Wrights design of the 
Imperial Hotel in Tokyo (Figure 2-1). 

During the planning of his ornate edifice, Wright felt many earthquakes and 
noted that the terror of temblors never left me as I was planning the 
building. He knew that the site of the hotel would be exceptionally dangerous 
in an earthquake because eight feet of topsoil overlaying 60 feet of soft mud 
would not offer firm support. To meet this threat, he introduced a number of 
innovations, including shallow foundations on broad footings, supported by small 
groups of concrete pilings along the foundation wall. Rather than unreinforced 
brick walls, the building had double-course walls composed of two outer layers 
of brick bonded in the middle, with a core of reinforcing bars set in concrete. 
He designed the first floor walls to be rigid and thick; the walls of higher 
floors tapered upwards and contained fewer windows. He topped the structure with 
a hand-worked green copper roof.

Wright was also among the first architects to appreciate that the mechanical 
systems in buildings, such as plumbing and wiring, could be hazards in 
earthquakes. To lessen this risk, he ran the hotel pipes and wires through 
trenches or hung them from the structure so that any disturbance might flex and 
rattle but not break the pipes and wiring. He also conceived the beautiful 
reflecting pool at the front of hotel as a reservoir of water for fire fighting.
Less than nine months after the opening of the Imperial Hotel, the Great 1923 
Kanto earthquake caused enormous devastation in the Tokyo area, shattering over 
5,000 buildings and creating a firestorm. The merit of Wrights reflecting pool 
became clear. The Imperial Hotel still stood after its battering in the 
earthquake, although the damage and cracking within the building was 
considerable. 

Nowadays, seismologists can offer the architect and engineer more reliable 
quantitative knowledge than in 1923 concerning the earthquake hazard at a 
particular site, and also the patterns and intensities of the earthquake waves 
that are likely to shake the structure. To a large extent this is due to recent 
availability of more instrumental recordings of intense seismic wave motions in 
various geological conditions, especially near to their fault sources.

The aim of this chapter is to provide some of the latest knowledge about 
earthquakes that may be most relevant to architectural design. The intent is 
that the description should serve architects when they discuss with their 
clients the appropriateness of certain designs, in relation to a seismic hazard. 
Toward this goal the discussion covers  faulting (the main cause of earthquakes) 
an explanation of the types of waves generated by the fault rupture, the effect 
of soils on the strong ground motions, and contemporary methods of estimating 
earthquake risk. 

References are also provided to a number of research papers and books for the 
architect who wants to pursue the subject more deeply. Several relevant 
addresses of web pages on earthquakes, of which there is a diverse and growing 
number, are also included.  

2.2	Observations of earthquakes	

2.2.1	Plate Tectonics and Seismicity

A coherent global explanation of the occurrence of the majority of earthquakes 
is provided by the geological model known as Plate Tectonics. The basic concept 
is that the Earths outermost part (called the lithosphere) consists of several 
large and fairly stable rock slabs called plates. The ten largest plates are 
mapped in Figure 2-2. Each plate extends to a depth of about 100-200 km and 
includes the Earths outermost rigid rocky layer, called the crust.

The moving tectonic plates of the Earths surface also provide an explanation of 
the various mechanisms of most significant earthquakes. Straining and fracturing 
of the regional crustal rocks result from collisions between adjacent 
lithospheric plates, from destruction of rocky slab-like plate as it descends or 
subducts into a dipping zone beneath island arcs, and from spreading out of the 
crust along mid-oceanic ridges. In the United States, the most significant 
subduction zone is the Cascadia Zone in western Washington state, where the Juan 
de Fuca Plate slides (or subducts) under the America Plate (Figure 2-2).  

Research indicates that ruptures along this zone have resulted in very large 
magnitude earthquakes about every 500-600 years .  The 1964 Alaska earthquake 
was in a subduction zone and was responsible for the greatest recorded  United 
States earthquake. The earthquakes in these tectonically active boundary regions 
are called interplate earthquakes. The very hazardous shallow earthquakes of 
Chile, Peru, the eastern Caribbean, Central America, Southern Mexico, 
California, Southern Alaska, the Aleutians the Kuriles, Japan, Taiwan, the 
Philippines, Indonesia, New Zealand, the Alpine-Caucasian-Himalayan belt are of 
plate-edge type. Earthquakes generated at depths down to 700 km also occur along 
plate edges by a mechanism yet unclear.

As the mechanics of the lithospheric plates have become better understood, long-
term predictions of the place and size of interplate earthquakes become 
possible. For example, many plates spread toward the subduction zones at long-
term geologic rates of from 2 to 5 cm (about one to two inches) per year. 
Therefore, in active arcs like the Aleutian and Japanese islands and subduction 
zones like Chile and western Mexico, the history of large earthquake occurrence 
can identify areas that currently lag in earthquake activity.
There is a type of large earthquake that is produced by slip along faults 
connecting the ends of offsets in the spreading oceanic ridges and the ends of 
island arcs or arc-ridge chains (see Figure 2-2). In these regions, plates slide 
past each other along what are called strikeslip, or transform faults. 
Considerable work has been done on the estimation of strong ground motion 
parameters for the design of critical structures in earthquake-prone countries 
with either transform faults or ocean-plate subduction tectonics, such as Japan, 
Alaska, Chile, Mexico, and the United States. Similar hazard studies have been 
published for the Himalaya, the Zagros (Iran), and Alpine regions all examples 
of mountain ranges formed by continent-to-continent collisions. Such collision 
zones are regions where very damaging earthquakes sometimes occur.

While simple plate-tectonic theory provides a general understanding of 
earthquakes and volcanoes, it does not explain all seismicity in detail, for 
within continental regions, away from boundaries, there are also large 
devastating earthquakes. These intraplate earthquakes can be found on nearly 
every continent (Yeats et al., 1997). The disastrous Bhuj (M = 7.7) earthquake 
in northeast India in the seismically active Kutch province was a recent example 
of such an intraplate earthquake (see Section 2.3.3 for an explanation of 
earthquake magnitude (M). In the United States, the most famous intraplate 
earthquakes occurred in 1811-1812 in the New Madrid area of Missouri, along the 
Mississippi River; another is thedamaging 1886 Charleston, South Carolina, 
earthquake. The Nisqually earthquake of 2001 that took place in Washington was a 
deep focus earthquake with a moment magnitude of 6.8.  However, because of its 
depth of focus (32 miles), structural damage to buildings was not widespread and 
modern buildings and those recently upgraded performed well.

Shallow-focus earthquakes (focus depth less than 70 km) wreak the most 
devastation, and they contribute about three-quarters of the total energy 
released in earthquakes throughout the world. In California, for example, all of 
the known damaging earthquakes to date have been shallow-focus. In fact, it has 
been shown that the great majority of earthquakes occurring in California 
originate from foci in the upper ten kilometers of the Earths crust, and only a 
few are as deep as 15-20 km, excepting those associated with subduction north of 
Cape Mendocino.

All types of tectonic earthquakes defined above are caused by the sudden release 
of elastic energy when a fault ruptures; i.e. opposite sides rapidly slip in 
opposite directions. This slip does work in the form of heat and wave radiation 
and allows the rock to rebound to a position of less strain.
Most moderate to large shallow earthquakes are followed, in the ensuing hours 
and even in the next several months, by numerous, usually smaller, earthquakes 
in the same vicinity. These earthquakes are called aftershocks, and large 
earthquakes are sometimes followed by very large numbers of them. The great Rat 
Island earthquake caused by subduction under the Aleutian Islands on 4 February 
1965 was, within the next 24 days, followed by more than 750 aftershocks large 
enough to be recorded by distant seismographs. Aftershocks are sometimes 
energetic enough to cause additional damage to already weakened structures. This 
happened, for example, a week after the Northridge earthquake of 17 January 1994 
in the San Fernando Valley, when some weakened structures sustained additional 
cracking from magnitude 5.5-6.0 aftershocks. A few earthquakes are preceded by 
smaller foreshocks from the source area, and it has been suggested that these 
can be used to predict the main shock, but attempts along this line have not 
proven statistically successful. 

Volcanoes and earthquakes often occur together along the margins of plates 
around the world that are shown in Figure 2-2.  Like earthquakes, there are also 
intraplate volcanic regions, such as the Hawaiian volcanoes in which earthquakes 
and volcanic activity are clearly physically related.

2.2.2	Earthquake Fault Types

The mechanical aspects of geological faults are the key factors in understanding 
the generation of strong seismic motions and modeling their different 
characteristics. Some knowledge of the fault type to be encountered at a site is 
useful to the architect because of the different types and intensities of motion 
that each fault type may generate. 

First, the geometry of fault-slip is important (see Figure 2-3). The dip of a 
fault is the angle that the fault surface makes with a horizontal plane, and the 
strike is the direction of the fault line exposed or projected at the ground 
surface relative to the north. A strike-slip or transform fault involves 
displacements of rock laterally, parallel to the strike. If, when we stand on 
one side of a fault and see that the motion on the other side is from left to 
right, the fault is right-lateral strike-slip. If the motion on the other side 
of the fault is from right to left, the fault is termed a left-lateral strike 
slip. Events of strike-slip type include the 1857 and 1906 San Andreas fault, 
California, earthquakes and more recently the 1996 Kobe, Japan (Mw = 6.9), 1999 
Izmit, Turkey (Mw =7.6, Figure 2-4), and 2002 Denali, Alaska (Mw =7.9), 
earthquakes.

The right-lateral displacement of the North Anatolian fault in Turkey from the 
1999 event is shown in Figure 2-4. Catastrophic damage to multi-story buildings 
both near and across the fault resulted from the fault motions. A lone standing 
building in the foreground demonstrates that variation in building construction 
is also a factor in the survivability of a structure.

A dip-slip fault is one in which the motion is largely parallel to the dip of 
the fault and thus has vertical components of displacement. There are two types 
of dip-slip faults: the normal and the reverse fault. 

A normal fault is one of dip-slip type in which the rock above the inclined 
fault surface moves downward relative to the underlying crust. Faults with 
almost vertical slip are also included in this category. The Borah Peak (Mw = 
7.3) earthquake in Idaho in 1983 is an example of a normal-type event that 
produced a scarp six feet high.

In a reverse fault, the crust above the inclined fault surface moves upward 
relative to the block below the fault. Thrust faults belong to this category but 
are generally restricted to cases when the dip angle is small. In blind thrust 
faults, the slip surface does not penetrate to the ground surface (for example, 
in the 1994 Northridge earthquake). 

For the common shallow crustal earthquakes, seismic ground motions differ 
systematically when generated by strike-slip, thrust, or normal mechanisms. 
Given the same earthquake magnitude, distance to the site, and site condition, 
the ground motions from thrust earthquakes tend to be (about 20-30 percent) 
larger than the ground motions from strike-slip earthquakes, and the ground 
motions from normal faulting earthquakes tend to be smaller (about 20 percent) 
than the ground motions from strike-slip earthquakes. For subduction earthquakes 
such as the 1964 Alaska (Mw = 9.2) event, the ground motions systematically 
differ from those generated by interface or intra-plate earthquakes. Again, for 
the same magnitude, distance, and site condition, the ground motions from intra-
plate earthquakes tend to be about 40 percent larger than the ground motions 
from inter-plate earthquakes.

Reverse-fault slips have the greatest range of size, because they can grow both 
in the strike and dip directions. In subduction zones, the largest reverse 
events occur in the depth range from 0-100 km, with lengths on the order of 
1,000 km. The 1960 Chile and 1964 Alaska mega-earthquakes (Mw = 9.5 and Mw = 
9.2, respectively) are examples of this type. The 1994 Northridge, California, 
earthquake, despite its moderate size (Mw = 6.7), inflicted considerable damage 
and casualties because of its location on a blind thrust beneath a heavily 
populated region. In most cases however, fault slip is a mixture of strike-slip 
and dip-slip and is called oblique faulting, such as occurred in the 1989 Loma 
Prieta (Mw  = 6.9) earthquake in central California. In the latter case also, 
the fault slip was not visible at the surface of the ground but was inferred 
from seismological recordings. Large scale thrusting of the ground surface was 
very evident along the Chelungpu fault in the 1999 Chi Chi earthquake (Mw = 7.6) 
in Taiwan (see Figure 2-5).

It is at once obvious that any description of seismicity requires a measure of 
earthquake size, for comparison between earthquakes and between seismic hazard 
zones. As in classical mechanics, a suitable quantity to characterize the 
mechanical work done by the fault rupture that generates the seismic waves is 
the mechanical moment. In these terms we can consider the seismic moment that 
is, as might be expected, proportional to the area of fault slip A multiplied by 
the slip distance D.

Fault offset a poses high risk for certain types of structures. When such 
structures, including dams and embankments, must be built across active faults, 
the design usually incorporates joints or flexible sections in the fault zone. 
The maximum horizontal offset in the 1906 San Francisco earthquake was about 18 
feet. 

2.2.3 Earthquake Effects

There are many earthquake effects related to the geology and form of the earth 
that are of significance for architects. In the most intensely damaged regions, 
the effects of severe earthquakes are usually complicated. The most drastic 
effects occur chiefly near he causative fault, where there is often appreciable 
ground displacement as well as strong ground shaking (e.g. Figure 2-4); at 
greater distance, noticeable earthquake effects often depend on the topography 
and nature of the soils, and are often more severe in soft alluvium and 
unconsolidated sediment basins. Some remarkable effects are produced in bodies 
of water such as lakes, reservoirs, and the sea.

l	Ground Shaking Intensity
Efforts to measure the size of an earthquake by rating microseismic data in the 
affected area go back to the 19th century. Before the invention of 
instrumentally based seismic magnitude, the most common historical scale rated 
the relative intensity of an earthquake. This measure is not capable of strict 
quantitative definition because seismic intensity at a particular point of the 
Earths surface depends on many factors, including the source moment M0, area of 
the rupture fault, the fault mechanism, the frequency-spectrum of wave energy 
released, the geological conditions, and the soils at a given site.

The most widely used scale historically was originated by Rossi and Forell in 
1878. A later modification developed by Mercalli in Italy, now termed the 
Modified Mercalli Intensity (MMI) scale, is suitable for conditions in the 
United States. Bolt (2003) describes the details of the various intensity 
measures. 

The geographical distribution of intensity is summarized by constructing 
isoseismal curves, or contour lines, which separate areas of equal intensity.  
The most probable position of the epicenter and the causative fault rupture is 
inside the area of highest intensity. An example of MMI curves for two moderate 
events is given in Figure 2-6. Clearly there can be large regional differences 
in MMI. Such variations in seismic wave attenuation are discussed in Section 
2.6.1.

Correlations have been worked out between measured characteristics of the 
seismic waves and the reported Modified Mercalli intensity. A common one is that 
between the maximum (peak) ground acceleration, A (centimeters per second 
squared), and the MM intensity, I. Such correlations are only broadly 
successful, particularly at the higher intensities. The description of the 
seismic waves for architectural and engineering purposes depends on a mixture of 
parameters, many of which are dependent on the frequency of the seismic waves. 
Nevertheless, because in many parts of the world instrumental measurements of 
ground motion are not available, rough seismic intensity remains popular as a 
descriptor as well as for great historical earthquakes. Peak Ground Acceleration 
is employed as a measure in the current USGS ShakeMaps program, for example: 
these are maps showing ground shaking intensities that are available on the 
internet within a few minutes of an earthquake occurrence (see Section 2.6).
A number of other hazards of a geological nature may be triggered by an 
earthquake occurrence.  These may at times cause severe damage and loss of life.

l	Landslides
Landslides, ground settlement, and avalanches occur widely with and without 
earthquakes as a cause. All require special architectural treatment. Landslides 
and avalanches occur on slopes of a variety of geological materials. For 
engineering works, the speed at which a landslide develops and moves is a most 
important feature. Few defenses are available against rapid unexpected 
movements, but those that move slowly over periods of months to years lend 
themselves to some precautionary measures. Zoning regulations based on localized 
geological studies are the most effective mitigation measures.
During an earthquake, a series of seismic waves shakes the ground in all 
directions, so that under the critical conditions of water saturation, slope, 
and soil type, even relatively low levels of ground acceleration can cause a 
landslide. Even if these dynamic accelerations last for only a short time, 
widespread sliding can occur on marginally stable slopes. During and following 
the 1971 San Fernando, California, earthquake, for example, thousands of 
landslides and rockfalls occurred in the San Gabriel Mountains and caused a 
prominent dust-cloud over the strongly shaken area for days. This was repeated 
during the nearby 1994 Northridge earthquake.

Another human catastrophe caused by an earthquake-triggered debris avalanche 
occurred in Peru on May 31, 1970. The earthquake of magnitude 7.7 stimulated a 
rock avalanche amounting to some 50 million cubic meters of rock, snow, ice, and 
soil that travelled 15 km from the north peak of Huascarn Mountain, buried the 
towns around Ranraharca and most of Yungay, and killed at least 18,000 people.
In many instances, smaller landslides and avalanches can be detected in advance 
by suitable instrumentation installed on the slope with the readings monitored 
at regular intervals. Means of control can then be applied in appropriate 
circumstances: for example, removing small volumes of material to relieve the 
load at the head of the slope and adding material to the toe can be accomplished 
by earth-moving equipment. For cuts that are man-made, local regulations or 
ordinances may need to be developed and enforced during construction in a 
vulnerable area. Slopes made of fill, for example, may be required to be no 
steeper than 1 vertical to 1-1/2 horizontal, and the fraction of the soil 
covering the slope must be carefully controlled. Drainage of water away from 
such slopes is usually specified.

l	Tsunamis and Seiches
The occurrence of an earthquake and a sudden offset along a major fault under 
the ocean floor, or a large submarine landslide, displaces the water like a 
giant paddle, thus producing powerful water waves at the ocean surface. When 
they reach a coastline, they may run up on land to many hundreds of meters. The 
elevation above the tide level (at the time of the tsunami) reached by the water 
is called the run-up height. This vertical distance is not the same as the 
tsunami water wave height offshore or the horizontal distance of water run-up 
from the normal water edge. 

There have been tsunamis in most oceans of the world, but most notably in the 
Pacific Ocean. The coastline of Hilo, Hawaii, has seen inundation several times, 
and the giant earthquake in Alaska in 1964 had a run-up height of six meters in 
Crescent City, California, killing several people. Near the fault motion, 119 
people drowned in Alaska.

A seismic sea wave warning system was set up in the Pacific after the 
devastating Aleutian tsunami of April 1, 1946. The tsunami warning center in 
Honolulu provides tsunami alerts and alerts local jurisdictions to issue 
warnings. 

The best disaster prevention measures for a tsunami-prone coast involve zoning 
that controls the types and sizes of buildings that, if any, are permitted. If a 
site has a high possibility of tsunami incursion, the designer should consider 
some of the design provisions against flood, such as elevating the building 
above an estimated waterline. Of course in the case of locally generated 
tsunami, provisions must also be made for the severe strong shaking.

Long-period movements of water can also be produced in lakes and reservoirs by 
large earthquakes. These oscillations of lake levels are termed seiches. The 
November 2003 Denali earthquake in Alaska generated seismic seiches in wells and 
lakes of the south central United States. In the 1971 San Fernando, California, 
earthquake water sloshed out of swimming pools, producing some risk.

l	Liquefaction
A notable hazard from moderate to large earthquakes is the liquefaction of 
water-saturated soil and sand produced by the ground shaking. In an earthquake, 
the fine-grained soil below the ground surface is subjected to alternations of 
shear and stress. In cases of low-permeability soils and sand, the water does 
not drain out during the vibration, building up pore pressure that reduces the 
strength of the soil.

Because earthquake shaking of significant amplitude can extend over large areas, 
and fine-grained soils in a saturated state are so widespread in their 
distribution, liquefaction has frequently been observed in earthquakes. In some 
cases, it is a major cause of damage and therefore is a factor in the assessment 
of seismic risk. Liquefaction in the 1964 Alaskan earthquake caused major 
disruptions of services and utilities and led to substantial building 
settlements and displacements. In the 1971 San Fernando, California, earthquake, 
liquefaction of soils in the San Fernando Dam caused a landslide in the upstream 
portion of the dam structure that almost resulted in a catastrophic dam failure. 
Widespread liquefaction resulted in severe damage after the 1811-1812 New Madrid 
and 1886 Charleston, South Carolina, earthquakes.

Many seismic regions have available liquefaction maps so that the risk of 
liquefaction at building sites can be assessed. Soil engineers have developed 
various technical methods of controlling liquefaction, the description of which 
goes beyond this chapter (see Chapter 3).

2.3 SEISMIC WAVES AND STRONG MOTION

2.3.1	Seismic Instrumental Recordings and Systems

Seismographs are instruments that are designed to record ground motions such as 
accelerations and displacements in earthquakes. Nowadays, technological 
developments in electronics have given rise to high-precision pendulum 
seismometers and sensors of both weak and strong ground motion. In these 
instruments, the electronic voltages produced by motions of a pendulum or the 
equivalent are passed through electronic circuitry to amplify the ground motion 
and digitize the signals for more exact measurements.

When seismic waves close to their source are to be recorded, special design 
criteria are needed. Instrument sensitivity must ensure that the relatively 
large amplitude waves remain on scale. For most seismological and engineering 
purposes, the wave frequency is high (1 to 10 Hz, i.e., cycles per second), so 
the pendulum or its equivalent can be small. For comparison, displacement meters 
need a pendulum with a long free period (many seconds).

Because many strong-motion instruments need to be placed at unattended sites for 
periods of months or years before a strong earthquake occurs, they usually 
record only when a trigger mechanism is actuated with the onset of seismic 
motion. Solid-state memories are now used with digital recording instruments, 
making it possible to preserve the first few seconds before the trigger starts 
the permanent recording. In the past, recordings were usually made on film 
strips, providing duration of up to a few minutes. 

In present-day equipment, digitized signals are stored directly on a memory 
chip, and are often telemetered to central recording sites in near real-time 
(several to tens of seconds). In the past, absolute timing was not provided on 
strong-motion records but only accurate relative time marks; the present trend, 
however, is to provide Universal (Greenwich Mean) Time - the local mean time of 
the prime meridian by means of special radio receivers or Global Positioning 
Satellite (GPS) receivers.

The prediction of strong ground motion and response of engineered structures in 
earthquakes depends critically on measurements of the locational variation of 
earthquake intensities near the fault. In an effort to secure such measurements, 
special arrays of strong-motion seismographs have been installed in areas of 
high seismicity around the world, both away from structures (free field) and on 
them (Figure 2-7). The seismic instrumentation of various types of buildings is 
clearly to be encouraged by architects, both for post-earthquake performance 
evaluation, future design modification and improved emergency response.
It is helpful for the user of strong-motion seismograms (called time 
histories) to realize that the familiar wiggly line graphic records are not 
the actual motion of the ground, but have been filtered in some way by both the 
recording instrument and by the agency providing the data (see Section 2.6). In 
most cases, however, for practical applications the architect or engineer need 
not be concerned about the difference.

2.3.2	Types of Earthquake Waves

In most instances of seismic ground motions in solid rock or soil, the waves 
involved are made up of four basic types of elastic waves that create the 
shaking that people feel and that causes damage in an earthquake. These waves 
are similar in many important ways to the waves observed in air, water, and 
elastic solids.

The first two types of waves travel through the body of the earth before 
arriving at the surface. The faster of these body waves is appropriately 
called the primary or P wave (Figure 2-8a). Its motion is the same as that of a 
sound wave in that, as it spreads out, it alternately pushes (compresses) and 
pulls (dilates) the rock. These P waves, just like acoustic waves, are able to 
travel through solid rock, such as granite and alluvium, through soils, and 
through liquids, such as volcanic magma or the water of lakes and oceans.
The second and slower seismic body wave through the earth is called the 
secondary or S wave or sometimes the shear wave (Figure 2-8b). As an S wave 
propagates, it shears the rocks sideways at right angles to the direction of 
travel. At the ground surface, the upward emerging S waves also produce both 
vertical and horizontal motions. Because they depend on elastic shear 
resistance, S waves cannot propagate in liquid parts of the earth, such as 
lakes. As expected from this property, their size is significantly weakened in 
partially liquefied soil. The speed of both P and S seismic waves depends on the 
density and elastic properties of the rocks and soil through which they pass. In 
earthquakes, P waves move faster than S waves and are felt first. The effect is 
similar to a sonic boom that bumps and rattles windows. Some seconds later, S 
waves arrive with their significant component of side-to-side shearing motion. 
As can be deduced from Figure 2-8, for upward wave incidence, the ground shaking 
in the S waves becomes both vertical and horizontal, which is the reason that 
the S wave motion is so effective in damaging structures.

The other two types of earthquake waves are called surface waves because their 
motion is restricted to near the earths surface. Such waves are analogous to 
waves in the ocean that do not disturb the water at depth. In a similar way, as 
the depth below the ground surface increases, the ground displacements of 
seismic surface waves decrease.

The first type of surface wave is called a Love wave (Figure 2-8c) Its motion is 
the same as that of S waves that have no vertical displacement; it moves the 
ground side to side in a horizontal plane parallel to the earths surface, but 
at right angles to the direction of propagation. The second type of surface wave 
is called a Rayleigh wave (Figure 2-8d). Like ocean waves, the particles of rock 
displaced by a Rayleigh wave move both vertically and horizontally in a vertical 
plane oriented in the direction in which the waves are traveling. The motions 
are usually in a retrograde sense, as shown by the arrows in Figure 2-8. Each 
point in the rock moves in an ellipse as the wave passes.

Surface waves travel more slowly than P and S waves and Love waves travel faster 
than Rayleigh waves in the same geological formation. It follows that as the 
seismic waves radiate outwards from the rupturing fault into the surrounding 
rocks, the different types of waves separate out from one another in a 
predictable pattern. However, because large earthquake fault sources have 
significantly extended slip surfaces (i.e., many tens of kilometers), the 
separation is often obscured by overlapping waves of different wave types at 
sites close to the fault. Examples of near-fault large amplitude time histories 
are shown in Figure 2-9.

As seismic body waves (the P and S waves), move through layers of rock or soil, 
they are reflected or refracted at the layer interfaces. To complicate matters 
further, whenever either one is reflected or refracted, some of the energy of 
one type is converted to waves of the other type. When the material stiffnesses 
differ from one layer to another, the layers act as wave filters that amplify 
the waves at some frequencies and deamplify them at others. 

It is important to note that when P and S waves reach the surface of the ground, 
most of their energy is reflected back into the crust, so that the surface is 
affected almost simultaneously by upward and downward moving waves. For this 
reason, considerable amplification of shaking typically occurs near the surface, 
sometimes doubling the amplitude of the upcoming waves.  This surface 
amplification enhances the input shaking to structures and is responsible for 
much of the damage produced at the surface of the earth. In contrast, in many 
earthquakes, mineworkers below ground report less shaking than people on the 
surface. Nowadays, it is routine for soil engineers to make allowance for the 
wave amplification effect as the input seismic waves pass upwards through the 
soil layer to the ground surface.

It should be noted that seismic S waves travel through the rocks and soils of 
the earth with both a shearing and a rotational component. The latter components 
of ground motion have important effects on the response of certain types of 
structures, and some building codes now take rotational ground motion into 
consideration.

Seismic waves of all types progressively decrease in amplitude with distance 
from the source.  This attenuation of waves varies with different regions in the 
United States. The attenuation of S waves is greater than that of P waves, but 
for both types attenuation increases as wave frequency increases. Ground motion 
attenuation can flatten and even reverse its downward trend due to strong 
reflected arrivals from rock interfaces. It has been shown that such reflections 
led to elevated ground motions in the 60-80 km distance range from the 1989 Loma 
Prieta, California, earthquake (i.e., in Oakland and San Francisco). Deposits of 
low velocity sediments in geological basins can also cause elevated levels of 
ground motions. 

For a more detailed discussion of seismic wave attenuation and theoretical wave 
amplitude, see Section 2.6.1.

The physical characteristics of seismic waves have been verified by many 
recordings at moderate (15-30 km) to larger distances from the wave source 
called the far-field, but are not adequate to explain important details of the 
heavy shaking near the source of an energetic earthquake called the near-field. 
As explained above, near a rupturing fault, the strong ground shaking consists 
of mixtures of seismic wave types that have not separated distinctly. Although 
this complication makes identification of P, S, and surface waves on strong 
motion records obtained near the rupturing fault difficult, there has been 
recent progress in this skill, based on correlations between actual recordings 
and theoretical modelling. This advance has made possible the computation of 
realistic ground motions at specified sites for engineering design purposes.
Three final points about seismic waves are worth emphasizing here: 

1.	Earthquake waves are much affected by soil elastic properties. For 
example, in weathered surface rocks, alluvium and water-saturated soil, the 
relative sizes of P, S, and surface waves can vary significantly, depending on 
wave frequency, as they propagate through the surficial non-homogenous 
geological structures. Under extreme conditions of large wave amplitude and 
special geotechnical properties, the linear elastic behavior breaks down and 
nonlinear effects occur. 

2.	Patterns of incoming seismic waves are modified by the three-dimensional 
nature of the underground geological structures. As mentioned above, 
instrumental evidence on this effect was obtained from recordings of the 1989 
Loma Prieta, California, earthquake. In this case, strong-motion recordings 
indicated that there were reflections of high-frequency S-waves from the base of 
the earths crust at a depth of about 25 km under the southern San Francisco 
Bay. Also, in this earthquake, large differences in the rock structure from one 
side of the San Andreas fault to the other produced variations in ground motion 
by lateral refraction of S waves. The effect produced significant S wave 
amplitude variation as a function of azimuth from the seismic source, in a 
period range of about 1 to 2 seconds. In addition, there was measurable 
scattering of S waves by separate alluvial basins in the south part of San 
Francisco Bay. Overall, the seismic intensity was enhanced in a region between 
San Francisco and Oakland, about 10 km wide by 15 km long. The observed damage 
and seismic intensity are well explained by these seismological results.

3.	It is important to explain the special seismic intensity enhancement in 
the near field of the earthquake source. Because of special features of 
engineering importance, this discussion of seismic wave patterns near to the 
fault source is given in the separate Section 2.4. As may be seen in Figure 2-
10, time histories of the seismic waves contain pulse-like patterns of motion of 
crucial importance to earthquake response of larger structures.

2.4. SEISMIC SOURCES AND STRONG MOTION

As has been discussed in the previous sections, seismic waves are generally 
generated by the sudden rupture of faults, but can also be initiated by other 
natural processes, such as pulsing of volcanic magma and landsliding. They can 
also be caused by man-made explosions and collapse of subterranean mines. The 
strength of S-wave radiation depends upon the mechanism of the source. In 
particular, fault rupture is an efficient generator of S waves, which are 
responsible for much of the demand of earthquakes on the built environment. The 
seismic wave amplitudes vary with azimuth from the source as a result of the 
orientation of the force couples that cause the fault rupture. The resulting 
pattern of radiation of all types of seismic waves may be described 
mathematically using the same terms used in defining the different types of 
faults (see Figure 2-3), i.e., in terms of the fault strike, dip, and direction 
of slip.

2.4.1	Earthquake Magnitude

The original instrumental measure of earthquake size has been significantly 
extended and improved in recent years. First, because the fundamental period of 
the now superseded Wood-Anderson seismograph is about 0.8 sec., it selectively 
amplifies those seismic waves with periods ranging from 0.5 to 1.5 sec. It 
follows that because the natural periods of many building structures are within 
this range, the first commonly used parameter, called the Richter magnitude (ML) 
based on this seismograph, remains of value to architects. Generally, shallow 
earthquakes have to attain Richter magnitudes of more than 5.5 before 
significant damage occurs, even near the source of the waves. It should be 
remembered that a one unit increase in magnitude indicates a ten-fold increase 
in the amplitude of the earthquake waves.

The definition of all magnitude scales entails that they have no theoretical 
upper or lower limits. However, the size (i.e., the seismic moment) of an 
earthquake is practically limited at the upper end by the strength of the rocks 
of the earths crust and by the area of the crucially strained fault source. 
Since 1935, only a few earthquakes have been recorded on seismographs that have 
had a magnitude over 8.0 (see Table 2-1). At the lower extreme, highly sensitive 
seismographs can record earthquakes with a magnitude of less than minus two. 
For reference, an architect may still encounter the following magnitude scales. 
1.	Surface Wave Magnitude (Ms) is based on measuring the amplitude of surface 
waves with a period of 20 sec. Surface waves with a period around 20 sec are 
often dominant on the seismograph records of distant earthquakes (epicentral 
distances of more than 1,000 km).

2.	Body Wave Magnitude (Mb) Because deep focus earthquakes have no trains of 
surface waves, only the amplitude of the recorded P wave is used. 
Nowadays, because of the shortcomings of ML, Mb, and to a lesser degree Ms in 
distinguishing between the size of the biggest earthquakes, the Moment Magnitude 
scale, Mw , has replaced earlier definitions.

Studies have shown that the Richter Magnitude (ML) scale progressively 
underestimates the strength of earthquakes produced by large fault ruptures. The 
upper-bound value for this scale is about ML = 7. The body wave magnitude (Mb) 
saturates at about the same point. In contrast, the surface-wave magnitude (Ms) 
that uses the amplitude of waves with periods of 20 seconds saturates at about 
Ms = 8. Its inadequacy in measuring the size of great earthquakes can be 
illustrated by comparing values for the San Francisco earthquake of 1906 and the 
great Chilean earthquake of 1960. Both earthquakes had a surface wave magnitude 
(Ms) of 8.3. However, the area that ruptured in the San Francisco earthquake was 
approximately 15 km deep and 400 km long, whereas the length that ruptured in 
the Chilean earthquake was equal to about half of the state of California. 
Clearly the Chilean earthquake was a much larger event (Figure 2-10).
The moment-magnitude scale (Mw) does not suffer from saturation for great 
earthquakes. The reason is that it is directly based on the forces that work 
over the area of the fault rupture to produce the earthquake and not on the 
amplitude and limited frequencies of specific types of seismic waves. Hence, as 
can be expected, when moment magnitudes were assigned to the 1906 San Francisco 
earthquake and the 1960 Chilean earthquake, the magnitude of the San Francisco 
earthquake dropped to 7.9, whereas the magnitude of the Chilean earthquake rose 
to 9.5. Ms and Mw for some great earthquakes are compared in Table 2-1.

2.4.2 Elastic Rebound and its Relationship to Earthquake Strong Ground Motion

The slip along the San Andreas fault that produced the 1906 earthquake was 
studied by H. F. Reid. He imagined a birds-eye view of a straight line drawn at 
a certain time at right angles across the San Andreas fault. As the tectonic 
force slowly works, the line bends, the left side shifting in relation to the 
right. The deformation amounts to about a meter in the course of 50 years or so. 
This straining cannot continue indefinitely; sooner or later the weakest rocks, 
or those at the point of greatest strain, break. This fracture is followed by a 
springing back or rebounding, on each side of the fracture.

This elastic rebound was believed by Reid to be the immediate cause of 
earthquakes, and his explanation has been confirmed over the years. Like a watch 
spring that is wound tighter and tighter, the more the crustal rocks are 
elastically strained, the more energy they store. When a fault ruptures, the 
elastic energy stored in the rocks is released, partly as heat and partly as 
elastic waves. These waves are the earthquake. A remarkable example of this 
phenomenon that produced striking offsets occurred in Turkey in the 1999 Izmit 
earthquake (Figure 2-4).  

Straining of rocks in the vertical dimension is also common. The elastic rebound 
occurs along dipping fault surfaces, causing vertical disruption in level lines 
at the surface and fault scarps. Vertical ground displacement too can amount to 
meters in dip-slip faulting (as in the 1999 Chi Chi, Taiwan, earthquake, 
faulting in Figure 2-5).

Observations show that fault displacement occurs over a continuum of rates from 
less than a second to very slow fault slip. Although the latter creep can pose 
significant hazard for structures built across such rupturing faults, these slow 
slips do not radiate elastic seismic waves. Indeed, the generation of strong 
seismic waves requires that the elastic rebound of the fault is rapid. The 
Lucerne record (Figure 2-9) for the Landers earthquake 3 km from the fault shows 
that elastic rebound (fling-step) occurred over about 7 seconds. This static 
offset arises from near-field waves and their amplitudes attenuate more rapidly 
than far-field body waves. Since this attenuation is strong, and the rise time 
of the fling-step increases with distance, large dynamic motions derived from 
this phenomenon are typically limited to sites very close to the fault. A time 
derivative of the fling-step produces a pulse-like velocity record (Figure 2-
10). For example the fling-step at the Lucerne site for the Landers earthquake 
was recorded 3 km from the fault trace, and the 3 m/s peak velocity recorded 
within 1 km of the fault for the 1999 Chi-Chi, Taiwan, earthquake (Table 2-2) 
had a significant contribution from the fling-step.

2.4.3 Source Directivity and its Effect on Strong Ground Motions

For structures near active faults an additional seismological source effect may 
be important in design in which the direction and speed of a rupture along a 
fault focuses wave energy, producing direction-dependent seismic wave 
amplitudes. This direction-dependent amplitude variation called directivity 
affects the intensity and damage potential of strong ground motions near and at 
moderate distances from the fault source. In contrast to large pulse-like 
dynamic motions derived from a fling-step, those due to directivity are results 
of the superposition or focusing of far-field body waves. Since waves distant 
from the fault attenuate less with distance than those nearby, directivity 
pulses with elevated motions can occur some distance from the fault. To keep 
these two effects separate, the terms directivity pulse and fling-step have 
been used for the rupture directivity and elastic rebound effects, respectively.
Directivity is a term that describes the focusing and defocusing of waves due to 
the direction of rupture with respect to the direction to a given site. 
Therefore it describes azimuthal variation in earthquake ground motion about the 
fault. The difference between the rupture direction and the direction to the 
site is related by an angle. Large ground accelerations and velocities are 
associated with small angles, since a significant portion of the seismic energy 
is channeled in the direction to the site.  Consequently, when a large urban 
area is located within the small angle, it will experience severe damage. 
Studies show that in the Northridge earthquake of 1994, the rupture propagated 
in the direction opposite from downtown Los Angeles and the San Fernando Valley, 
causing only moderate damage, whereas the collapsed SR-18/I5 highway interchange 
was in an area of small angle.  In the Kobe, Japan, earthquake of 1995, the 
rupture was directed towards the city of Kobe, resulting in severe damage.  The 
stations that lie in the direction of the earthquake rupture propagation will 
record shorter strong-motion duration than those located opposite to the 
direction of propagation.

Directivity can significantly affect strong ground motion by as much as a factor 
of 10, and methods are being developed to account for this effect through 
numerical simulation of earthquake ground motions, and by empirical adjustment 
of ground motion attenuation relationships. However, it is not clear how to 
incorporate directivity into methods for predicting ground motion in future 
earthquakes, because the angle between the direction of rupture propagation and 
the source to recording site and the slip history on the fault is not known 
before the earthquake. Studies that incorporate directivity in the analysis must 
therefore investigate many rupture scenarios to examine the range of possible 
motions.

2.5	Strong Ground Motion

As mentioned earlier, for architectural purposes it is important to know that 
near-fault ground motions often contain significant velocity wave pulses, which 
may be from fling in the near-fault, fault-parallel direction, or from 
directivity in the fault-normal direction extending a considerable distance from 
the ruptured fault. For strike-slip fault sources, they dominate the horizontal 
motion and may appear as single or double pulses, each with single or double-
sided amplitudes. The duration (period) of the main pulse may range from 0.5 
sec. to 5 sec. or more for the greatest magnitudes. These properties depend on 
the type, length, and complexity of the fault rupture. 

2.5.1	Duration of Strong Shaking

Field studies demonstrate that the duration of strong ground shaking is often a 
critical factor in the response of foundation materials and structures. There is 
no way to determine the duration of a design event and factor duration into 
current design codes. Soil response in particular can be strongly dependent on 
the increases in pore water pressure with repeated cyclic input. Also nonlinear 
degradation of damaged structures (also caused by long shaking and in large 
aftershocks) can lead to collapse.

2.5.2	Estimating Time Histories

Numerical modeling can be particularly helpful in predicting the effect of 
certain special geological structures on a hazard at a site. Consider, for 
example, the response of the Los Angeles alluvial basin to a large earthquake 
from slip of the San Andreas fault. A computer simulation was made in 1995 by 
Olsen et al. that gives wave motion for a three-dimensional numerical model, 
when the source is a magnitude 7.75 earthquake along the 170 km section of the 
San Andreas fault between Fort Tejon Pass and San Bernardino. The results are 
graphed in Figure 2-11. The wave propagation is represented as horizontal 
velocities of the ground parallel to the San Andreas fault.

The snapshots show that after 40 sec., ground motion in the basin begins to 
intensify, and 10 sec later the entire Los Angeles basin is responding to large 
amplitude surface waves. (The waves shown are spectrally limited to frequencies 
below 0.4 Hz. In an actual earthquake, the ground motions would contain much 
higher frequencies, but the effects would be similar.) The component of motion 
perpendicular to the fault strike is 25% larger than the parallel component near 
the fault due to the directivity of the rupture (see Section 2.4.2). This 
simulation predicted long-period peak ground velocities greater than 1 m/sec. at 
some areas in Los Angeles, even though the main trough of the basin is about 60 
km from the fault. Later analysis of the same region suggests that such computed 
amplitude factors (up to six in deeper parts of the basin) should be used by 
planners and designers as a guide only and with caution.

Instead of such synthetic models, quasi-empirical seismic strong ground motions, 
based on modified actual recordings of similar earthquakes, are now normally 
used to estimate seismic hazard. Two equivalent representations of the hazard 
are commonly considered together. The first is an estimate of the time-history 
of the ground motion appropriate to the site. The second is the response spectra 
(the spectral response of a damped single degree-of-freedom harmonic oscillator, 
see section 4.5.2) for the whole seismic motion at the site. These two 
representations of seismic hazard can be connected by appropriate 
transformations between the time and frequency descriptions of the earthquake.
In the simplest time-history representation, the major interest of architects 
and engineers in assessing the earthquake risk has traditionally been in the 
peak ground acceleration (PGA), velocity, and displacement as a function of 
frequency, or period. In recent work related to large and critical engineered 
structures, however, the pattern of wave motion has been recognized as crucial 
in structural response, because the nonlinear response of such structures is 
often dependent on the sequence of arrival of the various types of waves. In 
other words, damage would be different if the ground motion were run backwards 
rather than in the actual time sequence of arrival. The sequence (phasing) of 
the various wave types on the artificial seismograms can be checked from 
seismological knowledge of times of arrival of the P, S, directivity-pulse, 
fling, and surface waves. Only in this way can a realistic envelope of 
amplitudes in the time histories be assumed.

In the usual calculation, the initial step is to define, from geological and 
seismological information, the fault sources that are appropriate and dangerous 
for the site of interest. This fault source selection may be largely 
deterministic, based on prior experience, or largely probabilistic, and may be 
decided on grounds of acceptable risk. Next, specification of the propagation 
path of the strongest waves is made, as well as the P, S and surface wave 
velocities along the path. These speeds allow calculation of the appropriate 
delays in wave propagation between the source and the multi-support points of 
the structure and the angles of approach of the incident seismic waves.
The computation of realistic motions then proceeds as a series of nonlinear 
iterations, starting with the most appropriate observed strong-motion record 
available, called the seed motion, to a set of more specific time histories, 
which incorporate the seismologically defined wave patterns. The seed strong-
motion accelerograms are chosen to approximate the seismic source type (dip-
slip, etc.) and geological specifications for the region in question. (A set of 
suggested time histories for seed motions is listed in Table 2-2). Many sample 
digitized records can be downloaded using the Virtual Data Center (VDC) website 
of the Consortium of Organizations for Strong-Motion Observational Systems 
(COSMOS) (see section 2.10). The frequency content of the desired time-history 
is controlled by applying engineering constraints, such as a selected response 
amplitude spectrum. Such target spectra are obtained, for example, from previous 
engineering analysis and from earthquake building codes (see, e.g., IBC, 2003).

2.6.	SEISMIC HAZARD

2.6.1	Empirical Attenuation Curves

As has been outlined in the previous sections, the estimation of the earthquake 
hazard in a region or at a site requires the prediction of ground motions. The 
empirical estimation of seismic hazard curves is a necessary step. It follows 
that hazard calculations involve a number of assumptions and extrapolations. The 
common initial difficulty is ignorance of the actual seismic wave attenuation 
for the site in question, despite the recent publication of a variety of average 
curves for certain regions. The importance of attenuation factors in calculation 
of predicted ground motion at arbitrary distances has led to competing empirical 
attenuation forms based on available intensity measurements and geological 
knowledge.

Usually wave attenuation changes significantly from one geological province to 
another, and local regional studies are advisable to calibrate the parameters 
involved.

As mentioned in Section 2.4, although different measures of earthquake magnitude 
are still used, particularly with historical data, the moment magnitude (MW) is 
now usually adopted as a standard measure of size in attenuation statistics. 
Also, nowadays, some form of closest distance to the rupture is used as the 
distance parameter rather than epicentral or hypocentral distance. It is 
important to use the appropriate distance measure for a given attenuation 
relation. The most common source, ray path, and site parameters are magnitude, 
distance, style-of-fault, directivity, and site classification. Rupture 
directivity is defined in detail in Section 2.4.3 and is not discussed here. In 
some studies, additional parameters are used: hanging-wall flag, rupture 
directivity parameters, focal depth, and soil depth classification.
There are also differences in site classification schemes in different regions 
that make comparison of estimates of ground motions from alternative estimates 
difficult. Broad site categories such as rock, stiff-soil, and soft-soil 
are common and affect ground motions (Figure 2-12), but more quantitative site 
classifications based on the S-wave velocity, such as the average S-wave speed 
in the top 30 m, are now preferred. Most attenuation relations simply use a site 
category such as deep soil; however, this general category covers a wide range 
of soil depths from less than 100 m to several kilometers of sediments. Some 
attenuation relations use an additional site parameter to describe the depth of 
the sediment.

