1.

Introduction American Society of Civil Engineers

This report documents the results of a (ASCE) prepared the FEMA 356 report,
project for the Department of Homeland Prestandard and Commentary for the
Security’s Federal Emergency Seismic Rehabilitation of Buildings (the
Management Agency (FEMA) by the successor to FEMA 273/274), which was
Applied Technology Council (ATC) to published by FEMA in 2000. All of these
evaluate and improve the application of documents present similar approaches.
simplified inelastic analysis procedures for FEMA 273 and FEMA 356 use a procedure
use with performance-based engineering known as the Coefficient Method, and
methods for seismic design, evaluation, and ATC-40 details the Capacity-Spectrum
upgrade of buildings. Chapters 1 through 9 Method. The two approaches are essentially
summarize the developmental efforts and the same when it comes to generating a
results in concise language to facilitate “pushover” curve to represent the inelastic
application of the project findings in forcedeformation behavior of a building.
practice. Chapter 10 contains a summary They differ, however, in the technique used
and a practical application example using to calculate the inelastic displacement
the improved procedures. Supporting demand for a given representation of
information describing the project findings ground motion.The development of this
in detail are provided in the appendices. report was instigated by several factors. The
use of NSPs in engineering practice has
This document has been published in two accelerated since the publication of ATC-
formats: (1) a printed version, which 40 and FEMA 356. Consequently, there is
summarizes the developmental efforts and valuable information available on the
project findings and includes the practical application of these inelastic
application example (Chapters 1 through analysis procedures. In addition to
10), and (2) a complete version of the report experience with the initial application of
on CD-ROM (inside back cover), which these performance-based methods by
includes all of the material in the printed practicing professionals, ongoing research
version plus six appendices containing promises important modifications,
project results and findings. The printed improvements, and alternatives to current
version of the report is relatively brief to NSPs. There has also been a large national
facilitate use by design professionals. investment in performance- based
engineering, because of the tangible
1.1 Background prospect of vastly improving seismic design
During the past decade, significant progress practices. The future effective use of
has been made in performance-based performance-based engineering depends on
engineering methods that rely on nonlinear the continued development of reliable and
static analysis procedures (NSPs). In 1996, credible inelastic analysis procedures. The
ATC published the ATC-40 report, Seismic intent of the ATC-55 project has been to
Evaluation and Retrofit of Concrete gather the results of practical experience
Buildings, which was developed with and relevant research and to develop
funding from the California Seismic Safety guidance for improving the application of
Commission. In a larger project funded by nonlinear static analysis procedures to both
FEMA, ATC (under contract to the existing and new structures.
Building Seismic Safety Council) prepared
the FEMA 273 Guidelines for the Seismic 1.2 Project Purpose and Scope
Rehabilitation of Buildings, and the The purpose of the ATC-55 project was to
companion FEMA 274 Commentary, evaluate 6current NSPs, as described in
which werepublished in 1997 by FEMA. FEMA 356 and ATC-40 and to develop
Soon thereafter, the
improvements where feasible. The primary practicing structural engineers can apply
objectives were: the procedures appropriately. Finally, the
• to improve understanding of the inherent archival aspect recognizes that the
assumptions and theoretical underpinnings development of inelastic procedures will
of existing and proposed new simplified continue, and that it is important to record
analysis procedures; detailed information from the project for
• to recognize the applicability, limitations, future reference and use.
and reliability of various procedures;
• to develop guidelines for practicing The scope of the evaluation of inelastic
engineers on how to apply the procedures to analysis procedures and the development of
new and existing buildings; and recommendations for improvement, as
• to provide direction for researchers on presented in this document, focus on
issues to consider for future improvements nonlinear static procedures (NSPs). In light
of simplified inelastic analysis procedures. of the concerns identified by practicing
engineers and researchers, the document
Project activities also were guided by the specifically addresses the following
fact that engineers and researchers have questions:
similar concerns with respect to inelastic • How well do current NSPs predict
analysis procedures. Some of the more maximum global displacement (elastic plus
prominent issues considered are listed inelastic)?
below: • How well do current NSPs predict effects
• In some cases, different nonlinear static arising from the multiple-degree-of-
procedures produce significantly different freedom (MDOF) response of structures?
results for the same building model and • What modifications might be incorporated
ground motion representation. into NSPs to improve accuracy and to
• Current procedures for addressing the reduce uncertainty associated with the first
degradation of stiffness and strength in two questions?
structures are ambiguous and unclear.
