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

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]