For thrust faults, high-frequency ground motions on the upthrown block (hanging-
wall side of a thrust fault) are much larger than on the downdropped block 
(footwall). This increase in ground motions on the hanging wall side is in part 
an artifact of using a rupture distance measure, but may also be due to the 
dynamics of waves interacting with the dipping fault plane and the surface of 
the earth. If a site on the hanging wall and footwall are at the same rupture 
distance, the site on the hanging wall side is closer to more of the fault than 
the site on the footwall side. Such difference was marked in damage patterns to 
houses and other structures in the 1999 Chi Chi, Taiwan, earthquake (Mw = 7.6).
In the eastern U.S., incorporation of a variation in the distance slope of the 
attenuation relation to accommodate the increase in ground motions due to 
supercritical reflections from the base of the crust has been suggested. 
Typically, this result leads to a flattening of the attenuation curve at 
distances of about 100 km). This is most significant for regions in which the 
high activity sources are at a large distance from the site.

An important statistical issue in developing attenuation relations is the uneven 
sampling of the data from different earthquakes. For example, in some cases, an 
earthquake may have only one or two recordings (e.g., the 1940 El Centro event), 
whereas, some of the recent earthquakes have hundreds of recordings (e.g., the 
1999 Chi Chi earthquake). The use of statistical weights can reduce this uneven 
sampling problem. There are two extremes: give equal weight to each data point 
or give equal weight to each earthquake. The random-effects model seems best. It 
uses a weighting scheme that varies between giving equal weight to each 
earthquake and equal weight to each data point, depending on the distribution of 
the data. 

In addition to the median measure of ground motion, the standard deviation of 
the measured ground motion parameters is also important for either deterministic 
or probabilistic hazard analyses. Worldwide, it is common to use a constant 
standard deviation, but recently, several attenuation relations have attributed 
magnitude or amplitude dependence to the standard deviation. 

2.6.2 Probabilistic Seismic Hazard Analysis (PSHA) and Building Codes 

Probabilistic Seismic Hazard Analysis provides an estimate of the likelihood of 
hazard from earthquakes based on geological and seismological studies. It is 
probabilistic in the sense that the analysis takes into consideration the 
uncertainties in the size and location of earthquakes and the resulting ground 
motions that could affect a particular site. Seismic hazard is sometimes 
described as the probability of occurrence of some particular earthquake 
characteristic (such as peak ground acceleration)  For statistical reasons, 
these probabilities cover a range of values, and because risk involves values 
being greater than expected, the word exceedance has been coined as explained 
below.

Probabilistic analysis uses four basic steps in order to characterize the 
probable seismic hazard:

l	Identification of the seismic source or faults. 
This often includes the identification of surface faulting features that can be 
recognized as active. Seismic sources may be specified as site specific, for an 
active source region or, when geologic information is poor, for random 
occurrence of active faults in the study region. Once the faulting hazard is 
identified, earthquake occurrence statistics are compiled, which might be in the 
form of annual rates of seismic events or, in an active regions of known faults, 
more specific information provided by paleoseismic studies such as dating 
episodes of fault offsets. (Paleoseismology involves digging to expose the 
underground face of a fault, so that historic offsets can be made visible and 
material suitable for radio-carbon age dating can be obtained). The objective is 
to obtain a measure of the frequency of earthquakes within a given time period 
as a function of magnitude that may be expressed as a probabilistic statement 
(or mathematical likelihood) of the earthquake occurrence. 

l	Characterization of annual rates of seismic events.
As an example, if there is one magnitude 7 earthquake in a given region every 50 
years, then the annual rate of occurrence is 0.5. Commonly used maps to express 
probability are cast in terms of a 50-year return period, and are used to 
determine the ground motion values to be specified in building codes and used in 
seismic design. 
Since damaging ground motions can result from nearby moderate earthquakes as 
well as large distant earthquakes, the recurrence rates for each magnitude range 
must be determined.

l	Development of attenuation relationships 
Attenuation relationships and their uncertainty due to limited information must 
be developed so that the ground motion parameters for each of the sources 
developed in the first step can be related to the distance of the study site 
from them.

l	Combining factors
The annual recurrence and the attenuation are combined to determine the site-
specific hazard.

Until the 1990s,  seismic building  codes used a single map of the United States 
that divided the country into numbered seismic zones (0,1,2,3,4) in which each 
zone was assigned a single acceleration value in % g  which was used to 
determine seismic loads on the structure.

Starting in the 1970s, new hazard maps began to be developed on a probabilistic 
basis. In the 1994 NEHRP Recommended Provisions (FEMA 222A), two maps of the US 
were provided in an appendix for comment. They showed effective peak 
acceleration coefficients and effective peak velocityrelated coefficients by 
use of contour lines that designated regions of equal value. The ground motions 
were based on estimated probabilities of 10% of exceedance in various exposure 
times (50, 100 and 250 years). The 1997 Recommended Provisions (FEMA 302) 
provided the first spectral response maps to pass consensus ballot. This lead to 
the current maps which, with some revisions, are now used in the 2003 NEHRP 
Provisions (FEMA 450), the ASCE Prestandard and Commentary for the Seismic 
Rehabilitation of Buildings (FEMA 356), and the International Building Code. 
The probabilistic analysis is typically represented in maps in the form of a 
percentage probability of exceedance in a specified number of years.  For 
example, commonly used probabilities are a 10% probability of exceedance in 50 
years (a return period of about 475 years) and a 2% probability of exceedance in 
50 years (a return period of about 2,500 years). These maps show ground motions 
that may be equaled but are not expected to be exceeded in the next 50 years: 
the odds that they will not be exceeded are 90% and 98%, respectively. 
Seismic hazard probability maps are produced by the United States Geological 
Survey (USGS) as part of the National Seismic Hazard Mapping Project in Golden, 
Colorado. The latest sets of USGS of maps provide a variety of maps for Peak 
Ground Acceleration and Spectral Acceleration, with explanatory material, and 
are available on the USGS web site 

The USGS map shown in Figure 2-13 is a probabilistic representation of hazard 
for the coterminous United States.  This shows the spectral acceleration in %g 
with a 2% probability of exceedance in 50 years: this degree of probability is 
the basis of the maps used in the building codes.

The return period of 1 in 2,500 years may seem very infrequent, but this is a 
statistical value, not a prediction, so some earthquakes will occur much sooner 
and some much later.  The design dilemma is that if a more frequent earthquake - 
for example, the return period of 475 years - were used in the lower seismic 
regions, the difference between the high and low-probability earthquakes is a 
ratio of between 2 and 5. Design for the high-probability earthquake would be 
largely ineffective when the low-probability event occurred  
In practical terms, the building designer must assume that the large earthquake 
may occur at any time. Thus, use of the 2,500 return period earthquake in the 
lower seismic regions ensures protection against rare earthquakes, such as the 
recurrence of the 1811-1812 earthquake sequence in New Madrid, Missouri, or the 
1898 Charleston, South Carolina, earthquake.  It was judged that the selection 
of 2 per cent in 50 years likelihood as the maximum considered earthquake ground 
motion would result in acceptable levels of seismic safety for the nation.
The acceleration experienced by a building will vary depending on the period of 
the building, and in general short-period buildings will experience more 
accelerations than long-period buildings, as shown in the response spectrum 
discussed in section 4.5.3. The USGS maps recognize this phenomenon by providing 
acceleration values for periods of 0.2 seconds (short) and 1.0 seconds (long).  
These are referred to as spectral acceleration (SA), and the values are 
approximately what are experienced by a building (as distinct from the peak 
acceleration which is experienced at the ground). The spectral acceleration is 
usually considerably more than the peak ground accelerations, for reasons 
explained in Section 4.7.

Figure 2-14 shows 2%/50 year probability maps for the central and southern 
United States for 0.2 seconds, and Figure 2-15 shows a similar map for 1.0 
second spectral acceleration.  

These USGS probability maps provide the basis for USGS maps used in building 
codes that provide design values for spectral acceleration used by structural 
engineers to calculate the seismic forces on a structure. These design value 
maps differ by use of a maximum considered earthquake (MCE) for the regions. For 
most regions of the country the maximum considered earthquake is defined as 
ground motion with a uniform likelihood of exceedance of 2% in 50 years (a 
return period of about 2,500 years) and is identical to the USGS probability 
maps. However, in regions of high seismicity, such as coastal California, the 
seismic hazard is typically controlled by large-magnitude events occurring on a 
limited number of well-defined fault systems. For these regions, rather than 
using the 2% in 50-year likelihood, it is considered more appropriate to 
directly determine the MCE ground motions based on the characteristic 
earthquakes of those defined faults. 

The 2000 NEHRP Provisions and the 2003 IBC provide maps that show the MCE  for 
the Coterminous United States, California and Hawaii, the Utah region, Alaska, 
the Puerto Rico region and Guam. These maps are produced in black and white line 
with no color coding. A CD-ROM is available from FEMA, USGS, ACSF and IBC that 
includes a software package that can provide map values based on 
latitude/longitute or postal zip code.

Finally, the acceleration values shown on the maps are not used directly for 
design. Instead, they are reduced by 1/3; this value is termed the Design 
Earthquake (DE) and is the value used by engineers for design.  The reason for 
this is that engineers believe that the design provisions contain at least a 
margin of 1.5 against structural failure. MCE is inferred to provide collapse 
prevention level, while the actual design is done using the design earthquake 
(DE), which is 2/3 MCE for code-level life-safety protection-level. This belief 
is the result of the study of the performance of many types of buildings in 
earthquakes, mostly in California.

There have been numerous comments that the level of seismic hazard being used in 
the central and eastern United States results in design values that are 
unreasonably high.  As a result, a review and re-verification of the 2% in 50 
years ground-shaking probability for use as the MCE will be implimented. This 
study is being done as part of the 2008 NEHRP Recommended Provisions update 
process. 

2.6.3 Rapid Response: ShakeMaps
An important point in summarizing the present status of assessment of seismic 
strong ground motions is that in a number of countries, digital strong-motion 
systems linked to communication centers (telephone, wireless, or satellite) have 
now been installed. These provide processed observational data within a few 
minutes after shaking occurs. The USGS ShakeMap program produces a computer-
generated representation of ground shaking produced by an earthquake. (Figure 2-
16) The computation produces a range of ground shaking levels at sites 
throughout the region using attenuation relations that depend on distance from 
the earthquake source, and the rock and soil conditions through the region so 
that the observed strong ground motions can be interpolated. One format of the 
maps contours peak ground velocity and spectral acceleration at 0.3, 1.0, and 
3.0 seconds and displays the locational variability of these ground motion 
parameters.

Not only peak ground acceleration and velocity maps are computed using 
instrumental measurements, but by empirical correlations of the various scales, 
approximate Modified Mercalli Intensity estimates are also mapped. These maps 
make it easier to relate the recorded ground motions to the felt shaking and 
damage distribution. In a scheme used in the Los Angeles basin, the Instrumental 
Intensity map is based on a combined regression of recorded peak acceleration 
and velocity amplitudes (see Wald et al., 1999).

In 2001, such ShakeMaps for rapid-response purposes became available publicly on 
the Internet (see Section 2.10) for significant earthquakes in the Los Angeles 
region and the San Francisco Bay Area of California. Similar maps are available 
in other countries. Additionally, efforts are underway to combine near-real-time 
knowledge about the earthquake source process with the observed strong ground 
motions to produce maps that may better take into account the effects due to 
directivity. ShakeMaps represent a major advance not only for emergency 
response, but also for scientific and engineering purposes. Their evolution and 
improvement will no doubt be rapid.

2.7	Conclusions
The seismological methods dealt with in this chapter will no doubt be much 
extended in subsequent years. First, greater sampling of strong-ground motions 
at all distances from fault sources of various mechanisms and magnitudes will 
inevitably become available. An excellent example of seismic recording growth 
comes from the 1999 Chi-Chi, Taiwan, earthquake.

Another interesting recent case is the major Alaska earthquake of November 3, 
2002. This 7.9 magnitude earthquake was caused by rupture along the Denali fault 
for 200 km, with right-lateral offsets up to 10 m. A number of strong-motion 
records were obtained; the Trans-Alaskan oil pipeline did not suffer damage 
because of an innovative pipeline design combined with sophisticated knowledge 
of the seismology.

Second, more realistic 3D numerical models will solve the problem of the 
sequential development of the wave mixtures as the waves pass through different 
geological structures. Two difficulties may persist: the lack of knowledge of 
the roughness distribution along the rebounding fault and, in many places, the 
lack of quantitative knowledge of the soil, alluvium, and crustal rock 
variations in the region. For these reasons, probabilistic estimation as a basis 
of engineering decisions seems preferable.

Over the past decade, advances in digital seismometry have greatly reduced the 
recovery and computer processing time of recorded data, producing near-real time 
analysis products important for post-earthquake emergency response (Gee et al., 
1996; Dreger and Kaverina, 2000; Wald et al., 1999). Continuing improvements in 
technology are expected to further increase the amount of timely earthquake 
source and strong ground motion information. A recent significant advance in 
general motion measurement is correlation with precisely mapped co-seismic 
ground deformations, and efforts are currently underway to obtain and analyze 
these data in near-realtime. Networks of continuous, high-sample-rate Global-
Positioning-System (GPS) instruments will no doubt help greatly in future 
understanding of the source problem and the correct adjustment to strong-motion 
displacement records. 

A broad collection of standardized strong-motion time histories is now being 
accumulated in virtual libraries for easy access on the Internet. Such records 
will provide greater confidence in seismologically sound selection of ground 
motion estimates.

Additional instrumentation to record strong ground motion remains a crucial need 
in earthquake countries around the world. Such basic systems should measure not 
only free-field surface motions, but also downhole motions to record the wave 
changes as they emerge at the earths surface.
The Advanced National Seismic System (ANSS) program is a major USGS  and NEHRP 
initiative that provides accurate and timely information on seismic events.  It 
is working to unify seismic monitoring in the United States, and provides a 
framework to modernize instrumentation and revolutionize data availability for 
research, engineering and public safety. (For more information, see 
http:www.anss.org/.) 

In particular, many contemporary attenuation estimates for ground velocity and 
displacement will no doubt be improved as more recorded measurements are 
included, rendering earlier models obsolete. The statistical basis for 
separation of the probability distributions as functions of the various key 
parameters will become more robust. To keep abreast of changes, ground motion 
attenuation model information may be found at the USGS Earthquake Hazards 
Program website (see Section 2.10).

2.8	Acknowledgments
We thank the personnel of the U. S. Geological Survey for their efforts in 
preparing USGS Fact Sheet 017-03, from which several figures were obtained.
2.9 Cited and Other Recommended References
Abrahamson, N. A. and W. Silva, 1997, Empirical response spectral attenuation 
relations for shallow crustal earthquakes, Seism. Res. Letters, 68, 94-127.
Atkinson, G. M., and D. M. Boore, 1995, New ground motion relations for eastern 
North America, Bull. Seism. Soc. Am., 85, 17-30.
Bolt, B. A., 1975, The San Fernando earthquake, 1971. Magnitudes, aftershocks, 
and fault dynamics, Chap. 21, Bull. 196, Calif. Div. of Mines and Geol., 
Sacramento, Calif.
Bolt, B. A. 2003. Engineering seismology, In: Earthquake Engineering, Recent 
Advances and Applications, eds. Y. Bozorgnia and V. V. Bertero, CRC press. 
Florida.
Bolt, B. A., 2003, Earthquakes, 5th edition, W. H. Freeman: New York.
Comerio, M., 1998. Disaster Hits Home, University of California Press: Berkeley.
FEMA 450, NEHRP (National Earthquake Hazards Reduction Program) Recommended 
Provisions and Commentary for Seismic Regulations for New Buildings and Other 
Structures, 2003 Edition, Federal Emergency Management Agency, Washington, DC 
Gee, L. S., D. S. Neuhauser, D. S. Dreger, M. Pasyanos, R. A. Uhrhammer, and B. 
Romanowicz, 1996, Real-time seismology at UC Berkeley: The Rapid Earthquake Data 
Integration Project, Bull. Seism. Soc. Am., 86, 936-945.
Hanks, T C, and H.  Kanamori, 1979. A moment magnitude scale, Journ. Geophys. 
Res., 84, 2348-2350.
International Code Council, International Building Code (IBC), 2003, Falls 
Church, VA 
National Fire Protection Association, NFPA 5000, Building Construction and 
Safety Code , 2006,  , Quincy,MA
Olsen, K.B., R.J. Archuleta, and J.R. Matarese (1995). Three-dimensional 
simulation of a magnitude 7.75 earthquake on the San Andreas fault, Science, 
270, 1628-1632. 
Stein, R. S., 2003, Earthquake Conversations, Scientific American, Vol. 288, No. 
1, 72-79.
Yeats, R. S., C. R. Allan, K. E. Sieh, 1997. The Geology of earthquakes, Oxford 
University Press.
Yeats, R. S., 2001. Living with Earthquakes in California: A Survivors Guide, 
Oregon State University Press: Corvallis.
2.10 Web Resources
Consortium of Organizations for Strong-Motion Observational Systems COSMOS  
http://www.cosmos-eq.org
European Strong-Motion Database (ISESD)
http://www.isesd.cv.ic.ac.uk/
National Seismic Hazard Mapping Project, Golden, Colorado
http://geohazards.cr.usgs.gov/eq/
ShakeMaps   www.trinet.org/shake
Tsunami Warning Centers  http://www.prh.noaa.gov/pr/ptwc/
http://wcatwc.gov/
USGS Earthquake Hazards Program  http://earthquake.usgs.gov/


[End of Chapter 2]

Risk Management Series Designing for Earthquakes
A Manual for Architects
FEMA 454 / December 2006

SITE EVALUATION AND SELECTION BY RICHARD EISNER
3.1 INTRODUCTION
This chapter describes how earthquake hazards can affect site selection and 
planning, and the process for identification of site and regional factors that 
impact seismic design. Site selection is typically determined by initial land 
costs, land use criteria such as zoning, proximity to transportation, and 
utility infrastructure. Additional site location factors that should be 
considered include environmental and geotechnical site conditions that would 
impact building performance, and factors that influence structural design 
criteria that would impact costs and performance.  
The importance of a design team comprised of the client, architect, the 
geotechnical civil engineer and structural engineer is emphasized, and a process 
for geotechnical assessment of a site is identified. Regional factors of 
earthquake probabilities and ground motions are identified and reviewed at the 
project level. The interaction of the regional risk, building program, and 
client expectations is discussed in the context of performance objectives.  Site 
hazards are identified, and mitigation approaches are presented.
3.2 SELECTING AND ASSESSING BUILDING SITES IN EARTHQUAKE COUNTRY
In earthquake hazard areas, selection and evaluation of the site will be 
critical to meeting client expectations on project performance. Identification 
and analysis of the threat posed by earthquakes to a specific location or site 
are more complex and frequently less precise than analysis for hazards such as 
flood or wind, where information about frequency and intensity of events is well 
documented. For example, the threat of flood is defined by 100-year flood zones 
delineated by the National Flood Insurance Program1 (1% probability of being 
exceeded per year) and is mapped at the parcel level. A site is either in or out 
of the flood zone.  Areas at risk to earthquake damage encompass entire regions, 
not just the areas adjacent to faults. The zones of potential damage are not 
neatly defined or delineated. There are numerous factors in addition to shaking 
that will affect a buildings performance and its continued function. Therefore, 
at the onset of a project, a thorough examination should be undertaken of 
regional potential for earthquakes and the areas that will be damaged by ground 
faulting, ground shaking, subsidence and liquefaction, utility disruption and 
from the secondary hazards and impacts from earthquake-caused fires, floods, and 
hazardous materials releases.  
3.2.1 Performance Criteria, Site Selection, and Evaluation
Site selection criteria should be derived from the buildings program and 
performance-based design criteria. A simple project may only be designed to the 
minimum level of performance - life safety. Such a structure is only expected to 
protect the lives of occupants and may be so extensively damaged after a quake 
that it will have to be demolished.  With a small project where client 
performance criteria are limited, the site evaluation criteria will be focused 
on the immediate site environment, on-site hazards, and adjacent structures and 
land uses. Geotechnical investigations focus primarily on the site. Mitigation 
is usually accomplished by providing a setback to separate new construction from 
adjacent hazards and through design of the foundations and structure to meet the 
building code. Where the client has higher expectations of building performance, 
such as minimizing damage and maintaining business operations, the assessment 
will need to be more rigorous, and the scope of the site investigations will 
extend far beyond the property line to include all of the potential hazards 
that would influence continuity of operations, including land uses in proximity 
to the site and area access and egress, utility performance, the need for 
alternative lifeline capability (back-up generators, water and waste water 
processing and storage, alternate telecommunications, etc.).  
For facilities designed to performance-based criteria, including minimum 
disruption and continued operation, the location of the site within the region 
may play a critical role in meeting client expectations. The definition of 
site becomes the region within which the facility is located, and 
vulnerability assessments must examine both facilities and the connections the 
facilities have to raw materials, personnel and distribution to markets. This is 
a more holistic view of building design and vulnerability, that addresses 
disruption of operations and the economic impacts of disasters.
3.2.2 Building Program and Site Evaluation
The development of the program for the building and the definition of 
performance criteria are  iterative processes that take into account the needs 
of the client, the characteristics of the earthquake hazard, the characteristics 
of the site (or alternate sites) and availability and cost of engineering 
solutions to mitigate the hazard (Figure 3-1). If the client wishes the building 
to withstand a major earthquake without damage and be able to maintain 
operations, the program will establish both performance and site selection 
criteria to achieve their goal.  The program will also establish utility, 
access, and egress performance expectations that will influence location.
3.3 THE IMPORTANCE OF THE RIGHT TEAMGEOTECHNICAL ENGINEERING EXPERTISE
Understanding and incorporating the earthquake threat and its impact on a 
location or facility is a complex assessment process requiring an understanding 
of the earthquake hazard, how a site will respond to arriving ground motions, 
and how a structure will interact with the sites motions.  It is therefore 
essential that the client, architect and structural engineer retain the services 
of a Geotechnical Engineer to provide input to the assessment of alternate 
sites, and to assist in the structural design of the programmed facility.  
3.3.1 The Site Assessment Process
The California Geological Survey (CGS) provides guidance in the use of 
geotechnical and civil engineering expertise in Guidelines for Evaluating and 
Mitigating Seismic Hazards in California.2 The Guideline emphasizes the need for 
both geotechnical engineering to identify and quantify the hazard, and civil 
engineering to develop mitigation options for the architect and owner. Chapter 3 
of the Guideline provides recommended site investigations for assessing seismic 
hazards and is summarized below.
3.3.2 Geotechnical Report Content
The geotechnical investigation of the site is a vital resource to designer and 
structural engineer in designing and building an earthquake-resistant structure. 
The CGS recommends that a geotechnical report include the following data:
	Description of the proposed project location, topography, drainage, 
geology, and proposed grading.
	Site plan indicating locations of all tests.
	Description of the seismic setting, historic seismicity and location of 
closest seismic records used in site evaluation.
	Detail (1:24,000) geologic map of the site indicating pertinent geologic 
features on and adjacent to the site.
	Logs of all boring or other subsurface investigations.
	Geologic cross section of the site.
	Laboratory test results indicating pertinent geological data.
	Specific recommendations for site and structural design mitigation 
alternatives necessary to reduce known and/or anticipated geologic and seismic 
hazards.
3.3.3 Additional Investigations to Determine Landslide and Liquefaction
Additional tests may be necessary to determine if there is a potential for 
earthquake induced landslides and/or liquefaction.  These tests and procedures 
are identified in Recommendation Procedures for Implementation of California 
Department of Mines and Geology (CDMG) Special Publication 117:  Guidelines for 
Analyzing and Mitigating Liquefaction in California; and Recommended Procedures 
for Implementation of DMG Special Publication 117: Guidelines for Analyzing and 
Mitigating Landslide Hazards in California.3 
3.3.4 Information Sources for the Site Assessment Process
In evaluating or selecting a site, the objective will be to identify those 
natural and man-made forces that will impact the structure, and then to design a 
site plan and the structure to avoid or withstand those forces. It is necessary 
to start the site evaluation process with research of information available from 
local building and planning departments, the National Weather Service, FEMAs 
National Flood Insurance Program (NFIP), the United States Geological Survey, 
state geological surveys, university geology departments and published research, 
and a geotechnical engineering firm familiar with the region and sources of 
local information. Include, where available, hazard mapping zones of ground 
faulting, liquefaction, landslides and probabilistic assessments of ground 
motions.
Where sites are within a mapped hazard zone, a site-specific investigation 
should be conducted by a geotechnical engineer to identify or demonstrate the 
absence of faulting, liquefaction or landslide hazards. When a hazard is 
identified and quantified, recommendations for mitigation should be provided. 
The following information will assist in assessing the geotechnical hazards in a 
region or on a site:
	Topographic, geologic and soil engineering maps and reports, and aerial 
photographs
	Water well logs and agricultural soils maps.
	State hazard evaluations maps.
FEMAs Understanding Your Risks:  Identifying Hazards and Estimating Losses 
(FEMA 386-2)4 provides an excellent example of a hazard assessment process that 
can be adapted to your practice.  
When a site is outside a mapped hazard zone, ensure that proposed development 
and alterations to the site do not increase susceptibility to hazards (such as 
cuts and fills that increase ground water percolation or increase the likelihood 
of earthquake-induced landslides).
3.4 LOCAL GOVERNMENT HAZARD ASSESSMENTSDMA 2000
In 2000, Congress amended the Stafford Act (federal legislation that provides 
pre- and post-disaster relief to local and state governments), adding 
requirements that local governments, states and tribes identify and develop 
mitigation plans to reduce losses from natural hazards. The Disaster Mitigation 
Act of 2000 (DMA 2000)5 requires these governments to identify and map all 
natural hazards that could affect their jurisdictions.  Beginning in November 
2004, local and state governments were to be able to provide an architect or 
engineer with hazard and risk assessments for earthquakes, flooding, landslides, 
tsunami, and coastal erosion. The risk assessments6 are intended to be the basis 
for land use development decisions and for setting priorities for local and 
federal mitigation funding, but they will also provide a basis for initial site 
selection and evaluation.  
3.5 TOOLS for GETTING STARTED
As noted in Chapters 2 (Section 2.6.2) and 4, earthquakes produce complex 
forces, motions and impacts on structures.  Between the earthquake and the 
structure is the site, which determines how the building experiences the 
earthquake, and what secondary hazards are triggered by ground motions. These 
additional hazards include surface rupture or faulting; near-source effects of 
strong ground motions; ground failure and landslides, subsidence; and lateral 
spreading and liquefaction. In coastal regions, in areas within dam inundation 
zones, or areas protected by earth levies, flooding can occur as a result of dam 
or levy failure triggered by ground motions, or, in coastal areas, by 
earthquake-triggered tsunamis. Each of these primary and secondary hazards 
should be identified in the site assessment and mitigated where they would 
adversely impact building performance.  The following sections will elaborate on 
each of these site hazards and identify mitigation alternatives.  
3.5.1 Understanding Regional Earthquake Risk-Big Picture of Expected Ground 
Motions
There are a number of resources available that provide a regional view of the 
earthquake hazard. Overall assessments of risk are expressed as probabilities 
that mapped ground motions will exceed a certain level over a period of time. A 
common measure is the 10% probability that peak ground acceleration (violence of 
ground shaking) will be larger than the value mapped, over a 50-year period.  
These maps provide an assessment of the relative intensity of ground motions for 
a region.
  USGS 2002 Ground Motion Maps
The building code uses the Maximum  Considered Earthquake (MCE) maps which are 
based on the USGS Seismic Hazard Maps with a 2%  probability of being exceeded 
in about 2,500 years (figure 3-2). See section 2.6.2. These maps depict areas 
that have an annual probability of approximately 1 in 2,300 of the indicated 
peak ground acceleration being exceeded, and account for most known seismic 
sources and geological effects on ground motions. The areas of intense orange, 
red, brown and black are the most likely areas to experience violent ground 
shaking greater than 30% of the force of gravity in the next 50 years. The maps 
provide a general assessment of relative ground motions, but are at a scale that 
does not help in a site selection process. It is clear from the map, however, 
that violent ground motions are more likely in the coastal regions of 
California, Oregon and Washington, the Sierra Nevada range of California, and 
the Wasatch Range of Utah than in Colorado, Kansas and Oklahoma.
  State Survey Risk Maps
Many states provide geological data that can assist in assessing regional 
seismic risk. In California, for example, the CGS in cooperation with the USGS 
has taken the data from the above map and provided a more detailed set of 
regional maps. The map of the Bay Area (Figure 3-3) depicts the peak ground 
accelerations with a 2% probability of being exceeded in 50 years at a regional 
scale and combines probability of occurrence of large ground motions and soil 
and geological conditions that would amplify ground motions. The areas depicted 
in red through gray are the areas where the most violent ground shaking will 
most frequently occur.  These areas are adjacent to active faults capable of 
producing violent ground motions and areas where soils conditions will increase 
ground motions. Thus, areas in gray are adjacent to active faults and along the 
margins of the San Francisco Bay, where unconsolidated soils will amplify ground 
motions. It is important to note, however, that the map depicts probability of 
relative shaking and that damaging ground motions can occur anywhere in the 
region depicted on the map.  
Both the USGS and CGS depict the ground motions that are produced by all 
earthquakes on all faults that could influence a particular location. These maps 
can be extremely helpful to the architect and client in determining the relative 
risk of alternative sites and the trade-offs of location, vulnerability and 
offsetting costs for a structure that will resist ground motions.  
  HAZUS Earthquake Loss Estimates
The Federal Emergency Management Agency (FEMA) has developed a software program 
that can be used to estimate earthquake damage and losses at a regional level. 
HAZUS (Hazards United States) provides estimates of damage and losses to 
infrastructure such as highway bridges, electrical and water utilities, 
casualties and requirements for shelter for displaced households.  The quality 
of a HAZUS estimate of losses will depend on the detail of information input to 
the program.  Local soil data and building inventory will determine the accuracy 
of the loss estimates.  Estimates can be produced for specific faults, for 
specific scenarios, or for annualized losses over a period of years.  In each 
case, the loss estimate is helpful in understanding the risk context for a 
project -what damage and disruption will occur in the community surrounding the 
project. Below are two HAZUS maps (Figures 3-4 and 3-5) illustrating intensity 
of ground motions in PGA and Total Economic Losses, by census tract for a M7.5 
earthquake on the Hayward Fault in Alameda County, California. Similar estimates 
can be produced for other areas of the country where an earthquake threat would 
influence site selection.  While HAZUS is helpful in understanding regional 
vulnerability and patterns of damage and loss, it is not appropriate for 
assessment of damage on an individual building site.  
Information about HAZUS is available from your local and state emergency 
services office, and from FEMA at www.fema.gov/hazus/hz_index.shtm and 
www.hazus.org.
3.6 EARTHQUAKE HAZARDS TO AVOID
The most obvious manifestations of earthquakes are earthquake fault offset, 
liquefaction, landslides, and ground shaking (Figures 3-6 and 3-9).  Each of 
these hazards can be mitigated through careful site planning. Examples in this 
section are drawn from California, where a broad range of hazard identification 
and mitigation approaches is available.  Hazard data and land regulation 
practices vary from state to state and within states. A geotechnical consultant 
is your best source of local data.  
3.6.1	Earthquake Fault Zones
The United States Geological Survey and many state geological surveys produce 
maps of active earthquake faults - that is, faults that exhibit Holocene 
surface displacement or ruptured within the last 11,000 years. These maps 
depict faults where they have ruptured the ground surface, as fault movements 
usually recur in geologically weak zones. In California, the legislature 
mandated the mapping of active faults after the 1971 San Fernando earthquake 
(Alquist-Priolo Earthquake Fault Zoning Act)10. The Fault Hazard Zones Act maps 
are published by the state, and location of a site within a Fault Hazard Zone 
requires disclosure of the hazard at point of sale. Local governments are 
responsible for reviewing geologic reports and approving those reports before 
approving a project (Figure 3-7).  
Fault mapping is a continuing process of discovery, analysis, and mapping.  
However, it is important to note that not all earthquake faults rupture to the 
surface and not all earthquake faults are currently mapped. In the 1989 Loma 
Prieta earthquake (M6.7), the fault did not rupture to the surface, yet caused 
more than $6 billion in damage and resulted in more than 60 deaths. For some 
active faults, there may not be a surface manifestation indicating recent 
activity. Both the 1971 San Fernando and 1994 Northridge earthquakes occurred on 
blind thrust faults, where faulting did not reach the surface, so the hazard was 
not recognized until the earthquake occurred. Nonetheless, fault zones pose a 
clear danger to structures and lifelines, and where formally mapped or inferred 
from geologic reports, site plans should provide a setback to protect structures 
from fault movement.  
  Mitigating Fault Zone Hazards
The Alquist-Priolo Earthquake Fault Zoning Act12 provides reasonable guidance 
that should be applied in site selection and site design. Requirements include:
	Disclosure that a property is within a mapped Seismic Hazard Zone. The 
zones vary in width, but are generally  of a mile wide and are defined by 
turning points identified on the zone maps. Figure 3-8 shows a zone map that 
identifies active traces of the fault, the date of last rupture, and defines the 
fault zone within which special studies are required prior to development. The 
zone boundary, defined by turning points, encompasses known active traces of the 
fault and provides approximately 200 meters setback between the fault trace and 
the boundary line. Check with local government planning agencies for the most 
current maps.
	Local governments must require a geologic report for any project within 
the fault hazard zone to ensure that no structure is built across an active 
fault trace.
	No structures for human habitation shall be built within 50 feet of an 
identified fault trace (an exception is provided for single-family residential 
structures when part of a development of four or fewer structures).
In states where seismic hazard zones are not identified, the geologic report for 
a project should locate identified or suspected fault traces, and recommend 
mitigation measures, including those identified above, to reduce the risk posed
3.6.2 Ground Failure Due to Liquefaction 
Liquefaction occurs when water-saturated soils, sands, or gravels flow laterally 
or vertically like a liquid.  This occurs when earthquake ground motions shake 
the material until the water pressure increases to the point that friction 
between particles is lost, and the ground flows, losing its strength (Figure 3-
9). Liquefaction is most likely to occur where the soils are not consolidated 
(near rivers and streams, in basins, near coastlines and in areas of 
unconsolidated alluvium) and where ground water is within three to four meters 
of the surface. Liquefaction can occur at greater depths, resulting in large-
scale ground failure that can destroy pavement, underground utilities, and 
building foundations (Figure 3-10). The subsidence of Turnagain Heights in 
Anchorage during the 1964 earthquake is an example of deep-seated liquefaction 
and ground failure. When a soil liquefies, it can flow laterally, eject 
vertically as a sand boil, or result in subsidence and ground failure (Figure 3-
11).  
Sand boils and flows on the surface can displace and damage structures and 
utilities. Lateral liquefaction flows will result in subsidence, loss of 
foundation integrity, disruption of underground utilities and damage to 
structures resting on the soil surface, including roadways and utility 
structures. Liquefaction susceptibility and potential should be identified in 
the site geotechnical investigation, as explained in Section 3.3.3.  
  Liquefaction Hazard Zones
Liquefaction susceptibility can be determined from site geologic investigations 
and from a review of geologic and soil maps and water well and bore hole logs. 
In California, liquefaction potential mapping is part of the CGSs Earthquake 
Hazard Mapping Program. Liquefaction hazard zone maps have been completed for 
sections of the Los Angeles and San Francisco Bay Regions (Figures 3-12 and 3-
13).
Within an identified liquefaction hazard zone, maps of liquefiable soils, 
prepared by a geotechnical engineer, should identify the location and extent of 
cohesionless silt, sand, and fine-grained gravel in areas where the ground 
water is within 50 feet of the surface. Procedures for testing and criteria for 
determining liquefaction susceptibility are contained in Recommended Procedures 
for Implementation of DMG Special Publication 117:  Guidelines for Analyzing and 
Mitigating Liquefaction in California.
  Mitigation Options for Liquefiable Sites
There are structural solutions for mitigating liquefaction potential that 
address the design of foundation systems that penetrate the liquefiable layers. 
It should be noted that while it is frequently cost effective to design 
structures to withstand liquefaction, making access and egress routes, parking 
and storage facilities and above and underground utilities liquefaction-
resistant is prohibitively expensive. A whole-site solution may be more 
practical when site choice is limited and susceptibility is significant. See the 
mitigation approaches below.
  Location of the Structure
The simplest way to mitigate the potential of liquefaction is to avoid those 
locations in a region or on a site where the potential for ground failure is 
identified in the geotechnical investigation. Locate structures where ground 
water is low, where soils are compacted, and where soils are not homogeneous 
sands or gravels.
  Intervention on the Site	
While avoidance is the optimum solution, it is not always possible. Mitigating 
liquefaction potential involves changing the characteristics of the site. The 
following options are all costly and vary in extent of risk mitigation. Seek 
advice from geotechnical and civil engineering consultants about the most cost-
effective intervention.
	Site Compaction	
	On sites with unconsolidated soils, the response of the site can be 
improved by compacting the soil, compressing it so that soil particles are 
forced together, reducing water-filled voids and increasing the friction between 
soil particles.  
	Change Soil	
	The performance of the site can also be improved by excavation of the 
liquefiable soils and replacement with compacted hetero-geneous fill. By 
changing the soil, the susceptibility of the site to liquefaction will be 
significantly reduced. However, for both this approach and the compaction 
alternative, site performance is improved by construction of barriers to the 
infiltration of water so that the groundwater level of the site is lowered.

	Dewatering the Site	
	An alternative to reconstituting the site by replacing the soil is to 
dewater the site. This approach requires constructing wells to pump out and 
lower the ground water level to reduce liquefaction susceptibility. To reduce 
the demand for continuous pumping, dewatering should be combined with the 
construction of infiltration barriers. A back-up power source to ensure post-
disaster pump operations should be provided.
  Special Design Considerations	
As noted above, the potential for liquefaction of a site poses severe problems 
for maintenance of access and egress and performance of lifelines including 
power, telecommunications, water sewer and roadways. For facilities that are 
expected to be in continuous operation after disasters, redundant access to 
utility networks, multiple access and egress paths, and back-up power and 
communication systems should be provided.  Liquefaction potential may be 
difficult to assess, so a conservative approach to the design of continuous 
operation facilities is essential.

3.6.3 Areas of Intensified Ground Motions
Local geology, proximity to faults and soil conditions play significant roles in 
how earthquake forces impact a structure. The Loma Prieta earthquake (1989) 
provided a striking example of how local soils and regional geology can 
determine damage. Sixty miles from the earthquakes epicenter in the Santa Cruz 
Mountains, the soils determined the pattern of damage to the Cypress Viaduct in 
Oakland. As illustrated below, the damage corresponded to the quality of the 
ground.  On bedrock materials in the East Bay hills of Oakland, ground motions 
were small and there was little damage. On sandy and gravel soils between the 
East Bay hills and the San Francisco Bay, the amplitude of ground motions 
increased, but there were few collapsed structures. However, on the soft mud 
adjacent to the Bay, the amplitude of the ground motions and the duration of 
strong shaking increased. The Cypress Structure, where it passed from sand and 
gravel to soft mud collapsed (Figure 3-14).
A similar condition existed in the Marina District of San Francisco where soft 
soils liquefied, amplified motions and extended the duration of shaking until 
several structures collapsed, while elsewhere in San Francisco, on firmer 
ground, there was little or no damage.
Figure 3-15 was developed by the USGS and predicts amplification of ground 
motions based on soil types adjacent to San Francisco Bay. According to the 
USGS, this map shows the capability of the ground to amplify earthquake shaking 
in the communities of Alameda, Berkeley, Emeryville, Oakland, and Piedmont. The 
National Earthquake Hazards Reduction Program recognizes five categories of soil 
types and assigns amplification factors to each. Type E soils in general have 
the greatest potential for amplification, and type A soils have the least. These 
soil types are recognized in many local building codes. Records from many 
earthquakes show that ground conditions immediately beneath a structure affect 
how hard the structure shakes. For example, sites underlain by soft clayey soils 
tend to shake more violently than those underlain by rock. The map depicts the 
amplification potential at a regional scale, and it should not be used for site-
specific design. Subsurface conditions can vary abruptly, and borings are 
required to estimate amplification at a given location. 
3.6.4	Ground Failure, Debris Flows, and Land Slides
Potential for ground failure and landslides is determined by soil type, water 
content (degree of saturation), gradient (slope angle) and triggering events (an 
earthquake, excavation that upsets the site equilibrium, increase in water 
content resulting from irrigation or storm run-off).  Geotechnical 
investigations of the site and surrounding terrain are critical in determining 
site vulnerability.  
	Landslide Hazard Maps
The USGS and CGS have prepared Landslide Hazard Zone Maps for parts of northern 
and southern California (Figure 3-16).  The map depicts a section of the 
Oakland-Berkeley East Bay Hills, indicating areas where slope, soil type and 
seismic risk could trigger landslides.  Construction in the Landslide Hazard 
Zone requires an assessment by a geotechnical engineer.  
Downslope slides can undermine building foundations and cut off utilities and 
access, rendering a facility non-operational and/or structurally unsafe (Figure 
3-17).  The USGSs National Landslide Hazards Mitigation Strategy14 and the 
California Geological Surveys Recommended Procedures for Implementation of DMG 
Special Publication 117:  Guidelines for Analyzing and Mitigating Landslide 
Hazards in California15 offer guidance in determining landslide vulnerability 
and mitigation options.
  Mitigation Options
Foundation systems and structures can be designed to reduce damage from ground 
failure. The geotechnical engineer can provide recommendations for appropriate 
foundation design.
	Set-Back	
	The most failsafe option for mitigation is to locate structures and 
lifelines in parts of the site that are not at risk to slide damage. Set back 
structures from both the toe of an upslope and from the lip of a down slope. 
Allow separation to accommodate catch basins, debris diverters and barriers. 
Parking lots or storage areas can be designed and located to buffer structures 
from debris.  
	Drainage	
	Since water acts as a lubricant on slope-failure surfaces, it is critical 
that the site and its surroundings be well drained, that irrigation is limited, 
and that dewatering systems reduce subsurface hydrostatic (water pressure) 
pressures. Dewatering systems can either be passive (drains into slopes, French 
drains, top-of-slope catch basins) or active, providing pumping of subsurface 
water from sumps into a drain system. In both cases, continuous maintenance is 
essential to ensure reliable operation of the system. Emergency power may also 
be required for active drainage systems. Where storm water runoff must be 
managed on site, design of parking and landscaped areas should accommodate 
storage. Facility access procedures will need to address displacement of parking 
and limitations on access during periods the site is flooded.
	Redundant Infrastructure	
	Ground failure can severely disrupt utility and lifeline connections to a 
site. Where continued operations are essential to a client, connections to 
utility and transportation networks should be redundant, providing more than one 
means of connection, access, and egress. For telecommunications, redundancy 
would include dedicated connections to two different switching offices, planned 
to follow two different routes to the site. Multiple access and egress paths 
should also be provided. For facilities dependent on electrical power, multiple, 
dispersed connections to the grid, co-generation and/or emergency back-up power 
generation should be planned.  Where continuous operation is not essential, 
emergency back-up power should be provided to ensure safety, security and 
operation of environmental protection systems (such as heating and ventilating 
systems [HVAC], water pumps, security systems, evacuation and lighting systems, 
computer operations and data security.  Emergency power generation capacity 
should exceed minimum requirements to ensure adequate power for projected needs 
of essential systems. For facilities where a consistent quality-controlled 
supply of water is essential for operations, on-site storage and purification 
should be provided to meet operational needs until alternative sources can be 
secured.
3.7	OFF-SITE ISSUES THAT AFFECT SITE SELECTION
As noted previously, for facilities designed to performance-based criteria, 
including minimum disruption and continued operation, the location of the site 
within the region may play a critical role in mitigation options.  A 
vulnerability assessment should address issues of access to and egress from the 
site to regional transportation and communication systems, the robustness of 
utilities that support the site, and regional earthquake impacts that would 
affect site operations.
3.7.1	Access and Egress
For manufacturing and essential facilities where access and egress are essential 
for continued operations, siting decisions should address the vulnerability of 
access roads, freeways, public and private transit and transportation structures 
upon which business operations will depend.  Selection of a site that provides 
multiple or redundant access and egress is a good idea.  This approach will also 
be essential if facility operations or production is dependant on access by 
employees, raw materials, and delivery of products, be they manufactured goods 
or information, to markets.  For example, a number of manufacturing firms have 
relocated their manufacturing from California to other states where product 
manufacture and delivery would not be disrupted by earthquake damage to 
buildings, freeway structures, telecommunications, and the dislocation of 
employees.  
3.7.2	Infrastructure
We have become more dependent on infrastructure, particularly high-speed 
telephony for day-to-day business operations.  Most businesses are also totally 
dependent on electrical power from a regional grid, and water and waste-water 
disposal from offsite utilities.  In assessing the vulnerability of a facility 
expected to be operational immediately after a disaster, the client and designer 
should assess the reliability of these infrastructure systems and provide for 
redundancy and back-up systems. For a critical system such as telephony, data 
telemetry or just Internet access, redundancy should include multiple access or 
paths to primary utilities. For example, for critical telephony, redundancy 
would provide multiple paths to different telephone switching offices and 
satellite communications capability. For electric power, back-up generators and 
fuel storage (and contracts with suppliers to provide refueling until utility 
power is restored) to provide for continued operations would be essential. On-
site storage for wastewater would provide redundancy to a sewer system that may 
be damaged by power loss, earthquake damage or flood.
3.7.3	Adjacency	
Adjacent land uses may pose a threat to the continued operation of the proposed 
facility. Collapse-hazard structures can spill debris onto the site, damaging 
structures or blocking access and egress. Hazardous materials released upwind of 
the site may force evacuation and shut down of operations. Setbacks from 
adjacent land uses and separation from adjacent structures should be used to 
protect structures, access and areas of refuge and to protect against pounding. 
In addition, HVAC systems may require enhanced design to protect building 
occupants from hazardous materials plumes. 
3.8	EARTHQUAKE AND TSUNAMI HAZARDS
A tsunami is a rapid rise in coastal sea level caused by offshore earthquakes 
that displace the ocean bottom, earthquake-triggered or natural submarine 
landslides and slumps, volcanic eruptions, or very infrequently by meteor 
strikes. Tsunami waves have a very long wavelength and travel at approximately 
500 miles per hour in the open ocean. As they approach shallow waters, their 
speed and wavelength decreases, and their height increases dramatically. 
While coastal storm surge is well documented and understood, the impacts of 
tsunamis are not as commonly understood. Storm surge produces higher tides and 
pounding waves over a period of hours. Tsunamis, generated by distant 
earthquakes on the Pacific Rim, volcanic eruptions or undersea landslides on 
near-coast continental shelves, or by near-shore earthquakes, can typically 
cause unpredictable, high and rapidly changing tidal-like inundation from 1 to 
30 feet in height above the tide, carrying flood waters and debris inland in, 
cases up to 100 feet.  Tsunami wave arrival is usually, but not always, preceded 
by extreme tidal recession. The initial wave is usually followed by secondary 
tsunami waves for periods lasting up to eight hours. These secondary waves can 
be higher and carry debris from initial inundation, creating a lethal 
combination of inundation and battering.
Tsunamis are not limited to the Pacific Coast, Hawaii and Alaska.  Earthquakes 
and volcanic eruptions can generate tsunamis along the US southeast and gulf 
coasts and the Caribbean, with a remote possibility of volcanic and submarine 
landslides generating a tsunami that could affect the entire Atlantic coastline.
3.8.1 Special Considerations for Coastal Area   		 Site Assessment
Coastlines are dynamic. Beaches erode and migrate, and bluffs collapse as part 
of the natural process in the coastal zone. Earthquakes can accelerate this 
process. Site plans must address the dynamic nature of the beach-ocean 
interface, providing setbacks adequate to accommodate inevitable change. 
Dramatic changes in short periods of time frequently occur as a result of 
earthquakes, storms and tsunami. Designing structures to resist coastal forces 
of wind, flood, storm surge, earthquake, tsunami inundation and battering is a 
complex problem.
	Mitigating Tsunami and Coastal Surge Hazards
FEMAs Coastal Construction Manual (CCM) identifies a process for evaluation of 
flood hazards in coastal areas that is applicable to earthquake and tsunami 
forces as well. Alternatives include locating development above the coastal 
flood zone, orientating structures to reduce the profile presented to wave 
action, site-planning options for locating structures, parking and landscaping, 
altering the site and construction of flood protective structures.16  FEMA 
suggests the following critical Dos and Donts, edited, abridged and adapted 
from the California Coastal Commission:
	DO avoid areas that require extensive grading.
	DONT rely on engineering solutions to correct poor design and planning.
	DO identify and avoid or set back from all sensitive, unstable, and 
prominent land features.
	DONT overlook the effects of infrastructure location on the hazard 
vulnerability of building sites.
	DO account for all types of erosion (long-term, storm-induced, stream, and 
inlets).
	DO incorporate setbacks from identified high-hazard areas.
	DONT forget to consider future site and hazard conditions.
	DO use a multi-hazard approach to planning and design.
	DONT assume that engineering and architectural practices can mitigate all 
hazards.
	DO involve a team of experts with local knowledge and a variety of 
expertise in site evaluation and assessment.
	Specific CCM Recommendations for Site Planning
	Set back structures beyond the code or zoning minimums to provide an extra 
margin of safety. It is better to be conservative than to have to relocate a 
structure in the future. If a structure must be located at the minimum setback, 
it should be designed to be relocated.
	Set back structures from the lip of coastal bluffs. See the CCM for 
recommendations.
	Be aware of multiple hazards. In many coastal states, coastal structures 
are subjected to potential storm surge, tsunami, coastal erosion, debris flows, 
fires and earthquakes!
	Provide setbacks between buildings and erosion or flood control structures 
to permit maintenance, strengthening and subsequent augmentation.
In site planning, be aware that vegetation and buildings can become dislodged 
and be driven by wind and wave action into structures. Vegetation may serve to 
stabilize beach areas, but it may not be able to resist tsunamis. Also beware of 
land forms that may channel inundation into structures.
  Tsunami-Specific Mitigation	
In many coastal communities, tsunami inundation is included in the National 
Flood Insurance Program. Information concerning the potential for tsunami 
inundation and maps can be obtained from the local or state emergency management 
office, FEMA or from NOAA. Detailed inundation projections are being prepared 
for the coastlines of California, Oregon, Washington, Alaska and Hawaii. The 
potential for tsunami inundation may also exist for some areas in Puerto Rico 
and the Gulf and southern Atlantic coast states. Unfortunately, the history of 
tsunami events in the coastal United States is incomplete. The simplest solution 
is to avoid new construction in areas subject to tsunami inundation (as a 
surrogate, maps of areas that have historically been inundated by storm surge 
may be used). For example, the range of projected tsunami inundation for 
Californias open coast is from 33 to 49 feet(10 to 15 meters), with variation 
in estuaries and bays. In Oregon, Washington, Alaska and Hawaii, wave heights 
can be greater. The recommendations of the CCM should be followed for 
construction in areas where erosion, flooding, hurricanes and seismic hazards 
exist. In developing a site plan in an area with inundation potential, cluster 
structures in areas with the lowest risk - generally the highest section of the 
site.  
	Resources for Tsunami Mitigation 
The National Tsunami Hazard Mitigation Program of NOAA has prepared a number of 
resource documents to assist architects and planners in mitigating tsunami risk. 
The guide, Planning for Tsunami:  Seven Principles for Planning and Designing 
for Tsunami 17, provides general guidance to local elected officials and those 
involved in planning, zoning, and building regulation in areas vulnerable to 
tsunami inundation.  
As in other areas where flooding can occur, structures should be elevated above 
the expected tsunami inundation height. Energy-abating structures, earth berms, 
and vegetation can dissipate some of the energy of the incoming and receding 
waves, but they are not a failsafe solution. In areas where inundation is 
expected to be less than a meter, flood walls may protect structures from both 
surge and battering from debris (Figure 3-18).
Structures in low-lying areas should be designed so that the surge passes under 
or through the building, by elevation of the structure or by creating weak non-
structural walls, also known as break-away walls, perpendicular to expected 
waves (Figure 3-19). This would allow the waves and debris to pass through the 
structure. Buildings should be oriented perpendicular to wave inundation to 
provide the smallest profile to the wave. However, it is critical in areas that 
experience both tsunami and earthquakes that foundations and structures be 
designed to resist earthquake forces and the forces of water velocity, debris 
battering, and scouring and liquefaction of foundations and piles.
When program requirements such as orientation to view or site limitations 
necessitate building configurations that are parallel to incoming waves, 
attention to structural design is critical.
Construction in coastal zones of California, Oregon, Washington, Alaska, and 
Hawaii must accommodate both tsunami inundation and earthquake ground motions 
from large near-coast events.
Site planning in areas of flood, coastal surge, and tsunami must provide for 
rapid evacuation of occupants to high ground, or structures must be designed for 
vertical evacuation to floors above forecast flood levels (Figure 3-20). This 
requires careful engineering because there is currently no guidance available 
for determining loads. For communities subject to both flood and earthquake 
hazards, structures intended for vertical evacuation should be designed to 
seismic standards higher than life safety so they will be available after the 
earthquake to accommodate tsunami evacuees.
3.9	CONCLUSION
The success of a project in meeting the clients expectations begins with the 
right team of architects and  geotechnical, civil, and structural engineers. 
Understanding the seismic hazards in all of their direct and indirect 
manifestations is critical to success. Good engineering is not an excuse or a 
remedy for an inadequate evaluation of the site and design that does not 
mitigate the earthquake risk. As can be seen in the remainder of this 
publication, successful design is a team effort, and starts at the site.