• The predicted response of short-period The initial phase of the project, during early
structures seems to be extreme when 2001, focused on the identification and
compared with observed performance. refinement of important issues related to the
• Since they are based on single-degree-of- improvement of inelastic seismic analysis
freedom (SDOF) approximations, procedures. Activities included the
nonlinear static procedures may not reliably solicitation of input from researchers (see
predict important response parameters for Appendix A.) and practicing engineers (see
some multi degree of freedom (MDOF) Appendix B.). This information was used to
structures. formulate a plan for the subsequent phases
of the project, comprising the evaluation of
1.3 Report Scope, Organization and current procedures and the development of
Contents proposed improvements.
The document is intended to be useful from
the practical, educational, and archival Several analytical efforts formed the basis
standpoints. Its fundamental purpose is to for the evaluation of current procedures and
provide guidance that can be used directly the development of improvements. The first
by engineering practitioners. From an tested the accuracy of the Coefficient
educational perspective, the report is Method of FEMA 356 and the Capacity
intended to facilitate a basic conceptual Spectrum Method of ATC-40 in predicting
understanding of underlying principles, as global displacement demands, when
well as the associatedcapabilities and compared to responsehistory analysis of
limitations of the procedures, so that
SDOF oscillators. This effort is described in observed performance of such structures,
Chapter 3, with detailed results provided in including:
Appendix C. • models used to predict performance of
such structures commonly neglect many
During evaluations of both the Coefficient elements that contribute to their strength;
Method and Capacity Spectrum Method, it • fixed base models used to predict
became evident that important structural response neglect foundation
clarifications regarding strength flexibility, resulting in predictions of
degradation 5are applicable to both NSP smaller periods than that of the actual
approaches. This issue is addressed in structures;
Chapter 4. Improved procedures for use • stiff buildings will experience small
with the Coefficient Method are described displacements even at large ductility
in Chapter 5. Improved procedures for use demand and thus may experience only
with the Capacity Spectrum Method, are limited damage; and
described in Chapter 6. Supplementary • in addition to foundation flexibility, other
information and data on the equivalent soilstructure interaction effects can
linearization approach are provided in significantly reduce the response of some
Appendix D. Chapter 7 describes an stiff structures to ground shaking.
independent analysis that was implemented
to test the accuracy of the procedural In part, these effects can be addressed by
improvements described in Chapters 5 and more accurate analytical models that
6. Comparisons with results using the incorporate all structural and nonstructural
original procedures are provided. For many elements significant to structural response
years, researchers have observed that the as well as the flexibility of foundations.
predicted inelastic displacement response Soil-structure interaction effects are of
of oscillators, with periods in excess of particular importance. Chapter 8 describes
about 1 second, is often very similar to the analysis techniques for SSI effects that have
predicted displacement response of elastic been adapted for use with nonlinear static
oscillators having the same period. This has procedures and detailed supporting
led to the so called “equal displacement information on soilstructure interaction is
approximation.” provided in Appendix E. Multi-degree-of-
freedom effects are addressed in Chapter 9,
Researchers have also recognized that the which summarizes a comprehensive
predicted inelastic response of oscillators analysis of five example buildings to
with short periods, less than approximately illustrate the application and limitations of
0.5 seconds, are often significantly larger simplified techniques to account for MDOF
than the predicted response of elastic effects within current NSPs. Details are
structures of the same period, particularly if provided in Appendix F. Finally, Chapter
the structures are both very stiff and very 10 comprises a complete summary of the
weak. When this principle is applied using results of the efforts and the suggested
nonlinear analysis techniques to the improvements from a practical perspective.
performance of small, stiff buildings, such Chapter 10 concludes with a detailed
as those that comprise much of the building example application of the suggested
inventory in the United States, very poor improved procedures to a building
performance and extreme damage is often structure.
predicted. This has created a paradox, in
that such buildings have generally been
observed to experience limited damage in
past earthquakes. Several factorscontribute
to this conflict between predicted and