NOTES

1 FEMA National Flood Insurance Program.  Information available at 
www.FEMA.gov/fima/
2 CGS 2002, Guidelines for Evaluating and Mitigating Seismic hazards in 
California,SpecialPublication 117, California Geological Survey, Seismic Hazard 
Mapping Program, and State of California.  Available at 
http://gmw.consrv.ca.gov/shmp/SHMPsp117.asp
3 Southern California Earthquake Center, University of Southern California, Los 
Angeles.  Available at http://gmw.consrv.ca.gov/shmp/SHMPpgminfo.htm
4 FEMA 386-2 is available on-line at www.fema.gov
5 Section 322, Mitigation Planning, of the Robert T. Stafford Disaster Relief 
and Emergency Assistance Act, enacted by Section 104 of the Disaster Mitigation 
Act of 2000 (Public Law 106-390)
6 201.6(C)(2), 44 CFR PART 201, State and Local Plan Interim Criteria Under the 
Disaster Mitigation Act of 2000
7 Available at http://geohazards.cr.usgs.gov/eq/html/wus2002.html
8 Map available from California Seismic Safety Commission, Sacramento, CA
9 HAZUS was developed by FEMA through a cooperative agreement with the National 
Institute of Building Sciences.  Information is available at 
www.fema.gov/hazus/hz_index.shtm
10 California Public Resources Code, Division 2, Geology, Mines and Mining,     
	Chapter 7.5, Sections 2621-2630, Earthquake Fault Zoning.  Available at 
www.consrv.ca.gov/cgs/rghm/ap/chp_7_5.htm#toc
11 CGS 1999, Fault Rupture Hazard Zones in California, Special Publication   42. 
Available at ftp://ftp.consrv.ca.gov/pubs/sp/SP42.PDF
12 California Public Resources Code, Division 2. Geology, Mines and Mining, 
Chapter 7.5 Earthquake Fault Zones, as amended
13 G.R. Martin and M.Lew, e0ds,  Southern California Earthquake Center, 
University of Southern  California,1999
14 USGS Open File Report 00-450, 2000
15 ASCE and Southern California Earthquake Center, June 2002
16 FEMA, Costal Construction Manual, 3rd Edition, FEMA 55CD. Available from FEMA 
at www.fema.gov
17 NOAA NTHMP 2001, Designing for Tsunami: Seven Principles for Planning and 
Designing for Tsunami Hazards. Laurence Mintier, ed. Available from 
http://www.pmel.noaa.gov/tsunami-hazard/links.html#multi

[End of Chapter 3]

EARTHQUAKE EFFECTS ON BUILDINGS BY CHRISTOPHER ARNOLD
4.1 Introduction
This chapter explains how various aspects of earthquake ground motion affect 
structures and also how certain building attributes modify the ways in which the 
building responds to the ground motion. The interaction of these characteristics 
determines the overall seismic performance of the building: whether it is 
undamaged; suffers minor damage; becomes unusable for days, weeks, or months; or 
collapses with great loss of life.   
Explanations of some characteristics of ground motion are followed by 
descriptions of several material, structural, and building attributes that, by 
interacting with ground motion, determine the buildings seismic performancethe 
extent and nature of its damage.
This chapter uses the information on the nature of ground motion in Chapters 2 
and 3, and applies it to structures and buildings. Chapter 5, on seismic issues 
in architectural design, continues the exploration of design and construction 
issues that, in a seismic environment, determine building performance.
4.2 Inertial Forces and Acceleration
The seismic body and surface waves create inertial forces within the building. 
Inertial forces are created within an object when an outside force tries to make 
it move if it is at rest or changes its rate or direction of motion if it is 
moving. Inertial force takes us back to high school physics and to Newton's 
Second Law of Motion, for when a building shakes it is subject to inertial 
forces and must obey this law just as if it were a plane, a ship, or an athlete. 
Newton's Second Law of Motion states that an inertial force, F, equals mass, M, 
multiplied by the acceleration, A. (Figure 4-1)
Mass can be assumed as equivalent (at ground level) to the weight of the 
building, and so this part of the law explains why light buildings, such as wood 
frame houses, tend to perform better in earthquakes than large heavy ones - the 
forces on the building are less.  
The acceleration, or the rate of change of the velocity of the waves setting the 
building in motion, determines the percentage of the building mass or weight 
that must be dealt with as a horizontal force. 
Acceleration is measured in terms of the acceleration due to gravity or g 
(Figure 4-2). One g is the rate of change of velocity of a free falling body in 
space.  This is an additive velocity of 32 feet per second per second.  Thus, at 
the end of the first second, the velocity is 32 feet per second; a second later 
it is 64 feet per second, and so on.  When parachutists or bungee jumpers are in 
free fall, they are experiencing an acceleration of 1g.  A building in an 
earthquake experiences a fraction of a second of g forces in one direction 
before they abruptly change direction. 
Engineering creations (planes, ships, cars, etc.) that are designed for a 
dynamic or moving environment can accommodate very large accelerations.  
Military jet planes, for example, are designed for accelerations of up to 9g.  
At this acceleration, the pilot experiences 9 times the body weight pressing 
down on the organs and blacks out.  
A commercial airliner in fairly severe turbulence may experience about 20 
percent g (or 0.2g), although unbuckled passengers and attendants have been 
known to hit the ceiling as a result of an acceleration drop of over 1g. A 
fast moving train on a rough track may also experience up to about 0.2g.
Poorly constructed buildings begin to suffer damage at about 10 percent g (or 
0.1g).  In a moderate earthquake, the waves of vibration may last for a few 
seconds, and accelerations may be approximately 0.2g. For people on the ground 
or at the bottom of a building, the sensations will be very similar to those of 
the occupants of the plane in turbulence or passengers standing in the fast 
moving train: they feel unsteady and may need to grab onto something to help 
them remain standing.  Earthquakes cause additional alarm because when the 
shaking starts, those experiencing it do not know whether it will quickly end or 
is the beginning of a damaging and dangerous quake.  Short accelerations may, 
for a fraction of a second, exceed 1.0g.  In the Northridge earthquake in 1994, 
a recording station in Tarzana, five miles (8 km) from the epicenter, recorded 
1.92g. 
4.3	Duration, Velocity, and Displacement
Because of the inertial force formula, acceleration is a key factor in 
determining the forces on a building, but a more significant measure is that of 
acceleration combined with duration, which takes into account the impact of 
earthquake forces over time. In general, a number of cycles of moderate 
acceleration, sustained over time, can be much more difficult for a building to 
withstand than a single much larger peak. Continued shaking weakens a building 
structure and reduces its resistance to earthquake damage.  
A useful measure of strong-motion duration is termed the bracketed duration.  
This is the shaking duration above a certain threshold acceleration value, 
commonly taken as 0.05g, and is defined as the time between the first and last 
peaks of motion that exceeds this threshold value. In the San Fernando 
earthquake of 1971, the bracketed duration was only about 6 seconds. In both the 
Loma Prieta and the Northridge earthquakes, the strong motion lasted a little 
over ten seconds, yet caused much destruction. In the 1906 San Francisco 
earthquake, the severe shaking lasted 45 seconds, while in Alaska, in 1964, the 
severe motion lasted for over three minutes.   
Two other measures of wave motion are directly related to acceleration and can 
be mathematically derived from it.  Velocity, which is measured in inches or 
centimeters per second, refers to the rate of motion of the seismic waves as 
they travel through the earth. This is very fast.  Typically, the P wave travels 
at between 3 km/sec and 8 km/sec or 7,000 to 18,000 mph.  The S wave is slower, 
traveling at between 2 km/sec and 5 km/sec, or 4,500 mph to 11,000 mph. 
Displacement refers to the distance that points on the ground are moved from 
their initial locations by the seismic waves. These distances, except 
immediately adjacent to or over the fault rupture, are quite small and are 
measured in inches or centimeters. For example, in the Northridge earthquake, a 
parking structure at Burbank, about 18 miles (29 km) from the epicenter recorded 
displacements at the roof of 1.6 inches (4.0 cm) at an acceleration of 0.47g.  
In the same earthquake, the Olive View hospital in Sylmar, about 7.5 miles (12 
km) from the epicenter, recorded a roof displacement of 13.5 inches (34 cm) at 
an acceleration of 1.50g. 
The velocity of motion on the ground caused by seismic waves is quite slowhuge 
quantities of earth and rock are being moved. The velocity varies from about 2 
cm/sec in a small earthquake to about 60 cm/sec in a major shake. Thus, typical 
building motion is slow and the distances are small, but thousands of tons of 
steel and concrete are wrenched in all directions several times a second.
In earthquakes, the values of ground acceleration, velocity, and displacement 
vary a great deal in relation to the frequency of the wave motion. High
frequency waves (higher than 10 hertz) tend to have high amplitudes of 
acceleration but small amplitudes of displacement, compared to low-frequency 
waves, which have small accelerations and relatively large velocities and 
displacements.
4.4	Ground Amplification
Earthquake shaking is initiated by a fault slippage in the underlying rock. As 
the shaking propagates to the surface, it may be amplified, depending on the 
intensity of shaking, the nature of the rock and, above all, the surface soil 
type and depth.  
A layer of soft soil, measuring from a few feet to a hundred feet or so, may 
result in an amplification factor of from 1.5 to 6 over the rock shaking. This 
amplification is most pronounced at longer periods, and may not be so 
significant at short periods. (Periods are defined and explained in the next 
section, 4.5.1.) The amplification also tends to decrease as the level of 
shaking increases.  
As a result, earthquake damage tends to be more severe in areas of soft ground.  
This characteristic became very clear when the 1906 San Francisco earthquake was 
studied, and maps were drawn that showed building damage in relation to the 
ground conditions. Inspection of records from soft clay sites during the 1989 
Loma Prieta earthquake indicated a maximum amplification of long-period shaking 
of three to six times. Extensive damage was caused to buildings in San 
Francisco's Marina district, which was largely built on filled ground, some of 
it rubble deposited after the 1906 earthquake.
Because of the possibility of considerable shaking amplification related to the 
nature of the ground, seismic codes have some very specific requirements that 
relate to the characteristics of the site. These require the structure to be 
designed for higher force levels if it is located on poor soil. Specially 
designed foundations may also be necessary.
4.5 Period and Resonance
4.5.1 Natural Periods
Another very important characteristic of earthquake waves is their period or 
frequency; that is, whether the waves are quick and abrupt or slow and rolling. 
This phenomenon is particularly important for determining building seismic 
forces.
All objects have a natural or fundamental period; this is the rate at which they 
will move back and forth if they are given a horizontal push (Figure 4-3). In 
fact, without pulling and pushing it back and forth, it is not possible to make 
an object vibrate at anything other than its natural period.  When a child in a 
swing is started with a push, to be effective this shove must be as close as 
possible to the natural period of the swing.  If correctly gauged, a very small 
push will set the swing going nicely.  Similarly, when earthquake motion starts 
a building vibrating, it will tend to sway back and forth at its natural period. 
Period is the time in seconds (or fractions of a second) that is needed to 
complete one cycle of a seismic wave.  Frequency is the inverse of thisthe 
number of cycles that will occur in a secondand is measured in Hertz.  One 
Hertz is one cycle per second. 
Natural periods vary from about 0.05 seconds for a piece of equipment, such as a 
filing cabinet, to about 0.1 seconds for a one-story building.  Period is the 
inverse of frequency, so the cabinet will vibrate at 1 divided by 0.05 = 20 
cycles a second or 20 Hertz.  
A four-story building will sway at about a 0.5 second period, and taller 
buildings between about 10 and 20 stories will swing at periods of about 1 to 2 
seconds.  A large suspension bridge may have a period of around 6 seconds. A 
rule of thumb is that the building period equals the number of stories divided 
by 10; therefore, period is primarily a function of building height.  The 60-
story Citicorp office building in New York has a measured period of 7 seconds; 
give it a push, and it will sway slowly back and forth completing a cycle every 
7 seconds.  Other factors, such as the building's structural system, its 
construction materials, its contents, and its geometric proportions, also affect 
the period, but height is the most important consideration (Figure 4-4).  
The buildings period may also be changed by earthquake damage.  When a 
reinforced concrete structure experiences severe ground shaking, it begins to 
crack: this has the effect of increasing the structures period of vibration: 
the structure is softening.  This may result in the structures period 
approaching that of the ground and experiencing resonance, which may prove fatal 
to an already weakened structure.  The opposite effect may also occur: a steel 
structure may stiffen with repeated cycles of movement until the steel yields 
and deforms.
4.5.2 Ground Motion, Building Resonance, and Response Spectrum
When a vibrating or swinging object is given further pushes that are also at its 
natural period, its vibrations increase dramatically in response to even rather 
small pushes and, in fact, its accelerations may increase as much as four or 
five times.  This phenomenon is called resonance. 
The ground obeys the same physical law and also vibrates at its natural period, 
if set in motion by an earthquake.  The natural period of ground varies from 
about 0.4 seconds to 2 seconds, depending on the nature of the ground. Hard 
ground or rock will experience short period vibration. Very soft ground may have 
a period of up to 2 seconds but, unlike a structure, it cannot sustain longer 
period motions except under certain unusual conditions.  Since this range is 
well within the range of common building periods, it is quite possible that the 
pushes that earthquake ground motion imparts to the building will be at the 
natural period of the building.  This may create resonance, causing the 
structure to encounter accelerations of perhaps 1g when the ground is only 
vibrating with accelerations of 0.2g.  Because of this, buildings suffer the 
greatest damage from ground motion at a frequency close or equal to their own 
natural frequency.
The terrible destruction in Mexico City in the earthquake of 1985 was primarily 
the result of response amplification caused by coincidence of building and 
ground motion periods (Figure 4-5). Mexico City was some 250 miles from the 
earthquake focus, and the earthquake caused the soft ground in margins of the 
old lake bed under the downtown buildings to vibrate for over 90 seconds at its 
long natural period of around 2 seconds. This caused buildings that were between 
about 6 and 20 stories in height to resonate at a similar period, greatly 
increasing the accelerations within them. Taller buildings suffered little 
damage. This amplification in building vibration is very undesirable. The 
possibility of it happening can be reduced by trying to ensure that the building 
period will not coincide with that of the ground. Thus, on soft (long-period) 
ground, it would be best to design a short, stiff (short-period) building.
Taller buildings also will undergo several modes of vibration so that the 
building will wiggle back and forth like a snake (Figure 4-6).
However, later modes of vibration are generally less critical than the natural 
period, although they may be significant in a high-rise building. For low-rise 
buildings, the natural period (which, for common structures, will always be 
relatively short) is the most significant. Note, however, that the low-period, 
low- to mid-rise building is more likely to experience resonance from the more 
common short-period ground motion.  
4.5.3	Site Response Spectrum
From the above, it can be seen that buildings with different periods (or 
frequency responses) will respond in widely differing ways to the same 
earthquake ground motion. Conversely, any building will act differently during 
different earthquakes, so for design purposes it is necessary to represent the 
buildings range of responses to ground motion of different frequency content.  
Such a representation is termed a site response spectrum. A site response 
spectrum is a graph that plots the maximum response values of acceleration, 
velocity, and displacement against period (and frequency). Response spectra are 
very important tools in earthquake engineering.
Figure 4-7 shows a simplified version of a response spectrum. These spectra 
show, on the vertical ordinate, the accelerations, velocities and displacements 
that may be expected at varying periods (the horizontal ordinate). Thus, the 
response spectrum illustrated shows a maximum acceleration response at a period 
of about 0.3 secondsthe fundamental period of a midrise building. This shows 
how building response varies with building period: as the period lengthens, 
accelerations decrease and displacement increases. On the other hand, one- or 
two-story buildings with short periods undergo higher accelerations but smaller 
displacements. 
In general, a more flexible longer period design may be expected to experience 
proportionately lesser accelerations than a stiffer building.A glance at a 
response spectrum will show why this is so: as the period of the building 
lengthens (moving towards the right of the horizontal axis of the spectrum), the 
accelerations reduce. Currently our codes recognize the beneficial aspect of 
flexibility (long period) by permitting lower design coefficients. However, 
there is an exchange, in that the lower accelerations in the more flexible 
design come at the expense of more motion. This increased motion may be such 
that the building may suffer considerable damage to its nonstructural 
components, such as ceilings and partitions, in even a modest earthquake.
Seismic codes provide a very simple standardized response spectrum that is 
suitable for small buildings (Figure 4-8). The code also provides a procedure 
for the engineer to construct a more accurate response spectrum for the 
building, based on various assumptions. For larger buildings in which a 
geotechnical consultant provides information on the site characteristics and an 
estimate of ground motions, detailed response spectra will also be provided to 
assist the engineer in the calculation of forces that the building will 
encounter. 
The response spectrum enables the engineer to identify the resonant frequencies 
at which the building will undergo peak accelerations. Based on this knowledge, 
the building design might be adjusted to ensure that the building period does 
not coincide with the site period of maximum response. For the site 
characteristics shown, with a maximum response at about 0.3 seconds, it would be 
appropriate to design a building with a longer period of 1 second or more. Of 
course, it is not always possible to do this, but the response spectrum shows 
clearly what the possible accelerations at different periods are likely to be, 
and the forces can be estimated more accurately. Information gained from a 
response spectrum is of most value in the design of large and high structures. 
How does one "tune" a building, or change its period, if it is necessary to do 
so? One could change the natural period of a simple structure such as a flag 
pole by any or all combinations of the methods shown in Figure 4-9:
  Changing the position of the weight to a lower height
  Changing the height of the pole
  Changing the sectional area or shape of the pole
  Changing the material
  Altering the fixity of the base anchorage 

There are analogous possibilities for buildings, though the building is much 
more complex than the simple monolithic flagpole:   

	Tune the building by ensuring that the structural characteristics of a 
design are compatible with those of the site as the preliminary design is 
developed. 
	Incorporate devices in the structure that dissipate energy and change the 
response characteristics. These devices are discussed in Chapter 7. 
	After the Mexico City earthquake of 1985, a number of damaged buildings 
had their upper floors removed to lower their period and reduce their mass, thus 
reducing the likelihood and consequences of resonance.  
4.6 Damping 
If a structure is made to vibrate, the amplitude of the vibration will decay 
over time and eventually cease. Damping is a measure of this decay in amplitude, 
and it is due to internal friction and absorbed energy. The nature of the 
structure and its connections affects the damping; a heavy concrete structure 
will provide more damping than a light steel frame.  Architectural features such 
as partitions and exterior faade construction contribute to the damping.  
Damping is measured by reference to a theoretical damping level termed critical 
damping. This is the least amount of damping that will allow the structure to 
return to its original position without any continued vibration. For most 
structures, the amount of damping in the system will vary from between 3 percent 
and 10 percent of critical. The higher values would apply to older buildings 
(such as offices and government buildings) that employed a structure of steel 
columns and beams encased in concrete together with some structural walls, which 
also had many heavy fixed partitions (often concrete block or hollow tile), and 
would have high damping values. The lower values would apply to a modern office 
building with a steel-moment frame, a light metal and glass exterior envelope, 
and open office layouts with a minimum of fixed partitions.  
The main significance of damping is that accelerations created by ground motion 
increase rapidly as the damping value decreases. The response spectra shown in 
Figure 4-10 show that the peak acceleration is about 3.2g for a damping value of 
0 %, 0.8g for a damping value of 2 % and a value of about  0.65g for a value of 
10 %.  
Tables are available that indicate recommended damping values, and the damping 
characteristics of a structure can be fairly easily estimated.  Response spectra 
generally show acceleration values for 0, 2, 5, and 10 % damping. A damping 
value of zero might be used in the design of an simple vibrator, such as a flag 
pole or a water tank supported on a single cantilever column. For typical 
structures, engineers generally use a value of 5 % critical.
Damping used to be regarded as a fixed attribute of buildings, but in recent 
years a number of devices have been produced that enable the engineer to 
increase the damping and reduce the building response.  This greatly increases 
the designers ability to provide a tuned response to the ground motion.  
4.7 Dynamic Amplification
It was early observed and calculated that for most structures, the structural 
movement is greater than that of the ground motion.  The increase of the 
structural movement over that of the ground motion is commonly referred to as 
dynamic amplification.  This amplification is caused when energy is reflected 
from the P and S waves when they reach the earths surface, which is 
consequently affected almost simultaneously by upward and downward moving waves 
(See Chapter 2, Section 2.3.2). The extent of amplification also varies 
depending on the dynamic properties of the structure and the characteristics of 
the initial earthquake ground motion encountered.  The important engineering 
attributes of the structure are:
	The period of vibration of the structure
	The damping properties of the structure 
For typical earthquake motions and for structures having the common damping 
value of 5 percent damping and a period range of 0.5 seconds to 3.3 seconds, the 
dynamic amplification factor would be about 2.5, which is a significant 
increase. For higher damping values, the amplification factor is reduced.
4.8	Higher Forces and Uncalculated Resistance
Even if a building is well damped and is unlikely to resonate, it may be 
subjected to forces that are much higher than the computed forces for which it 
was designed. For those familiar with the assumptions of design for vertical 
loads and the large factors of safety that are added into the calculations, this 
may seem surprising. Why is this so?
The answer is that to design a building for the very rare maximum conceivable 
earthquake forces, and then to add a factor of safety of two or three times as 
is done for vertical loads, would result in a very expensive structure whose 
functional use would be greatly compromised by massive structural members and 
structural walls with very limited openings: the ordinary building would 
resemble a nuclear power plant or a military bunker. 
Experience has shown, however, that many buildings have encountered forces far 
higher than they were designed to resist and yet have survived, sometimes with 
little damage. This phenomenon can be explained by the fact that the analysis of 
forces is not precise and deliberately errs on the conservative side so that the 
building strength is, in reality, greater than the design strength. In addition, 
the building often gains additional strength from components, such as 
partitions, that are not considered in a structural analysis. Some structural 
members may be sized for adequate stiffness rather than for strength, and so 
have considerable reserve strength. Materials often are stronger in reality than 
the engineer assumes in his calculations. Finally, seismically engineered 
structures have an additional characteristic that acts to provide safety in the 
event of encountering forces well beyond the design threshold: this is the 
important property of ductility. Taken together, these characteristics, though 
not all explicit, provide a considerable safety factor or uncalculated 
additional resistance. 
4.9	Ductility
The gap between design capacity (the theoretical ability of a building to 
withstand calculated forces) and possible actual forces is, finally, largely 
dealt with by relying on the material property of ductility. This is the 
property of certain materials (steel in particular) to fail only after 
considerable inelastic deformation has taken place, meaning that the material 
does not return to its original shape after distortion. This deformation, or 
distortion, dissipates the energy of the earthquake.
This is why it is much more difficult to break a metal spoon by bending it than 
one made of plastic. The metal object will remain intact, though distorted, 
after successive bending to and fro while the plastic spoon will snap suddenly 
after a few bends. The metal is far more ductile than the plastic (Figure 4-11).
The deformation of the metal (even in the spoon) absorbs energy and defers 
absolute failure of the structure. The material bends but does not break and so 
continues to resist forces and support loads, although with diminished 
effectiveness. The effect of earthquake motion on a building is rather like that 
of bending a spoon rapidly back and forth: the heavy structure is pushed back 
and forth in a similar way several times a second (depending on its period of 
vibration).
Brittle materials, such as unreinforced masonry or inadequately reinforced 
concrete, fail suddenly, with a minimum of prior distortion. The steel bars 
embedded in reinforced concrete can give this material considerable ductility, 
but heavier and more closely spaced reinforcing bars and special detailing of 
their placement are necessary. 
 Ductility and reserve capacity are closely related: past the elastic limit (the 
point at which forces cause permanent deformation), ductile materials can take 
further loading before complete failure.  In addition, the member proportions, 
end conditions, and connection details will also affect ductility.  Reserve 
capacity is the ability of a complete structure to resist overload, and is 
dependent on the ductility of its individual members.  The only reason for not 
requiring ductility is to provide so much resistance that members would never 
exceed elastic limits.
Thus, buildings are designed in such a way that in the rare case when they are 
subjected to forces higher than those required by a code, the materials and 
connections will distort but not break.  In so doing, they will safely absorb 
the energy of the earthquake vibrations, and the building, although distorted 
and possibly unusable, is at least still standing. 

4.10 Strength, Stiffness, Force Distribution and Stress Concentration	 

4.10.1 Strength and Stiffness	
Strength and stiffness are two of the most important characteristics of any 
structure.  Although these two concepts are present in non-seismic structural 
design and analysis, the distinction between strength and stiffness is perhaps 
most critical, and its study most highly developed, in structural engineering 
for lateral forces.
Sufficient strength is necessary to ensure that a structure can support imposed 
loads without exceeding certain stress values.  Stress refers to the internal 
forces within a material or member that are created as the structural member 
resists the applied load. Stress is expressed in force per unit area (for 
example, pounds per square inch).  
Stiffness is measured by deflection, the extent to which a structural member, 
such as a floor, roof, or wall structure, bends when loaded. Deflection is 
generally expressed as a fraction of length of the member or assembly. For 
gravity loads, this is usually the only aspect of stiffness that is of concern.  
When floor joists are designed for a house, for example, it is often deflection 
rather than strength that dictates the size of the joiststhat is, the depth of 
the joists is determined by how much they will bend under load rather than by 
whether they can safely support the floor loads. Typically, an unacceptable 
amount of bending (in the form of an uncomfortable springy feeling to 
occupants) will occur well before the joists are stressed to the point at which 
they may break because of overload.  
To ensure sufficient strength and stiffness, codes such as the International 
Building Code (IBC) provide stress and deflection limits that are not to be 
exceeded for commonly used materials and assemblies.  For example, interior 
partitions shall be designed to resist all loads to which they are subjected, 
but not less than a force of 5 pounds per square foot applied perpendicular to 
the walls. In addition, the deflection of such walls under a load of 5 pounds 
per square foot shall not exceed 1/240 of the span for walls with brittle 
finishes and 1/120 of the span for walls with flexible finishes.  
Most designers are familiar with deflection in this sense, and have an intuitive 
feel for this quality.  In seismic design, deflection of vertical structural 
members, such as columns and walls, is termed drift. Analogous to the deflection 
of horizontal members, limitations on drift may impose more severe requirements 
on members than the strength requirements. Story drift is expressed as the 
difference of the deflections at the top and bottom of the story under 
consideration: this is also often expressed as a ratio between the deflection 
and the story, or floor-to-floor height (Figure 4-12). Drift limits serve to 
prevent possible damage to interior or exterior walls that are attached to the 
structure and which might be cracked or distorted if the structure deflects too 
much laterally, creating racking forces in the member. Thus the IBC requires 
that drift be limited in typical buildings to between 0.02 and 0.01 times the 
building height, depending on the occupancy of the building. For a building that 
is 30 feet high, drift would be limited to between 3.6 inches and 7.2 inches 
depending on the building type.      
When the earthquake-induced drift is excessive, vertical members may become 
permanently deformed; excessive deformation can lead to structural and 
nonstructural damage and, ultimately, collapse.
  
Thus strength and stiffness are two important characteristics of any structural 
member. Two structural beams may have approximately equal material strengths and 
be of similar shape but will vary in their stiffness and strength, depending on 
how they are oriented relative to the load.This concept can be easily understood 
by visualizing the flexibility of a narrow, deep beam placed where it has to 
support a load: the extent of deflection will depend on whether the load is 
placed on the beams flat surface or on its edge (Figure 4-13).
4.10.2 Force Distribution and Stress Concentration
In seismic design, there is another very important characteristic of stiffness, 
besides that of deflection.  The simple solution of determining the overall 
lateral force on the building by multiplying the building weight by its 
acceleration has already been discussed.  But the engineer needs to know how 
this force is allocated to the various resisting structural elements that must 
be designed: each shares some proportion of this overall force. The answer is 
that the force is distributed in proportion to the relative stiffness of the 
resisting members.  In other terms, the applied forces are attracted to and 
concentrated at the stiffer elements of the building.  Thus the engineer must 
calculate the stiffness of the resisting elements to ascertain the forces that 
they must accommodate.
The relative rigidities of members are a major concern in seismic analysis.  As 
soon as a rigid horizontal element or diaphragm, such as a concrete slab, is 
tied to vertical resisting elements, it will force those elements to deflect the 
same amount.  If two elements (two frames, walls, braces, or any combination) 
are forced to deflect the same amount, and if one is stiffer, that one will take 
more of the load. Only if the stiffnesses are identical can it be assumed that 
they share the load equally.  Since concrete slab floors or roofs will generally 
fit into the "rigid diaphragm" classification, and since it is unusual for all 
walls, frames, or braced frames to be identical, the evaluation of relative 
rigidities is a necessary part of most seismic analysis problems in order to 
determine the relative distribution of the total horizontal force to the various 
resisting elements. 
The reason why forces are related to the stiffness of the resisting elements can 
be understood by visualizing a heavy block supported away from a wall by two 
short beams. Clearly, the thick, stiff beam will carry much more load than the 
slender one, and the same is true if they are turned 90 degrees to simulate the 
lateral force situation (Figure 4-14).
An important aspect of this concept in relation to column lateral stiffness is 
illustrated in Figure 4-15. In this figure the columns have the same cross 
section, but the short column is half the length of the long one. 
Mathematically, the stiffness of a column varies approximately as the cube of 
its length. Therefore, the short column will be eight times stiffer instead of 
twice as stiff and will be subject to eight times the horizontal load of the 
long column. Stress is concentrated in the short column, while the long column 
is subject to nominal forces.  
In a building with members of varying stiffness, an undue proportion of the 
overall forces may be concentrated at a few points of the building, such as a 
particular set of beams, columns, or walls, as shown at the top of Figure 4-15. 
These few members may fail and, by a chain reaction, bring down the whole 
building. People who are in the building demolition business know that if they 
weaken a few key columns or connections in a building, they can bring it down. 
An earthquake also tends to "find" these weak links. 
This condition has serious implications for buildings with column or shear walls 
of different length.  In designing a structure, the engineer tries to equalize 
the stiffness of the resisting elements so that no one member or small group of 
members takes a disproportionate amount of the load.  If this cannot be done - 
if their size and stiffness vary for architectural reasons, for example -- then 
the designer must make sure that stiffer members are appropriately designed to 
carry their proportion of the load.
A special case of this problem is that sometimes a short-column condition is 
created inadvertently after the building is occupied.  For example, the space 
between columns may be filled in by a rigid wall, leaving a short space for a 
clerestory window. Such a simple act of remodeling may not seem to require 
engineering analysis, and a contractor may be hired to do the work: often such 
work is not subject to building department reviews and inspection.  Serious 
damage has occurred to buildings in earthquakes because of this oversight 
(Figure 4-16).
4.11 Torsional Forces
The center of mass, or center of gravity, of an object is the point at which it 
could be exactly balanced without any rotation resulting.  If the mass (or 
weight) of a building is uniformly distributed (in plan), the result is that the 
plan's geometric center will coincide with the center of mass.  In a building, 
the main lateral force is contributed by the weight of the floors, walls, and 
roof, and this force is exerted through the center of mass, usually the 
geometric center of the floor (in plan). If the mass within a floor is uniformly 
distributed, then the resultant force of the horizontal acceleration of all its 
particles is exerted through the floor's geometric center.  If the resultant 
force of the resistance (provided by shear walls, moment frames, or braced 
frames) pushes back through this point, dynamic balance is maintained. 
Torsional forces are created in a building by a lack of balance between the 
location of the resisting elements and the arrangement of the building mass.  
Engineers refer to this as eccentricity between the center of mass and the 
center of resistance, which makes a building subjected to ground motion rotate 
around its center of resistance, creating torsion - a twisting action in plan, 
which results in undesirable and possibly dangerous concentrations of stress  
(Figure 4-17).
In a building in which the mass is approximately evenly distributed in plan 
(typical of a symmetrical building with uniform floor, wall and column masses) 
the ideal arrangement is that the earthquake resistant elements should be 
symmetrically placed, in all directions, so that no matter in which direction 
the floors are pushed, the structure pushes back with a balanced stiffness that 
prevents rotation from trying to occur. This is the reason why it is recommended 
that buildings in areas of seismic risk be designed to be as symmetrical as 
possible. In practice, some degree of torsion is always present, and the 
building code makes provision for this.
4.12 Nonstructural Components
For many decades, seismic building codes focused exclusively on the structure of 
the buildingthat is, the system of columns, beams, walls, and diaphragms that 
provides resistance against earthquake forces.  Although this focus remains 
dominant for obvious reasons, experience in more recent earthquakes has shown 
that damage to nonstructural components is also of great concern. In most modern 
buildings, the nonstructural components account for 60 to 80 percent of the 
value of the building (Figure 4-18). Most nonstructural components are fragile 
(compared to the building structure), easily damaged, and costly to repair or 
replace (Figure 4-19).
The distinction between structural and nonstructural components and systems is, 
in many instances, artificial. The engineer labels as nonstructural all those 
components that are not designed as part of the seismic lateral force-resisting 
system. Nature, however, makes no such distinction, and tests the whole 
building. Many nonstructural components may be called upon to resist forces even 
though not designed to do so.
The nonstructural components or systems may modify the structural response in 
ways detrimental to the safety of the building. Examples are the placing of 
heavy nonstructural partitions in locations that result in severe torsion and 
stress concentration, or the placement of nonstructural partitions between 
columns in such a way as to produce a short column condition, as described in 
Section 4.10 and illustrated in Figure 4-16. This can lead to column failure, 
distortion, and further nonstructural damage. Failure of the fire protection 
system, because of damage to the sprinkler system, may leave the building 
vulnerable to post-earthquake fires caused by electrical or gas system damage.
4.13 Construction Quality
One other characteristic that applies to any structure may be obvious, but must 
be emphasized:  the entire structural system must be correctly constructed if it 
is to perform well. Lateral forces are especially demanding because they 
actively attempt to tear the building apart, whereas vertical loads (with the 
exception of unusual live loads such as automobiles or large masses of people) 
sit still and quiet within the materials of the building. 
The materials of the seismically resistant structure must have the necessary 
basic strength and expected properties, but most importantly, all the structural 
components must be securely connected together so that as they push and pull 
against one another during the earthquake, the connections are strong enough to 
transfer the earthquake forces and thereby maintain the integrity of the 
structure.This means that detailed design and construction of connections are 
particularly important. 
The correct installation of reinforcing steel and anchors in concrete 
structures; the correct design, fabrication and installation of connection 
members in steel structures; and correct nailing, edge clearances and 
installation of hold-downs in wood framing are all critical. For nonstructural 
components, critical issues are the maintenance of correct clearances between 
precast concrete cladding panels, at seismic separation joints, and between 
glazing and window framing, and the correct design and installation of bracing 
of heavy acceleration-sensitive components such as tanks, chillers, heavy 
piping, electrical transformers and switch gear. 
Quality control procedures must be enforced at all phases of design and 
construction, including material testing and on-site inspection by qualified 
personnel. The earthquake is the ultimate testing laboratory of the construction 
quality of the building.
4.14 CONCLUSION	
This chapter has discussed a number of key characteristics of earthquake ground 
shaking that affect the seismic performance of buildings.  In addition, a number 
of building characteristics have been reviewed that, together with those of the 
ground, determine the buildings seismic performance: how much damage the 
building will suffer.  These characteristics are common to all buildings, both 
new and existing, and all locations.  
The building response to earthquake shaking occurs over the time of a few 
seconds. During this time, the several types of seismic waves are combining to 
shake the building in ways that are different in detail for each earthquake. In 
addition, as the result of variations in fault slippage, differing rock through 
which the waves pass, and the different geological nature of each site, the 
resultant shaking at each site is different.  The characteristics of each 
building are different, whether in size, configuration, material, structural 
system, method of analysis, age, or quality of construction: each of these 
characteristics affects the building response.  
In spite of the complexity of the interactions between the building and the 
ground during the few seconds of shaking there is broad understanding of how 
different building types will perform under different shaking conditions.  This 
understanding comes from extensive observation of buildings in earthquakes all 
over the world, together with analytical and experimental research at many 
universities and research centers. 
Understanding the ground and building characteristics discussed in this chapter 
is essential to give designers a feel for how their building will react to 
shaking, which is necessary to guide the conceptual design of their building.  
The next chapter continues this direction by focusing on certain architectural 
characteristics that influence seismic performance - either in a positive or 
negative way.  
4.15 References 
Building Seismic Safety Council, NEHRP Recommended Provisions for Seismic 
Regulations for New Buildings and Other Structures, Part 2 Commentary, FEMA 450,  
2003, Washington, DC.
International Code Council, International Building Code 2003 (IBC), Birmingham, 
AL.
Stratta, James, Manual of Seismic Design,1987, Prentice-Hall, Englewood Cliffs, 
NJ

4.16	to find out more
MCEER, Center for Earthquake Engineering Research.  mceer@buffalo.edu
Bolt, B. A., Earthquakes (4th edition) W. H. Freeman and Company New York N. Y. 
2002.
Gere J. M. and Shah, H.C.,1984 Terra non Firma, Stanford Alumni Association, 
Stanford CA. 
Levy M. and Salvadori M.,Why Buildings Stand Up, W. W. Norton and Company, New 
York, NY. 1990.
Levy M. and Salvadori M.,Why the Earth Quakes, W. W. Norton and Company, New 
York, NY. 1997.
Levy M. and Salvadori M.,Why Buildings Fall Down, W. W. Norton and Company, New 
York, NY 2002.


[End of Chapter 4]



SEISMIC ISSUES IN ARCHITECTURAL DESIGN BY CHRISTOPHER ARNOLD
5.1 INTRODUCTION
This chapter uses the information in the preceding chapter to explain how 
architectural design decisions influence a buildings likelihood to suffer 
damage when subjected to earthquake ground motion. The critical design decisions 
are those that create the building configuration, defined as the buildings size 
and three dimensional shape, and those  that introduce detailed complexities 
into the structure, in ways that will be discussed later.
In sections 5.2 to 5.5, the effects of architectural design decisions on seismic 
performance are explained by showing a common structural/architectural 
configuration that has been designed for near optimum seismic performance and 
explaining its particular characteristics that are seismically desirable. In 
Section 5.3, the two main conditions created by configuration irregularity are 
explained. In Section 5.4, a number of deviations from these characteristics 
(predominantly architectural in origin) are identified as problematical from a 
seismic viewpoint. Four of these deviations are then discussed in more detail in 
Section 5. 5 both from an engineering and architectural viewpoint, and 
conceptual solutions are provided for reducing or eliminating the detrimental 
effects. Section 5.6 identifies a few other detailed configuration issues that 
may present problems.
Section 5.7 shows how seismic configuration problems originated in the universal 
adoption of the International Style in the twentieth century, while Section 
5.8 gives some guidelines on how to avoid architectural/structural problems. 
Finally, Section 5.9 looks to the future in assessing todays architectural 
trends, their influence on seismic engineering, and the possibility that seismic 
needs might result in a new seismic architecture.
5.2 THE BASIC SEISMIC structural SYSTEMS
A buildings structural system is directly related to its architectural 
configuration, which largely determines the size and location of structural 
elements such as walls, columns, horizontal beams, floors, and roof structure. 
Here, the term structural/architectural configuration is used to represent this 
relationship.
5.2.1	The Vertical Lateral Resistance Systems 
Seismic designers have the choice of three basic alternative types of vertical 
lateral forceresisting systems, and as discussed later, the system must be 
selected at the outset of the architectural design process. Here, the intent is 
to demonstrate an optimum architectural/structural configuration for each of the 
three basic systems. The three alternatives are illustrated in Figure 5-1.
These basic systems have a number of variations, mainly related to the 
structural materials used and the ways in which the members are connected. Many 
of these are shown in Chapter 7: Figures 7-2, 7-3, 7-11A and 7-11b show their 
comparative seismic performance characteristics. 
	Shear walls
	Shear walls are designed to receive lateral forces from diaphragms and 
transmit them to the ground. The forces in these walls are predominantly shear 
forces in which the material fibers within the wall try to slide past one 
another. To be effective, shear walls must run from the top of the building to 
the foundation with no offsets and a minimum of openings.
	Braced frames
	Braced frames act in the same way as shear walls; however, they generally 
provide less resistance but better ductility depending on their detailed design. 
They provide more architectural design freedom than shear walls.  
	There are two general types of braced frame: conventional concentric and 
eccentric. In the concentric frame, the center lines of the bracing members meet 
the horizontal beam at a single point.    
	In the eccentric braced frame, the braces are deliberately designed to 
meet the beam some distance apart from one another: the short piece of beam 
between the ends of the braces is called a link beam. The purpose of the link 
beam is to provide ductility to the system: under heavy seismic forces, the link 
beam will distort and dissipate the energy of the earthquake in a controlled 
way, thus protecting the remainder of the structure (Figure 5-2).
  Moment-resistant frames
	A moment resistant frame is the engineering term for a frame structure 
with no diagonal bracing in which the lateral forces are resisted primarily by 
bending in the beams and columns mobilized by strong joints between columns and 
beams.  Moment-resistant frames provide the most architectural design freedom.
These systems are, to some extent, alternatives, although designers sometimes 
mix systems, using one type in one direction and another type in the other.  
This must be done with care, however, mainly because the different systems are 
of varying stiffness (shear-wall systems are much stiffer than moment-resisting 
frame systems, and braced systems fall in between), and it is difficult to 
obtain balanced resistance when they are mixed.  However, for high-performance 
structures,) there is now increasing use of dual systems, as described in 
section 7.7.6. Examples of effective mixed systems are the use of a shear-wall 
core together with a perimeter moment-resistant frame or a perimeter steel-
moment frame with interior eccentric-braced frames. Another variation is the use 
of shear walls combined with a moment-resistant frame in which the frames are 
designed to act as a fail-safe back-up in case of shear-wall failure.
The framing system must be chosen at an early stage in the design because the 
different system characteristics have a considerable effect on the architectural 
design, both functionally and aesthetically, and because the seismic system 
plays the major role in determining the seismic performance of the building.  
For example, if shear walls are chosen as the seismic force-resisting system, 
the building planning must be able to accept a pattern of permanent structural 
walls with limited openings that run uninterrupted through every floor from roof 
to foundation.
5.2.2  Diaphragmsthe Horizontal Resistance  System
The term diaphragm is used to identify horizontal-resistance members that 
transfer lateral forces between vertical-resistance elements (shear walls or 
frames).  The diaphragms are generally provided by the floor and roof elements 
of the building;  sometimes, however, horizontal bracing systems independent of 
the roof or floor structure serve as diaphragms. The diaphragm is an important 
element in the entire seismic resistance system (Figure 5-3).
The diaphragm can be visualized as a wide horizontal beam with components at its 
edges, termed chords, designed to resist tension and compression: chords are 
similar to the flanges of a vertical beam (Figure 5-3A)
A diaphragm that forms part of a resistant system may act either in a flexible 
or rigid manner, depending partly on its size (the area between enclosing 
resistance elements or stiffening beams) and also on its material.  The 
flexibility of the diaphragm, relative to the shear walls whose forces it is 
transmitting, also has a major influence on the nature and magnitude of those 
forces. With flexible diaphragms made of wood or steel decking without concrete, 
walls take loads according to tributary areas (if mass is evenly distributed). 
With rigid diaphragms (usually concrete slabs), walls share the loads in 
proportion to their stiffness (figure 5-3B).
Collectors, also called drag struts or ties, are diaphragm framing members that 
collect or drag diaphragm shear forces from laterally unsupported areas to 
vertical resisting elements (Figure 5-3C).
Floors and roofs have to be penetrated by staircases, elevator and duct shafts, 
skylights, and atria. The size and location of these penetrations are critical 
to the effectiveness of the diaphragm. The reason for this is not hard to see 
when the diaphragm is visualized as a beam. For example, it can be seen that 
openings cut in the tension flange of a beam will seriously weaken its load 
carrying capacity.  In a vertical load-bearing situation, a penetration through 
a beam flange would occur in either a tensile or compressive region. In a 
lateral load system, the hole would be in a region of both tension and 
compression, since the loading alternates rapidly in direction (Figure 5-3D). 
5.2.3  Optimizing the Structural/Architectural Configuration
Figure 5-4 shows the application of the three basic seismic systems to a model 
structural/architectural configuration that has been designed for near optimum 
seismic performance. The figure also explains the particular characteristics 
that are seismically desirable.
Building attributes: 
  Continuous load path. 
Uniform loading of structural elements and no stress concentrations. 
  Low height-to base ratio 
Minimizes tendency to overturn.
  Equal floor heights
Equalizes column or wall stiffness, no stress concentrations.
  Symmetrical plan shape 
Minimizes torsion.
   Identical resistance on both axes
Eliminates eccentricity between the centers of mass and resistance      and 
provides balanced resistance in all directions, thus minimizing torsion.
	Identical vertical resistance 
No concentrations of strength or weakness.
   Uniform section and elevations 
Minimizes stress concentrations.
   Seismic resisting elements at perimeter 
 Maximum torsional resistance.		
   Short spans 
Low unit stress in members, multiple columns provide redundancy -loads can be 
redistributed if some columns are lost.
   No cantilevers 
Reduced vulnerability to vertical accelerations.
	No openings in diaphragms(floors and roof) 
Ensures direct transfer of lateral forces to the resistant elements.
In the model design shown in Figure 5-4, the lateral force resisting elements 
are placed on the perimeter of the building, which is the most effective 
location; the reasons for this are noted in the text. This location also 
provides the maximum freedom for interior space planning. In a large building, 
resistant elements may also be required in the interior. 
Since ground motion is essentially random in direction, the resistance system 
must protect against shaking in all directions. In a rectilinear plan building 
such as this, the resistance elements are most effective when placed on the two 
major axes of the building in a symmetrical arrangement that provides balanced 
resistance.  A square plan, as shown here, provides for a near perfectly 
balanced system.
Considered purely as architecture, this little building is quite acceptable, and 
would be simple and economical to construct.  Depending on its exterior 
treatment - its materials, and the care and refinement with which they are 
disposed- - it could range from a very economical functional building to an 
elegant architectural jewel. It is not a complete building, of course, because 
stairs, elevators, etc., must be added, and the building is not spatially 
interesting.  However, its interior could be configured with nonstructural 
components to provide almost any quality of room that was desired, with the 
exception of unusual spatial volumes such as spaces more than one story in 
height. 
In seismic terms, engineers refer to this design as a regular building.  As the 
building characteristics deviate from this model, the building becomes 
increasingly irregular.  It is these irregularities, for the most part created 
by the architectural design, that affect the buildings seismic performance. 
Indeed many engineers believe that it is these architectural irregularities that 
contribute primarily to poor seismic performance and occasional failure.  
5.3 The Effects of Configuration Irregularity 
Configuration irregularity is largely responsible for two undesirable 
conditions-stress concentrations and torsion. These conditions often occur 
concurrently.
5.3.1 Stress Concentrations
Irregularities tend to create abrupt changes in strength or stiffness that may 
concentrate forces in an undesirable way. Although the overall design lateral 
force is usually determined by calculations based on seismic code requirements, 
the way in which this force is distributed throughout the structure is 
determined by the building configuration.
Stress concentration occurs when large forces are concentrated at one or a few 
elements of the building, such as a particular set of beams, columns, or walls. 
These few members may fail and, by a chain reaction, damage or even bring down 
the whole building. Because, as discussed in Section 4.10.2, forces are 
attracted to the stiffer elements of the building, these will be locations of 
stress concentration.
Stress concentrations can be created by both horizontal and vertical stiffness 
irregularities. The short-column phenomenon discussed in Section 4.10.2 and 
shown in Figure 4-14 is an example of stress concentration created by vertical 
dimensional irregularity in the building design. In plan, a configuration that 
is most likely to produce stress concentrations features re-entrant corners: 
buildings with plan forms such as an L or a T.) A discussion of the re-entrant 
corner configuration will be found in Section 5.5.4. 
The vertical irregularity of the soft or weak story types can produce dangerous 
stress concentrations along the plane of discontinuity. Soft and weak stories 
are discussed in Section 5.5.1. 
5.3.2 Torsion
Configuration irregularities in plan may cause torsional forces to develop, 
which contribute a significant element of uncertainty to an analysis of building 
resistance, and are perhaps the most frequent cause of structural failure.

As described in Section 4.11 and shown in Figure 4-17, torsional forces are 
created in a building by eccentricity between the center of mass and the center 
of resistance. This eccentricity originates either in the lack of symmetry in 
the arrangement of the perimeter-resistant elements as discussed in Section 
5.5.3., or in the plan configuration of the building, as in the re-entrant-
corner forms discussed in Section 5.5.4. 
5.4 Configuration Irregularity in the Seismic Code
Many of the configuration conditions that present seismic problems were 
identified by observers early in the twentieth century. However, the 
configuration problem was first defined for code purposes in the 1975 Commentary 
to the Strucural Engineers Association of California (SEAOC) Recommended Lateral 
Force Requirements (commonly called the SEAOC Blue Book). In this section over 
twenty specific types of irregular structures or framing systems were noted as 
examples of designs that should involve further analysis and dynamic 
consideration, rather than the use of the simple equivalent static force method 
in unmodified form. These irregularities vary in importance in their effect, and 
their influence also varies in degree, depending on which particular 
irregularity is present. Thus, while in an extreme form the re-entrant corner is 
a serious plan irregularity, in a lesser form it may have little or no 
significance. The determination of the point at which a given irregularity 
becomes serious was left up to the judgment of the engineer.  
Because of the belief that this approach was ineffective, in the 1988 codes a 
list of six horizontal (plan) and six vertical (section and elevation) 
irregularities was provided that, with minor changes, is still in todays codes. 
This list also stipulated dimensional or other characteristics that established 
whether the irregularity was serious enough to require regulation, and also 
provided the provisions that must be met in order to meet the code. Of the 12 
irregularities shown, all except one are configuration irregularities; the one 
exception refers to asymmetrical location of mass within the building. The 
irregularities are shown in Figures 5.5 and 5.6. The code provides only 
descriptions of these conditions; the diagrams are added in this publication to 
illustrate each condition by showing how it would modify our optimized 
configuration, and to also illustrate the failure pattern that is created by the 
irregularity.

For the most part, code provisions seek to discourage irregularity in design by 
imposing penalties, which are of three types:
	Requiring increased design forces.
	Requiring a more advanced (and expensive) analysis procedure.
	Disallowing extreme soft stories and extreme torsional imbalance in high 
seismic zones.   
It should be noted that the code provisions treat the symptoms of irregularity, 
rather than the cause.  The irregularity is still allowed to exist; the hope is 
that the penalties will be sufficient to cause the designers to eliminate the 
irregularities.  Increasing the design forces or improving the analysis to 
provide better information does not, in itself, solve the problem.  The problem 
must be solved by design.
The code-defined irregularities shown in Figures 5-5 and 5-6 serve as a 
checklist for ascertaining the possibility of configuration problems. Four of 
the more serious configuration conditions that are clearly architectural in 
origin are described in more detail in the sections below.  In addition, some 
conceptual suggestions for their solution are also provided, as it may not be 
possible totally to eliminate an undesirable configuration.  
5.5 Four Serious Configuration Conditions
Four configuration conditions (two vertical and two in plan) that originate in 
the architectural design and that have the potential to seriously impact seismic 
performance are:
	Soft and weak  stories
	Discontinuous shear walls
	Variations in perimeter strength and stiffness
	Reentrant corners


5.5.1 Soft and Weak Stories (Code Irregularities Types V1 and V5)  
 The problem and the types of condition
The most prominent of the problems caused by severe stress concentration is that 
of the soft story. The term has commonly been applied to buildings whose 
ground-level story is less stiff than those above. The building code 
distinguishes between soft and weak stories.  Soft stories are less stiff, 
or more flexible, than the story above; weak stories have less strength. A soft 
or weak story at any height creates a problem, but since the cumulative loads 
are greatest towards the base of the building, a discontinuity between the first 
and second floor tends to result in the most serious condition.  
The way in which severe stress concentration is caused at the top of the first 
floor is shown in the diagram sequence in Figure 5-7. Normal drift under 
earthquake forces that is distributed equally among the upper floors is shown in 
Figure 5-7A. With a soft story, almost all the drift occurs in the first floor, 
and stress concentrates at the second-floor connections (Figure 5-7B). This 
concentration overstresses the joints along the second floor line, leading to 
distortion or collapse (Figure 5-7C).
Three typical conditions create a soft first story (Figure 5-8). The first 
condition (Figure 5-8A) is where the vertical structure between the first and 
second floor is significantly more flexible than that of the upper floors. (The 
seismic code provides numerical values to evaluate whether a soft-story 
condition exists). This discontinuity most commonly occurs in a frame structure 
in which the first floor height is significantly taller than those above, so 
that the cube law results in a large discrepancy in stiffness (see Section 
4.10.2 and Figure 4-13).   
The second form of soft story (Figure 5-B) is created by a common design concept 
in which some of the vertical framing elements do not continue to the 
foundation, but rather are terminated at the second floor to increase the 
openness at ground level. This condition creates a discontinuous load path that 
results in an abrupt change in stiffness and strength at the plane of change. 
Finally, the soft story may be created by an open first floor that supports 
heavy structural or nonstructural walls above (Figure 5-8C). This situation is 
most serious when the walls above are shear walls acting as major lateral force-
resisting elements. This condition is discussed in Section 5.5.2, since it 
represents an important special case of the weak- and soft- story problem.
Figure 5-9 shows the Northridge Meadows apartment building after the Northridge 
(Los Angeles) earthquake of 1994. In this building, most of the first floor was 
left open for car parking, resulting in both a weak and flexible first floor. 
The shear capacity of the first-floor columns and the few walls of this large 
wood frame structure were quite inadequate, and led to complete collapse and 16 
deaths. 
Figure 5-10 shows another apartment house in Northridge in which two stories of 
wood frame construction were supported on a precast concrete frame.  The frame 
collapsed completely. Fortunately there were no ground floor apartments, so the 
residents, though severely shaken, were uninjured. 
 Solutions
The best solution to the soft and weak story problem is to avoid the 
discontinuity through architectural design. There may, however, be good 
programmatic reasons why the first floor should be more open or higher than the 
upper floors. In these cases, careful architectural/structural design must be 
employed to reduce the discontinuity.  Some conceptual methods for doing this 
are shown in Figure 5-11.
Not all buildings that show slender columns and high first floors are soft 
stories. For a soft story to exist, the flexible columns must be the main 
lateral force-resistant system.  
Designers sometimes create a soft-story condition in the effort to create a 
delicate, elegant appearance at the base of a building.  Skillful 
structural/architectural design can achieve this effect without compromising the 
structure, as shown in Figure 5-12.  The building shown is a 21-story apartment 
house on the beach in Vina del Mar, Chile. This building was unscathed in the 
strong Chilean earthquake of 1985.   
5.5.2 Discontinuous Shear Walls (Code Type Irregularity V5)
 The problem and the types of condition
When shear walls form the main lateral resistant elements of a structure, and 
there is not a continuous load path through the walls from roof to foundation, 
the result can be serious overstressing at the points of discontinuity. This 
discontinuous shear wall condition represents a special, but common, case of the 
soft first-story problem.    
The discontinuous shear wall is a fundamental design contradiction: the purpose 
of a shear wall is to collect diaphragm loads at each floor and transmit them as 
directly and efficiently as possible to the foundation. To interrupt this load 
path is undesirable; to interrupt it at its base, where the shear forces are 
greatest, is a major error. Thus the discontinuous shear wall that terminates at 
the second floor represents a worst case of the soft first-floor condition. A 
discontinuity in vertical stiffness and strength leads to a concentration of 
stresses, and the story that must hold up all the rest of the stories in a 
building should be the last, rather than the first, element to be sacrificed.
Olive View Hospital, which was severely damaged in the 1971 San Fernando, 
California, earthquake, represents an extreme form of the discontinuous shear 
wall problem. The general vertical configuration of the main building was a 
soft two-story layer of rigid frames on which was supported a four story 
(five, counting penthouse) stiff shear wall-plus-frame structure (Figures 5-13, 
5-14). The second floor extends out to form a large plaza. Severe damage 
occurred in the soft story portion. The upper stories moved as a unit, and moved 
so much that the columns at ground level could not accommodate such a high 
displacement between their bases and tops, and hence failed. The largest amount 
by which a column was left permanently out-of-plumb was 2 feet 6 inches (Figure 
5-15). The building did not collapse, but two occupants in intensive care and a 
maintenance person working outside the building were killed.
 Solutions
The solution to the problem of the discontinuous shear wall is unequivocally to 
eliminate the condition. To do this may create architectural problems of 
planning or circulation or image. If this is so, it indicates that the decision 
to use shear walls as resistant elements was wrong from the inception of the 
design. If the decision is made to use shear walls, then their presence must be 
recognized from the beginning of schematic design, and their size and location 
made the subject of careful architectural and engineering coordination early. 
5.5.3 Variations in Perimeter Strength and Stiffness (Code Type P1)
  The problem and the types of condition
As discussed in Section 4.11, this problem may occur in buildings whose 
configuration is geometrically regular and symmetrical, but nonetheless 
irregular for seismic design purposes.
A buildings seismic behavior is strongly influenced by the nature of the 
perimeter design. If there is wide variation in strength and stiffness around 
the perimeter, the center of mass will not coincide with the center of 
resistance, and torsional forces will tend to cause the building to rotate 
around the center of resistance.  
Figure 5-16 shows an apartment house in Via del Mar, Chile, following the 
earthquake of 1985.  The city is an ocean resort, and beach-front apartments are 
designed with open frontage facing the beach.  This small seven-story 
condominium building had only three apartments per floor, with the service areas 
and elevator concentrated to the rear and surrounded by reinforced concrete 
walls that provided the seismic resistance. The lack of balance in resistance 
was such that the building rotated around its center of resistance, tilted 
sharply, and nearly collapsed. The building was subsequently demolished.  
A common instance of an unbalanced perimeter is that of open-front design in 
buildings, such as fire stations and motor maintenance shops in which it is 
necessary to provide large doors for the passage of vehicles.  Stores, 
individually or as a group in a shopping mall, are often designed as boxes with 
three solid sides and an open glazed front (Figure 5-17).
The large imbalance in perimeter strength and stiffness results in large 
torsional forces. Large buildings, such as department stores, that have 
unbalanced resistance on a number of floors to provide large window areas for 
display are also common. A classic case of damage to a large store with an 
unbalanced-perimeter resistance condition was that of the Penneys store in the 
Alaska earthquake of 1964 (Figure 5-18).
  Solutions
The solution to this problem is to reduce the possibility of torsion by 
endeavoring to balance the resistance around the perimeter.  The example shown 
is that of the store front. A number of alternative design strategies can be 
employed that could also be used for the other building type conditions noted 
(Figure 5-19).
The first strategy is to design a frame structure of approximately equal 
strength and stiffness for the entire perimeter.  The opaque portion of the 
perimeter can be constructed of nonstructural cladding, designed so that it does 
not affect the seismic performance of the frame.  This can be done either by 
using lightweight cladding or by ensuring that heavy materials, such as concrete 
or masonry, are isolated from the frame (Figure 5-19A).
A second approach is to increase the stiffness of the open facades by adding 
sufficient shear walls, at or near the open face, designed to approach the 
resistance provided by the other walls (Figure 5-19B).

A third solution is to use a strong moment resisting or braced frame at the open 
front, which approaches the solid wall in stiffness. The ability to do this will 
depend on the size of the facades; a long steel frame can never approach a long 
concrete wall in stiffness. This is, however, a good solution for wood frame 
structures, such as small apartment buildings, or motels with ground floor 
garage areas, or small store fronts, because even a comparatively long steel 
frame can be made as stiff as plywood shear walls (Figure 5-19C).
The possibility of torsion may be accepted and the structure designed to have 
the capacity to resist it, through a combination of moment frames, shear walls,) 
and diaphragm action. This solution will apply only to relatively small 
structures with stiff diaphragms designed in such a way that they can 
accommodate considerable eccentric loading (Figure 5-19D). 
Manufacturers have recently produced prefabricated metal shear walls, with high 
shear values, that can be incorporated in residential wood frame structures to 
solve the house-over-garage problem.  
5.5.4 	Re-entrant Corners (Code Type Irregularitiy  H5) 
  The problem and the types of condition
The re-entrant corner is the common characteristic of building forms that, in 
plan, assume the shape of an L, T, H, etc., or a combination of these shapes 
(Figure 5-20).
There are two problems created by these shapes. The first is that they tend to 
produce differential motions between different wings of the building that, 
because of stiff elements that tend to be located in this region, result in 
local stress concentrations at the re-entrant corner, or notch.  
The second problem of this form is torsion. Which is caused because the center 
of mass and the center of rigidity in this form cannot geometrically coincide 
for all possible earthquake directions. The result is rotation. The resulting 
forces are very difficult to analyze and predict.  Figure 5-21 shows the 
problems with the re-entrant-corner form. The stress concentration at the 
notch and the torsional effects are interrelated. The magnitude of the forces 
and the severity of the problems will depend on:
	The characteristics of the ground motion
	The mass of the building
	The type of  structural systems
	The length of the wings and their aspect ratios (length to width 
proportion)
	The height of the wings and their height/depth ratios
Figure 5-22 shows West Anchorage High School, Alaska, after the 1964 earthquake. 
The photo shows damage to the notch of this splayed L-shape building.   Note 
that the heavy walls have attracted large forces. A short column effect is 
visible at the column between the two bottom windows which have suffered classic 
X shaped shear-failure cracking and the damage at the top where this highly 
stressed region has been weakened by the insertion of windows.
Re-entrant corner plan forms are a most useful set of building shapes for urban 
sites, particularly for residential apartments and hotels, which enable large 
plan areas to be accommodated in relatively compact form, yet still provide a 
high percentage of perimeter rooms with access to air and light.
These configurations are so common and familiar that the fact that they 
represent one of the most difficult problem areas in seismic design may seem 
surprising.  Examples of damage to re-entrant-corner type buildings are common, 
and this problem was one of the first to be identified by observers. 
The courtyard form, most appropriate for hotels and apartment houses in tight 
urban sites, has always been useful; in its most modern form, the courtyard 
sometimes becomes a glass-enclosed atrium, but the structural form is the same.
  Solutions
There are two basic alternative approaches to the problem of re-entrant-corner 
forms: structurally to separate the building into simpler shapes, or to tie the 
building together more strongly with elements positioned to provide a more 
balanced resistance (Figure 5-23). The latter solution applies only to smaller 
buildings.
Once the decision is made to use separation joints, they must be designed and 
constructed correctly to achieve the original intent.  Structurally separated 
entities of a building must be fully capable of resisting vertical and lateral 
forces on their own, and their individual configurations must be balanced 
horizontally and vertically.
To design a separation joint, the maximum drift of the two units must be 
calculated by the structural consultant.  The worst case is when the two 
individual structures would lean toward each other simultaneously; and hence the 
sum of the dimension of the separation space must allow for the sum of the 
building deflections.
Several considerations arise if it is decided to dispense with the separation 
joint and tie the building together.  Collectors at the intersection can 
transfer forces across the intersection area, but only if the design allows for 
these beam-like members to extend straight across without interruption. If they 
can be accommodated, full-height continuous walls in the same locations are even 
more effective. Since the portion of the wing which typically distorts the most 
is the free end, it is desirable to place stiffening elements at that location.
The use of splayed rather than right angle re-entrant corners lessens the stress 
concentration at the notch (Figure 5-24). This is analogous to the way a rounded 
hole in a steel plate creates less stress concentration than a rectangular hole, 
or the way a tapered beam is structurally more desirable than an abruptly 
notched one.
5.6 Other Architectural/Structural Issues 

5.6.1 Overturning: Why Buildings Fall Down, Not Over
Although building mass or weight was discussed as part of the F = MA equation 
for determining the horizontal forces, there is another way in which the 
buildings weight may act under earthquake forces to overload the building and 
cause damage or even collapse. 
Vertical members such as columns or walls may fail by buckling when the mass of 
the building exerts its gravity force on a member distorted or moved out of 
plumb by the lateral forces. This phenomenon is known by engineers as the P e or 
P delta effect, where P is the gravity force or weight, and e or delta is 
the eccentricity or the extent to which the force is offset. All objects that 
overturn do so as a result of this phenomenon (Figure 5-25).  
The geometrical proportions of the building also may have a great influence on 
whether the P-delta effect will pose a problem, since a tall, slender building 
is much more likely to be subject to overturning forces than a low, squat one. 
It should be noted, however, that if the lateral resistance is provided by shear 
walls, it is the proportions of the shear walls that are significant rather than 
those of the building as a whole. 
However, in earthquakes, buildings seldom overturn, because structures are not 
homogeneous but rather are composed of many elements connected together; the 
earthquake forces will pull the components apart, and the building will fall 
down, not over.  Strong, homogeneous structures such as filing cabinets, 
however, will fall over. A rare example of a large steel-frame building collapse 
is that of the Pio Suarez apartments in the Mexico City earthquake of 1985. Of 
the three nearly identical buildings, one collapsed, one was severely damaged, 
and the third suffered moderate damage.  The structures had asymmetrical lateral 
bracing at their perimeters, and the steel frames were poorly detailed and 
buckled (Figure 5-26).
The collapse of the Cypress Freeway in Oakland, California, in the Loma Prieta 
earthquake (though a viaduct rather than building) was a rare example of a low-
rise structural collapse (Figure 5-27),
5.6.2 Perforated Shear Walls
Another undesirable condition is when a shear wall is perforated by aligned 
openings for doors , windows and the like, so that its integrity may be 
compromised. Careful analysis is necessary to ensure that a continuous load path 
remains without a significant loss of horizontal shear capacity. Some types of 
perforated shear wall with unaligned openings have performed well (Figure 5-28).
5.6.3 Strong Beam, Weak Column
Structures are commonly designed so that under severe shaking, the beams will 
fail before the columns.  This reduces the possibility of complete collapse.  
The short-column effect, discussed in Section 4.10.2, is analogous to a weak-
column strong-beam condition, which is sometimes produced inadvertently when 
strong or stiff nonstructural spandrel members are inserted between columns.  
The parking structure shown in Figure 5-29 suffered strong-beam weak-column 
failure in the Whittier, California, earthquake of 1987.
5.6.4 Setbacks and Planes of Weakness
Vertical setbacks can introduce discontinuities, particularly if columns or 
walls are offset at the plane of the setback.  A horizontal plane of weakness 
can be created by the placement of windows or other openings that may lead to 
failure, as in this building in the Kobe, Japan, earthquake of 1995 (Figure 5-
30).
5.7 IRREGULAR CONFIGURATIONS: A TWENTIETH CENTURY PROBLEM
The foregoing discussion has identified irregular architectural/structural 
forms that can contribute to building damage or even collapse. These 
irregularities are present in many existing buildings, and the ways in which 
they affect seismic performance need to be understood by building designers so 
that dangerous conditions are not created. The irregular-configuration problem 
was made possible by nineteenth-century structural technology and created by 
twentieth-century architectural design.   
5.7.1 A New Vernacular: the International Style and its Seismic Implications	
The innovation of the steel and reinforced concrete frame at the end of the 
nineteenth century enabled buildings to be freed from the restrictions imposed 
by load-bearing masonry. However, until the early years of the twentieth 
century, western architectural design culture dictated a historical style even 
when totally new building types, such as railroad stations or skyscrapers, were 
conceived. The architectural forms used were all derived from the engineering 
imperatives of load-bearing masonry structure: these masonry-devised forms 
survived well into the twentieth century, even when buildings were supported by 
concealed steel frames, and arches had become stylistic decoration (Figure 5-
31).
This historicism came under attack early in the century  from a number of avant-
garde architects, predominantly in Europe, who preached an anti-historical dogma 
in support of an architecture that they believed more fully represented the 
aspirations and technology of a new age. Later, this movement was termed the 
International Style. 
This revolution in architectural aesthetics had many dimensions: aesthetic, 
technical, economic and political. One result was to give aesthetic validity to 
a highly economical, unadorned, rectilinear box for almost all building 
functions. The international style preached the aesthetic enjoyment of the 
delicacy and slenderness that the steel or concrete frame structure had made 
possible. 
The prototype of the international style was exemplified in the Pavillon Suisse 
in Paris in 1930 (Figure 5-32).
As architects and engineers began to exploit the aesthetics of the building 
frame, the seeds of seismic configuration problems were sown.  In its earliest 
forms the style frequently created buildings that were close to our ideal 
seismic building configuration. However, the style often had a number of 
characteristics not present in earlier frame and masonry buildings that led to 
poor seismic performance. These were:
	Elevation of the building on stilts or pilotis 
	This had attractive functional characteristics, such as the ability to 
introduce car parking under the building, or the building could be opened to the 
public and its visitors in ways that were not previously possible. It was 
attractive aesthetically: the building could appear to float airily above the 
ground.
	However, without full understanding of the seismic implications of 
vertical structural discontinuity, designers often created soft and weak 
stories.
	The free plan and elimination of interior-load bearing walls
	Planning freedom was functionally efficient and aesthetically opened up 
new possibilities of light and space.
	However, the replacement of masonry and tile partitions by frame and 
gypsum board greatly reduced the energy absorption capability of the building 
and increased its drift, leading to greater nonstructural damage and possible 
structural failure.
	The great increase in exterior glazing and the invention of the light-
weight curtain wall 
	The curtain wall was a significant feature of the new vernacular and was 
subject to continuous development and refinement.  At one end, it became the 
most economical method of creating an exterior faade; at the other end it led 
to the apparently frameless glass walls and double-skin energy-efficient curtain 
walls of today.   Like free interior planning, the light exterior cladding 
greatly reduced the energy-absorption capability of the building and increased 
its drift. 
The post-World War II years saw worldwide explosive urban development, and the 
new aesthetic, because of its lack of ornamentation, simple forms, and emphasis 
on minimal structure, was very economical.  This ensured its widespread 
adoption. Unfortunately, seismic design, particularly the need for ductility - 
as it related to the new, spare, framed buildings - was inadequately understood. 
Thus the aesthetics and economies of the international style in vogue from about 
the 50s to the 70s has left the worlds cities  with a legacy of poor seismic 
configurations that presents a serious problem in reducing the earthquake threat 
to our towns and cities.
Configuration irregularities often arise for sound planning or urban design 
reasons and are not necessarily the result of the designers whim (or 
ignorance). The problem irregularities shown in Figures 5-5 and 5-6 represent 
structural/architectural errors that originate in the architectural design as 
the result of a perceived functional or aesthetic need. The errors can be 
avoided through design ingenuity, and mutual understanding and a willingness to 
negotiate design issues between the architect and engineer. The architect needs 
to understand the possible implications of the design, and the engineer needs to 
embrace the design objectives and participate in them creatively.  
5.8 Designing for Problem Avoidance
Regardless of building type, size, or function, it is clear that the attempt to 
encourage or enforce the use of regular configurations is frequently not going 
to succeed; the architects search for original forms is very powerful.  The 
evolution and recent trends in formal invention are shown in Figure 5-38 in 
Section 5.9.2.  
The seismic code, as illustrated in Figures 5-5 and 5-6, is oriented towards 
everyday economical building and goes a modest route of imposing limited 
penalties on the use of irregular configurations in the form of increased design 
forces and, for larger buildings, the use of more advanced analytical methods; 
both these measures translate into cost penalties Only two irregularities are 
banned outright: extreme soft stories and extreme torsion in essential buildings 
in high seismic zones. This suggests a strategy that exploits the benefits of 
the ideal configuration but permits the architect to use irregular forms when 
they suit the design intentions.
5.8.1 Use of Regular Configurations
A design that has attributes of the ideal configuration should be used when:
	The most economical design and construction is needed, including design 
and analysis for code conformance, simplicity of seismic detailing, and 
repetition of structural component sizes and placement conditions. 
	When best seismic performance for lowest cost is needed.
	When maximum predictability of seismic performance is desired.

5.8.2 Designs for Irregular Configurations
When the design incorporates a number of irregularities the following procedures 
should be used:
	A skilled seismic engineer who is sympathetic to the architects design 
intentions should be employed as a co-designer from the outset of the design.
	The architect should be aware of the implications of design irregularities 
and should have a feel for the likelihood of stress concentrations and torsional 
effects (both the cause and remedy of these conditions lie in the 
architectural/structural design, not in code provisions).
	The architect should be prepared to accept structural forms or assemblies 
(such as increased size of columns and beams) that may modify the design 
character, and should be prepared to exploit these as part of the aesthetic 
language of the design rather than resisting them.
	The architect and engineer should both employ ingenuity and imagination of 
their respective disciplines to reduce the effect of irregularities, or to 
achieve desired aesthetic qualities without compromising structural integrity.
	Extreme irregularities may require extreme engineering solutions; these 
may be costly, but it is likely that a building with these conditions will be 
unusual and important enough to justify additional costs in materials, finishes, 
and systems.
	A soft or weak story should never be used: this does not mean that high 
stories or varied story heights cannot be used, but rather that appropriate 
structural measures be taken to ensure balanced resistance. 
5.9 Beyond the international Style: TOWARDS a Seismic Architecture?
Most owners desire an economical and unobtrusive building that will satisfy the 
local planning department and look nice but not unusual.  However, as noted 
above, the occasional aspiration for the architect to provide a distinctive 
image for the building is very powerful and is the source of continued evolution 
in architectural style and art. This thrust is allied to todays marketing 
demand for spectacular forms. The history of architecture shows that design 
innovation has its own life, fed by brilliant form-givers who provide prototypes 
that keep architecture alive and exciting as an art form. Thus, like economics, 
architectural design has its supply- and demand-sides that each reinforce one 
another.   
The International Style still exists as a vernacular and can range from everyday 
economical buildings to refined symbols of prestige. But there are now many 
competing personal styles.  Have the tenets of good seismic design played any 
role in determining their characteristics? Is it possible that future 
architectural stylistic trends might seek inspiration in seismic design as an 
aesthetic that matches the exigencies of physics and engineering with visual 
grace and intrigue?  
5.9.1 The Architects Search for Forms  Symbolic and Metaphorical
The aesthetic tenets of the International Styleparticularly the metal/glass 
cubistic buildingbegan to be seriously questioned by the mid-1970s.  This 
questioning finally bore fruit in an architectural style known broadly as post-
modern. Among other characteristics, post-modernism embraced:
	The use of classical forms, such as arches, decorative columns, pitched 
roofs in nonstructural ways and generally in simplified variations of the 
original elements
	The revival of surface decoration on buildings 
	
   A return to symmetry in configuration
In seismic terms, these changes in style were, if anything, beneficial. The 
return to classical forms and symmetry tended to result in regular 
structural/architectural configurations, and almost all of the decorative 
elements were nonstructural.  An early icon of post-modernism, the Portland, 
Oregon, office building, designed by Michael Graves (Figure 5-33) used an 
extremely simple and conservative structural system.  Indeed, this building, 
which created a sensation when completed, has a structural/architectural 
configuration that is similar to the model shown in Figure 5-33. The sensation 
was all in the nonstructural surface treatments, some proposed exterior statues, 
and in its colors.  
A conventionally engineered steel or concrete member that was supporting the 
building could be found inside every classical post-modern column.   It is clear 
that an interest in seismic design or structure in general had no influence on 
the development of post-modernism; it was strictly an aesthetic and cultural 
movement.  
At the same time that post-modernism was making historical architectural style 
legitimate again, another style began to flourish, to some extent in complete 
opposition.  This style (originally christened hi-tech) returned to the 
celebration of engineering and new industrial techniques and materials as the 
stuff of architecture.  This style originated primarily in Europe, notably in 
England and France, and the influence of a few seminal works, such as the 
Pompidou Center in Paris (Figure 5-34). 
Although seismic concerns had no influence on the origin and development of this 
style, it is relevant here because it revived an interest in exposing and 
celebrating structure as an aesthetic motif.
Post-modernism died a quick death as an avant-garde style, but it was important 
because it legitimized the use of exterior decoration and  classically derived 
forms. These became common in commercial and institutional architecture (Figure 
5-35). The notion of decorating the economical cube with inexpensive 
simplified historic or idiosyncratic nonstructural elements has become 
commonplace.  
At the same time, in much everyday commercial architecture, evolved forms of the 
International Style still predominate, to some extent also representing 
simplified (and more economical) forms of the high-tech style. Use of new 
lightweight materials such as glass fiber-reinforced concrete and metal-faced 
insulated panels has a beneficial effect in reducing earthquake forces on the 
building, though provision must be made for the effects of increased drift on 
nonstructural components or energy-dissipating devices used to control it.
5.9.2	New Architectural Prototypes Today
The importance of well-publicized designs by fashionable architects is that they 
create new prototypical forms.  Architects are very responsive to form and 
design, and once a new idiom gains credence, practicing architects the world 
over begin to reproduce it. Todays New York corporate headquarters high-rise 
becomes tomorrows suburban savings and loan office, as shown in Figures 5-36 
and 5-37.   
Today, however, unlike the era of the International Style and the adoption of 
modern architecture, there is no consensus on a set of appropriate forms. At 
present, spectacular architectural design is in fashion and sought after by 
municipalities, major corporations, and institutions. So, it is useful to look 
at todays cutting-edge architecture, because among it will be found the 
prototypes of the vernacular forms of the future. 
Figure 5-38 shows the evolution of the architectural form of the high-rise 
building from the 1920s to today. There is a steady evolution in which the 
international style dominates the scenes from about 1945 to 1985.  For a brief 
interlude, post-modern architecture is fashionable, in company with high-tech. 
Towards the end of the century, architectural forms become more personal and 
idiosyncratic, and evolution is replaced by competition.  The first five years 
of the millenium have seen the emergence of a number of very personal styles, 
from the jagged forms of Liebskind to the warped surfaces of Gehry. The Foster 
office in London pursues its own in-house evolution of high-tech design. 
In general, todays high-rise buildings remain vertical, and have direct load 
paths, and their exterior walls are reasonably planar.  Some high-rise towers 
have achieved a modest non-verticality by the use of nonstructural components. A 
more recent development is that of the torqued tower, as in the Freedom Tower 
at the World Trade Center and Santiago Calatravas Turning Torso tower in 
Malmo, Sweden, shown in Figure 5-38. For very tall buildings, it is claimed that 
these twisted forms play a role in reducing wind forces, besides their visual 
appeal, but their forms are not of significance seismically. 
In lower buildings, where there is more freedom to invent forms than in the high 
rise, planning irregularities (and corresponding three-dimensional forms) are 
now fashionable that go far beyond the irregularities shown in Figure 5-6.  
Figure 5-39 shows the extraordinary range of plan forms for art museums 
conceived by four of todays most influential architects.
Highly fragmented facades now abound, serving as metaphors for the isolated and 
disconnected elements of modern society.  Often-repeated design motifs include 
segmental, undulating, or barrel-vaulted roofs and canopies, and facades that 
change arbitrarily from metal and glass curtain wall to punched-in windows. 
In all this ferment, there is much originality and imagination, and often high 
seriousness. It remains to be seen whether any of these forms become attractive 
to the typical practitioner and their more conservative clients; however, 
indications of the influence of some of these motifs can now be discerned in 
more commonplace buildings along the highways and in schools and universities 
(Figure 5-40).
One may question the extent to which architectural trends look as if they will 
increase or decrease the kinds of configuration irregularities that manifested 
themselves in the international style era. The answer appears to be that they 
will increase, because much new architecture is clearly conceived independently 
of structural concerns or in the spirit of theatrical set design, with the 
engineer in the role of an enabler rather than collaborator. 
5.9.3 Towards an Earthquake Architecture
In the search for meaning in architecture that supersedes the era of 
International Style and the superficialities of fashion exemplified by much of 
post-modernism and after, perhaps architects and engineers in  the seismic 
regions of the world might develop an earthquake architecture.  One approach 
is an architecture that expresses the elements necessary to provide seismic 
resistance in ways that would be of aesthetic interest and have meaning beyond 
mere decoration. Another approach is to use the earthquake as a metaphor for 
design.
5.9.4 Expressing the Lateral-Force Systems
For the low and midrise building, the only structural system that clearly 
expresses seismic resistance is the use of exposed bracing. There are historical 
precedents for this in the half-timbered wood structures of medieval Germany and 
England. This was a direct and simple way of bracing rather than an aesthetic 
expression, but now these buildings are much prized for their decorative 
appearance. Indeed, the half-timbered style has become widely adopted as an 
applied decorative element on U.S. architecture, though for the most part at a 
modest level of residential and commercial design. 
Two powerful designs in the 1960s, both in the San Francisco Bay Area, used 
exposed seismic bracing as a strong aesthetic design motif. These were the Alcoa 
Office Building and the Oakland Colisem, both designed in the San Francisco 
office of Skidmore, Owings and Merrill (Figure 5-41).    
In spite of these two influential designs and others that used exposed wind 
bracing, the subsequent general trend was to de-emphasize the presence of 
lateral-resistance systems. Architects felt that they conflicted with the desire 
for purity in geometric form, particularly in glass box architecture, and also 
possibly because of a psychological desire to deny the prevalence of 
earthquakes. However, in the last two decades it has become increasingly 
acceptable to expose lateral-bracing systems and enjoy their decorative but 
rational patterns (Figure 5-42).
This new acceptability is probably due to boredom with the glass cube and the 
desire to find a meaningful way of adding interest to the faade without 
resorting to the applied decoration of post-modernism.  In addition, greater 
understanding of the earthquake threat has led to realization that exposed 
bracing may add reassurance rather than alarm.  
Exposed bracing is also used as an economical retrofit measure on buildings for 
which preservation of the faade appearance is not seen as important.  A 
possible advantage of external bracing is that often the building occupants can 
continue to use the building during the retrofit work, which is a major economic 
benefit; however, see Chapter 8.5.3.1 for further discussion of this point.  
External bracing retrofits have also sometimes had the merit of adding visual 
interest to a number of dull 1960s rectilinear type facades (Figure 5-43).
The movement towards exposed seismic bracing has some parallels with the 
aesthetic movement of exposing buildings mechanical systems.  Designers who had 
become bored with expanses of white acoustical ceiling realized that mechanical 
systems, particularly when color-coded, were of great visual interest and also 
intrigued those who are fascinated by mechanical systems and devices.  Another 
parallel with seismic design is that, when mechanical systems were exposed, 
their layout and detailing had to be much more carefully designed and executed, 
from an aesthetic viewpoint.  In a similar way, exposed bracing has to be more 
sensitively designed, and this has seen the development of some elegant design 
and material usage (Figure 5-44).
New innovations, such as base isolation and energy absorbing devices, have 
sometimes been exploited for aesthetics and reassurance. The designers of an 
early and ingenious base isolated building in New Zealand (the Union House 
office building in Auckland) not only exposed its braced-frame, but also made 
visible its motion-restraint system at its open first-floor plaza (Figure 5-45). 
Experiments in linking the rationality of structure to the poetics of form and 
surface are shown in Figure 5-46, which shows two schemes for advanced systems 
of perimeter bracing that, if exposed, are perhaps livelier than conventional 
concealed bracing. The left hand figure shows a 60 story structure with 10 story 
braced super frame units, restrained by periodic two story moment frame clusters 
with hydraulic dampers. The right-hand figure shows a 48 story moment frames 
with random offset toggle hydraulic dampers. The apparent random character of 
the bracing is based on the load patterns within the structure.
The intent is to exploit an interest in structural expression and its forms, and 
create a code that can be read by anyone that has a sense of how lateral 
forces operate and must be resisted.  
5.9.5	The Earthquake as a Metaphor
A more theoretical use of the earthquake as a design inspiration is that of 
designing a building that reflects the earthquake problem indirectly, as a 
metaphor. This approach is rare, but has some interesting possibilities for 
certain building types, such as seismic engineering laboratories. 
One of the few executed examples of this approach is the Nunotani Office 
Building in Tokyo. The architect, Peter Eisenman of New York, says that the 
building represents a metaphor for the waves of movement as earthquakes 
periodically compress and expand the plate structure of the region (Figure 5-
47).
A listing of ideas for this metaphorical approach has been suggested as part of 
a student design project at the architecture school, Victoria University, New 
Zealand (Table 5.1). Figure 5-48 shows a student project in which damage is used 
as a metaphor, following the example of the Nunotani Building.
The architect/artist Lebbeus Woods has created imaginary buildings in drawings 
of extraordinary beauty that explicitly use the representation of seismic forces 
as a theme (Figure 5-49)
In his project Radical reconstruction, Woods was inspired by the 1995 Kobe 
earthquake to explore the implications of building destruction.  Of his many 
drawings and paintings inspired by San Francisco, Woods has written that these 
projects explore the possibilities for an architecture that in its conception , 
construction and inhabitation comes into new and potentially creative 
relationships not only with the effects of earthquakes, but more critically with 
the wider nature of which they are a part. 
The expression of seismic resistance and the metaphor of the earthquake could 
yet provide a rich creative field for a regional architecture that derives at 
least some of its aesthetic power from the creation of useful and delightful 
forms that also celebrate the demands of seismic forces and the way they are 
resisted. 
5.10	Conclusion
This chapter has focused on the basic seismic structural systems in relation to 
architectural configurations, and has looked at architectural design from 
through a seismic filter. This shows that many common and useful architectural 
forms are in conflict with seismic design needs.  To resolve these conflicts the 
architect needs to be more aware of the principles of seismic design and the 
engineer needs to realize that architectural configurations are derived from 
many influences, both functional and aesthetic.  The ultimate solution to these 
conflicts depends on the architect and engineer working together on building 
design from the outset of the project and engaging in knowledgeable negotiation. 
Trends in architectural taste suggest that for the engineer to expect to 
convince the architect of some of the conventional virtues of seismic design 
such as simplicity, symmetry and regularity is only realistic for projects in 
which economy and reliable seismic performance are paramount objectives.  When 
the architect and the client are looking for high style design the forms will 
probably be irregular, unsymmetrical and fragmented.  The wise and successful 
engineer will enjoy the challenges. New methods of analysis will help, but 
engineers must also continue to develop their own innate feeling for how 
buildings perform, and be able to visualize the interaction of configuration 
elements that are quite unfamiliar.
5.11	References
International Code Council, International Building Code, Birmingham AL, 2003
Lebbeus Woods: Radical Reconstruction, Princeton Architectural Press, New York, 
NY, 1997
Andrew Charleson and Mark Taylor: Earthquake Architecture Explorations, 
Proceedings, 13thWorld Conference on Earthquake Engineering, Vancouver, BC 2004
Mark Taylor, Julieanna Preston and Andrew Charleson, Moments of Resistance, 
Archadia Press, Sydney, Australia, 2002
5.12	To find out more
Christopher Arnold, Architectural Considerations (chapter 6), The Seismic Design 
Handbook, Second Edition ( Farzad Naeim, ed.) Kluver Academic Publishers, 
Norwell, MA 2001
Terence Riley and Guy Nordenson, Tall Buildings, The Museum of Modern Art, New 
York, NY, 2003
Sheila de Vallee, Architecture for the Future, Editions Pierre Terrail, Paris, 
1996
Maggie Toy, ed. Reaching for the Sky, Architectural Design, London, 1995
Yukio Futagawa, ed, GA Document A serial chronicle of modern architecture, A.D.A 
Edita, Tokyo, published periodically 

[End of Chapter 5]




THE REGULATION OF SEISMIC DESIGN by Christine Theodoropolous
6.1 INTRODUCTION 				 
This chapter presents an overview of the development of seismic design codes in 
the United States. It includes a discussion of concepts underlying performance-
based seismic design and addresses how seismic provisions in current model 
building codes can inform architectural design decisions. 
Readers can find an explanation of the basis for hazard maps that are used to 
define code-specified design earthquake parameters in Chapter 2. Chapter 3 
discusses the seismic zonation principles used to generate seismic design 
regulations that restrict property use or require site-specific design 
approaches. Chapter 5 explains how building codes classify building 
configurations for seismic design purposes. Chapter 8 discusses the regulatory 
environment for seismic-design involving existing buildings. 
Although building codes have evolved substantially in the last hundred years, 
they still reflect history in the way that they are organized and used. Code 
sections that originated as fire mitigation measures precede the sections 
containing the structural provisions that include seismic design requirements. 
Sections pertaining to modern building systems, such as plumbing and electrical 
systems, are published as separate volumes. In conventional building design 
practice, architects take primary responsibility for addressing fire-related 
provisions. Although the use of fire-resistive design consultants is becoming 
common in certain kinds of institutional and commercial building designs, 
architects using these consultants will still develop initial conceptual designs 
in accordance with the principles that underlie fire-resistive code measures. 
The same is not commonly true in seismic design. Although building configuration 
and other design features, determined by the architect, are known to impact the 
structural and nonstructural performance of buildings in earthquakes, architects 
do not usually consult the sections of the code where the seismic design 
requirements are addressed. Codes governing seismic design were established in 
the second half of the twentieth century, primarily by structural engineers, and 
reflect the increasing specialization and disciplinary division between 
architectural and engineering practice. Few architects practicing today take 
responsibility for structural aspects of seismic design, and many consult with 
engineers on the design of nonstructural architectural elements that are 
regulated by seismic code provisions. However, architects routinely make design 
decisions that impact the use and interpretation of seismic design codes. 
6.2 EARTHQUAKES AND CODE ACTION	
Historically, seismic design provisions were added to codes in response to the 
lessons learned from earthquake damage. Although the evolution of technical 
understanding of building performance has guided the development of these 
provisions, code action has been driven primarily by political rather than 
technical advances. Communities with well-developed political mechanisms for 
addressing public safety have tended to pioneer code developments, but the long 
periods between damaging earthquakes have made it easy for communities to forget 
to follow through with efforts begun in the aftermath of disasters. In addition, 
the political and technical complexities inherent in extracting lessons from 
earthquakes have made it difficult to achieve consensus on appropriate code 
measures. 
6.2.1 Early 20th Century 
Despite the destruction of 27,000 buildings and fatalities estimated by the USGS 
(United States Geological Survey) and others to be between 700 and 2,100, the 
1906 San Francisco earthquake did not stimulate an explicit code response. A 
wind-load requirement was implemented and was assumed to be sufficient to resist 
earthquake forces. Post-earthquake investigators reported that 80% to 95% of 
damage in the most affected areas of the city was caused by fire, with only 5% 
to 20% of the damage caused by shaking; the event was interpreted as a great 
fire rather than a great earthquake (Figure 6-1).
Differences in building performance based on construction, configuration, and 
soils conditions were observed during and after the earthquake, but there was no 
systematic investigation of the performance of anti-seismic systems voluntarily 
implemented by engineers in the last four decades of the 19th century, or of the 
code requirement implemented in 1901 to provide bond iron through the wythes of 
brick walls. Performance observations of selected steel-frame and concrete 
structures were used to justify the removal of some code restrictions concerning 
building height. In the aftermath of the San Francisco earthquake, engineers 
awareness of seismic risk increased, resulting in voluntary efforts at seismic-
resistant design, but codes did not specifically direct the design community to 
address earthquake related hazards.  

By the 1920s, the mechanisms for implementing seismic regulations for buildings 
in California were in place. The professional dialog about earthquake design had 
become more public since 1906, and post-earthquake investigators who examined 
field conditions after the 1925 Santa Barbara Earthquake called for code action 
(Figure 6-2).
In 1927, the Pacific Coast Building Officials Conference (precursor to ICBO, the 
International Conference of Building Officials) included an appendix of optional 
seismic design provisions in the first edition of the Uniform Building Code 
(UBC). A lateral load requirement was set at 7.5% of the building weight with an 
increase to 10% for sites with soft soils. This established the first version of 
the equivalent lateral force procedure still used in seismic codes today (Box 
1). At the same time, some California cities began to adopt mandatory seismic 
design provisions in response to advocacy from citizens. 
In the late 1920s the Structural Engineers Association of California (SEAOC) 
formed to address concerns about access to technical information and 
professional practice issues. For the next several decades, SEAOCs volunteer 
efforts would yield significant contributions to California codes and ultimately 
assume a leading role in the development of U.S. seismic codes.
6.2.3 Mid-Century Codes and the Introduction of Statewide Regulations
A large number of school buildings constructed of unreinforced masonry were 
severely damaged in the 1933 Long Beach earthquake (Figure 6-3). A public outcry 
for safer schools resulted in intense efforts by California legislators to 
quickly enact legislation requiring seismic design provisions. The Field Act of 
1933 transferred the responsibility for approving plans and supervising 
construction of public schools to the State Division of Architecture. It 
included lateral force requirements that varied according to the type of 
structural system. In the same year, the Riley Act created a mandatory seismic 
design coefficient for all buildings in the State of California and prevented 
construction of new unreinforced masonry buildings. The provisions of the Field 
and Riley Acts were developed in a simple form for implementation and did not 
reflect the latest developments in engineering understanding. It was not until 
1943 that the Los Angeles Building Code adopted the first provisions in the 
United States that accounted for building height and flexibility. 
The Long Beach earthquake provided the stimulus for state-mandated seismic 
design provisions that began the process of coalescing independent local efforts 
and assuring that minimum standards were enforced throughout California. 
However, the seismic provisions of the UBC did not become the standard in 
California until 1960. Before that time, many local jurisdictions added their 
own seismic design provisions to the Riley Act requirements. 
In the late 1940s SEAOC responded to the inadequacy of seismic design codes by 
embarking upon work that would form the basis for the first edition of the 
Recommended Lateral Force Requirements and Commentary, also known as the SEAOC 
Blue Book. These recommended seismic design provisions for new buildings were 
included in the 1961 UBC. The Blue Book, published from 1959 through 1999, 
continued to evolve with major re-evaluations after significant earthquakes. 
After 1960, the development of seismic design codes typically began with 
provision proposals initiated by the Blue Book that were later incorporated into 
the UBC. New systems, such as ductile moment frames, were incorporated into the 
code incrementally as they came into use. Inclusion of similar provisions in the 
other model codes in the United States followed the UBCs lead but occurred 
somewhat later. The UBC, used most extensively in the west, initiated national 
trends in earthquake-resistant code requirements.
The 1971 San Fernando earthquake occurred less than a decade after the 
extensively studied 1964 Alaska earthquake, and confirmed findings of greater-
than-expected damage to engineered buildings meeting code provisions (Figure 6-
4). Extensive data on the performance of newer building systems was collected. 
Observations stimulated research on the influence of reinforcement patterns on 
the strength and deformation of concrete structural components. Evidence for the 
effects of building configuration and relative stiffness of structural elements 
as well as the performance of structural connections spurred revisions to code 
requirements. 
Although the 1976 UBC increased building strength requirements and adopted the 
concept of increasing strength according to occupancy type, design forces 
remained only a fraction of the actual forces experienced by buildings during 
ground shaking (Figure 6-5). As discussed in chapter 4, building codes permit 
designs for less-than-expected earthquake forces to facilitate relatively simple 
linear analysis and design procedures because such designs, when coupled with 
detailing requirements also stipulated in codes, have proven successful in past 
earthquakes. In 1973, in an effort to assist the application of current 
technological developments to structural engineering practice, SEAOC established 
the Applied Technology Council (ATC) to translate engineering research into 
usable design information. 
In the aftermath of earthquakes of the 1960s and 70s, the need for more 
stringent building standards at the state level was recognized. The California 
Hospital Act, passed soon after the San Fernando earthquake, mandated more 
stringent building standards, plan checking, and inspection for hospitals under 
the direction of the Office of Statewide Health Planning and Development, with 
the intent of improving patient protection and maintaining building use through 
the regulation of non-structural as well as structural design.
6.2.4 Late 20th Century: the Move toward New Model Building Codes
By the mid 1970s, the need for federal seismic design standards, coupled with 
the structural engineering professions interest in significantly updating code 
content and streamlining its organization, shifted code development from 
regional to national efforts.  A series of projects to develop national 
guidelines for seismic design began in the mid70s. This effort provided the 
groundwork that led to the publication: ATC-3-06, Tentative Provisions for the 
Development of Seismic Regulations for Buildings. In 1977, the U.S. Congress 
passed the National Earthquake Hazards Reduction Act. In the following year, 
FEMA established the National Earthquake Hazard Reduction Program (NEHRP) and 
the Building Seismic Safety Council (BSSC) to develop the first nationally 
applicable seismic design guidelines for new buildings. 
A major milestone was reached in 1985 when FEMA published the first version of 
the NEHRP Recommended Provisions for Seismic Regulations of Buildings and Other 
Structures. The Provisions were originally conceived to be a resource document, 
rather than a code, but the format and language of the Provisions conform to 
conventional code language. The NEHRP Provisions had major national influence, 
and both the BOCA  (Building Officials Code Administrators) National Code and 
the SBCCI (Southern Building Code Congress International) Standard Code adopted 
requirements based on the NEHRP Provisions in 1991 and 1992 respectively. The 
seismic provisions of current building codes are largely based on the NEHRP 
Provisions, supplemented by industry materials association standards. A 
significant difference between the NEHRP Provisions and earlier model codes was 
the introduction of provisions that related design forces to the characteristics 
of the ground motion of the site. This part of the Provisions required designers 
to consider dynamic effects and resulted in larger than previously considered 
forces and building deformations for some kinds of structures. Most important 
architecturally was the inclusion of regulations that identified building 
configuration as a factor in determining acceptable engineering analysis methods 
and selecting structural systems. 
The extent of damage to buildings and the larger-than-expected economic losses 
caused by the 1989 Loma Prieta earthquake galvanized the need for public 
involvement in identifying acceptable building performance (Figure 6-6). After 
Loma Prieta, many building owners discovered that they misunderstood the 
protection provided by the codes and had made investment decisions based on 
unrealistic expectations about building performance and the likelihood of 
damaging earthquakes. A post-earthquake economic recession in Northern 
California alerted government officials to the potential economic cost of large 
earthquakes. The insurance industry responded with a lack of confidence in the 
insurability of the building stock. For those concerned with economic impacts, 
the life safety intent of building codes was no longer a sufficient standard.  
People who made decisions that affected their communities seismic risk needed 
to better understand the relationship between code compliance and building 
performance. At the same time, engineers were developing methods for predicting 
building performance with greater accuracy. The combined effect of these 
developments stimulated an increased interest in performance-based codes and 
design guidelines.  
The 1994 Northridge earthquake surprised the engineering community by severely 
damaging several recently constructed steel moment frame buildings (Figure 6-7). 
It confirmed the need for experimental research as part of the code development 
process. It also reinforced many of the lessons learned by the public in Loma 
Prieta. Damage patterns in Northridge revealed the uncertainties associated with 
earthquakes, and provided examples of how buildings designed to resist the 
larger equivalent lateral forces required by more recent codes do not always 
perform better than older buildings. Ductile moment-resisting connections used 
in steel frames showed unexpected failures due to the complex behavior of welded 
beam-column connections.  
In the aftermath of the Northridge earthquake, FEMA sponsored an extensive study 
of the steel moment connection problem. SAC, a joint venture of SEAOC, ATC and 
CUREE (Consortium of Universities for Research in Earthquake Engineering) 
conducted the work and produced a series of publications containing guidelines 
for engineering practice. The guidelines introduced pre-qualified connection 
designs and more explicit requirements for substantiating proposed designs with 
experimental research. The need for analytical techniques that could predict 
performance more accurately was identified. Many of the recommendations were 
later incorporated into building codes (Figure 6-8).
After the Loma Prieta and Northridge earthquakes, disaster-stricken communities 
raised questions about the nature of code protection, and design professionals 
began to question their own performance expectations for buildings designed to 
meet code requirements. Government agencies and representatives at the local, 
state, and national levels became concerned with the potential economic cost of 
large earthquakes. At the same time, innovative structural systems designed to 
improve performance, such as seismic isolation, became more commonly used. 
In this climate, U.S. model code associations, the ICBO, the SBCCI , and BOCA, 
joined to form the International Code Council (ICC). By the year 2000, the UBC 
and NEHRP seismic design provisions were merged in the first edition of the 
International Building Code (IBC).  Shortly after, NFPA (National Fire 
Protection Association) undertook a building code development process that 
produced the NFPA 5000 Building Code with seismic design requirements also based 
on the NEHRP Provisions. As local jurisdictions adopt these new model codes 
design teams, working on projects in regions with lower levels of seismicity, 
have been required to address more stringent seismic design regulations than has 
been the case in the past, particularly on sites with less than optimal soils 
conditions. NEHRP requires site-specific soil data in order to determine the 
seismic design category of a building. According to NEHRP, a building project in 
Atlanta on soft soil may be required to have the same level of seismic design as 
a building in California. More extensive geotechnical investigations of the 
building site can be used to determine the appropriate code-defined soil 
classification and provide the design team with critical information that will 
affect seismic design requirements specified by the code.
In the 1990s, FEMA sponsored the development of guidelines for the seismic 
evaluation and rehabilitation of existing buildings that introduced methods that 
would inform the future conceptual basis of codes for new buildings. Nonlinear 
analysis, an analytical method that integrates the deformation of a structure 
into the analysis of a structural design, was identified as an essential tool 
for some seismic design applications and the concept of performance goals was 
introduced. This work produced several publications that have had a significant 
impact on code development: FEMA 273, NEHRP Guidelines of the Seismic 
Rehabilitation of Buildings, supplemented by FEMA 274, a Commentary, and FEMA 
276, Example Applications.  FEMA 356, Prestandard and Commentary for the Seismic 
Rehabilitation of Buildings converted the guidelines into code language. They 
are discussed in Chapter 8. Many of FEMAs seismic evaluation and rehabilitation 
concepts have been adopted in the International Existing Building Code.
By the end of the decade, engineering organizations put forward several 
proposals for code action that called for a more unified, performancebased 
approach to the regulation of seismic design for new buildings. (See section 6.4 
for a discussion of performance-based seismic design.) The SEAOC Vision 2000: 
Performance-Based Seismic Engineering of Buildings called for a code development 
project that would create a more coherent rationale for seismic code provisions, 
drawing upon new analytical methods and incorporating an approach that 
incorporates performance predictions into the design process. The Earthquake 
Engineering Research Center (EERC) summarized the issues surrounding building 
performance in FEMA 283, Performance-Based Seismic Design of Buildings: an 
Action Plan for Future Studies. The Earthquake Engineering Research Institute 
(EERI) produced FEMA 349, Action Plan for Performance-Based Seismic Design, and 
FEMA has now contracted for the development of performance-based seismic design 
guidelines. 
6.2.5 Current Status of Seismic Code Development
The introduction of IBC and NFPA 5000 has assisted the move toward standardizing 
seismic code regulations in the U.S., but design professionals express concern 
that the pace of code developments has not kept up with the professions needs. 
Debates about the value of simplicity versus the value of reliability continue. 
Critics argue that codes have become unnecessarily complex and fail to 
communicate the intent behind code provisions. There is concern that code 
procedures can be overly restrictive in ways that discourage sound design 
strategies. There is also concern that, in some cases, designs that meet the 
code may be inadequate. These issues are being aired and addressed in the 
dialog that surrounds the code revision process and in the work currently 
underway to develop methods of performance-based design.
6.3 CODE INTENT

6.3.1 The Purpose of Earthquake Code Provisions
The IBCs stated purpose is:
	to establish the minimum requirements to safeguard the public health, 
safety and general welfare through structural strength, means of egress, 
facilities, stability, sanitation, adequate light and ventilation, energy 
conservation and safety to life and property from fire and other hazards 
attributed to the built environment and to provide safety to fire fighters and 
emergency responders during emergency operations. 
The primary intent of all seismic code provisions is to protect the life safety 
of building occupants and the general public through the prevention of 
structural collapse and nonstructural life-threatening hazards during an 
earthquake. However, it is generally acknowledged that seismic code provisions 
are also intended to control the severity of damage in small or moderate 
earthquakes. In large earthquakes, damage is expected; engineers rely on the 
mechanisms provided by damage to contribute to a structures damping capacity. 
The SEAOC Blue Book states that code-designed buildings should be able to:
1.	Resist a minor level of earthquake ground motion without damage;

2.	Resist a moderate level of earthquake ground motion without structural 
damage, but possibly experience some nonstructural damage;

3.	Resist a major level of earthquake ground motion having an intensity equal 
to the strongest either experienced or forecast for the building site, without 
collapse, but possibly with some structural as well as nonstructural damage. 
Codes also recognize that some structures are more important to protect than 
others. The NEHRP Provisions stated purpose is:
1.	To provide minimum design criteria for structures appropriate to their 
primary function and use, considering the need to protect the health, safety, 
and welfare of the general public by minimizing the earthquake-related risk to 
life, and

2.	To improve the capability of essential facilities and structures 
containing substantial quantities of hazardous materials to function during and 
after design earthquakes. 

6.3.2 Conflicts Between Intent, Expectations, and Performance
Codes do not explicitly address economic intent. Members of the general public 
who believe earthquake-resistant design should provide them with usable 
buildings after an earthquake often misunderstand the term meets code. Design 
professionals are responsible to convey seismic design performance expectations 
to decision makers. Clear communication between engineers and building owners is 
important and the architects role as a facilitator of the dialog between 
owners, and members of the design team is critical to promoting a shared 
understanding that can form the basis for appropriate design decisions. This 
communication is complicated by the indirect and somewhat unpredictable 
relationship between code compliance and building performance.  The NEHRP 
Provisions state:
	The actual ability to accomplish these goals depends upon a number of 
factors including the structural framing type, configuration, materials, and as-
built details of construction.
Local site factors and the variations that can occur with different combinations 
of structural systems and materials, construction methods and building 
configuration cause differences in building performance. Factors influencing 
performance can be subtle and the cause of damage difficult to determine. 
Earthquakes have produced different damage patterns in apparently identical 
adjacent buildings. The professional judgment of the design team, the extent of 
code compliance, and level of plan review also affect building damage.  For 
example, school buildings constructed under the quality control regulations of 
the Field Act have consistently outperformed other buildings designed to meet 
similar code provisions.
Given the complexity of building performance in earthquakes, it is readily 
apparent that many past code improvements which increased the lateral base 
shear (and therefore the strength of buildings to resist a static lateral load) 
did not have a directly proportional effect on building performance. There are 
too many other factors involved.  Although more sophisticated analysis methods 
can produce better performance predictions, seismic performance of buildings can 
be more accurately expressed as a probabilistic rather than as an absolute 
phenomenon. For example, in a Modified Mercalli level  IX  earthquake (defined 
in Chapter 2), it is expected that less than one percent of the midrise concrete 
shear-wall buildings designed to meet the seismic code minimums of the 1991 UBC 
will collapse.  Five percent of them will experience extensive structural and 
nonstructural damage, and thirty percent will experience moderate (primarily 
nonstructural) damage. Seventy percent will have minor or no damage. At present, 
performance data from real earthquakes or analysis results is not extensive 
enough to generate reliable probabilistic scenarios for all of the existing and 
commonly designed combinations of building configurations and structural 
systems. However, as this information becomes available, it is likely to be 
reflected in future codes.
Codes are put forward as minimum standards, but it is common practice for 
designers and owners to aim to meet rather than exceed code minimums as way to 
control project costs. As owners have become more aware of the limitations of 
seismic codes, there is greater interest in electing to build to higher 
standards. This is particularly true for corporate and institutional building 
owners who self-insure or include risk management and loss projections in their 
planning process. As owners raise concerns about acceptable levels of risk, 
there is an increased need to be able to predict building performance and relate 
that performance to design standards.
6.4 PERFORMANCE BASED SEISMIC DESIGN 

6.4.1 Prescriptive Design, Performance Design, and the Code
Performance-based design is fundamentally different from prescriptive design. 
Prescriptive codes describe what to dothe goal is to achieve a particular 
design outcome that meets the intent of the code. For example, Table 2305.3.3 of 
the IBC states that the maximum aspect ratio (height to length) for shear walls 
sheathed with particleboard is 3.5:1. The intent behind this provision is to 
insure adequate sheathing of elements used as shear walls and to prevent 
unrealistically high tie-downs from overturning moments. In contrast, a 
performance approach for the same structure would describe the intent of the 
code in a way that allows the designer to decide how the intent is met.  In the 
case of the wood frame building above, the code might stipulate that the lateral 
movement, or drift, of each floor with respect to the floor below may not exceed 
2.5% of the floor-to-floor height.  It is then up to the designer to decide how 
to achieve this outcome. If the designer chooses to use shear walls that are 
more slender than prescribed by the code, then the designer will need to 
demonstrate, using a rational basis acceptable to the building official, that 
the slender shear walls, as well as the rest of the structure, will meet 
performance requirements.  
The distinction between performance and prescriptive methods is complicated by 
the fact that performance can be a relative concept. For example, a designer 
studying a concrete-frame building with shear walls could view the maximum story 
drift permitted by codes for this structural system as a prescriptive 
requirement.  The reason the code limits drift is to prevent the negative 
consequences of large lateral movements. These include p-delta effects that 
compromise column integrity, and the concentrated stresses at connections, and 
higher levels of nonstructural damage caused by excessive movements of the 
structural frame. If a designer can address these issues through a performance-
based approach, the drift need not be limited.
Prescriptive code provisions have some advantages. They have been shown to be 
reliable for meeting life safety objectives in the U.S. The performance of 
school buildings in recent California earthquakes substantiates this. They can 
be applied consistently even in cases where design judgment is difficult or 
where the designer or building official is inexperienced with alternative design 
methods. But prescriptive codes do not readily support innovative or alternative 
approaches to seismic design that provide equal or better building performance. 

6.4.2 Definitions of Performance-Based Seismic Design
The term performance based seismic design, as currently used, has multiple 
definitions. It is used to refer to a design approach that meets the life safety 
and building performance intents of the code, while providing designers and 
building officials with a more systematic way to get at the alternative design 
option currently available in codes. In this regard, performance-based seismic 
design facilitates innovation and makes it easier for designers to propose new 
building systems not covered by existing code provisions or to extend the use of 
existing systems beyond code limitations. For example, a designer may propose to 
use a given structural system for a building that is taller than code permits 
for that system. The designer would provide the building official with a 
performance-based rationale that shows how the design will meet the intent 
behind the codes height limit. 
Performance-based design is also used to refer to a design approach that 
identifies and selects a performance level from several performance options.  
The current version of the IBC can be called performance-based because it 
incorporates distinctions between performance goals for different building uses. 
But the term is more commonly used when referring to performance options that 
exceed code minimums or in cases when buildings are expected to remain 
operational after a disaster.
From a technical perspective, performance-based design has a different 
definition. It is a design approach that provides designers with tools to 
achieve specific performance objectives such that the probable performance of a 
structure could be reliably predicted. Current codes do not aim to do this. 
Their goal is to achieve a minimum standard, based on life safety, for most 
structures and an immediate post-earthquake occupancy standard for facilities 
essential for post-earthquake recovery. Code requirements have evolved over time 
to insure that a reasonable effort has been made to meet the minimum standard, 
but they do not yield performance predictions. The expected performance level of 
a building that meets current seismic codes is highly variable and undefined by 
the code. However, the seismic engineering community is now exploring code 
development options that create a more explicit link between design approach and 
code content.

6.4.3 Implementing Performance-Based Seismic Design
At the present time, code development efforts related to seismic design are 
focusing on incorporating performance-based design concepts. It is proposed that 
future codes would establish frameworks that would assist designers as they 
provide building owners with a clear choice between a minimum standard and 
specified higher performance levels. This work includes an effort to translate 
recent advances in engineering understanding of building performance in past 
earthquakes, laboratory tests, and structural analysis methods to design 
guidelines. FEMA, seeking to improve code reliability and facilitate design to 
higher standards, is funding a longer term effort to develop performance-based 
design guidelines as initially outlined in FEMA 349. The project scope includes 
the development of structural, nonstructural, and risk management guidelines 
supported by a development plan and a stakeholders guide.  
The shift to a performance-based seismic design code will have other broader 
impacts on building design practice. Although architects are generally not 
concerned with the details of applying the seismic code and routinely delegate 
this aspect of code compliance to the structural engineer, a performance-based 
seismic code will involve the architect more directly in the seismic design 
process.  Architects working with such a code must become very familiar with the 
definition of performance levels, and the economic implications of the choice of 
level. Seismic design concerns are also likely to figure more explicitly in the 
pre-design phase of the architectural design process when project objectives are 
identified, which often takes place before the involvement of the structural 
engineer. In the traditional role of coordinator of the design team and primary 
contact for the client, the architect will facilitate the dialog surrounding 
performance-based seismic design decisions. As the primary deliverer of 
information to the client, the architect will need to include more extensive 
discussions of technical and economic aspects of seismic design than are 
presently necessary with a single standard code.  In projects with alternative 
administrative structures, such as design-build, architects will experience 
increased interaction related to seismic design decisions. 
As performance-based seismic design methods come into use, the technical 
challenges inherent in devising structures for architectural schemes that 
require structural irregularities may increase. As discussed in Chapter 5, 
buildings with regular configurations perform more predictably than those with 
irregular configurations.  Engineers attempting to meet performance levels 
specified in a code may become more reluctant to provide performance assurances 
for irregular structures proposed by the architect. The pressure to predict 
performance accurately could result in an overly conservative approach to 
building configuration. Communication and creative collaboration between the 
architect and the engineer will become more critical, particularly during the 
initial conceptual phases of the building design process.
The shift to a performance-based approach will also impact the format, 
enforcement, and implementation of codes. Current codes do not specify design 
methods or imply a design philosophy. They presume that designers will determine 
the design approach. A performance-based code suggests that the criteria 
associated with performance objectives would emerge from an articulated 
conceptual framework for seismic design. The format of codes would reflect that 
framework. Enforcing a code that specifies multiple levels of design and 
performance would require a more complex procedure than currently used. 
Implementation of a performance-based seismic design code suggests a departure 
from the traditional evolutionary model of code development to a redesign of the 
code document. This process would require the support of elected officials and 
the public. Its success will depend upon increased public understanding of the 
behavior of structures in earthquakes, the limitations of current codes, and the 
rationale behind a performance-based approach. 
A performance-based seismic design code format could provide a unified basis for 
comparison of design alternatives that give decision makers a consistent means 
of quantifying risk. That basis will enable design professionals to respond to 
the needs of all decision makers and stakeholders concerned with seismic design, 
including owners, lenders, insurers, tenants, and communities at large. 
Performance-based design methods are already being used for this purpose, but a 
code-specified framework could assist decision makers who wish to compare design 
alternatives and project types.
The need for simplified methods for lower risk building project types will 
remain, and performance-based seismic design would ultimately provide a basis 
for the development of new prescriptive code requirements. These requirements 
would be as simple or simpler to apply as current code provisions, but would 
have a unified conceptual basis that would provide more predictable performance 
outcomes. It would be easier for designers to relate code provisions to code 
intent and eliminate some of the obstacles to seismic design innovation. It 
would also allow designers to combine performance-based and prescriptive methods 
in a more consistent manner.  
6.5 seismic design provisions

6.5.1 Code-Defined Parameters
Seismic design provisions vary in complexity, depending upon the risk associated 
with the project type. As building risk increases and structural design demands 
become greater, code provisions expand to include a greater number of parameters 
that support more sophisticated analytical techniques. Table 6.1 illustrates how 
these code-defined parameters can be influenced by architectural design 
decisions, thereby impacting seismic design requirements. 
6.5.2 Performance Levels
In performance-based seismic design, a decision-making team that includes 
designers as well as stakeholders makes choices between alternate performance 
levels. To do this, the team must consider the appropriate level of seismic 
hazard to which the building should be designed as well as the acceptable risk 
that would guide building performance expectations. Implicit in this process is 
an evaluation of costs and benefits.  Performance levels must be articulated 
qualitatively and technically. Qualitative performance objectives are stated 
according to the needs of stakeholders. These objectives include life safety, 
ability to use a building for shelter after an event, ability to continue to 
produce services or income at the site after an event, and the costs associated 
with repair, loss of use, and loss of contents. 
Technical performance levels translate qualitative performance levels into 
damage states expected for structural and nonstructural systems. As defined in 
Table 6-2, the SEAOC Vision 2000 document proposes four qualitative performance 
levels. Figure 6-9 shows how the NEHRP Provisions relate these performance 
levels to probabilistic seismic hazard levels and occupancy categories called 
Seismic Use Groups (SUG). SUGs classify structures according to risk and 
importance. SUG I is the category assigned to most structures. SUG II structures 
have high-occupancy levels with restricted egress. SUG III structures are 
essential facilities for post-earthquake recovery or facilities that contain 
hazardous materials. The performance criteria for each group is progressively 
more stringent. 
6.5.3 Performance-Based Seismic Engineering
The technical definition of a performance level must specify building 
performance in terms that can be verified by an analysis of the proposed 
building design.  As the design team responds to the specific needs of 
individual stakeholders, multiple performance objectives will be identified, and 
multiple technical parameters will be used to establish acceptance criteria. The 
identification and implementation of technical performance criteria requires the 
ability to predict, with reasonable reliability, the behavior of a building 
subject to an earthquake hazard. Recent advances in analytical tools and 
computational capabilities have advanced the art of earthquake performance 
prediction and made the development of performance-based approaches to 
engineering design more possible now than in the recent past.  
Seismic engineering analysis is the art of translating understandings of seismic 
hazards into predictions of building behavior in a way that can inform building 
design.  It is used in the conceptual design phase to evaluate alternative 
structural systems and configurations, in the schematic design phase to refine 
the layout of the selected structural system, and in the design development 
phase to determine the details of construction.  Throughout the design process, 
engineering analysis is used to verify that the proposed design will perform at 
an acceptable level and satisfy building code requirements. Engineers select 
analysis methods based on the type of structural behavior being examined and the 
need for information about that behavior. Simple methods can be useful for 
conceptual design even when complex methods are required for design development. 
In a given building design project, multiple analysis methods will be used to 
verify design decisions that range from the selection of a structural system to 
the selection of a bolt diameter.  
In performance-based seismic engineering practice, performance implications of 
some design decisions have to be predicted with a greater degree of accuracy 
than others. Designers aim to use the simplest, most cost-effective method 
appropriate for the design task at hand. It takes a combination of theoretical 
knowledge and engineering judgment to select appropriate analytical methods. 
Routine design of small regular buildings of conventional construction can be 
accomplished with simple analytical methods, whereas large, irregular buildings 
are likely to require a more complex analysis. Designers will sometimes choose 
to use more than one analysis method as a way to examine different aspects of a 
structures behavior. Although the code does not specify approaches to seismic 
design, it does restrict the use of certain analytical methods to insure that 
they are used appropriately.  From an architects perspective, this means that 
some combinations of building parameters will require more costly engineering 
analysis methods.
6.5.4 Engineering Analysis Methods 
Seismic analysis methods can be divided into two groupslinear procedures and 
nonlinear procedures. Linear procedures are by far the most common and are used 
in the majority of seismic design applications for new buildings. The term 
linear refers to the assumption that there is a constant proportional 
relationship between structural deformation and the forces causing it. Engineers 
are familiar with linear analysis because it is the same as used for analysis of 
beams, girders, and columns for gravity systems. Most prefer linear seismic 
design for the same reason. However, the linear forces used and building 
deformations are not those expected in an earthquake and have been somewhat 
arbitrarily developed to result in buildings that are adequately resistant to 
life-threatening earthquake damage. A nonlinear approach adds another layer of 
analytic complexity. It takes into account the changing stiffness of various 
elements and the overall structure during the shaking. Structural elements are 
designed by examining deformations rather than forces. Although nonlinear 
analysis can be used to design new buildings, it is more commonly used to 
predict seismic response for evaluation and retrofit of existing structures.  
Linear analysis can be static or dynamic. The IBCs equivalent lateral force 
procedure, shown in Box 1, is a simple version of a linear static analysis. A 
linear static procedure is well suited to buildings with regular configurations 
that have a response to ground motion that is dominated by the back and forth 
swaying of a structure called the fundamental mode. Linear models can also be 
dynamic. The dynamic version of a linear model, shown in Box 2, is known as a 
modal analysis. It is based on an idealized site response spectrum and takes 
into account motions that are influenced by higher mode shapes providing more 
information about a buildings behavior under seismic loads. Because it is 
linear, the displacements expected under different modes can be added together 
to identify critical design behaviors.  A modal analysis is frequently used to 
create a more accurate picture of how irregular structures would perform. 
Seismic codes allow and, in some cases, require designers to substitute a modal 
analysis for static methods.
Box 2. Linear Dynamic Analysis
A linear dynamic analysis is useful for evaluating irregular or dynamically 
complex buildings.  An irregular building is defined as having a distribution of 
mass or stiffness of the structure that is nonuniform and is often created in 
buildings that that have complex space planning requirements or asymmetrical 
configurations. Dynamic complexity is common in flexible structural systems. 
Flexibility is greatly influenced by the selection of structural system and 
building height.  Flexible buildings tend to have a significant response to 
higher mode shapes. Mode shapes are movement patterns that occur naturally in 
structures that have been set in motion by ground shaking. The diagram below 
compares the shapes of the first three modes. 
Designers use linear dynamic analysis to determine the degree of influence each 
mode shape will have on a structures performance. The importance of higher 
modes depends on the relationship between the fundamental mode of the structure 
and the dynamic-ground shaking characteristics of the site.  Designers express 
mode shape influence in terms of the percent of building mass assigned to a 
particular mode. If the building mass vibrates primarily in the first or 
fundamental mode, a static analysis is permitted by the code. Although linear 
dynamic analysis methods are becoming routine in engineering practice, they are 
more complicated because they require detailed information about ground motion. 
When linear dynamic analysis is used to meet code requirements or check code 
conformance, the structure-ground shaking interaction is usually modeled using a 
response spectrum. The IBC code includes design response spectra, a procedure 
for developing a design structure curve based on the general design response 
spectrum show below.
An alternative and significantly more complex method for modeling ground 
shaking, called a time history analysis, examines modal response using actual 
ground motion data.  The code requires that time history analyses consider 
several different ground motion records to insure that the structure response is 
sufficiently representative to account for future unknown ground motion 
patterns.


Nonlinear analysis methods can also be static or dynamic. The pushover 
analysis shown in Box 3 is a static nonlinear analysis technique that offers the 
potential for increased accuracy at the expense of increased complexity. It 
requires multiple iterations of a static analysis that can account for the 
effects of yielding elements and help designers visualize how building behavior 
transforms as damage states progress. It is often used to identify and control 
the weak links that initiate structural failure mechanisms.
A nonlinear dynamic analysis or time history analysis is the most sophisticated 
and time-consuming method requiring a detailed knowledge of building properties 
and ground motions. It is relatively new to design practice and is used in the 
research and experimental stages of design applications. The IBC accepts this 
analysis procedure, provided that a design review is performed by an independent 
team of qualified design professionals. 
In general, nonlinear methods are not useful if the intent of the design is to 
verify compliance with minimum code provisions.  Nonlinear methods generate a 
significantly greater quantity of analytical results that require substantially 
more engineering effort to produce and interpret. These methods are more 
appropriately used to determine estimates of performance with higher reliability 
than standard code expectation or to analyze the combined effects of new 
retrofit components and existing systems. If the intent of the analysis is to 
verify code compliance, linear analysis methods will suffice. Future codes are 
likely to include explicit guidelines for the use of nonlinear methods as they 
become a more routine part of conventional engineering practice. 
Box 3. Nonlinear Static Pushover Analysis
A pushover analysis is a nonlinear static method that accounts for the way 
structures redesign themselves during earthquakes. As individual components of a 
structure yield or fail, the earthquake forces on the building are shifted to 
other components. A pushover analysis simulates this phenomenon by applying 
loads until the weak link in the structure is found and then revising the model 
to incorporate the changes in the structure caused by the weak link. A second 
iteration is then performed to study how the loads are redistributed in the 
structure. The structure is pushed again until the second weak link is 
discovered. This process continues until the yield pattern for the whole 
structure under seismic loading is identified. 
A pushover analysis is only useful for evaluating nonlinear structures for which 
the fundamental mode dominates and is not suitable for certain irregular or 
dynamically complex structures. These would need to be designed using a 
nonlinear dynamic method. Some building structures are inherently linear because 
they lack redundancy or exhibit brittle modes of failure. These structures can 
be modeled sufficiently accurately using linear analysis methods. A sequence of 
nonlinear events used in a pushover analysis is shown below for a single-story, 
two-bay reinforced concrete frame with one bay of steel cross bracing. The 
sequence is shown numbered in the figure.
In current engineering practice, pushover analysis is more commonly used to 
evaluate the seismic capacity of existing structures and appears in several 
recent guidelines for seismic design concerning existing buildings. It can also 
be a useful design tool for the performance-based design of new buildings that 
rely on ductility or redundancies to resist earthquake forces. 
To implement this method reliably, designers must be able to make a sufficiently 
accurate model that will reflect inelastic properties and potential deformation 
mechanisms of the structure. This requires a more sophisticated understanding of 
the structures behavior than linear methods.  Nonlinear analysis results 
produce more data and can be difficult to interpret. The current IBC code treats 
nonlinear analysis as an alternate method, but nonlinear approaches are likely 
to become integrated into the code as the code evolves in the future.
6.6 Nonstructural Codes
The IBC 2003 seismic code deals with the problem of reducing damage to 
nonstructural components in two ways.  The first is to impose limits on the 
horizontal drift or deflection of the main structure.  This is because 
nonstructural damage (such as glass breakage or fracturing of piping) may occur 
because the building structure is too flexible, causing racking in wall panels, 
partitions, and glazing framing.  Flexible structures are economical because the 
code allows for considerably reduced forces to be used in the design; however, 
this approach solves the structural problem at the expense of the nonstructural 
components.  Although this approach can still be used, the imposition of drift 
limits ensures that the flexibility of the structure will not be such that 
excessive nonstructural damage results.
The second approach is to assign force values based on acceleration to the 
critical nonstructural components and their connections to ensure that they will 
be strong enough to resist seismic accelerations in their own right or as the 
result of attachment to the structure.  The analysis procedure is similar to, 
although less complex than, the procedure for determining the equivalent lateral 
force on the main structure. Modifications to the basic F=Ma equation include 
coefficients for importance, component amplification, component response 
modification, and the height of the component within the structure.  All these 
modifiers increase the design forces relative to the spectral accelerations 
derived from the hazard maps.
In a way similar to that used to determine the forces on the main structure, the 
required performance characteristics for nonstructural components are related to 
three different Seismic Use Groups based on the use and occupancy of the 
building.  
Several exemptions are made in the IBC seismic code:
1.   All components in Seismic Design Category A structures are exempt because 
of the lower seismic effects on these items.
2.	All architectural components (except parapets supported by bearing or 
shear walls) in Seismic Design Category B structures are exempt if they have an 
importance factor of 1.00, which indicates that they are not a life safety 
threat.  (The importance factor is a 1.5 multiplier for components that are 
needed to function after the earthquake for life safety, that contain hazardous 
contents, or are large storage racks open to the public.). The importance factor 
is selected by the engineer, based on criteri in the code and consultation with 
the building official.
3.	Mechanical and electrical components, Seismic Design Category B.
4.	Mechanical and electrical components, Seismic Design Category C when the 
components importance factor is 1.00. 
5.	All Mechanical and electrical components in all seismic design categories 
that weigh less than 400 pounds, are mounted 4 ft. or less above the floor, have 
an importance factor of 1.00, are not critical to the continued operation of the 
building, and include flexible connections between the components and associated 
ductwork, piping and conduit. 
6.	Mechanical and electrical components in Seismic Design Categories C, D and 
F that weigh 20 pounds or less, the importance factor is 1.00, and flexible 
connections are provided.
Of all the elements of the building envelope, heavy precast concrete wall 
cladding panels attached to steel or reinforced concrete-frame structures 
require the most design and construction attention to ensure seismic safety.  
These typically span from floor-to-floor: horizontal drift or deformation of the 
building structural frame can create considerable racking forces in panels that 
are rigidly attached at top and bottom, resulting in damage or possible drop-
off.  Therefore, the attachment of these panels must permit differential 
movement of the floors without transmitting racking forces to the panels.  This 
is achieved by special detailing of the connection of panels to structure.
Seismic codes require that heavy panels accommodate movement either by sliding 
or ductile connections. In high seismic zones sliding connections are rarely 
used, because of the possibility of incorrect adjustments when bolts are used, 
jamming or binding due to unwanted materials left after installation, and 
jamming due to geometrical change of the structural frame under horizontal 
forces. 

The need for disassociating the heavy panel from the frame has a major impact on 
connection detailing. As a result, a connection commonly termed push-pull has 
been developed, primarily in California, which provides, if properly engineered 
and installed, a simple and reliable method of de-coupling the panel from the 
structure. The generic connection method consists of supporting the panel by 
fixed bearing connections to a structural element at one floor to accommodate 
the gravity loads, and using ductile tie-back connections to a structural 
element at an adjoining floor (Figure 6-10).
Recent developments in nonstructural seismic codes include a performance-based 
design approach, comparable to that used for structural design. This approach 
was first defined in the NEHRP Guidelines for the Seismic Rehabilitation of 
Buildings (FEMA 273,274) and subsequently in FEMA 356, Prestandard and 
Commentary for the Seismic Rehabilitation of Buildings. This will be replaced in 
2006 by a new ASCE standard, ASCE 41. 
6.7 CONCLUSION 
The continually transforming content of seismic design codes reflects the 
evolution of design practice as it takes place in changing technical and 
political contexts. As the shift to a performance-based approach takes place, 
designers are raising questions about possible impacts on design practice. How 
will a performance-based seismic code affect professional liability? How will it 
affect the cost of professional design services? Will it be particularly 
difficult or expensive for small firms or inexperienced owners to implement? 
Will it really help designers manage uncertainty? Without real data on real 
buildings in real earthquakes to confirm our performance predictions, how 
confident can we be that our performance-based design methods work?  
As the dialog surrounding building code development continues, new insights are 
emerging, particularly the recognition that this process needs the attention of 
all stakeholders concerned with the built environment. For architects, the new 
codes and the performance-based concepts behind them will require greater 
involvement in seismic design decisions. As architects help owners investigate 
the feasibility of proposed building projects and lead their clients through the 
design process, they will need to be aware of the interaction between design 
decisions and seismic design regulations. 
6.8 references

ACSE, 2003. Minimum Design Loads for Buildings and Other Structures, prepared 
and published by the American Society of Civil Engineers, Reston, Virginia.

ASCE, 2000. Prestandard and Commentary for the Seismic Rehabilitation of 
Buildings, prepared by the American Society of Civil Engineers, published by the 
Federal Emergency Management Agency, FEMA 356, Washington, D.C. 

ATC, Communicating with Owners and Managers of New Buildings on Earthquake Risk, 
prepared by the Applied Technology Council for the Federal Emergency Management 
Agency, FEMA 389, Washington, D.C. 

ATC/BSSC, 2003, NEHRP Guidelines for the Seismic Rehabilitation of Buildings, 
prepared by the Applied Technology Council for the Building Seismic Safety 
Council, published by the Federal Emergency Management Agency, FEMA 273, 
Washington, D.C. 

ATC/BSSC, 1997. NEHRP Commentary on the Guidelines for the Seismic 
Rehabilitation of Buildings, prepared by the Applied Technology Council for the 
Building Seismic Safety Council, published by the Federal Emergency Management 
Agency, FEMA 274, Washington, D.C.

BOCA, National Building Code, published by the Building Officials and Code 
Administrators, Country Hills, Illinois.
 
BSSC, 2004. NEHRP Recommended Provisions for Seismic Regulations for New 
Buildings and Other Structures, Part I: Provisions, and Part II, Commentary, 
2000 Edition, prepared by the Building Seismic Safety Council, published by the 
Federal Emergency Management Agency, Publications, FEMA 450, Washington, DC. 

EERC, 1996. Performance-Based Seismic Design of Buildings: An Action Plan for 
Future Studies, prepared by the Earthquake Engineering Research Center and 
published by the Federal Emergency Management Agency, FEMA 283, Washington D.C.
EERI, 2000. Action Plan for Performance Based Seismic Design, prepared by the 
Earthquake Engineering Research Institute and published by the Federal Emergency 
Management Agency, FEMA 349, Washington D.C.

ICBO, 1997. Uniform Building Code,1997 edition prepared and published by the 
International Council of Building Officials, Whittier, California.

ICC, 2002, 2003. International Building Code, prepared and published by the 
International Code Council, Falls Church, Virginia.

IEBC, 2003. International Existing Builing Code, published by the International 
Code Council, Falls Church, Virginia.

NPFA, 2002. NFPA 5000 Building Construction and Safety Code, prepared and 
published by the National Fire Protection Agency, Quincy, Massachusetts. 

SBCCI, Standard Building Code, published by the Southern Building Code Congress 
International, Birmingham, Alabama.

SEAOC, 1999. Recommended Lateral Force Requirements and Commentary, prepared by 
the Structural Engineers Association of California, published by the 
International Conference of Building Officials, Whittier, California. 

SEAOC, 1995.Vision 2000: Performance Based Seismic Engineering of Buildings, 
Structural Engineers Association of California, Sacramento, California. 

[End of Chapter 6]



SEISMIC DESIGN--PAST, PRESENT AND FUTURE BY ERIC ELSESSER
7.1 INTRODUCTION
Design of any building is a challenge for architects and engineers, and the 
challenge is made more complex by providing for earthquake resistance. During 
the past 100 years, seismic design philosophy and details have progressed from 
simply considering earthquakes to be the same as wind loads, to a sophisticated 
understanding of the phenomenon of the earthshaking that induces a building 
response. 
This chapter covers the 100-year history of seismic structural systems as 
developed by engineers and architects, ranging from simple to sophisticated 
solutions. Basic structural behavior is outlined; guidance for selecting a good 
structural system is suggested, and the following issues are discussed:
  Scale and size of buildings and structural components
  The impact of building configuration
  Force verses energy
  Drift or movement
  Structural mechanisms (passive to active)
  Costs and post-earthquake repair costs
7.2 A BRIEF SUMMARY OF 100 YEARS OF   	STRUCTURAL SEISMIC DESIGN
Seismic exposure has extended over many centuries, but systematic seismic design 
has occurred only over the past 100 years, especially in California since the 
1906 San Francisco earthquake.
A group of thoughtful and creative engineers, responding to the observed damage 
in the 1906 earthquake, started to study, conceive, and design a progression of 
structural solutions to solve the earthquake response problem. This creative 
work has extended over a 100-year period, and continues today. A brief 
progression of key milestones in this seismic design history follows:
   Initial seismic designs for buildings were based on wind loads, using static 
force concepts. This approach started in the late 1800s and lasted to the mid-
1900s.
   After the 1906 San Francisco earthquake, concepts of building dynamic 
response gained interest, and in the early 1930s, initial studies of structural 
dynamics with analysis and models were initiated at Stanford University. This 
approach ultimately led to a design approach that acknowledged the importance of 
building periods and dynamic rather than static design concepts.
   Dynamic design concepts were enhanced by the acceleration spectra method used 
for design as developed by Professor Housner at the California Institute of 
Technology (See Chapter 4, Section 4.5.3).
   While analysis methods were being developed, engineers needed additional 
knowledge about nonlinear behavior of structural components. Substantial testing 
of materials and connection assemblies to justify actual behavior were 
undertaken from 1950 through 1990 at numerous universities (University of 
California, Berkeley; University of Illinois; University of Michigan; University 
of Texas; etc.).
   Since 1980 to the present, sophisticated computer analysis programs have been 
and continue to be developed to facilitate design of complex structural systems 
and the study of nonlinear behavior.
In the past 70 years substantial change and progress have taken place, not only 
in California but also over the entire United States, so that concepts and 
systems can now be utilized that previously could only be dreamt about.
7.3 HISTORIC AND CURRENT STRUCTURAL- SEISMIC SYSTEMS
7.3.1 Early Structural Systems-Pre-1906 
         San Francisco Earthquake
San Francisco in 1906 had a varied building stock with a few basic structural 
systems widely represented. Almost all common residential buildings were of 
light-frame wood construction, and most performed well in the earthquake, except 
for those on poor, weak soils or those with unbraced lower story walls. Most 
small- to medium-sized business buildings (about five to six stories in height) 
were constructed with brick masonry-bearing walls, using wood-framed floors and 
roofs. These buildings had a variable performance, with upper stories 
experiencing partial collapse and masonry walls typically showing shear cracks 
to varying degrees. Tall buildings, constructed during the previous 10 to 15 
years (prior to 1906), utilized a steel frame to support gravity loads and 
provided unreinforced brick/stone perimeter walls which served to provide 
lateral load resistance. These buildings generally performed well during the 
earthquake. Most buildings, when subjected to the firestorm after the 
earthquake, did not do well.
The general conclusion following the 1906 earthquake was that a steelframed 
building designed to support gravity loads and surrounded with well-proportioned 
and anchored brick walls to resist earthquake forces was a superior structural 
system, and it was commonly adopted by the design profession.
7.3.2 The Early Years (1906  1940)
Immediately after the 1906 earthquake, when reconstruction and new buildings 
became essential, a variety of new structural concepts were adopted. Brick 
masonry infill walls with some reinforcing were used, and steel frames were 
designed to carry lateral loads using one of the following ideas: knee bracing, 
belt trusses at floors to limit drift, rigid-frame moment connections using 
column wind-gussets, or top and bottom girder flange connections to columns.
As concrete construction became popular after 1910, concrete momentframe 
buildings together with shear walls, emerged for industrial and lower height 
commercial buildings. Concrete slowly replaced brick as a structural cladding 
after 1930, and buildings commonly used a light steel frame for floor support 
with a complete perimeter concrete wall system for lateral loads.
7.3.3 The Middle Years (1945  1960)
Immediately after World War II, construction of large projects started again. 
New ideas were common, and some refinement of framing systems for tall buildings 
was proposed and adopted.
Expressive structural systems were studied and used, but they were usually 
covered from view with conventional exterior finishes.
The transition from riveted connections to high-strength bolted joints occurred 
in the 1950s. By 1960, another steel connection change was starting to occur; 
girder flanges welded directly to columns to create moment frame connections. 
Because engineers initially did not trust the limited use of moment frames, 
structural designs were conservative, with substantial redundancy created by 
utilizing complete moment-frame action on each framing grid, in each direction.
7.3.4 The Mature Years (1960  1985)
The 25-year period from 1960 to 1985 represents the mature years, in that 
substantial projects were completed using the concepts of either ductile moment 
frames or concrete shear walls.
The structural engineering profession accepted the validity of 1) ductile 
concrete-moment frames, 2) ductile shear walls, or 3) ductile welded steel-
moment frames as the primary structural system for resisting lateral loads. The 
primary design activity became optimization of the system or, in other words, 
how few structural elements would satisfy the minimum requirements of the 
building codes. Substantial connection tests were carried out at university 
laboratories to justify this design approach.
7.3.5 The Creative Years (1985  2000)
After the damage caused by the 1989 Loma Prieta earthquake (San Francisco Bay 
Area) and the 1994 Northridge earthquake (Los Angeles), the structural 
engineering profession began to ask itself about actual earthquake performance. 
Would real performance differ from the solution obtained by simple compliance 
with the building code? This investigative process defined many issues, and one 
of the most important was the dissipation of seismic energy by the building 
structure. The pursuit of this issue led engineers to the consideration of dual 
systems and seismic isolation to limit lateral displacement. 
Several significant solutions have been developed using the dual-system concept 
with stable cyclic seismic behavior:
Each of these systems is part of an overall framing concept. These dual and 
stable mechanisms represent the current search for reliable seismic performance.
7.4 BACKGROUND AND PROGRESSION OF STRUCTURAL-SEISMIC CONCEPTS
The progression of seismic systems selected by structural engineers has resulted 
from three factors:
1.   Study of Past Earthquakes
	Learning from past earthquake performance: Successful seismic structural 
systems continue to be used; unsuccessful systems are eventually abandoned. New 
and better ideas frequently flow from observed earthquake damage.
2. 	Research Data
	New ideas for structural concepts are frequently developed jointly by 
design engineers and university research laboratories. These systems are 
physically tested and analytically studied.
3. 	Building Codes
	Finally, structural systems, that are listed in building codes eventually 
are used by many engineers as approved. The problem with code concepts, in 
these times of rapidly changing systems, is that codes are created about 5 to 10 
years after new ideas are developed, so that codes may no longer be current or 
at the cutting edge of new thinking; overly specific codes may tend to stifle 
and delay new ideas.

7.4.1 Development of Seismic Resisting Systems
Over a 100-year period, seismic resisting systems have developed substantially. 
The use of San Francisco buildings as a typical measure of the evolution 
provides a good summary of the past and present, and some indication of the 
future.
A summary of individual buildings gives a clue as to thinking. The plot of 
systems (Figure 7-1) connects the concepts and indicates the progression of 
ideas.
7.4.2 Pictorial History of Seismic Systems
The following pages provide a visual history of key features in the evolution of 
seismic systems developed and utilized in San Francisco and other Western 
regions of high seismicity.
7.5 COMMENTARY ON STRUCTURAL FRAMEWORKS
Both the steel-frame and concrete-frame buildings previously tabulated can be 
summarized on the basis of a primary framing system, considering both frameworks 
and structural cladding.
7.5.1 Steel Building Frameworks
It can be seen from the previous tabulation that steel frameworks have 
progressed from a simple steel frame, braced laterally by unreinforced masonry, 
to complete moment frames with full lateral load resistance. However, the 1994 
Northridge earthquake in southern California created serious doubts as to the 
integrity of welded moment frames. Actually, several years before the 1994 
earthquake, thoughtful structural engineers recognized the advantages of dual 
structural systems for the structural redundancy required to resist large 
earthquakes. The engineers had two separate tasks. The first task was to 
minimize the structure for economy, and the second task was to provide a secure 
load path to protect the structure.
A review of the basic eight steel-frame concepts tabulated in Figure 7-2 
indicates that they are all reasonable, but some have substantially more 
redundancy than others:
1.   The simple 1890-1920 steel framework with unreinforced brick infill: The 
system has vertical support with an infill system that allows brick joint 
movement for energy dissipation. It is a good inexpensive system that allows for 
simple repair of brick after an earthquake.
2.   The 1910-1930 column-to-girder gusset-plate connection with nominally 
reinforced concrete infill walls: A good low-cost steel riveted detail with 
concrete providing stiffness for controlling lateral drift.
3.   The 1910-1940 trussed girder wind brace providing inexpensive drift control 
of the frame: The encasing concrete also provided substantial lateral stiffness, 
and forced the steel column sections to actually be stronger and stiffer and to 
create girder yielding, a good contemporary concept.
4.   The 1920-1940 knee-braced moment frame with concrete encasement provided a 
nominally stiff frame system.
5.   The 1930-1970 riveted (or bolted) top and bottom girder fixity to the 
column created a steel-moment frame. The concrete fire-proofing encasement on 
buildings through 1960 enhanced the moment-frame stiffness.
6.   The 1950-1970 top and bottom bolted haunched girder moment frame provided 
inexpensive girder stiffness and was especially strong if encased in cast-in-
place concrete.
7.   The more recent 1970-2000 all welded girder moment frame relied only on the 
steel system for seismic resistance and produced the most flexible of steel 
frames. These steel systems were not encased in concrete and were clad only with 
precast concrete, metal panels, or glass, not providing added structural 
strength or stiffness.
      After the Northridge earthquake, these conventionally welded frames were 
seen to be vulnerable, providing far less ductility than anticipated. A major 
FEMA-funded study has attempted to find solutions to this very significant 
problem. The current solutions tend to be expensive to fabricate.
8.   The 1995-2000 steel-moment frames with a dual system of dampers, unbonded 
braces, or eccentric-braced frames, all clad with light-weight materials, appear 
to be good solutions.
The engineering profession had progressed fairly slowly until the early 1980s 
from the basic framing concepts that were first evolved in the early 1900s. When 
the concerns about seismic performance and energy dissipation became paramount, 
researchers and design engineers investigated mechanisms and configurations to 
supplement the basic rectangular grid framing in use for over 100 years.
7.5.2 Concrete Building Frameworks  
Reinforced concrete framing became popular in California immediately after the 
1906 earthquake. The significant framing systems that evolved are shown in 
Figure 7-3.
1.   The early years, about 1908 to 1915, featured nonductile frameworks of 
reinforced concrete column-girder moment frames. The earliest utilized wood 
floor infills between concrete floor girders, followed rapidly by all-concrete 
floors. It was not until the early 1960s that nonductile frames were recognized 
as an unsafe collapse mechanism, because of inadequate shear capacity, or 
because of poorly confined concrete.
2.   Early steel frames for gravity load support were initially encased in 
masonry cladding, soon to be changed to reinforced concrete frame infill walls, 
1910 - 1930. This was a simple change which added definable strength around the 
steel.
3.   Soon after the bare concrete frames were developed, they were infilled with 
concrete shear walls for lateral stability, 1910 - 1930. Not all of these 
buildings were well conceived structurally, and because of the ease of adding or 
deleting walls, torsional problems became common for this type of building.
4.   Significant reinforced concrete shear-wall buildings acting as bearing 
walls evolved in the1920s, without moment frames.
5.   It was not until the late 1960s and early 1970s, after substantial research 
at the University of Illinois, that the benefit of confined concrete columns and 
ductile concrete frames was recognized and adopted for dependable seismic 
resistance.
6.   In the 1970s and 1980s, a dual system of ductile concrete moment frames 
coupled with confined concrete shear walls was recognized. This concept works 
best with a perimeter moment frame and an interior shear wall core, or the 
reverseusing a perimeter ductile wall and an interior ductile moment frame. 
Finally, a dependable concrete dual system evolved.
7.   Shear walls coupled together with yielding shear links were developed in 
New Zealand in the 1980s after successful tests carried out by Park and Paulay 
in 1975. This coupled-wall system is another excellent example of creative 
research used to develop a low-cost mechanism for seismic energy dissipation.
7.6 System Characteristics
Selecting a good structure requires engineering common sense. Common sense 
requires understanding the earthquake motion and its demands, and understanding 
the structural behavior of the individual systems available. There are 
differences of scale (small versus large), differences between elastic and 
inelastic behavior; and differences of dynamic responses and seismic energy 
dissipation. Structural and architectural configurations (such as regular versus 
irregular forms) are also significant in the performance. The many variables 
often make it difficult to select an appropriate system. The building code lists 
numerous structural systems, but it does not provide guidance in the selection 
of a system, and the many systems are not equal in performance.
Key performance issues are elastic behavior, inelastic behavior, and the related 
cyclic behavior resulting from pushing a structure back and forth. This behavior 
should be stable, nondegrading, predictable, and capable of dissipating a large 
amount of seismic energy.
 7.6.1 Elastic DesignLinear Systems
The simple building code approach to seismic design requires diminishing an 
acceleration spectra plot by use of an R value. Elastic design is expressed by 
an R value, which is used to modify the acceleration spectral value to a simple 
seismic design force (Figure 7-4). This is a simple but frequently questionable 
method. It does not consider performance, nonlinear cyclic behavior, or most 
importantenergy dissipation.
7.6.2 Post-Elastic DesignNonlinear Drift
Inelastic design is a better indication of realistic lateral drift or deflection 
that results from real earthquake motions (Figure 7-5). Nonlinear drift impacts 
structural and nonstructural behavior. For significant seismic energy 
dissipation, the drift should be large, but for favorable nonstructural or 
content behavior this drift should be small. A building with a large but 
unstable structural drift will collapse. A building with a limited or small 
structural drift generally will not dissipate significant seismic energy without 
significant damage. 
7.6.3 Cyclic Behavior
A good measure of seismic performance is stable cyclic hysteretic behavior. The 
plot of load vs. deformation of an element, for motion in both directions, 
represents cyclic behavior (Figure 7-6). If the load curves are full, 
undiminished, without necking down. they represent a stable system that is 
ductile and has sufficient capacity to deliver a constant level of energy 
dissipation during the shaking imposed by an earthquake. Degrading cyclic 
systems  may be acceptable if they degrade slowly and in a predictable manner.
7.6.4 Performance-Based Seismic Design
Years of physical testing and corresponding analytical studies have provided a 
greatly increased understanding of earthquakes, materials and assemblies. Recent 
seismic performance design has forced engineers to look at the full range of 
structural behavior, from linear to nonlinear to failure.
Good, dependable seismic systems behave with a stable, nondegrading behavior; 
others do not. The ones that are not stable diminish in capacity with continuing 
cycles, until they have little capacity to dissipate seismic energy. If the 
earthquake continues for a long duration (over 10 to 15 seconds), the structure 
can become weak and unstable, and may fail.
The single conventional system with only one means of resisting seismic forces 
is especially vulnerable to long-duration earthquakes, because when that system 
degrades, no alternative exists. This concern has evolved for high-performance 
structures into the concept of utilizing multiple-resisting structural systems 
that act progressively, so that the overall structural capacity is not 
significantly diminished during the earthquake.
7.6.5 Nonlinear Performance Comparisons
Considering only strength and displacement (but not energy dissipation), six 
alternative structural systems are represented by seismic performance pushover 
plots for comparison for a specific four-story building, but  a useful example 
of a common low- to mid-rise building type (Figure 7-7).  For other building 
heights and types, the values will vary. Each system was designed and sized for 
code compliance, all with a lateral drift of less than one inch. However, the 
nonlinear displacement (or drift) for each alternative system is significantly 
different:
  The braced frame (BF) needs to reach the 5% damped spectral curve, but the 
system fails at a drift of about one-and-a-half inches, far below the target of 
5%t damping. A possible solution would be to design the brace for about four 
times the code value; excessively expensive, this solution deals only with 
strength, not with seismic energy. 
    The shear wall (SW) needs to reach the 10% damped or spectral curve, but can 
only reach a drift of about two inches, about half of the required drift. Again, 
significant over-design might solve the strength problem, but may not solve the 
energy issue.
 The eccentric braced frame (EBF) needs to reach the 10% damped curve at a drift 
of about 5.5 inches, which exceeds its ductility and would create excessive 
deformation. The solution is to add more capacity.
  The steel-moment frame (MF) without a supplemental system needs to reach a 5%t 
damped curve at a drift of about nine to ten inches, far in excess of its 
capacity. The only solutions are to over-design the MF or to add a supplemental 
system.
  The steel-moment frame with passive dampers (MF + Dampers) needs to reach the 
20% damped spectra curve which it does at a drift of about four inches. This is 
a reasonable solution.
  The base-isolation solution (BI) satisfies the demand at a drift from 10 to 12 
inches, which it can easily do without damag,e because most of the lateral drift 
occurs in the isolation itself.
7.6.6 Energy Dissipation
During the duration of ground shaking, seismic energy is stored in a 
structure. If that input energy can be stored and dissipated without damage to 
the structure, the behavior is favorable. If, however, the structure cannot 
store the energy, the system may rupture with either local or catastrophic 
behavior (Figure 7-8A). Earthquake energy accumulates during ground shaking so 
that the total energy continually increases. If the structure can safely 
dissipate energy as rapidly as it is input to the structure, no problem occurs. 
In fact, the structure can store some of the energy due to ground shaking, and 
safely dissipate the balance after the earthquake stops. If the structure cannot 
keep up with the energy input, the structure may suffer minor cracking, or 
rupture, or major failure (Figure 7-8B).
7.6.7 The Design ApproachForce vs. Energy
Searching for the perfect system with conventional solutions has been limited to 
date, because seismic forces are used that are based on accelerations, and then 
the resulting lateral drift or movements of the structure are reviewed to check 
the behavior of both structural and nonstructural elements, such as rigid 
cladding. This process has an inherent conflict.
Seismic accelerations are used because a design force, F, from the relation 
F=Ma, can easily be obtained where M is the building mass, and a is the ground 
acceleration.
For the typical stiff-soil site, the larger the acceleration a, the larger the 
seismic force F. The larger the force F, the stronger the structure is; the 
stronger the structure, the stiffer the structure. The stiffer the structure is, 
the higher the seismic acceleration, and so on. A strong, stiff structure 
appears to be good conceptually. The lateral drift (or horizontal displacement) 
is at a minimum, which is also good. However, the contradiction of this approach 
comes from seismic energy dissipation, which is the fundamental need and 
characteristic of good seismic design; energy dissipation comes from large 
displacements, not small displacements.
Large displacements are needed to dissipate seismic energy, and small 
displacements are needed for lateral drift control to protect cladding, glazing, 
and interior systems. This produces a conflict for most of the classic 
conventional structural concepts and normal nonstructural components.
The most useful seismic systems are those that have predictable, stable, 
nondegrading cyclic behavior. Contemporary structures with these characteristics 
are base-isolation systems; moment frames with dampers; shear-link systems, such 
as coupled shear walls and eccentric-braced frames; and other dual-resistance 
systems with built-in redundancy.
The most useful nonstructural components are those that accommodate large 
lateral movements without failure.
7.7 THE SEARCH FOR THE PERFECT SEISMIC SYSTEM
The 100-year review of seismic systems indicates a slow development of 
structural solutions. In fact, most new development occurs after a damaging 
earthquake. Perhaps this slow periodic development is due to the need to discern 
whether previous ideas were successes or failures. The following topics 
represent steps and ideas in the search for the perfect system.
7.7.1 Structural Mechanisms
There have been periodic significant conceptual breakthroughs in structural 
thinking. For example, Dr. Ian Skinner and his team, working at the New Zealand 
National Laboratory, developed in 1976 a set of energy dissipating concepts 
suitable for use in seismic protection. Among the most notable concepts was the 
practical use of elastomeric isolation bearings for global protection of 
complete structures. Some of the other concepts developed in New Zealand have 
also been utilized in building design. Figure 7-9 illustrates the set of 
mechanisms published in 1980.
Many of these relatively simple mechanisms were new to the design professions 
and to constructors; consequently, they represented unknown processes and 
unknown costs. Development and education are the keys to the acceptance and 
adoption of these systems. Perhaps a few well-publicized, prototype projects 
would familiarize the design professions and constructors with the details and 
costs of these good ideas. To date, this has only been done with the flexural 
plate and the lead-rubber isolation bearing.
7.7.2 Semi-Active and Active Dampers
Recent research and design work in Japan and the United States has focused on 
approaches for structural control during actual wind storms and earthquakes. 
These approaches have been summarized by Hanson and Soong, and can be divided 
into three groups: passive systems, such as base isolation and supplementary 
energy dissipation devices; active systems that require the active participation 
of mechanical devices whose characteristics are used to change the building 
response; and hybrid systems that combine passive and active systems in a manner 
such that the safety of the building is not compromised, even if the active 
system fails.The current systems being studied are characterized by devices that 
control properties and behaviors. Some of these systems are in limited use; 
others are still in development.
The goal of these devices is to respond actively to the variable character of 
wind and earthquake displacements, velocity, accelerations, etc., by adding 
damping or altering stiffness. This controlled behavior will provide the needed 
resistance to respond to ever-changing earthquakes. However, the challenge with 
semi-active or active control systems is maintaining their behavior trouble-free 
over an extended period of time - specifically over many years. These concepts, 
although very appealing, will require some time to perfect and bring to market.
7.7.3 Cost-Effective Systems
The most important measure of good earthquake-resistant design is the impact on 
the structure after the earth has stopped shaking. With little building damage, 
repair costs are low. With significant damage or collapse, repair or replacement 
costs are high. The measure of success in seismic design is selection of a 
structure that will suffer minimum damage, and with corresponding low post-
earthquake repair cost. The behavior of each structural system differs with 
earthquake ground motions, soil types, duration of strong shaking, etc. Our past 
observations of damage yields the best measure of future repair costs. Systems 
with stable cyclic behavior, good energy dissipation, and controlled interstory 
drift will yield low repair costs (Figure 7-10).
The above five attributes, which are tabulated in Figures 7-11A and 7-11B for 
the various significant seismic structural systems, show that a range of 
possibilities is indicated.  These tables are presented for discussion, and the 
evaluations stated are the authors opinion, based on observations, analysis, 
and common goals for seismic performance.  The green colored boxes are favorable 
conditions; the red tend to be unfavorable. The favorable structural systems 
will do the following:
   Possess stable cyclic behavior
   Control lateral drift
   Dissipate seismic energy without failure
   Create a low post-earthquake repair cost
The design reduction value R, discussed above in section 7.6.1,  does not 
necessarily correlate with performance. The R value was a consensus value 
developed for conventional elastic design. With the advent of performance design 
based on nonlinear evaluation the R value serves only as a rough estimate of 
system behavior, but not a realistic estimate of performance.
7.7.4 Avoiding the Same Mistakes
Architects and engineers learn from their detailed investigations of past 
earthquake damage and can document the significant issues and lessons that can 
be learned about particular structural problems. Some problems occur because of 
inappropriate building or structural configuration, some because of brittle, 
nonductile structural systems, some because the building or structure could not 
dissipate sufficient seismic energy, and some because of excessive loads caused 
by dynamic resonance between the ground shaking and the building. Figure 7-12 
illustrates some classic problems.
Why, with all our accumulated knowledge, does all this failure continue? 
Buildings tend to be constructed essentially in the same manner, even after an 
earthquake. It takes a significant effort to change habits, styles, techniques 
and construction. Sometimes bad seismic ideas get passed on without too much 
investigation and modification.
The challenge for architects, engineers, and constructors is to ask why so much 
damage occurred, and what can be done to correct the problem. Understanding the 
basic seismic energy demand is a critical first step in the search for 
significant improvement. Not all structural systems, even though they are 
conventional and commonplace, will provide safe and economical solutions.
7.7.5 Configurations Are Critical
Configuration, or the three-dimensional form of a building, frequently is the 
governing factor in the ultimate seismic behavior of a particular structure. 
Chapter 5 covers conventional configuration issues where conventional 
rectangular grids are used in building layout, design and construction. However, 
contemporary architectural design is changing, in large part because the 
computer allows complex graphic forms and analyses to be generated and easily 
integrated into a building design. 
The resulting irregular, random, free-form grids and systems have just begun to 
be explored from the structural engineering viewpoint. They are frequently 
rejected because of various cost issues, and because of unproven real earthquake 
behavior. 
The potential for optimizing seismic resistance with respect to structural 
configuration is an obvious direction for the future. Structural form should 
follow the needs. How can we define seismic needs? 
Buildings must dissipate energy; how does one configure a structure to dissipate 
energy? There are natural forms and design concepts that act as springs, rocking 
mechanisms, flexural stories, yielding links, articulated cable-restrained 
configurations, pyramid forms, cable anchors, etc. Any system that can dissipate 
seismic energy without damage is a candidate. Figure 7-13 illustrates some 
special concepts utilizing building configuration to dissipate energy.
7.7.6 Common-Sense Structural Design-Lessons Learned
The simple way to reduce seismic demand within a structure is to understand the 
actual demand. An earthquake is a dynamic phenomenon with all its classic 
characteristics. If one can reduce the effective damaging character of the 
earthquake, the behavior of a structure or building will be significantly 
improved. The following five issues can significantly reduce earthquake damage 
and related costs.
  Select the Appropriate Scale
The size and scale of the building should determine the appropriate structural 
solution. It is common sense to use a light braced frame for a small structure, 
such as one- or two-story wood or light-gauge steel systems. The light seismic 
system is compatible with the light building mass. As the building mass 
increases with the use of heavy concrete floors, the mass increases by a factor 
of 8 to 12 times. This would imply a much heavier seismic frame or brace (Figure 
7-14), 8 to 12 times stronger per floor, and if the building grows from two to 
ten stories, the total mass increases from two to 80-120 times. 
The same frame or brace which works well at two stories no longer works well at 
ten stories, because structural components when simply scaled up no longer 
behave in the same fashion. For instance, a light steel section with 3/8-inch 
flanges is ductile, but no longer has the same ductility when the flanges are 
two inches thick. The thick sections and welds are now subjected to high shears 
and are prone to failure. 
Some structural systems are very forgiving at small sizes but not at a large 
scale. Alternative structural systems must be used for the larger, more 
demanding buildings. The appropriate systems must not degrade in strength and 
should have ample sustainable damping. It is critical that scale be considered, 
even though it is not considered by the seismic building codes. This scale issue 
requires study, observation, and common sense, but the issue of appropriate 
scale continually confronts designers. Careful, unbiased research is necessary.
  Reduce Dynamic Resonance
It is important to significantly reduce the dynamic resonance between the 
shaking ground and the shaking building, and to design the structure to have a 
period of vibration that is different from that of the ground period. The 
difference is simply illustrated with a classic resonance curve (Figure 7-15).
The relationship between the building period and spectral acceleration of a real 
earthquake varies between a site with firm soil and a site with soft soil 
(Figure 7-16).
If the building has a period of vibration, T1, corresponding to the peak ground 
acceleration, the most severe demand occurs. Shift the period, T2,  by altering 
the structural system to lengthen the period (firm soil) or shorten or lengthen 
the period T2 (soft soil). 
This requires design effort by both the architect and engineer, but the result 
can be a significant reduction in the seismic response.
  Increase Damping
It is valuable to significantly increase the structural system damping. Damping 
reduces vibration amplitude similar to the hydraulic shock absorbers in an 
automobile, and damping reduces the structural demand (Figure 7-17), as 
illustrated in the spectral acceleration plot.
Significant damping can be introduced into a structure by 1) adding a passive 
damping system (hydraulic, friction, etc.); 2) utilizing a fractured concrete 
member such as a shear-link serving to couple two walls; 3) fracturing a 
concrete shear wall; 4) utilizing a seismic isolation system (with 10 to 20% 
damping), or 5) utilizing a tuned-mass damper (a challenging solution for most 
buildings). An increase of damping by using a non-distinctive system is a most 
positive solution for reducing seismic demand.
  Provide Redundancy
It is also important to add redundancy or multiple load paths to the structure 
to improve seismic resistance. After experience in many earthquakes and much 
study and discussion, the engineering profession has generally concluded that 
more than a single system is the ideal solution for successful seismic 
resistance. If carefully selected, multiple systems can each serve a purpose; 
one to add damping and to limit deflection or drift, the other to provide 
strength. Multiple systems also serve to protect the entire structure by 
allowing failure of some elements without endangering the total building.
An informative sketch of a classic redundant-framing concept, with frames on 
each grid line, versus a contemporary multiple system with two types of framing, 
one for strength, the other for damping, is shown in Figure 7-18. The current 
dual systems now being developed and utilized are a significant improvement over 
the historic single seismic resisting systems.
  Energy Dissipation
Solving the seismic energy dissipation problem is the ultimate test of good 
earthquake-resistant design. Since large building displacement is required for 
good energy dissipation, while minimum displacement is required to protect the 
many brittle non-structural components in the building, only one seismic 
resisting system adequately solves both aspects of this problemseismic base 
isolation. 
However, seismic isolation is not suited for all building conditions: 
specifically, tight urban sites adjacent to property lines (a movement problem), 
and tall buildings over 12 stories (a dynamic resonance problem). Cost of 
construction is another consideration (about a 1 to 2% premium). Seismic 
isolation is an ideal solution for irregular buildings and unusual or creative 
building forms that are difficult to solve with conventional structural systems.
Other dual systems can also solve the energy dissipation problem. Six seismic 
systems are outlined in Figure 7-19. Each system has a cyclic load-deformation 
loop which is full and nondegrading. Most importantly, each has a predicted 
minor-to-moderate post-earthquake damage, with a correspondingly low post-
earthquake repair cost. This fact makes these good seismically sustainable 
buildings, expanding the meaning of green building.
7.8 CONCLUSIONS
We must continue to develop and use increasingly realistic analysis methods to 
design new buildings and to modify existing structures. This is not a simple 
task because of the numerous variables: duration of shaking, frequency content 
of the seismic motion, displacement, velocity, acceleration, direction of the 
earthquake pulse, proximity to the active fault, and soil amplification effects. 
In addition to the ground effects, the structure has its own variables: size; 
shape; mass; period of vibration; irregularities in shape, stiffness, and 
strength; and variation in damping. 
After the past 100 years of seismic design, the perfect architectural/structural 
solution is still elusive, but with creative thought, testing, and computers, 
the problem is more transparent, and we can now more rapidly study variables 
than in the recent past. Design work today has become part research, part 
invention, and common sense, and we are on the threshold of creative 
breakthrough.
7.9 REFERENCES
1.  Park, R. and Paulay, T, 1975. Reinforced Concrete Structures, Wiley
Interscience.
2.  Paulay, T. and Priestly, N, 1992. Seismic Design of Reinforced Concrete and 
Masonry Buildings, Wiley. 
3.  Hanson, R. and Soong, T.T, 2001. Seismic Design with Supplementary Energy 
Dissipation Devices, Earthquake Engineering Research Insitute, Oakland CA.

[End of Chapter 7]



EXISTING BUILDINGS--EVALUATION AND RETROFIT BY WILLIAM HOLMES
8.1	INTRODUCTION
It is widely recognized that the most significant seismic risk in this country 
resides in our existing older building stock.  Much of the country has enforced 
seismic design for new buildings only recently; even on the West Coast, seismic 
codes enforced in the 1960s and even into the 1970s are now considered suspect. 
Although there are sometimes difficulties in coordinating seismic design 
requirements with other demands in new construction, the economical, social, and 
technical issues related to evaluation and retrofit of existing buildings are 
far more complex.
8.1.1 Contents of Chapter
This chapter describes the many issues associated with the risk from existing 
buildings, including common building code provisions covering older buildings, 
evaluation of the risks from any one given building and what levels of risk are 
deemed acceptable, and methods of mitigation of these risks through retrofit. A 
FEMA program to provide methods to mitigate the risk from existing buildings has 
been significant in advancing the state of the art, and this program is 
described in some detail, particularly the model building types used in most, if 
not all, of the FEMA documents.
8.1.2 Reference to Other Relevant Chapters
The basic concepts used for seismic design or estimation of seismic performance 
are the same for any building. Thus, the principles described in Chapters 4, 5, 
and 7 are applicable for older, potentially hazardous buildings. The development 
of seismic systems as seen through examples of buildings in the San Francisco 
Bay Area is particularly relevant to the issues covered in this chapter, because 
systems typically evolved due to poor performance of predecessors.
Nonstructural systems in buildings create the majority of dollar loss from 
buildings in earthquakes, although the quality of structural performance affects 
the level of that damage. Seismic protection of nonstructural systems, both for 
design of new buildings and for consideration in older buildings, is covered in 
Chapter 9.
8.2  BACKGROUND
In every older building, a host of deficiencies is identified as the state of 
the art of building design and building codes advances. Code requirements change 
because the risk or the expected performance resulting from the existing 
provisions is deemed unacceptable. Deficiencies are commonly identified due to 
increased understanding of fire and life safety, disabled access, hazardous 
materials, and design for natural hazards. Thus, it is not surprising that many 
of the older buildings in this country are seismically deficient, and many 
present the risk of life-threatening damage. It is not economically feasible to 
seismically retrofit every building built to codes with no or inadequate seismic 
provisions, nor is it culturally acceptable to replace them all.  These 
realities create a significant dilemma: How are the buildings that present a 
significant risk to life safety identified? How is the expected performance 
predicted for older buildings of high importance to businesses or for those 
needed in emergency response? How can we efficiently retrofit those buildings 
identified as high risk?
The term seismic deficiency is used in this chapter as a building characteristic 
that will lead to unacceptable seismic damage. Almost all buildings, even those 
designed to the latest seismic codes, will suffer earthquake damage given strong 
enough shaking; however, damage normally considered acceptable should not be 
expected in small events with frequent occurrence in a given region, and should 
not be life threatening. Damage may be judged unacceptable due to resulting high 
economic cost to the owner or due to resulting casualties. Therefore, conditions 
that create seismic deficiencies can vary from owner to owner, from building to 
building, and for different zones of seismicity.  For example, unbraced, 
unreinforced brick masonry residential chimneys are extremely vulnerable to 
earthquake shaking and should be considered a deficiency anywhere that shaking 
is postulated. On the other hand, unreinforced brick masonry walls, infilled 
between steel frame structural members, are expected to be damaged only in 
moderate to strong shaking and may not be considered a deficiency in lower 
seismic zones. Seismic deficiencies identified in this chapter generally will 
cause premature or unexpected damage, often leading to threats to life safety, 
in moderate to strong shaking. Buildings in regions of lower seismicity that 
expect Modified Mercalli Intensity (MMI) levels of not more than VII, or peak 
ground accelerations (PGA) of less than 0.10g (g = acceleration of gravity), may 
need special consideration.
Of course, every building with completed construction is existing. However, 
the term existing building has been taken to mean those buildings in the 
inventory that are not of current seismic design. These groups of buildings, 
some of which may not be very old, include buildings with a range of probable 
performance from collapse to minimal damage. In this chapter, the term existing 
building is used in this context.
8.2.1 Changes in Building Practice and Seismic Design Requirements Resulting in 
Buildings that are Currently Considered Seismically Inadequate
Chapter 7 documents in detail how building systems have evolved in the San 
Francisco Bay Area. This evolution was probably driven more by fire, economic, 
and construction issues than by a concern for seismic performance, at least in 
the first several decades of the twentieth century, but many changes took place. 
Similarly, Chapter 6 gives a brief history of the development of seismic codes 
in the United States. It is clear that for many reasons, building construction 
and structural systems change over time. In the time frame of the twentieth 
century, due to the rapid increase in understanding of the seismic response of 
buildings and parallel changes in code requirements, it should be expected that 
many older buildings will now be considered seismically deficient.
Seismic codes in this country did not develop at all until the 1920s, and at 
that time they were used voluntarily. A mandatory code was not enforced in 
California until 1933. Unreinforced masonry (URM) buildings, for example, a 
popular building type early in the twentieth century and now recognized as 
perhaps the worst seismic performer as a class, were not outlawed in the zones 
of high seismicity until the 1933 code, and continued to be built in much of the 
country with no significant seismic design provisions until quite recently. 
Figure 8-1 shows an example of typical URM damage. The first modern seismic 
codes were not consistently applied until the 1950s and 1960s, and then only in 
the known regions of high seismicity. Of course, not all buildings built before 
seismic codes are hazardous, but most are expected to suffer far more damage 
than currently built buildings
Even buildings designed to modern seismic codes may be susceptible to high 
damage levels and even collapse. Our understanding of seismic response has grown 
immensely since the early codes, and many building characteristics that lead to 
poor performance were allowed over the years. For example, concrete buildings of 
all types were economical and popular on the West Coast in the 1950s and 1960s. 
Unfortunately, seismic provisions for these buildings were inadequate at the 
time, and many of these buildings require retrofit. Highlights of inadequacies 
in past building codes that have, in many cases, created poor buildings are 
given below.
	Changes In Expected Shaking Intensity and Changes in Zoning
	Similar to advancements in structural analysis and the understanding of 
building performance, enormous advancements have been made in the understanding 
of ground motion, particularly since the 1950s and 1960s. The seismicity (that 
is, the probability of the occurrence of various-sized earthquakes from each 
source) of the country, the likely shaking intensity from those events depending 
on the distance from the source and the local soil conditions, and the exact 
dynamic nature of the shaking (the pattern of accelerations, velocities, or 
displacements) are all far better understood. These advancements have caused 
increases in seismic design forces from a factor of 1.5 in regions very near 
active faults (on the West Coast) to a factor of 2 to 3 in a few other areas of 
the country (e.g. Utah; Memphis, Tennessee). The damage to the first Olive View 
Hospital (Figure 8-2), in addition to other issues, was a result of inadequate 
zoning.
	Changes in Required Strength or Ductility
	As discussed in Chapter 4, the required lateral strength of a seismic 
system is generally traded off with the ductility (the ability to deform 
inelasticallynormally controlled by the type of detailing of the components and 
connections) of the system.  Higher strength requires lower ductility and vice 
versa. The most significant changes in codesreflecting better understanding of 
minimum requirements for life safetyare general increases in both strength and 
ductility. Many building types designed under previous seismic provisions, 
particularly in the 1950s, 1960s, and 1970s, are now considered deficient, 
including most concrete-moment frames and certain concrete shear walls, steel-
braced frames, and concrete tilt-ups.
	Other buildings designed with systems assumed to possess certain ductility 
have been proven inadequate. Figure 8-3 shows typical steel moment-frame damage 
in the Northridge earthquake caused by brittle behavior in a structural system 
previously thought to be of high ductility.
	Recognition of the Importance of Nonlinear Response
	Historically, a limited amount of damage that absorbed energy and  
softened the building, thus attracting less force, was thought o reduce seismic 
response. Although this is still true, it is now recognized that the extent and 
pattern of damage must be controlled. Early codes required the design of 
buildings for forces three to six times less than the elastic demand (the forces 
that the building would see if there was no damage), assuming that the 
beneficial characteristics of damage would make up the difference.  
Unfortunately, buildings are not uniformly damaged, and the change in structural 
properties after damage (nonlinear response) often will concentrate seismic 
displacement in one location. For example, if the lower story of a building is 
much more flexible or weaker than the stories above, damage will concentrate at 
this level and act as a fuse, never allowing significant energy absorption from 
damage to the structure above. This concentrated damage can easily compromise 
the gravity load-carrying capacity of the structure at that level, causing 
collapse. Similarly, concrete shear walls were often discontinued at lower 
floors and supported on columns or beams. Although the supporting structure was 
adequately designed for code forces, the wall above is often much stronger than 
that and remains undamaged, causing concentrated and unacceptable damage in the 
supporting structure. 
	A final example of this issue can be seen by considering torsion. As 
explained in Chapter 4, Section 4.11, torsion in a building is a twisting in 
plane caused by an imbalance in the location of the mass and resisting elements. 
Older buildings were often designed with a concentration of lateral strength and 
stiffness on one endan elevator/stair tower, for exampleand a small wall or 
frame at the other end to prevent torsion. However, when the small element is 
initially damaged, its strength and stiffness changes, and the building as a 
whole may respond with severe torsion.
	Current codes contain many rules to minimize configurations that could 
cause dangerous nonlinear response, as well as special design rules for elements 
potentially affected (e.g., columns supporting discontinuous shear walls). Olive 
View Hospital featured a weak first story in the main building, causing a 
permanent offset of more than one foot and near collapse; discontinuous shear 
walls in the towers caused a failure in the supporting beam and column frame, 
resulting in complete overturning of three of the four towers. Figure 8-4 shows 
a typical tuck-under apartment building in which the parking creates a weak 
story. 
8.2.2 Philosophy Developed for Treatment of  Existing Buildings
Building codes have long contained provisions to update life-safety features of 
buildings if the occupancy is significantly increased in number or level of 
hazard (transformation of a warehouse to office space, for example). As early as 
the mid-1960s, this concept started to be applied to seismic systems. Many older 
buildings contained entire structural systems no longer permitted in the code 
(e.g., URM, poorly reinforced concrete walls), and it quickly became obvious 
that 1) these components could not be removed, and 2) it was impractical and 
uneconomical to replace all older buildings. The new code could therefore not 
be applied directly to older buildings, and special criteria were needed to 
enable adaptive reuse while meeting the need to protect life safety of the 
occupants.  In some cases, an entirely new and code-complying lateral system was 
installed, while leaving existing, now prohibited, construction in place. (This 
procedure was used in many school buildings in California after the Field Act 
was passed in 1933up until the school seismic safety program was essentially 
completed in the 1960s.) This procedure proved very costly and disruptive to the 
building and was thought to discourage both improved seismic safety and general 
redevelopment.
A philosophy quickly developed suggesting that existing buildings be treated 
differently from new buildings with regard to seismic requirements. First, 
archaic systems and materials would have to be recognized and incorporated into 
the expected seismic response, and secondly, due to cost and disruption, seismic 
design force levels could be smaller. The smaller force levels were rationalized 
as providing minimum life safety, but not the damage control of new buildings, a 
technically controversial and unproven concept, but popular. Commonly existing 
buildings were then designed to 75% of the values of new buildingsa factor that 
can still be found, either overtly or hidden, in many current codes and 
standards for existing buildings.
Occasionally, early standards for existing buildings incorporated a double 
standard, accepting a building that passed an evaluation using 75% of the code, 
but requiring retrofits to meet a higher standard, often 90% of the code.
8.2.3 Code Requirements Covering Existing Buildings
As the conceptual framework of evaluation and retrofit developed, legal and code 
requirements were also created. These policies and regulations can be described 
in three categories; active, passive, and post-earthquake. Active policies 
require that a defined set of buildings meet given seismic criteria in a certain 
time framewithout any triggering action by the owner. For example, all bearing-
wall masonry buildings in the community must meet the local seismic safety 
criteria within ten years. Passive policies require minimum seismic standards in 
existing buildings only when the owner triggers compliance by some action
usually extensive remodeling, reconstruction, or addition. Post-earthquake 
policies developed by necessity after several damaging earthquakes, when it 
became obvious that repairing an obviously seismically poor building to its pre-
earthquake condition was a waste of money. It then became necessary to develop 
triggers to determine when a building could simply be repaired and when it had 
to be repaired and retrofitted as well.
 Passive Code Provisions  
As noted above, the development of requirements to seismically update a building 
under certain conditions mimicked economic and social policies well-established 
in building codes. Namely, the concept crystallized that if sufficient resources 
were spent to renew a building, particularly with a new occupancy, then the 
building should also be renewed seismically.  Seismic renewal was defined as 
providing life safety, but not necessarily reaching the performance expected 
from a new building. A second kind of triggerthat could be termed trigger of 
opportunityhas also been used in some communities. These policies try to take 
advantage of certain conditions that make seismic improvements more palatable to 
an owner, such as retrofit of single-family dwellings at point of sale or 
requiring roof diaphragm upgrades at the time of re-roofing.
Triggers based on alterations to the building are by far the most common and 
will be discussed further here. These policies are somewhat logical and 
consistent with code practice, but they created two difficult socioeconomic-
technical issues that have never been universally resolved. The first is the 
definition of what level of building renewal or increase in occupancy-risk 
triggers seismic upgrading. The second is to establish the acceptable level of 
seismic upgrading.
Most typically, the triggering mechanisms for seismic upgrade are undefined in 
the code and left up to the local building official. The Uniform Building Code, 
the predecessor to the IBC in the western states, waffled on this issue for 
decades, alternately inserting various hard triggers (e.g., 50% of building 
value spent in remodels) and ambiguous wording that gave the local building 
official ultimate power. The use of this mechanism, whether well defined in 
local regulation or placed in the hands of the building official, ultimately 
reflects the local attitude concerning seismic safety. Aggressive communities 
develop easily and commonly triggered criteria, and passive or unaware 
communities require seismic upgrade only in cases of complete reconstruction or 
have poorly defined, easily negotiated triggers. For more specific information 
on seismic triggers in codes, see the accompanying sidebar.

Box 1 Seismic Triggers in Codes
Events or actions that require owners to seismically retrofit their buildings 
are commonly called triggers. For example, in many communities, if an owner 
increases the occupancy risk (as measured by number of occupants, or by use of 
the building), they must perform many life-safety upgrades, including seismic 
ones. However, for practical and economic reasons, seldom does this trigger 
require conformance with seismic provision for new buildings, but rather with a 
special life-safety level of seismic protection, lower than that used for new 
buildings.
The code with the longest history in high-seismic regions, the Uniform Building 
Code (UBC), has long waffled on this issue. Besides the traditional code life-
safety trigger based on clear-cut changes in occupancy, this code over the years 
has included provisions using hard triggers based on the cost of construction, 
and soft language that almost completely left the decision to the local building 
official. The last edition of this code, the 1997 UBC, basically allowed any 
(non-occupancy related) alteration as long as the seismic capacity was not made 
worse.
The codes and standards that will replace the UBC are based on a federally 
funded effort and published by FEMA as the NEHRP Provisions.  These codes 
include the International Building Code (IBC), the National Fire Protection 
Agency (NFPA) and ASCE 7, a standard covering seismic design now ready for 
adoption by the other codes. This family of regulations has a common limit of a 
5% reduction in seismic capacity before full compliance is required.  This 
reduction could be caused by an increase in mass (as with an addition) or a 
decrease in strength (as with an alteration that places an opening in a shear 
wall). Full compliance in this case is defined as compliance with the provisions 
for new buildings that do not translate well to older buildings. It is unclear 
how this will be interpreted on the local level.
Many local jurisdictions, however, have adopted far more definitive triggers for 
seismic retrofit. San Francisco is a well-known example, perhaps because the 
triggers are fairly elaborately defined and because they have been in place for 
many years. In addition to the traditional occupancy-change trigger, San 
Francisco requires conformance with seismic provisions specially defined for 
existing buildings when substantial nonstructural alterations are done on 2/3 or 
more of the number of stories within a two-year period, or when substantial 
structural alterations, cumulative since 1973, affect more than 30% of the floor 
and roof area or structure.
The City of Portland, Oregon requires a seismic upgrade of URMs when the cost of 
construction exceeds $30/sf for a one story building, or $40/sf for buildings 
greater than two stories.
Although most jurisdictions leave this provision purposely loose, some also have 
adopted definitive triggers based on cost of construction, on the particular 
building type, and on various definitions of significant structural change.  
Government, in some cases, has been much more aggressive in setting triggers to 
activate seismic retrofit, perhaps to create a lawful need for funds which 
otherwise would be difficult to obtain. The state of California has set a 
definitive list of seismic triggers for state-owned buildings: a) alteration 
cost exceeding 25% of replacement cost; b) change in occupancy; c) reduction of 
lateral load capacity by more than 5% in any story; d) earthquake damage 
reducing lateral load capacity by more than 10% at any story.
The federal government likewise, in RP 6, [NIST, 2002], also has definitive 
triggers: a) change in occupancy that increases the buildings importance or 
level of use; b) alteration cost exceeding 50% of replacement cost; c) damage of 
any kind that has significantly degraded the lateral system; d) deemed to be a 
high seismic risk; and e) added to federal inventory though purchase or 
donation.
The regulations and policies governing any building, private or public, which 
will be significantly altered, should be researched in the planning stage to 
understand the effective seismic triggers, written or understood.

When seismic improvement is triggered, the most common minimum requirement is 
life safety consistent with the overall code intent. However, the use of 
performance-based design concepts to establish equivalent technical criteria is 
a recent development and is not yet universally accepted. As indicated in the 
last section, the initial response to establishing minimum seismic criteria was 
to use the framework of the code provisions for new buildings with economic and 
technical adjustments as required. These adjustments included a lower lateral 
force level (a pragmatic response to the difficulties of retrofit), and special 
consideration for materials and systems not allowed by the provisions for new 
buildings. (Unreinforced masonry, for example, was not only prohibited as a 
structural system in zones of high seismicity, but also could not be used in a 
building at all.) Use of a lateral force level of 75% of that required for new 
buildings became fairly standard, but the treatment of archaic materials is 
highly variable from jurisdiction to jurisdiction.
Many local retrofit provisions are gradually being replaced by national 
guidelines and standards for seismic evaluation and retrofit (e.g. ASCE 31, 
2003; FEMA 356, 2000, etc.). In addition, performance based seismic design is 
enabling a more direct approach to meeting a communitys minimum performance 
standardsalthough this requires the policy-makers to decide what the minimum 
performance standard should be, a difficult task that crosses social, economic, 
and technical boundaries.
In summary, both the passive triggers for seismic retrofit and the design or 
performance criteria are often ill-defined and, at best, highly variable between 
jurisdictions. Design professionals should always determine the governing local, 
state, or federal regulations or policies when designing alterations or remodels 
on existing buildings.  
  Active Code Provisions 
Active code provisions result from policy decisions of a jurisdiction to reduce 
the community seismic risk by requiring seismic upgrading of certain buildings 
known to be particularly vulnerable to unacceptable damage. For the most part, 
these provisions are unfunded mandates, although low-interest loan programs have 
been developed in some cases. These risk reduction programs usually allow owners 
a lengthy period to perform the retrofit or to demolish the buildingsten years 
or more.  The standard for retrofit is also normally included in the law or 
regulation and is typically prescriptive, although performance-based design 
options are becoming more acceptable.
Two large-scale examples of active seismic code provisions were started by the 
state of California. The first was a program to reduce the risk from URM 
buildings. The state legislature, lacking the votes to simply require mitigation 
throughout the state, instead passed a law (SB 547-1986) that required local 
jurisdictions to develop inventories of these buildings in their area, to notify 
the owners that their building was considered hazardous, and to develop a 
community-wide hazard reduction plan.  Although not required to do so, most 
jurisdictions chose as their hazard reduction plan to pass active code 
ordinances giving owners of the buildings ten or so years to retrofit them. Over 
10,000 URM buildings have been brought into compliance with these local 
ordinances, most by retrofit, but some by demolition (SSC, 2003).
The second program, created by SB 1953 in 1994 following the Northridge 
earthquake, gave California hospital owners until 2030 to upgrade or replace 
their hospitals to comply with state law governing new hospital buildings. The 
programs intention is to enable buildings to be functional following an 
earthquake. This law affected over 500 hospitals and over 2,000 buildings 
(Holmes, 2002). Although compliance is ongoing, this law has been problematic 
due to the high cost and disruption associated with retrofitting hospital 
buildings, and the highly variable economic condition of the health system as 
well as individual facilities.
Other examples include local ordinances to retrofit tilt-up buildings, less 
controversial because of the clear high vulnerability and low retrofit cost of 
these buildings. Similar to investigating local regulations regarding triggers, 
it is also wise to determine if any existing building planned for alterations is 
covered by (or will be covered in the foreseeable future) a requirement to 
retrofit. It is generally acknowledged that seismic improvements are easier to 
implement when done in association with other work on the building.
  Post-Earthquake Code Provisions
Following a damaging earthquake, many buildings may be closed pending 
determination of safety and necessary repairs. A lack of clear repair standards 
and criteria for re-occupancy has created controversy and denied owners use of 
their buildings after most damaging earthquakes. Assuming that the earthquake 
itself is the ultimate judge of seismic acceptability, many communities may take 
the opportunity in the post-earthquake period to require strengthening of 
buildings that are apparently seismically deficient due to their damage level. 
However, implementation of this theory incorporating conservative policies that 
require many retrofits may delay the economic recovery of the community. On the 
other hand, standards for repair and/or strengthening which are not conservative 
could lead to equal or worse damage in the next earthquake. It has also been 
observed that owners of historic, rent-controlled, or otherwise economically 
controlled buildings may have an incentive to demolish damaged buildings to the 
detriment of the community at large.
Traditionally, communities (building departments) have used color codes for 
several or all of the following categories of buildings following an earthquake:
A.   Undamaged; no action required. If inspected at all, these buildings will be 
Green-tagged.
B.   Damaged to a slight extent that will only require repair of the damage to 
the original condition. These buildings will generally be Green-tagged, but the 
category could also include some Yellow-tags.
C.   Damaged to a greater extent that suggests such seismic weaknesses in the 
building that the overall building should be checked for compliance with minimum 
seismic standards. This will often require overall retrofit of the building. 
These buildings will generally be Red-tagged, but the category could also 
include some Yellow tags.
C1. (A subcategory of C). Damaged to an extent that the building creates a 
public risk that requires immediate mitigation, either temporary shoring or 
demolition. The ultimate disposition of these buildings may not be determined 
for several months.  These buildings will all be Red-tagged.
The most significant categorization is the differentiation between B and C. The 
difference to an owner between being placed in one category or the other could 
be an expense on the order of 30%-50% of the value of the building, reflecting 
the added cost of retrofit to that of repair.  Earthquakes being rare, few 
communities have been forced to create these policies, but a few have. Oakland, 
California, prior to the Loma Prieta earthquake, set a trigger based on the loss 
of capacity caused by the damage. If the damage was determined to have caused a 
loss of over 10% of lateral force capacity, then retrofit was triggered.  Los 
Angeles and other southern California communities affected by the Northridge 
earthquake used a similar standard, but the 10% loss was applied to lines of 
seismic resistance rather than the building as a whole. These code regulations, 
although definitive, are problematic because of the technical difficulty of 
determining loss of capacity, particularly to the accuracy of 1% (a 1% change 
can trigger a retrofit).
The importance of this issue has been magnified by interpretation of federal 
laws that creates a tie between reimbursement of the cost of repair of certain 
local damage to the pre-existence and nature of these local damage triggers. 
Owners and designers of older existing buildings should be aware of such 
triggers that could affect them should they suffer damage from an earthquake. In 
some cases, it may be prudent for an owner to voluntarily retrofit a vulnerable 
building to avoid the possibility of being forced to do it in a post-earthquake 
environment as well as to possibly avoiding a long closure of the building.
8.3	 THE FEMA PROGRAM TO REDUCE THE  SEISMIC RISK FROM EXISTING BUILDINGS
In 1985, the Federal Emergency Management Agency (FEMA) recognized that the 
principal seismic risk in this country came from the existing building stock, 
the majority of which was designed without adequate seismic provisions. 
Following a national workshop that identified significant issues and potential 
educational and guideline projects that FEMA could lead, a program was launched 
that is still ongoing.  In addition to providing education and technical 
guidelines in the area of high-risk existing buildings, other FEMA programs were 
also significant in enabling communities to understand and mitigate their 
seismic risk, most notably the development of the regional loss-estimating 
computer program, HAZUS.  Most of these activities are documented as part of the 
FEMA yellow book series (so known because of its distinctive yellow covers), 
well known to engineers in this country and, in fact, around the world. 
Unfortunately, these documents are less known to architects, although many of 
them contain useful insights into not only the issues surrounding seismic 
evaluation and retrofit of existing buildings, but also into all aspects of 
seismic design. 
8.3.1	FEMA-Sponsored Activity for Existing Buildings
Following is a summary of selected FEMA-sponsored projects beginning in the late 
1980s.  A full listing is given in FEMA 315, Seismic Rehabilitation of 
Buildings, Strategic Plan 2005.
	Rapid Visual Screening
	FEMA 154: Rapid Visual Screening of Buildings for Potential Seismic 
Hazards, 1988, updated 2001
	A method to enable an efficient first sorting of selected buildings into 
an adequately life-safe group and a second group that will require further 
evaluation. The evaluation was intended to be performed on the street in an hour 
or less per building. The first task is to assign the building to a predefined 
model building type and then identify additional characteristics that could 
refine the seismic vulnerability. The method has proven useful to efficiently 
generate an approximate mix of buildings that will properly characterize a 
communitys vulnerability, but not to definitely rate individual buildings, due 
to the difficulty of identifying significant features from the street. Generally 
it is necessary to obtain access to the interior of a building, or, more 
commonly, it is even necessary to review drawings to confidently eliminate older 
buildings as potentially hazardous.
	Evaluation of Existing Buildings
	FEMA 178: NEHRP Handbook for Seismic Evaluation of Existing Buildings, 
1989 and FEMA 310: Handbook for the Seismic Rehabilitation of Buildings: a 
Prestandard.
	A widely used guide to determine if individual buildings meet a nationally 
accepted level of seismic life safety. This method requires engineering 
calculations and is essentially prescriptive, which facilitates consistency and 
enables enforceability. This life-safety standard was adopted by, among others, 
the federal government and the state of California in certain programs. The 
prescriptive bar may, however, have been set too high, because very few older 
buildings pass.  Since its original development, FEMA 178 has been refined and 
republished as FEMA 310, and finally was adopted as a Standard by the American 
Society of Civil Engineers as ASCE 31 in 2003.
	Techniques Used in Seismic Retrofit
	FEMA 172: NEHRP Handbook of Techniques for Seismic Rehabilitation of 
Existing Buildings, 1992
	In recognition of the lack of experience in seismic upgrading of buildings 
of most of the countrys engineers and architects, this document outlined the 
basic methods of seismically strengthening a building, including conceptual 
details of typically added structural elements. The material recognizes the FEMA 
model building types, but is primarily organized around strengthening of 
structural components, perhaps making the material less directly accessible.The 
document also preceded by several years the publication of the analytical tools 
to design retrofits (FEMA 273, see below). For whatever reason, the publication 
went relatively unused, despite the fact that it contains useful information, 
particularly for architects unfamiliar with seismic issues.
	Financial Incentives
	FEMA 198: Financial Incentives for Seismic Rehabilitation of Hazardous 
Buildings, 1990
	To encourage voluntary seismic upgrading, this document described the 
financial incentives to do so, ranging from tax benefits to damage avoidance.
	Development of Benefit-Cost Model
	FEMA 227: A Benefit-Cost Model for the Seismic Rehabilitation of 
Buildings, 1992
	Due to the expected high cost of seismic rehabilitation, the need to 
provide a method to calculate the benefit-cost ratio of seismic retrofit was 
identified early in the program. This requires estimation of financial losses 
from earthquake damage resulting from a full range of ground-shaking intensity.  
Financial losses include direct damage to structural and nonstructural systems 
as well as business interruption costs.  A controversial feature was the 
optional inclusion of the value of lives lost in the overall equation. The 
project was primarily to develop the model rather than to provide new research 
into expected damage or casualty rates. Thus, approximate relationships 
available at the time were used.  However, the documentation concerning 
contributing factors to a benefit-cost analysis is quite complete, and a 
computerized functional spreadsheet version of the method was developed. 
	Although the use of benefit-cost analysis never became popular in the 
private sector, trials of the method indicated that very low retrofit costs, 
high business-interruption losses, or high exposure-to-casualty losses are 
required to result in a positive benefit-cost ratio. For example rehabilitation 
of tilt-up buildings is usually fairly inexpensive and usually proves cost 
effective.  Similarly, buildings with high importance to a business or with high 
occupancy in areas of high seismicity also result in positive results. Despite 
the apparent overall results from this program, considerable rehabilitation 
activity continued, both in conditions expected to yield a positive benefit-cost 
and in other conditions (in many cases due to extreme importance given to life 
safety).
	Typical Costs of Seismic Rehabilitation
	FEMA 156: Typical Costs for Seismic Rehabilitation of Existing Buildings, 
1988, 2nd edition, 1994.
	The second edition of this document collected case histories of 
constructed rehabilitation and completed reports judged to have realistic costs. 
A database was created to separate costs by primary influence factors: model 
building types, rehabilitation performance objectives, and seismicity. A serious 
difficulty in collection of accurate data was the inevitable mixing of pure 
rehabilitation costs and associated costs such as life-safety upgrades, the 
American Disabilities Act (ADA), and even remodels. Although a large amount of 
data was collected, there was not nearly enough to populate all combinations of 
the factors. Nevertheless, a method was developed to use the data to make 
estimates of costs for given situations. The major problem was that the 
coefficient of variation of rehabilitation costs, for any given situation, is 
very high due to high variability in the extent of seismic deficiencies. The 
information collected is probably most useful to estimate costs for large 
numbers of similar buildings where variations will average out. Use of the 
method to accurately estimate the cost of a single building is not recommended, 
although even the ranges given could be useful for architects and engineers not 
familiar with retrofit issues.
	Technical Guidelines for Seismic Rehabilitation
	FEMA 273: NEHRP Guidelines for the Seismic Rehabilitation of Buildings, 
1997 and FEMA 356 Prestandard and Commentary for the Seismic Rehabilitation of 
Buildings.
	This document, developed over five years by over 70 experts, was the 
culmination of the original program. Previously, the most common complaint from 
engineers and building officials was the lack of criteria for seismic retrofit. 
FEMA 273 incorporated performance-based engineering, state-of-the-art nonlinear 
analysis techniques, and an extensive commentary to make a significant 
contribution to earthquake engineering and to focus laboratory research on 
development of missing data. The document broke away from traditional code 
methods and in doing so, faced problems of inconsistency with the design of new 
buildings. Improvements were made in a follow-up document, FEMA 356, but the 
practical results from use of the method indicate that considerable judgment is 
needed in application. Work is continuing to improve analysis methods and in 
methods to predict the damage level to both components of a building and to the 
building as a whole. Even given these difficulties, the document has become the 
standard of the industry.
	Development of a Standardized Regional Loss Estimation MethodologyHAZUS
	National Institute of Building Sciences, Earthquake Loss Estimation 
Methodology, Technical Manual (for latest release).
	A good summary of this development is contained in a paper by Whitman, et 
al. in EERI Earthquake Spectra, vol 13, no 4. FEMA also maintains a HAZUS 
website, www.fema.gov/hazus.  
	The development of a standardized regional loss estimation methodology was 
not in the original FEMA plan to reduce risks in existing buildings. However, 
this development has had a major impact in educating local officials about their 
seismic risks and estimating the level of risk around the country in a standard 
and comparable way.
	In 1990, when the development of HAZUS began, the primary goals were to 
raise awareness of potential local earthquake risks, to provide local emergency 
responders with reasonable descriptions of post-earthquake conditions for 
planning purposes, and to provide consistently created loss estimates in various 
regions to allow valid comparison and analysis. The loss estimation methodology 
was intended to be comprehensive and cover not only building losses, but also 
damage to transportation systems, ports, utilities, and critical facilities. A 
technically defensible methodology was the goal, not necessarily an all-
encompassing software package. When it became obvious that the methodology was 
far more useful and could be more consistently applied as software, HAZUS was 
born. The program uses census data and other available physical and economic 
databases to develop, on a first level of accuracy, a model of local conditions. 
Expected or speculated seismic events can be run and losses estimated. Losses 
include direct damage, business interruption, and casualties, as well as loss of 
utilities, loss of housing units, and many other parameters of use to emergency 
planners. The building inventory uses the FEMA model building types and an 
analysis method closely tied to FEMA 273, linking HAZUS to other FEMA-sponsored 
work regarding existing buildings.
	Subsequent to the original development activity, HAZUS was expanded to 
create loss estimates for wind and flood.  
	Incremental Rehabilitation
	FEMA 395, Incremental Seismic Rehabilitation of School Buildings (K-12), 
2003.
	This is the first in a series of manuals that FEMA (U.S. Department of 
Homeland Security) intends to develop for various occupancy types including, for 
example, schools, hospitals, and office buildings.  The concept is based on the 
fact that seismic strengthening activities are more efficiently accomplished in 
conjunction with other work on the building, and such opportunities should be 
identified and exploited even if only part of a complete rehabilitation is 
accomplished. This is perhaps most applicable to K-12 school buildings because 
of their relatively small size and ongoing maintenance programs.  FEMA model 
building types are again used to categorize potential opportunities in different 
conditions. As is pointed out in the manual, this technique has to be applied 
with care to avoid an intermediate structural condition that is worse than the 
original.

8.3.2	The FEMA Model Building Types
Most of these developments were part of the integrated plan developed in 1985.  
As such, FEMA coordinated the projects and required common terminology and 
cross-references.
The most successful and virtually standard-setting effort was the creation of a 
set of model building types to be used for the characterization of existing 
buildings. The model building types are based primarily on structural systems 
rather than occupancy, but have proven extremely useful in the overall program.  
Model building types are defined by a combination of the gravity-load carrying 
system and the lateral-load carrying system of the building. Not every building 
type ever built in the country and certainly not the world is represented, but 
the significant ones are, and the relative risks of a community can well be 
represented by separating the local inventory into these types. Of course, there 
was no attempt to represent every modern building type because they are not 
considered hazardous buildings.  However, with minor sub-categorization that has 
occurred with successive documents, the majority of buildings, new or old, now 
can be assigned a model building type.  The test of the usefulness came with the 
successful development of HAZUS using the model building type because this 
program needed a reasonably simple method to characterize the seismic 
vulnerability of inventories of buildings across the country.
Currently, no single FEMA document contains a graphic and clear description of 
the model building types, although engineers can generally determine the correct 
category.  Because of the ubiquitous FEMA-developed documents, guidelines, and 
standards regarding existing buildings, and their common use by engineers, such 
descriptions are included here to facilitate communication with architects. The 
types are illustrated on pages 8-23 through 8-31. Table 8-3, at the end of the 
chapter, presents a summary of the performance charactoristics and commom 
rehabilitation techniques.  
8.4 SEISMIC EVALUATION OF EXISTING BUILDINGS
Not all older buildings are seismically at risk. If they were, the damage from 
several earthquakes in this country, including the 1971 San Fernando and the 
1984 Northridge events, would have been devastating, because much of the 
inventory affected was twenty or more years old. Often, strong ground shaking 
from earthquakes significantly damages building types and configurations well 
known to be vulnerable, and occasionally highlights vulnerabilities previously 
unrealized. For example, the Northridge earthquake caused damage to many wood-
frame buildingsmostly apartmentsand relatively modern steel moment-frame 
buildings, both previously considered to be of low vulnerability. It is natural 
to catalogue damage after an earthquake by buildings with common 
characteristics, the most obvious characteristic being the construction type, 
and the secondary characteristic being the configuration. Both of these 
parameters are central to processes developed to identify buildings especially 
vulnerable to damage before the earthquake. In fact, the categorization of 
damage by building type is primarily what led to the development of the FEMA 
Model Building Types discussed in Section 8.3.2.
However, only in the most vulnerable building types does damage occur relatively 
consistently.  For example, at higher levels of shaking, the exterior walls of 
unreinforced masonry bearing-wall buildings have relatively consistently fallen 
away from their buildings in many earthquakes, ever since this building type was 
built in large numbers in the late 19th century. More recently, a high 
percentage of pre-Northridge steel moment-frame buildings have received damage 
to their beam-column connections when subjected to strong shaking.  Even in 
these cases, the damage is not 100% consistent and certainly not 100% 
predictable.  In building types with less vulnerability, the damage has an even 
higher coefficient of variation.  Engineers and policymakers, therefore, have 
struggled with methods to reliably evaluate existing buildings for their seismic 
vulnerability.
As discussed in Section 8.2, the initial engineering response was to judge older 
buildings by their capacity to meet the code for new buildings, but it became 
quickly apparent that this method was overly conservative, because almost every 
building older than one or two code-change cycles would not complyand thus be 
considered deficient.  Even when lower lateral force levels were used, and the 
presence of archaic material was not, in itself, considered a deficiency, many 
more buildings were found deficient than was evidenced in serious earthquake 
damage.  Thus, policymakers have generally been successful in passing active 
retrofit provisions (see Section 8.2.3) only in the most vulnerable buildings, 
such as URM and tilt-ups, where damage has been significant and consistent, and 
individual building evaluation is not particularly significant.
The evaluation of existing buildings typically starts with identification of the 
building type and damaging characteristics of configuration (e.g., soft story).  
This can be done rapidly and inexpensively but, except for a few vulnerable 
building types, is unreliable when taken to the individual building level.  
Engineers and code writers have also developed intermediate levels of evaluation 
in which more characteristics are identified and evaluated, many by calculation.  
In the last decade, more sophisticated methods of analysis and evaluation have 
been developed that consider the nonlinear response of most structures to 
earthquakes and very detailed material and configuration properties that will 
vary from building to building.
 8.4.1 Expected Performance by Building Type
As previously mentioned, damage levels after earthquakes are collected and 
generally assigned to bins of common characteristics, most commonly the level of 
shaking, building material and type, and configuration.  Combined with numerical 
lateral-force analysis of prototype buildings, this information can be analyzed 
statistically.  The three primary parameters -  building type, shaking level, 
and damage level - are often displayed together in a damage probability matrix 
similar to Table 8-1.  The variability of damage is such that for any shaking 
level, as shown in the columns, there is normally a probability that some 
buildings will be in each damage state.  The probabilities in these tables can 
be interpreted as the percentage of a large number of buildings expected to be 
in each damage state, or the chances, given the shaking level, that an 
individual building of this type with be damaged to each level.
Statistical information such as this is used in several ways:
	Identification of clearly vulnerable or dangerous buildings to help 
establish policies of mitigation
Many extremely vulnerable building types or components can be identified by 
observation without statistical analysis, including URM, soft-story tuck-under 
apartment buildings, the roof-to-wall connection in tilt-up buildings, 
residences with cripple wall first-floor construction, and connections of pre-
Northridge steel moment-frames.  The clearly and more consistently dangerous 
building types have often generated enough community concern to cause the 
creation of policies to mitigate the risks with retrofit.  For a combination of 
reasons, URMs and tilt-ups currently are the targets of the most active 
mitigation policies.
	Earthquake Loss Estimation
Regional earthquake loss estimates have been performed for forty or more years 
to raise awareness in the community about the risks from earthquakes and to 
facilitate emergency planning.  Given an approximate distribution of the 
building inventory and a map of estimated ground motion from a given earthquake, 
damage-probability matrices (or similar data) can be used to estimate damage 
levels to the building stock.  From the damage levels, economic loss, potential 
casualties, and business interruption in a community can be estimated.
Starting in 1991, FEMA began a major program to develop a standard way of 
performing such loss estimations to facilitate comparative loss estimates in 
various parts of the country.  This program resulted in a computer program, 
HAZUS, described briefly in Section 8.3.1.
	Formal Economic Loss Evaluations (e.g. Probable Maximum Loss or PML)
Since consensus loss relationships became available (ATC, 1985), a demand has 
grown to include an estimate of seismic loss in due-diligence studies done for 
purchase of buildings, for obtaining loans for purchase or refinance, or for 
insurance purposes.  An economic loss parameter, called Probable Maximum Loss, 
has become the standard measuring stick for these purposes.  The PML for a 
building is the pessimistic loss (the loss suffered by the worst 10% of similar 
buildings) for the worst shaking expected at the site (which gradually became 
defined as the shaking with a 500-year return period, similar to the code design 
event).  Although a detailed analysis can be performed to obtain a PML, most are 
established by building type and a few observable building characteristics.  
Because of the high variability in damage and the relatively incomplete 
statistics available, PMLs are not very reliable, particularly for an individual 
building.
	Rapid Evaluation
As foreseen by FEMAs original plan for the mitigation of risks from existing 
buildings, a rapid evaluation technique should be available to quickly sort the 
buildings into three categories: obviously hazardous, obviously acceptable, and 
uncertain.  The intent was to spend less than two hours per building for this 
rapid evaluation.  Under the plan, the uncertain group would then be evaluated 
by more detailed methods.  The results of FEMAs development efforts, FEMA 154 
(Section 8.3.1) is fairly sophisticated but, because of the large amount of 
unknown building data that is inherent in the system, for an individual 
building, is unreliable.  The sorting method is probably quite good for 
estimating the overall vulnerability of a community because of the averaging 
effect when estimating the risk of many buildings.
8.4.2 Evaluation of Individual Buildings
Engineers have been seismically evaluating existing buildings for many years, 
whether by comparing the conditions with those required by the code for new 
buildings, by using some local or building-specific standard (e.g. URMs), or by 
using their own judgment.  These methods are still used, as well as very 
sophisticated proprietary methods developed within private offices, but the 
majority of evaluations are now tied in some way to the general procedures of 
ASCE 31-03, Seismic Evaluation of Existing Buildings (ASCE, 2003), that in 2003 
became a national standard.  There are three levels of evaluation in the 
standard called tiers, which, not accidentally, are similar to the standard of 
practice prior to the standardization process.  These levels of evaluation are 
briefly described below, as well as similar methods that fall in the same 
categories.
However, before beginning a seismic evaluation, particularly of a group of 
buildings, it is logical to assume that buildings built to modern codes must 
meet some acceptable standard of life safety.  Due to the large number of older 
buildings, the effort to eliminate some from consideration resulted in several 
well-known milestone years.  First it was compliance with the 1973 Uniform 
Building Code (UBC) or equivalent.  After study and reconsideration of 
relatively major changes made in the 1976 UBC, this code was used as a 
milestone.  Primarily caused by life-threatening damage to various building 
types in the 1994 Northridge earthquake and subsequent code changes, a 
relatively complex set of milestone years was developed.  ASCE 31 contains such 
a set of code milestone years for each of the several codes used in this country 
over the last thirty years.  Although ASCE 31 suggests that compliance with 
these codes is only a recommended cut-off to not require evaluations for life 
safety, in all but very unusual situations, the table can be accepted.  This 
table, Table 3-1 of ASCE 3,1 is reproduced as Figure 8-7.
	Initial Evaluation (ASCE 31 Tier 1)
The ASCE Tier 1 evaluation is similar to FEMAs Rapid Evaluation in that it is 
based on the model building type and certain characteristics of the building.  
The significant difference is that structural drawings, or data equivalent to 
structural drawings, are required to complete the evaluation, and the evaluation 
will take several days rather than several hours.  After identifying the 
appropriate FEMA Building Type, a series of prescriptive requirements are 
investigated, most of which do not require calculations.
If the building is found to be noncompliant with any requirement, it is 
potentially seismically deficient.  After completing the investigation of a 
rather exhaustive set of requirements, the engineer reviews the list of 
requirements with which the building does not comply, and decides if the 
building should be categorized as noncompliant or deficient.  A conservative 
interpretation of the method is that any single noncompliance is sufficient to 
fail the building, but most engineers exercise their judgement in cases of 
noncompliance with only a few requirements. Historically, this method has 
developed with the pass/fail criterion of life safety, but the final ASCE 
Standard includes criteria for both Life Safety and Immediate Occupancy, a 
performance more closely related to continued use of the building.  Because of 
the importance associated with the Immediate Occupancy performance level, a 
building cannot pass these requirements with only a Tier 1 analysis.
	Intermediate Evaluation (ASCE 31 Tier 2)
The ASCE intermediate level of evaluation, called Tier 2, is similar in level of 
effort of historical nonstandardized methods.  Normally, an analysis of the 
whole building is performed and the equivalents of stress checks are made on 
important lateral force-resisting components.  This analysis is done in the 
context and organization of the set of requirements used in Tier 1, but the 
process is not unlike seismic analysis traditionally performed for both 
evaluation and design of new buildings.  ASCE 31 includes the requirements for 
both the LifeSafety and Immediate Occupancy performance levels for a Tier 2 
Evaluation.
   Detailed Evaluation (ASCE 31 Tier 3)
The most detailed evaluations are somewhat undefined because there is no ceiling 
on sophistication or level of effort.  The most common method used in Tier 3 is 
a performance evaluation using FEMA 356, Prestandard and Commentary for the 
Seismic Rehabilitation of Buildings (FEMA, 2000), based on simplified nonlinear 
analysis using pushover analysis (see Chapter 6).  This method approximates the 
maximum lateral deformation that the building will suffer in a design event, 
considering the nonlinear behavior created by yielding and damage to components.  
The level of deformation of individual components is compared with standard 
deformations preset to performance levels of Collapse Prevention, Life Safety, 
and Immediate Occupancy, with the probable damage state of the building as a 
whole set at that level of the worst component.  Efforts are being made to more 
realistically relate the damage states of all the components to a global damage 
state.  This method can be used either to determine the probable damage state of 
the building for evaluation purposes, or to check the ability of a retrofit 
scheme to meet a target level.
With the advancement of computer capability and analysis software, nonlinear 
analysis techniques are constantly being improved.  The ultimate goal, although 
not expected to be an everyday tool in the near future, is to simulate the 
movements of the full buildings during an entire earthquake, including the 
constantly changing properties of the structural components due to yielding and 
damage.  The overall damage to various components is then accumulated and the 
global damage state thereby surmised.
8.4.3 Other Evaluation Issues
There are several other issues associated with seismic evaluation that should be 
recognized.  Only three will be discussed here.  First is the data required to 
perform competent evaluations at the various levels, as discussed above.  
Second, it is important to understand the performance expectation of the pass-
fail line for various evaluation methodologies.  Last, the reliability (or lack 
thereof) of the methods and of evaluation and/or performance prediction in 
general,should be recognized.
  Data Required for Seismic Evaluation
Obviously, for methods depending on the FEMA building type, the building type 
must be known.  In fact, there are other similar classifications of building 
types also used to define building performance at the broadest level.  Using 
data as discussed in paragraph 8.4.1, crude expectations of performance and 
therefore comparative evaluation can be completed.  Most such systems, however, 
are refined by age, physical condition of the building, configuration, and other 
more detailed data, when available.  Most rapid evaluation methods, based on 
building type and very basic building characteristics, do not require structural 
drawings.  Responsible evaluators will insist on a site visit (in many cases to 
make sure the building is still there, if nothing else).
The more standardized evaluation methods discussed in paragraph 8.4.2 
essentially require drawings.  If detailed structural drawings are not 
available, simple evaluations of some model building types (wood buildings, 
tilt-ups, and sometimes URM) can be performed based on layout drawings or from 
data prepared from field visits.  However, when reinforced concrete, reinforced 
masonry, or structural steel is a significant part of the structure, it is most 
often economically infeasible to reproduce as-built drawings.  Practically in 
those cases, with rare exceptions, the building is deduced to be in 
nonconformance and, as a retrofit, a new seismic system is introduced to render 
the unknowns of the existing structure insignificant.  Even in those cases, 
however, extensive field work is necessary to produce enough structural data to 
create a reasonable set of construction documents.
If original structural drawings are available that are confirmed to be 
reasonably accurate from spot checks in the field, most evaluation techniques 
can be employed.  However, material properties are often not included on the 
drawings and must be deduced from the era of construction.  Deterioration can 
also affect the material properties of several building types.  Often the 
potential variability in the analysis due to different possible combinations of 
material properties requires in-situ testing of material properties.  The 
techniques for this testing are well established, but cost and disruption to 
tenants are often an issue.
As explained in Chapter 3, Site Evaluation and Selection, many areas of the 
country are mapped in detail for seismic parameters related to design, although 
such parameters continue to be investigated and updated.  When warranted, site-
specific studies can be performed to obtain timely and locally derived data.  
However, other seismic site hazards, such as liquefaction, landslide, and 
potential surface fault rupture, are less well mapped and may require a site-
specific study, if there is reason to suspect their potential at a site.  
Perhaps a less obvious important characteristic of a site is the detail of 
adjacent structures.  Particularly in urban settings, adjacent buildings often 
have inadequate separation or are even connected to the building to be 
evaluated.  Although legal issues abound when trying to deal with this issue, it 
is unrealistic to analyze and evaluate such a building as if it were 
freestanding.  Formal evaluation techniques, such as ASCE 31, have addressed 
this issue, at least for buildings that are not connected, by highlighting the 
conditions known to potentially produce significant damage.  First, if floors do 
not align between adjacent buildings and pounding is expected, the stiff floor 
from one building could cause a bearing wall or column in the adjacent building 
to collapse.  Secondly, if buildings are of significantly different height, the 
interaction from pounding has been observed to cause damage.  See Figure 8-8.
  Performance Objectives and Acceptability
Traditionally, evaluation techniques have been targeted at determining if a 
building is adequately life safe in an earthquake, similar to code goals for new 
buildings.  However, as discussed in section 8.2.2, a standard different and 
less than that used for new buildings evolved, but was still termed life safety. 
Only with the development of performance-based engineering did evaluation 
methods aimed at other performance standards emerge.  Even life safety has 
proven to be amorphous over the years and often has been defined by the 
evaluation technique du jour.  Seismically, life safety is a difficult concept, 
due to the huge potential variation in ground motion and the many sources of 
damage that could cause injury or death.  However, the term is well embedded in 
public policy and continues to persist in seismic codes and standards.
FEMA, in sponsoring the development of FEMA 273 (and later FEMA 356), wanted a 
more specific definition of a suitable goal for seismic safety, and thus the 
Basic Safety Objective (BSO) was defined.  This performance objective consists 
of two requirements: the building would provide life safety for the standardized 
code event and, in addition, the building would not collapse in the Maximum 
Considered Event (MCE), a very rare event now defined by code.  Since these FEMA 
documents are non-mandatory (unless locally adopted), the BSO has not become a 
widely accepted standard (the BSO also includes mandatory nonstructural minimum 
requirements, which also may delay its wide acceptance).
Chapter 6 contains a detailed discussion of performance-based engineering, which 
is gaining acceptance for evaluations at any level. But performance 
characterization in various forms has been used for some time, primarily to set 
policy.  Such policies require descriptions of various performance levels, even 
if the technical ability to define or predict the various levels often lagged 
behind.  Table 8-2 shows several such performance descriptions, many developed 
decades ago, which have been used to set policymost concentrating on life 
safety.  The table is set up to approximately equilibrate levels of performance 
across horizontal lines.
The first columns in Table 8-2 describe a system used by the University of 
California.  GOOD, the best performance, is defined as the equivalent life 
safety as that provided by the code for new buildingsbut without consideration 
of monetary damage.  The next level below was set at the acceptable level for 
evaluation, while retrofits are required to meet the GOOD level.
The next column, labeled DSA, is a Roman numeral system developed by the 
California Division of the State Architect for use with state-owned buildings.  
Each level has a description of damage and potential results of damage 
(building not reocccupied for months) but no reference to engineering 
parameters.  The state used an acceptance level of IV, but set the goal for 
retrofits to III.
The levels described in the next columns come from one of the early developments 
of performance based earthquake engineering, Vision 2000, developed by the 
Structural Engineers Association of California.  It is a relatively 
comprehensive scale using five primary descriptions of damage, each with a 
plus and minus, resulting in ten levels.  
Finally, to indicate a perhaps more commonly recognized standard of performance, 
are the three occupancy tagging levels of Red, Yellow, and Green used for 
emergency evaluation immediately after damaging earthquakes.
  Reliability of Seismic Evaluations
The most significant characteristic in the design of buildings for earthquakes 
is the variability of ground motions.  Not only do magnitudes and locations 
vary, but also the effects of fault rupture, wave path, and local site soils 
create a literally infinite set of possible time histories of motion.  Studies 
have shown that time histories within a common family of parameters used for 
design (response spectrum) can produce significantly different responses.  This 
variation normally dominates over the scatter of results from analysis or 
evaluation techniques.
However, codes for new buildings can require many limitations of material, 
lateral system, configuration, and height that will reasonably assure acceptable 
performance, particularly the prevention of collapse.  These same limitations 
can seldom be applied to existing buildings, so the variation of actual 
performance is expected to be much larger.  In addition, the cost of retrofit is 
often high, and attempts have been made to avoid unnecessary conservatism in 
evaluation methodologies.  It is probable, therefore, that a significant number 
of buildings may fail to perform as evaluated, perhaps in the range of 10% or 
more.  No comprehensive study has been made to determine this reliability, but 
ongoing programs to further develop performance-based seismic engineering are 
expected to estimate the variability of evaluation results and refine the 
methods accordingly.
8.5 SEISMIC REHABILITATION OF EXISTING BUILDINGS
There are many reasons why buildings might be seismically retrofitted, including 
renovations that trigger a mandatory upgrade, a building subjected to a 
retroactive ordinance, or the owner simply wanting (or needing) improved 
performance.  The reason for the upgrade may influence the technique and 
thoroughness of the work, because owners faced with mandatory upgrade may seek 
out the least expensive, but approvable, solution, whereas an owner needing 
better performance will more likely be willing to invest more for a solution 
that addresses their particular concerns.  There are many other factors that 
shape a retrofit solution, such as the type of deficiency present, if the 
building is occupied, and the future use and aesthetic character of the 
building.
The continuing improvement of analysis techniques and the emergence of 
performance-based design are also having a large effect on retrofit schemes, by 
enabling engineers to refine their designs to address the specific deficiencies 
at the desired performance level.  In many cases, however, the retrofits are 
becoming controlled by the brittleness of existing components that must be 
protected from excess deformation with systems that may be stronger and/or 
stiffer than those used for new buildings.  Some older retrofits, done to 
prescriptive standards or using now-outdated strengthening elements borrowed 
from new building designs, may themselves be deficient, depending on the desired 
performance.  Seismic retrofit analysis, techniques, and components, similar to 
new building technology, are not static, and applications should be regularly 
reviewed for continued effectiveness.
8.5.1 Categories of Rehabilitation Activity
In most cases, the primary focus for determining a viable retrofit scheme is on 
vertically oriented components (e.g. column, walls, braces, etc.) because of 
their significance in providing either lateral stability or gravity-load 
resistance.  Deficiencies in vertical elements are caused by excessive inter-
story deformations that either create unacceptable force or deformation demands.  
However, depending on the building type, the walls and columns may be adequate 
for seismic and gravity loads, but the building is inadequately tied together, 
still forming a threat for partial or complete collapse in an earthquake.  It is 
imperative to have a thorough understanding of the expected seismic response of 
the existing building, and all of its deficiencies to design an efficient 
retrofit scheme.  There are three basic categories of measures taken to retrofit 
a building:
1)	Modification of global behavior, usually decreasing deformations (drifts);
2)	Modification of local behavior, usually increasing deformation capacity;
3)	Connectivity, consisting of assuring that individual elements do not 
become detached and fall, assuring a complete load path, and assuring that the 
force distributions assumed by the designer can occur.
The types of retrofit measures often balance one another, in that employing more 
of one will mean less of another is needed.  It is obvious that providing added 
global stiffness will require less deformation capacity for local elements (e.g. 
individual columns), but it is often less obvious that careful placement of new 
lateral elements may minimize a connectivity issue such as a diaphragm 
deficiency.  Important connectivity issues such as wall-to-floor ties, however, 
are often independent and must be adequately supplied. 
 Modification of Global Behavior
Modification to global behavior normally focuses on deformation, although when 
designing to prescriptive standards, this may take the form of adding strength. 
Overall seismic deformation demand can be reduced by adding stiffness in the 
form of shear walls or braced frames.  Addition of moment frames is normally 
ineffective in adding stiffness.  New elements may be added or created from a 
composite of new and old components.  Examples of such composites include 
filling in openings of walls and using existing columns for chord members for 
new shear walls or braced frames. 
Particular ground motions have a very specific deformation demand on structures 
with various periods, as discussed in Chapter 4.  Given an equal period of 
vibration, this deformation will occur, whether distributed over the height of 
the building or concentrated at one floor.  If one or more inter-story drifts 
are unacceptable, it may be possible to redistribute stiffness vertically to 
obtain a more even distribution of drift.  A soft or weak story is an extreme 
example of such a problem.  Such stories are usually eliminated by adding 
strength and stiffness in such a way as to more closely balance the stiffness of 
each level, and thus evenly spread the deformation demand over the height of the 
structure.
Seismic isolation is the supreme example of the concept of redistribution of 
deformation.  Essentially all deformation is shifted to bearings, placed at the 
isolation level, that are specifically designed for such response.  The bearings 
limit the response of the superstructure, which can be designed to remain 
essentially undamaged for this maximum load.  The feasibility of providing 
isolation bearings that limit superstructure accelerations to low levels not 
only facilitates design of superstructures to remain nearly elastic, but also 
provides a controlled environment for design of nonstructural systems and 
contents.
Global deformations can also be controlled by the addition of passive energy 
dissipation devices, or dampers, to the structure.  Although effective at 
controlling deformations, large local forces may be generated at the dampers 
that must be transferred from the device to structure and foundation, and the 
disruptive effect of these elements on the interior of the building is no 
different than a rigid brace.
  Modification of Local Behavior
Rather than providing retrofit measures that affect the entire structure, 
deficiencies also can be eliminated at the local component level.  This can be 
done by enhancing the existing shear or moment strength of an element, or simply 
by altering the element in a way that allows additional deformation without 
compromising vertical-load carrying capacity.
Given that in most cases, that certain components of the structure will yield 
(i.e., become inelastic), some yielding sequences are almost always benign: 
beams yielding before columns, bracing members yielding before connections, and 
bending yielding before shear failure in columns and walls.  These relationships 
can be determined by analysis and controlled by local retrofit in a variety of 
ways.  Columns in frames and connections in braces can be strengthened, and the 
shear capacity of columns and walls can be enhanced to be stronger than the 
shear that can be delivered.
Concrete columns can be wrapped with steel, concrete, or other materials to 
provide confinement and shear strength.  Concrete and masonry walls can be 
layered with reinforced concrete, plate steel, and other materials.  Composites 
of glass or carbon fibers and epoxy are becoming popular to enhance shear 
strength and confinement in columns, and to provide strengthening to walls. 
Another method to protect against the collapse risk posed by excess drift is to 
provide a supplementary gravity support system for elements that might be 
unreliable at expected high-deformation levels.  For example, supplementary 
support for concentrated wall-supported loads is a requirement in California 
standards for retrofit of unreinforced masonry buildings.  In several cases, 
supplementary support has also been used in concrete buildings.
Lastly, deformation capacity can be enhanced locally by uncoupling brittle 
elements from the deforming structure, or by removing them completely.  Examples 
of this procedure include placement of vertical saw cuts in unreinforced masonry 
walls to change their behavior from shear failure to a more acceptable rocking 
mode, and to create slots between spandrel beams and columns to prevent the 
column from acting as a short column prone to shear failure.
 Connectivity
Connectivity deficiencies are within the load path: wall out-of-plane connection 
to diaphragms; connection of diaphragm to vertical lateral force-resisting 
elements; connection of vertical elements to foundation; connection of 
foundation to soil.  A complete load path of some minimum strength is always 
required, so connectivity deficiencies are usually a matter of degree.  A 
building with a complete but relatively weak or brittle load path might be a 
candidate for retrofit by seismic isolation to simply keep the load below the 
brittle range.
The only location in the connectivity load path at which yielding is generally 
allowed is the foundation/structure interface.  Allowing no movement at this 
location is expensive and often counterproductive, as fixed foundations transfer 
larger seismic demands to the superstructure.  Most recently developed retrofit 
guidelines are attempting to provide simplified guidance to the designer on how 
to deal with this difficult issue and minimize foundation costs.
8.5.2 Conceptual Design of a Retrofit Scheme for an Individual Building
There are many specific methods of intervention available to retrofit designers, 
as previously discussed.  The selection of the specific type of element or 
system is dependent on local cost, availability, and suitability for the 
structure in question.  Any system used to resist lateral load in new buildings 
can also be used for retrofit.  It is thus an extensive task to develop 
guidelines for such selection.  In addition, as in the design of a new building, 
there is usually a choice of where to locate elements, although it is generally 
more restrictive in existing buildings.  However, in the end, there are 
nonseismic issues associated with each building or project that most often 
control the specific scheme to be used.
The solution chosen for retrofit is almost always dictated by building  user-
oriented issues rather than by merely satisfying technical demands.  There are 
five basic issues that are always of concern to building owners or users: 
seismic performance, construction cost, disruption to the building users during 
construction (often translating to a cost), long-term affect on building space 
planning, and aesthetics, including consideration of historic preservation.
All of these characteristics are always considered, but an importance will 
eventually be put on each of them, either consciously or subconsciously, and 
these weighting factors invariably will determine the scheme chosen. 
	Seismic performance
	Prior to the emphasis on performance-based design, perceived qualitative 
differences between the probable performance of difference schemes were used to 
assist in choosing a scheme.  Now, specific performance objectives are often set 
prior to beginning development of schemes.  Objectives that require a very 
limited amount of damage or continued occupancy will severely limit the 
retrofit methods that can be used and may control the other four issues.
	Construction cost
	Construction cost is always important and is balanced against one or more 
other considerations deemed significant.  However, sometimes other economic 
considerations, such as the cost of disruption to building users, or the value 
of contents to be seismically protected, can be orders of magnitude larger than 
construction costs, thus lessening its importance.
	Disruption to the building users during construction
	Retrofits are often done at the time of major building remodels, and this 
issue is minimized.  However, in cases where the building is partially or 
completely occupied, this parameter commonly becomes dominant and controls the 
design.
	Long-term effect on building space planning
	This characteristic is often judged less important that the other four and 
is therefore usually sacrificed to satisfy other goals.  In many cases, the 
planning flexibility is only subtly changed.  However, it can be significant in 
building occupancies that need open spaces, such as retail spaces and parking 
garages.
	Aesthetics
	In historic buildings, considerations of preservation of historic fabric 
usually control the design.  In many cases, even performance objectives are 
controlled by guidelines imposed by preservation.  In non historic buildings, 
aesthetics is commonly stated as a criterion, but in the end is often 
sacrificed, particularly in favor of minimizing cost and disruption to tenants.
These parameters can merely be recognized as significant influences on the 
retrofit scheme or can be used formally to compare schemes.  For example, a 
comparison matrix can be developed by scoring alternative schemes in each 
category and then applying a weighting factor deduced from the owners needs to 
each category.
Figure 8-9 describes the evolution of a retrofit scheme based on several changes 
in the owners weighting of these five characteristics.
8.5.3 Other Rehabilitation Issues
  Inadequate recognition of disruption to occupants
It is unfortunately common for the extent of interior construction and 
disruption to be underestimated.  In many cases, occupants who were originally 
scheduled to remain in place are temporarily movedat a significant increase in 
cost of the projector the work is required to be done in off-hours, also a 
premium cost.  Figures 8-10 and 8-11 indicate the level of construction 
intensity often required in retrofit.
Similarly, exterior solutions, where strengthening elements are placed on the 
outside of the building are often more disruptive and noisier than anticipated 
and often require collector members to be placed on each floor within the 
building.  Figure 8-12 shows the result of an exterior retrofit of adding towers 
on the outside of a building that, in fact, did not cause a single lost day of 
occupancy.  High-strength steel rods were epoxied into horizontal cores, drilled 
twenty feet into the existing concrete beams to form the needed collectors.
 Collateral required work
As previously mentioned, retrofit work is often performed in conjunction with 
other remodeling or upgrading activities in a building.  Such work normally 
triggers other mandatory improvements to the building, such as ADA compliance or 
life safety updatingall of which add cost to the project.  However, even when 
seismic retrofit is undertaken by itself, the costs of ADA compliance, removal 
of disturbed hazardous material, and possibly life safety upgrades must be 
considered.
8.5.4	Examples
It is impossible to include examples that show the full range of structural 
elements and configurations used in seismic retrofit.  There are definitely 
patterns, usually driven by economics or avoidance of disruption to occupants, 
but depending on the particular mix of owner requirements, as discussed in 
Section 8.5.2, thoughtful architects and engineers will always come up with a 
new solution.
Photographs of retrofit buildings, although often interesting, seldom can tell 
the full story of the development of the scheme, and if the majority of retrofit 
elements are inside or hidden, tell almost nothing.  Some photos are shown here, 
but are not intended to demonstrate the full range of buildings that have 
successfully undergone seismic retrofit or the full range of solutions to 
individual problems.  In addition, due to limited space, only one or two points 
are made with each photo, rather than a full case study.
8.6   SPECIAL ISSUES WITH HISTORIC BUILDINGS
Seismic evaluation and retrofit of historic buildings generate complex public 
policy issues for which few general rules can be identified.  Restoration, or 
renovations of large and important historic buildings usually have considerable 
public and jurisdictional oversight, in addition to employing an experienced 
design team that includes a special historic preservation consultant.  The 
control and oversight for less important buildings that have historic status at 
some level, or that may qualify for such status, are highly variable.  Designers 
are cautioned to locally investigate approval procedures for alterations on such 
buildings as well as seismic requirement, for them.
8.6.1	Special Seismic Considerations
It has been recognized in most areas of high seismicity that local public policy 
concerning seismic retrofit triggers must include special considerations for 
historic buildings.  As discussed in Section 8.2, initial seismic safety 
criteria for existing buildings were focused on requirements for new buildings, 
which were marginally appropriate for most older buildings, but completely 
inappropriate for historic buildings.  Special allowances were therefore created 
for archaic materials that were not allowed in new buildings, and the overall 
seismic upgrade level was lowered to reduce work that could compromise historic 
integrity and fabric.  These kinds of technical criteria issues have been 
somewhat mitigated by the completion of FEMA 356 and the emergence of 
performance-based earthquake engineering, because consideration of archaic 
materials and fine-tuning of performance levels are now part of the normal 
lexicon.
8.6.2	Common Issues of Tradeoffs
Many buildings in this country that qualify for historic status are not 
exceptionally old and can be made commercially viable.  The changes that are 
needed for successful adaptive reuse will often conflict with strict 
preservation guidelines, and compromises are needed in both directions to 
achieve a successful project that, in the end, could save the building from 
continuing decay and make it more accessible to the public.  These tradeoffs 
occur in many areas of design, but seismic upgrading work often requires 
interventions that are not needed for any other reason.  These interventions 
often fall under historic preservation guidelines that call for clear 
differentiation of new structural components, or that discourage recreation of 
historic components that are removed.  As previously indicated, there are no 
rules for these conditions, and the most appropriate solution for each case must 
be determined individually.
Another common conflict is between current preservation of historic fabric and 
future preservation of the building due to the chosen seismic performance level. 
Typically, a better target performance in the future, possibly preventing 
unrecoverable damage, requires more seismic renovation work now.  Most historic 
preservation codes allow lower expected seismic performance to reduce 
construction work and minimize damage. Like many seismic policies, there have 
not been enough earthquakes with seismically damaged historic buildings to test 
this general philosophy.  In an ever-growing number of cases of important 
buildings, this dilemma has been addressed using seismic isolationwhich by 
reducing loading to the superstructure, reduces required construction work and 
also reduces expected damage in future earthquakes. Typically, however, 
installing isolation into an existing building is expensive and may require a 
significant public subsidy to make viable. Several high-profile city hall 
buildings such as San Francisco, Oakland, Berkeley (after the 1989 Loma Prieta 
earthquake), and Los Angeles (after the 1994 Northridge earthquake) have been 
isolated, with FEMA assistance as part of post earthquake damage repairs.
8.6.3 Examples of Historical Buildings
The following illustrations show samples of seismic retrofit of historic 
buildings with brief descriptive notes.  Complete discussion of the preservation 
issues and rehabilitation techniques of each case would be extensive and cannot 
be included here.
8.7	Conclusion
Table 8-3 summarizes common seismic deficiencies stemming from various site and 
configuration characteristics as well as those that might be expected in each 
FEMA model building type.  See Section 8.2 for a discussion of seismic 
deficiency as used in this chapter and this table.  Also included in Table 8-3 
are retrofit measures that are often used for each situation.

8.8  References

8.8.1 References from Text
American Society of Civil Engineers (ASCE), Seismic Evaluation of Existing 
Buildings, ASCE 31-03, 2003.
Applied Technology Council (ATC), Earthquake Damage Evaluation Data for 
California, ATC 13, 1985.
California Seismic Safety Commission (CSSC), Status of the Unreinforced Masonry 
Builidng Law, 2003 Report to the Legislature, SSC 2003-03, 2003.
Federal Emergency Management Agency (FEMA), Prestandard and Commentary for the 
Seismic Rehabilitation of Buildings, FEMA 356, 2000.
Holmes, William T., Background and History of the California Hospital Seismic 
Safety Program, Proceedings, Seventh National Conference on Earthquake 
Engineering, Boston, 2002, Earthquake Engineering Research Institute.
Hoover, Cynthia A., Seismic Retrofit Policies: An Evaluation of Local Practices 
in Zone 4 and Their Application to Zone 3, 1992, Earthquake Engineering Research 
Institute.
National Institute of Standards and Technology (NIST), Standards of Seismic 
Safety for Existing Federally Owned or Leased Buildings, ICSSC RP 6, 2002.
U. S. Department of Interior (Interior a), The Secretary of the Interiors 
Standards for Rehabilitation, Department of Interior Regulations, 36 CFR 67, U. 
S Department of the Interior, National Park Service, Preservation Assistance 
Division, Washington, D.C.
U. S. Department of Interior (Interior b), Guidelines for Rehabilitating 
Historic Buildings, U. S Department of the Interior, National Park Service, 
Preservation Assistance Division, 1977, Washington, D.C.
8.8.2 To Learn More
Applied Technology Council, Seismic Evaluation and Retrofit of Concrete 
Buildings, Report No. SSC 96-01, California Seismic Safety Commission, 1996. 
A contemporary of FEMA 356 that features many of the same methods and 
performance terminology. This document contains a good description of retrofit 
strategies.

Earthquake Engineering Research Institute Ad Hoc Committee on Seismic 
Performance, Expected Seismic Performance of Buildings, SP 10, Earthquake 
Engineering Research Institute, 1994.
This document was published slightly before FEMA 356 and thus contains slightly 
different performance terminology.  However, photo examples and extensive 
description of damage states are contained.  In addition, estimates are given 
for the approximate number of various buildings that would be expected to be in 
various damage states for different ground motion intensities.

Federal Emergency Management Agency, Region 1, Safeguarding Your Historic Site, 
Boston, MA.
This document contains an extensive bibliography covering renovation and repair 
of existing buildings.

Freeman, John R.  Earthquake Damage and Earthquake Insurance.  McGraw-Hill Book 
Company, Inc., 1932, New York.
Extremely interesting from a history standpoint, this book Includes discussion 
of seismology, geotechnical engineering, structural engineering, codes, and loss 
estimation, and excellent history and available data on earthquakes up to 1932.

Holmes, William T., Risk Assessment and Retrofit of Existing Buildings, 
Proceedings Twelve World Conference on Earthquake Engineering, Auckland, New 
Zealand, 2000.
This paper contains a more technically oriented description of the methods of 
FEMA 356 and strategies for design of retrofit systems.

[End of Chapter 8]



NONSTRUCTURAL DESIGN PHILOSOPHY BY GARY McGAVIN
9.1 INTRODUCTION
This chapter builds on the knowledge previously presented and discusses the 
issue of seismic design for nonstructural components and systems.  Initially, 
the primary purpose of seismic design was the desire to protect life safety. 
Buildings were designed so that the occupants could safely exit the facility 
following a damaging earthquake. Damage to buildings has always been allowed by 
the code, even to the extent that the building might need to be demolished 
following the event, even if correctly designed according to the seismic code. 
Until very recent years, with minor exceptions, nonstructural design has been 
minimally required by the model building codes.  
The lack of attention to nonstructural systems and their increasing complexity 
have resulted in the majority of dollar losses to buildings in recent 
earthquakes.  These losses are the result both of the direct cost of damage 
repair and of functional disruption while repairs are undertaken. Today, good 
seismic design requires that both structural and nonstructural design be 
considered together from the outset of the design process. Figures 9-1 and 9-2 
illustrate such a design.
Landers Elementary School was constructed and occupied just prior to the 
magnitude 7.3 Landers earthquake in 1992.  The building was situated just 0.4 
mile from approximately 10 feet of horizontal offset along the fault trace and 
experienced severe shaking. Nonstructural damage was minor and included cracked 
stucco and dislodged suspended ceiling tiles in the multipurpose room. In the 
magnitude 6.5 Big Bear earthquake, which occurred three hours after the Landers 
event, at the school a water line broke and a hot water heater restraining strap 
failed due to the incorrect use of lag bolts (too short and not anchored into 
the studs).  The hot water heater remained upright and functional.
As more building owners recognize the necessity to remain operational following 
a major event, architects will be called upon to provide designs that go beyond 
the minimal code requirements for life safety and exiting.  As our existing 
stock of older buildings is seismically retrofitted, the nonstructural 
components and systems must also be seismically retrofitted to the same level as 
the structure.	
9.2 WHAT IS MEANT BY THE TERM NONSTRUCTURAL
Nonstructural systems and components within a facility are all those parts of a 
building that do not lie in the primary load-bearing path of the building and 
are not part of the seismic resisting system.  In general, they are designed to 
support their own weight, which is then transferred to the primary structural 
system of the building.  The number and complexity of nonstructural systems and 
components far outnumber the structural components of a building. Figure 9-3 
shows the basic structural and nonstructural systems.  
While nonstructural components are not intended to contribute to seismic 
resistance, nature does not always respect this distinction. Rigid nonstructural 
walls spanning between structural columns will change the local stiffness of the 
structural system and alter its rsponse, possibly creating a stress 
concentration. Partitions may suddenly be called upon to perform a supporting 
role, as seen in Figure 9-4. Conversely, structural members may act in a 
nonstructural manner if, for example, the contractor omits placing the steel 
reinforcing in a reinforced concrete wall.
The number and complexity of nonstructural systems and components is very large. 
A typical broad categorization includes the following:
	Architectural
	Electrical
	Mechanical
	Plumbing
	Communications
	Contents and Furniture
A more specific list of nonstructural components based on the International 
Building Code (IBC) are categorized as architectural, mechanical, and electrical 
and is shown below. The IBC also provides design coefficients for each category 
that are applied to the component to establish the design seismic force.

9.2.1   Architectural Components

Interior nonstructural walls and partitions
Cantilever elements
    Parapets
    Chimneys
Exterior nonstructural wall elements and connections
    Light wall elements (metal insulated panels)
    Heavy wall elements (precast concrete)
       Body of panel connections
       Fasteners of the connecting systems
Veneer
    Limited deformability elements
    Low deformability elements
Penthouse (separate from main building structure)
Ceilings
    Suspended 
    Attached to rigid sub-frame
Cabinets
    Storage cabinets and laboratory equipment
Access floors
Appendages and ornamentation
Signs and billboards
Other rigid components
Other flexible components
9.2.2   Mechanical and Electrical Components

General mechanical
    Boilers and furnaces
    Pressure vessels freestanding and on skirts
    Stacks
    Large cantilevered chimneys
Manufacturing and process machinery
    General 
    Conveyors (nonpersonnel)
Piping system
    High deformability elements and attachments
    Limited deformability elements and attachments
    Low deformability elements and attachments
HVAC system equipment
    Vibration isolated
    Nonvibration isolated
    Mounted in-line with ductwork
Elevator components
Escalator components
Trussed towers (freestanding or guyed)
General electrical
    Distributed systems (bus ducts, conduit, cable trays)
    Equipment
Lighting fixtures
    Surface mounted to structure
    Suspended from structure
    Supported by suspended ceiling grid, surface mounted, or hung 	    from 
suspended ceiling

9.2.3 Consequences of Inadequate Nonstructural Design
Historically, the seismic performance of nonstructural systems and components 
has received little attention from designers. The 1971 San Fernando earthquake 
alerted designers to the issue mainly because well-designed building structures 
were able to survive damaging earthquakes while nonstructural components 
suffered severe damage. It became obvious that much more attention had to be 
paid to the design of nonstructural components. Some investigators have 
postulated that nonstructural system or component failure may lead to more 
injury and death in the future than structural failure. 
The following are the basic concerns for nonstructural system/component failure:
	Direct threat to life
	Indirect threat to life
	Loss of building function (loss of revenue and service)
	High repair costs 
9.3 NONSTRUCTURAL SEISMIC DESIGN AND NORMAL SEISMIC DESIGN
Designing for earthquakes has historically been the domain of the structural 
engineer. This publication has shown elsewhere what forces are brought to bear 
on buildings, how the building can be expected to respond due to the earthquake 
event, what effects the local soils have on the building, and how the building 
will transmit seismic forces from the foundation up through the structure of the 
building. By definition, the nonstructural systems and components of the 
building are attached to the buildings primary structure or, in the case of 
furniture and unsecured equipment/contents, rest unattached on the floors of the 
building. Seismic forces are generally amplified as they travel up from the 
foundation through the building to the top of the structure. These increased 
forces are transmitted to the nonstructural components at their interface with 
the structure. Many nonstructural systems and components are often very 
flexible, in contrast to the relatively rigid building structure. This 
flexibility often leads to a much higher level of excitation than the buildings 
primary structure.
9.4 EFFECTS OF IMPROPER NONSTRUCTURAL DESIGN
There are a number of objects that can directly cause either death or injury if 
they are not properly designed for restraint (Figures 9-5 and 9-6).  These 
injuries are generally due to falling hazards, such as large sections of plaster 
ceilings, HVAC registers, lights, filing cabinets, etc. There are also indirect 
threats to life and injury due to nonstructural failures. These might include 
the inability of occupants to safely exit a building due to damaged materials 
strewn across the stairs in exit stairwells (Figure 9-6).  
Although is this example was not a result of an earthquake, the MGM Grand Hotel 
Las Vegas fire in the 1980s serves to illustrate the complexity of nonstructural 
design. The fire activated the emergency power supply. The building construction 
included re-entrant corners that required a large seismic joint between building 
units. This joint in effect provided a chimney within the building that was not 
air tight between the floors.  The continued operation of the emergency power 
supply unit caused the asphyxiation of several occupants, since the HVAC system 
had its fresh air intake close to the exhaust of the emergency power. Thus, 
occupants died due to smoke inhalation and carbon monoxide poisoning in part due 
to the seismic design of the nonstructural systems. 
In some cases, nonstructural failure can cause a loss of function of the 
building. While this may not be critical for some building occupancies, it is 
very undesirable in others such as hospitals, emergency operations centers, 
police stations and communications centers. Unfortunately, failures are often 
caused by system or component interactions. In addition, more and more owners of 
commercial and industrial facilities are recognizing the need for continued 
operation in order to reduce financial loss following a damaging earthquake. 
Hospitals have a need for both continued function and reduction of economic 
loss. The owner-supplied equipment and contents within a hospital are often 
significantly more valuable than the building itself. Medications and bandages 
that are soaked due to flooding from broken fire sprinkler lines cannot be used 
when they are most needed. The sophisticated equipment within a hospital will 
take more time to repair, and be more costly, if damaged, than the equipment in 
an office building or a school.	
9.5 DAMAGE TO NONSTRUCTURAL SYSTEMS AND COMPONENTS
A lack of attention to detail during the design process is the most likely cause 
of damage to nonstructural systems and components in a moderate to severe 
earthquake. This damage poses a threat to the building occupants and may cause 
the owner significant losses in downtime and repairs. Examples in Table 9-1 
illustrate failures in earthquakes that resulted from inadequate nonstructural 
design.
Nonstructural design philosophy based on the analysis and design of individual 
components can lead to certain nonstructural failures in moderate and severe 
earthquakes. The new Olive View Hospital was rendered nonoperational in the 
Northridge earthquake due to such a philosophy. The building was a replacement 
for the previous hospital that was so badly damaged structurally in the 1971 
earthquake that it needed to be razed. The new replacement hospital was designed 
as a state-of-the-art facility, and as such, it should have remained operational 
during the 1994 earthquake. Figure 9-7 shows one of several systems interaction 
failures that caused the closure of the hospital. The building structural system 
supported the ceiling system and the fire sprinkler system. The codes require a 
component approach to seismic qualification for acceptance. The individual 
components that are analyzed include the ceiling, the lights set in the ceiling, 
the HVAC system that passes through the ceiling plane, and the main fire 
sprinkler feed pipe. The Olive View failure occurred when the building structure 
responded in one manner to the earthquake, the ceiling system and the systems 
that it supported shook in another manner, and finally the sprinkler system 
responded in a third manner. The result was a significant bending moment on the 
sprinkler drops when the ceiling impacted them, causing cross-thread bending at 
the joint where the drop connected to the main line. Each of the components was 
appropriately attached to the building as called for by the code. By not 
allowing either a flexible joint at the fire line drop or providing a larger 
hole where the sprinkler penetrates the ceiling plane, the failure of the system 
was virtually guaranteed.
Many of the failures found in the 1994 Northridge earthquake were a result of 
systems incompatibilities. It has been long realized that for building 
structures to survive an earthquake, there must be structural systems 
compatibility. Few designers would doubt the need for wall systems and roof 
systems to respond together in an earthquake. Yet, with respect to nonstructural 
considerations, the interactive nature of these systems has not been fully 
recognized, and thus, a $1.50 sprinkler pipe failure closed a hospital. Table 9-
2 shows examples of failures and success in the design of system interactions.
Systems need to be identified and have a seismic designation and qualification 
program just as an individual component. Facilities with sophisticated seismic 
qualification programs such as the Trans-Alaska Pipeline and nuclear power 
plants have always looked toward qualifying the entire nonstructural systems as 
well as the individual components.This type of procedure is especially important 
where specific functions must be maintained, such as with an emergency power 
supply in a hospital.
Moving equipment such as reciprocating pumps need isolation so as to not 
interject unwanted vibrations into the building structure. This isolation needs 
to be snubbed in order to limit the lateral excursions of the equipment.
9.6 DESIGN DETAILS FOR NONSTRUCTURAL DAMAGE REDUCTION
This section shows some examples of conceptual details for a number of typical 
nonstructural components.  These are intended to give an indication of 
appropriate design approaches and should not be used as construction 
documentation.  It will be seen that these approaches mostly consist of 
providing adequate support and  supplementary lateral bracing, or isolating 
nonstructural components from the building structure to reduce undesirable 
interaction between nonstructural and structural elements.  The isolation issue 
is discussed in more detail in Section 9.7
9.6.1	Precast Concrete Cladding Panels
Figure 9-8 shows a typical push-pull connection for a precast panel that spans 
between floors.  The bottom connection provides bearing; the top connection uses 
a steel rod that is designed to bend under lateral drift.  The rod must be 
strong enough, however, to resist out-of-plane wind loads.  The bearing 
connections may also be located at the top and the flexible connection at the 
bottom.
Figure 9-9 shows a typical layout of the supports for a story height panel and 
spandrel panels, and diagrams the lateral structural movement that must be 
accommodated.
9.6.2	Suspended Ceilings
Figure 9-10 shows typical suspended-ceiling bracing.  Diagonal bracing by wires 
or rigid members: spacing should not be more than 144 sq. ft. The vertical strut 
is recommended for large ceiling areas in high seismic zones; it may be provided 
by a piece of metal conduit or angle section. 
9.6.3	Lighting Fixtures 
Heavy fluorescent light fixtures inserted in suspended ceilings must be 
supported independently, so that if the grid fails, the fixture will not fall. 
Figure 9-11 shows a lighting fixture with two safety wires located at the 
diagonal. For heavy fixtures, four wires must be provided.  Suspended fixtures 
must be free to swing without hitting adjoining components.
9.6.4	Heavy (Masonry) Full-Height Non load Bearing Walls 
Heavy partitions, such as concrete block, should be separated from surrounding 
structure to avoid local stiffening of the structure and to avoid transmitting 
racking forces into the wall. Figure 9-12 shows two approaches for providing 
sliding or ductile connections at the head of full-story masonry partitions.
9.6.5	PartialHeight Masonry Walls
Figure 9-13 shows an overhead bracing system for a partial height wall.  The 
bracing used should have some degree of lateral flexibility so that structural 
deflections parallel to the wall do not transmit forces into the system.  Where 
vertical deflections due to dead loads, live loads, and seismic forces could 
occur, a slotted hole as shown, or some similar provision, should be made to 
prevent vertical loading of the wall.
9.6.6	Partial-Height Metal Stud Walls
Metal stud partitions that terminate at a suspended ceiling should be braced 
independently to the building structure, as shown in Figure 9-14.  Normal office 
height partitions can be braced by a single diagonal angle or stud brace. 
9.6.7	Parapet Bracing
Heavy parapets should be braced back to the roof structure.  This is a typical 
problem with unreinforced masonry buildings, which often have large unsupported 
parapets.  Figure 9-15 shows bracing for an existing masonry parapet; the roof 
structure should also be securely tied to the wall (not shown).
9.6.8	Sheet Metal Ductwork
Figure 9-16 shows a typical support and bracing system for large ductwork in a 
high seismic zone. The seismic code specifies the size of ducts and length of 
support that require seismic bracing.
9.6.9	Piping
Figure 9-17 shows typical bracing for large diameter piping. The seismic code 
specifies the types and diameter of piping, and length and type of hanger, that 
require seismic bracing. 
9.6.10	Vibration-Isolated Equipment
Equipment mounted on spring vibration isolators needs to be fitted with 
snubbers that limit lateral motion to prevent the equipment toppling off the 
isolators and suffering damage (Figure 9-18). The frequency of the isolation 
system is usually such that the motion of the equipment is greatly increased by 
earthquake forces. The snubbers are faced with resilient material that cushions 
any impacts that may occur. Detailed guidelines for the design of seismic 
restraints for mechanical, electrical, and duct and pipe are in FEMA 
publications 412, 413 and 414.
9.6.11	Emergency Power Equipment
Batteries for emergency power need positive restraint.  Figure 9-19 shows a 
custom designed rack, constructed from steel sections, to support and restrain a 
set of batteries. The batteries are also strapped to the rack for positive 
restraint.  Alternative emergency power sources, such as gas or oil, need 
flexible utility connections and restrained equipment. 
9.6.12	Tall Shelving
Tall shelves, such as library shelves, are often heavily loaded and acceleration 
sensitive.  They need longitudinal bracing and attachment to the floor. The top 
bracing should be attached to the building structure and strong enough to resist 
buckling when the heavy shelves attempt to overturn (Figure 9-20).
9.6.13	Gas Water Heaters
Gas water heaters need restraint to prevent the heater tank from toppling and 
breaking the gas connection, causing a fire risk.  Figure 9-21 shows a domestic 
hot water heater installation. A flexible gas connection is desirable but not 
essential if the tank is well restrained. The bottom restraint can be provided 
by an additional strap, or by securely bolting the base support to the floor.
9.7 THE NEED FOR Systems Design 
Following a damaging earthquake, whether large or small, the public expects to 
be able continue to use many building types. The building types that have the 
mandate for some post-earthquake operation tend to be essential facilities, such 
as acute care hospitals and those buildings where the owners see a clear 
financial benefit for continued operation.  Unfortunately, the expectations of 
the performance of our essential facilities are often not realized, and a number 
of modern healthcare facilities close with all too much regularity following 
even moderate earthquakes.
Over the years, codes have become more and more sophisticated with respect to 
structural seismic integrity.  Unfortunately, there remains a lack of 
understanding by many with respect to building function, in which the 
nonstructural systems play the key role.  The philosophy of code implementation 
carried out via the model codes, including the International Building Code 
(IBC), is based on the seismic provisions developed in the National Earthquake 
Hazard Reduction Program (NEHRP) and the Uniform Building Code (UBC), which uses 
the Structural Engineers Association of California recommendations. Both require 
simple component anchorage, which does not address function to the necessary 
level, especially for essential  facilities.  
Other design professions outside the building industry have long recognized that 
there is a need for systems design when continued function is necessary. The 
practice of systems design can be witnessed in aerospace design, naval design, 
nuclear power design, weapons design, and even race car design.  As an example, 
few would accept rides on modern aircraft if it was not believed that the 
aircraft had been designed from a systems engineering point of view.  It would 
not be acceptable for the landing gear to pinch the hydraulic brake lines when 
the wheels fold into the fuselage.  
Systems failures in health care facilities throughout the impacted area in the 
Northridge earthquake prompted the California Seismic Safety Commission to 
sponsor Senator Alfred Alquists Hospital Seismic Safety Act of 1994.  This 
legislation was clear in its direction to the industry and design professionals 
to maintain the operability of health care facilities following future damaging 
earthquakes.  The legislation called for consideration of systems design. 
Hospitals and other essential facilities complying with the legislation will 
have a lower risk of failure in future earthquakes.
The California Hospital Seismic Safety Act of 1994 took the further step of 
identifying building contents within the design parameters.  Prior to the 
implementation of this legislation, building contents had been left out of the 
qualification procedures in almost all codes, unless they met certain criteria.  
Without consideration of building contents, hospitals continue to be more 
vulnerable to failure due to earthquake shaking.  
9.8 WHO is responsible for DESIGN?
There is no clear answer for the responsibility of many nonstructural design 
issues.  In order to help designers determine who is responsible for 
nonstructural issues for both systems and components, Table 9-3 is provided as a 
guide.  Architects may wish to use this as a guide in establishing contractual 
relationships with their consultants prior to beginning design.  It is certainly 
helpful to all design professionals to know who is responsible for specific 
tasks.  It should be noted that there are many cases where design 
responsibilities are not clear, even when a responsibility chart such as that 
below is used.  The architect, as the design professional in charge, must ensure 
that the assigned responsibilities are clearly defined.
9.9 NONSTRUCTURAL CODES
The early 1970s saw the first inclusion for nonstructural provisions other than 
walls, parapets, and chimneys. The provisions have grown to include a wide 
variety of nonstructural components and building systems since the mid-1970s, 
but the seismic codes have yet to recognize the need for qualification of owner-
supplied equipment that is not fixed to the building.  The codes have also yet 
to come to grips with systems qualification and continued performance for 
facilities.  A discussion of the philosophy of codes for nonstructural 
components and systems is presented in Chapter 6, Section 6.6. 
9.10 METHODS OF SEISMIC QUALIFICATION
Qualification involves the acceptance of components and systems for use in a 
seismic environment and compliance with code requirements. There are numerous 
methods by which seismic qualification can be realized.  Each method has a 
narrow window of applications for effective seismic qualification.  These are:
	Design Team Judgment
	Prior Qualification
	Physical Tests
	Mathematical Analysis
	Static Equivalent Analysis
	Dynamic Analysis

9.10.1 Design Team Judgment
Design team judgment is a valuable resource in any seismic qualification 
program.  An inappropriate selection of equipment or qualification method by the 
design team may lead to nonstructural system failures during an earthquake.  The 
design team needs to meet early in the programming, schematic, and design 
development phases to discuss the various systems to be used in a facility 
design.  Just as in preliminary design the architect is interested in how deep 
structural members need to be for floor spans, and how much room is needed above 
the finished ceiling and below the structural system for HVAC and lighting, the 
architect and engineers need to discuss how movement of the various systems will 
impact each other in a sizeable earthquake.  Early appropriate building 
configuration decisions by the design team will have a great impact on the 
success of a building structure in an earthquake.  Similar appropriate decisions 
by the design team early in the design process will have a profound impact on 
the design of nonstructural systems and components.  Following the early 
discussions of the design team, more quantitative methods of qualification can 
be utilized, as discussed in the following sections.
9.10.2 Prior Qualification
In some cases, a product or system can be shown to have a previous qualification 
procedure that exceeds the code requirements.  This might include equipment that 
has been designed for shipboard installation, where the accelerations expected 
and durations of those accelerations far exceed the code requirements of a 
simple static mathematical model.  Such equipment is arguably considered to be 
qualified by prior experience.  Other examples might include a component 
manufacturer that has had their equipment tested on seismic shaking equipment 
that provides a stringent-enough test to envelope the specific installation, 
providing a prior qualification.  When available, the manufacturers will provide 
test results and/or reports detailing the testing program for the building 
officials review.
Appropriate detailing employing prior qualification is often used by the 
architect.  Consider, for example, suspended ceiling systems.  Most architects 
use suspended ceiling details that have proven over time to be effective, either 
through prior seismic experience or prior analysis.  The familiar 45o splay 
wires at areas of interval, such as 144 sq. ft., are such an example.  There is 
no need for the architect to recalculate the forces in the resisting wires for 
each new application.  Choosing the appropriate detail is often sufficient.  
There are, however, limits on standard details, and the architect should 
always review each detail to be used for the specific application.
9.10.3  Mathematical Analysis and Other 		
There are two basic forms of analysis.  The first is a simple static equivalent 
analysis.  This is the method most suited for most simple architectural problems 
for nonstructural design.  The second is the more complex dynamic analysis which 
requires costly engineering analysis and is seldom used for nonstructural 
components. In place of, or in combination with, physical testing the latter may 
be used.
These methods of qualification are generally not going to be employed by 
architects in normal designs.  In general, they are better suited for 
product/system manufacturers and researchers that want to show either a wide 
range of possible applications or to confirm predicted responses.  All of these 
qualification methods are expensive and time consuming.
9.11 SOME MYTHS REGARDING NONSTRUCTURAL   DESIGN
  My Engineers take care of all my seismic design
Many architects believe that seismic design is controlled in total by their 
engineers and they should not be involved in the conceptualization or the 
coordination of seismic design. As discussed throughout this work by the various 
authors, successful seismic design begins and ends with the architect. It is 
true that the engineers may control the details of many components within the 
facility, but it is the architect who must understand the interrelationship 
between the various systems within the facility for successful performance 
during and after an earthquake.  
  My building is base isolated  I dont need to worry about the nonstructural 
components
Building base isolation in general reduces the effects of horizontal motions 
within a building, but it does not eliminate them. The architect and design team 
should be aware of the limitations of base isolation and special conditions, 
such as the need for utilities to accommodate large lateral movement where they 
enter the building above or below ground.
  Window films protect windows from breakage in an earthquake
If properly applied, window films can reduce some glass breakage. The film needs 
to be taken all the way to the edge of the glass.  Often film to be applied to 
the glazing, while it is mounted in its frame, and is then cut with a razor 
blade against the mullions and muttons that score the glass, making it 
vulnerable to breakage during violent shaking.  As the film ages, it can lose 
its adhesion characteristics.  If films are used, they should be cross-ply films 
and have undergone aging tests to predict how they will outgas and to what 
extent they will become brittle.
   My building in San Bernardino survived the 1994 Northridge earthquake  it 
is earthquake proof
San Bernardino (or any other appropriate location) was a long distance from the 
1994 Northridge earthquake.  While distance does not always guarantee safety 
(San Francisco was approximately 60 miles from the focus of the Loma Prieta 
earthquake), in general being a substantial distance from the earthquake will 
lessen the effects of the earthquake on the building and its nonstructural 
components.  Nonstructural failures are commonly seen at greater distances than 
structural failures.  This is especially the case where the building components 
are not designed for the earthquake environment. 
   Vertical motions in earthquakes do not need to be considered for 
nonstructural design
Historically, the model earthquake codes paid little attention to the vertical 
component of shaking generated by earthquakes.  As a rule of thumb, the maximum 
vertical ground motion is generally 60 to 70% of the maximum horizontal ground 
motion.   While it may be unnecessary to consider the vertical motions of the 
structure as a whole, this is often not the case with nonstructural design.  The 
model codes have little reference to vertical acceleration design requirements 
for nonstructural components.  The building, usually due to its architectural 
configuration, can act as an amplifier for both horizontal and vertical motions.  
Therefore, even though the code most often does not require vertical design 
resistance, the designer must be cognizant of the implications of vertical 
motions during an earthquake and their potential effects (Figure 9-22).
9.12 WHAT CAN THE ARCHITECT DO TO DECREASE NONSTRUCTURAL DAMAGE?
Not only does the architect have the obligation to coordinate the overall design 
of the building, but the architect is also responsible for the basic seismic 
safety of the design.
The architect should guide the other design professionals in the design 
decisions, rather than simply turning the design over to the project engineers.  
Since many nonstructural issues involve the intermixing of several engineering 
professions, the architect should understand how each system will react with the 
other building systems.  The architect needs to be able to visualize the system, 
its components, and how they will interact in an earthquake, strong winds, fire, 
etc.  The architect should sit down with the consulting engineers early and 
often, beginning with a discussion of the earthquake performance objectives for 
the facility, to permit each of the disciplines to see the potential for 
interactions between systems and components.  Office standards should be 
developed for the interfacing of systems and components.
Simple designs make design life easier.  The current vogue for complex shapes in 
architectural design increase the complexity of the nonstructural systems.  This 
increase in complexity decreases the architects ability to visualize how 
systems and components will respond and interact.
9.13 THE COMPLEXITY OF RETROFITTING EXISTING BUILDINGS 
The nonstructural implications of retrofitting existing buildings can be very 
complex.  In many cases, the structural seismic retrofit may be compromised due 
to historic codes or other considerations.  In some cases, buildings have grown 
over time, which often means that nonstructural systems pass through more than 
one time era of construction.  These interfaces may make the nonstructural 
retrofit very difficult.  California architects and engineers are faced with a 
particular difficulty in this issue, based on the 1994 law requiring the 
upgrading of all existing acute care hospitals by 2030 (SB1953).  In fact, the 
task may be essentially impossible, and following the expense of the retrofits, 
hospitals may yet run a high risk of seismic failure due to nonstructural 
systems/component failures.  
A better solution than expecting performance out of systems that are difficult 
if not impossible to retrofit might be a hospital lifeboat.  The lifeboats 
could be self-contained, factory-built modular buildings sized to accommodate 
the expected emergency population needing attention following a major 
earthquake.  The lifeboats would be permanently stationed on the health care 
campus, ready for operation as needed.  These lifeboats could be provided at a 
fraction of the cost of the thorough retrofits currently required by SB1953, 
saving California money and providing a much higher degree of confidence that 
the hospitals will be operational for emergency services following an 
earthquake.
For facility types other than acute care facilities, the design team needs to 
identify how the existing building will react to the new structural 
improvements, and how these will impact the nonstructural elements of the 
facility.  The design professional may be required to determine which 
systems/components can fail and simply protect the occupants from falling 
hazards.
9.14 CONCLUSIONS
The largest immediate strides in resisting the impacts of nonstructural failures 
in future earthquakes will come from designers who understand the implications 
of systems design.  Next, there will be great increases in nonstructural seismic 
resistance by designers implementing the keep-it-simple philosophy.  Apart from 
that, there will be great strides in structural systems that will result in 
reduced-motion inputs at the nonstructural system/component interfaces.  These 
include base isolation as it is currently being employed.  Another future 
structural improvement that will likely improve nonstructural seismic 
performance is the increasing use of both active and passive seismic dampers.  
These systems show promise for significantly decreasing building motions that in 
turn will decrease nonstructural damage.  Some active dampers under development 
will be able to respond to the earthquake in almost real time.  
On the distant horizon is the transformation of the building structure from its 
seemingly rigid skeleton to a skeleton with a muscle system similar to an 
organism.  Shape-memory materials such as nitinol can be fabricated to act like 
muscles.  The nitinol reduces its size when heated, rather than expanding like 
most construction materials.  These building muscles can then be either 
tightened or relaxed with electrical input (heat), so that the building achieves 
balance during a seismic event in much the same way that our bodies can remain 
balanced when we stand on a moving bus or train.  As these shape-memory 
materials reduce the effects of acceleration on the building as a whole, they 
will reduce the large acceleration inputs on the nonstructural 
systems/components.  Shape-memory construction has been successfully utilized in 
aerospace design in recent years.  It will take considerable time for it to be 
successfully used in building design, although some limited research has been 
reported.
9.15 REFERENCES
2001. California Building Code based on the 1997 Uniform Building Code, 
International Conference of Building Officials, Whittier, CA.
2003 International Building Code, International Code Council, Country Club 
Hills, IL
FEMA 74, Reducing Risks of Nonstructural Earthquake Damage, FEMA, Washington 
D.C. 1994.
EQE, Structural Concepts and Details for Seismic Design, Lawrence Livermore 
National Laboratory, Livermore, CA. 1991.
Mason Industries Inc., Seismic Restraint Guidelines, Mason Industries Inc, 
Hauppauge, NY. 1999.

[End of Chapter 9]



10.1 INTRODUCTION
Earthquakes are only one of several hazards to which buildings are vulnerable.  
The two other significant natural hazards are floods and high winds, including 
tornadoes.  These hazards are extreme variants of benign natural processes.  
Earthquakes represent a highly accelerated instance of the slow adjustment that 
the earth makes as it cools.  High winds and tornadoes are an exaggerated form 
of the pleasant winds and breezes that freshen our everyday existence.   
Devastating floods are the result of excessive localized rainfall that, when 
normal, is necessary for the provision of water supply and the nurturing of 
plant life.  These natural hazards are not aberrations, are not malicious, and 
are part of natures order.
The traditional hazard to which buildings are vulnerable is fire, and history 
abounds with urban fire disasters right up to the present day.  Fire disasters 
are usually due to human errors and carelessness, but are sometimes originated 
by earthquakes and made more lethal by winds  (Figure10-1). Fire, however, when 
properly controlled, is an important contributor to human comfort.  Sometimes, 
of course, a fire disaster is the result of malicious intent.   The newest 
hazard - that of a terrorist attack on a building - is malicious, but is also an 
extreme form of everyday circumstance that we have come to accept: criminality. 
Building design has long recognized the need for locks and, more recently, 
remote cameras and sensing devices are also designed to prevent criminal - and 
now terrorists - from gaining entry to our buildings1
DESIGN FOR EXTREME HAZARDS BY CHRISTOPHER ARNOLD
10.2 MULTIHAZARD DESIGN SYSTEM INTERACTIONS
This publication focuses on design against earthquakes, but the other hazards 
must also be assessed.   Each of them has differing levels of riski.e., the 
probabilities and consequences of an event.  Some, such as earthquakes, floods, 
and high winds, are specific to certain regions.  The risks of terrorism are 
still uncertain compared to those of natural hazards that have a long history of 
statistical and scientific observation and analysis. Fires are more pervasive 
than any of the natural hazards.   However, design against fire has long been 
built into our building codes, in the form of approved materials, fire-resistant 
assemblies, exiting requirements, the width and design of stairs, the dimensions 
of corridors, and many other issues. 
An important aspect of designing against a single hazard such as earthquakes is 
the extent to which the design methods may reinforce or conflict with those 
necessary for protection against other hazards.  Multi-hazard design involves a 
risk assessment of all hazards at a programmatic stage to ensure that protection 
measures are not in conflict. Ideally, the measures used would focus on 
reinforcement rather than conflict, so that the overall risk management plan 
enables the cost of construction to be reduced.  
To assist the reader in evaluating the interactions between protective design 
methods, Table 10-1 summarizes the effects that seismic design measures may have 
on performance of the building in relation to other hazards. 
The horizontal rows show the five primary hazards. The vertical rows show 
methods of protection for the building systems and components that have 
significant interaction, either reinforcement or conflict. These methods are 
based on commonly accepted methods of risk reduction for the three main natural 
hazards, together with fire protection methods, and the methods for 
security/blast protection presented in FEMA 426, Reference Manual to Mitigate 
Potential Terrorist Attacks against Buildings, and FEMA 430, Site Design 
Guidance to Mitigate Potential Terrorist Attacks. 
The comments in this matrix are not absolute restrictions or recommendations, 
but rather are intended to provoke thought and further design integration.  
Reinforcement between hazards may be gained, and undesirable conditions and 
conflicts can be resolved by coordinated design between the consultants, 
starting at the inception of design.
Table 10-1 provides information to help the reader develop a list of 
reinforcements and conflicts for the particular combination of hazards that may 
be faced. Development of lists such as these can be used to structure initial 
discussions on the impact of multi-hazard design on the building performance and 
cost that, in turn, guide an integrated design strategy for protection.  The 
system and component heading list is similar to that used for the building 
security assessment checklist in FEMA 426, Reference Manual to Mitigate 
Potential Terrorist Attacks against Buildings.


[End of Chapter 10]

[End of FEMA 454]