Development of next-generation, performance-based, seismic design guidelines

Since October 2001, the Applied Technology Council (ATC), with funding from the Federal Emergency Management Agency (FEMA), has been engaged in a major, multi-year project to develop Next-Generation Performance-Based Seismic Design Guidelines for New and Existing Buildings (ATC-58 Project). The project is being conducted in two phases. Under Phase 1, which is scheduled to be completed in September 2010, a methodology for assessing the probable seismic performance of individual buildings in future earthquakes is being developed. In Phase 2, which has not yet been fully scoped, but is anticipated to require at least five years, performance-based seismic design procedures and guidelines will be developed. The purpose of the Phase 2 procedures and guidelines is to assist engineers in designing buildings to meet desired performance goals, and to assist stakeholders in taking advantage of the benefits of performance-based design.

During the first five years of the project, ATC has assembled a team of experts to conduct the project; conducted several workshops; developed a Program Plan, which has been recently published as the FEMA 445 Report titled, Next-Generation Performance-Based Seismic Design Guidelines, Program Plan for New and Existing Buildings; and commenced development of the principal Phase 1 product, Guidelines for Seismic Performance Assessment of Buildings.

FEMA envisions that the performance-based seismic design procedures and guidelines being developed under the project will eventually be incorporated into existing established seismic design resource documents, such as the National Earthquake Hazards Reduction Program’s (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, for new construction, and the successor documents to the FEMA 356 Prestandard and Commentary for the Seismic Rehabilitation of Buildings, for existing buildings.

Performance-based design: An overview

Performance-based seismic design explicitly evaluates how a building is likely to perform given the potential hazard it is likely to experience, considering uncertainties inherent in the quantification of potential hazard and uncertainties in assessment of the actual building response. It permits design of new buildings or upgrade of existing buildings with a realistic understanding of the risk of casualties, occupancy interruption, and economic loss that may occur as a result of future earthquakes.

It also establishes a vocabulary that facilitates meaningful discussion between stakeholders and design professionals on the development and selection of design options. It provides a framework for determining what level of safety and what level of property protection, at what cost, are acceptable to building owners, tenants, lenders, insurers, regulators, and other decision makers based upon the specific needs of a project.

In contrast to prescriptive design approaches in existing building code procedures for seismic design, performance-based design provides a systematic methodology for assessing the performance capability of a building, system, or component. It can be used to verify the equivalent performance of alternatives, deliver standard performance at a reduced cost, or confirm higher performance needed for critical facilities.

First-generation procedures introduced in the FEMA 273 NEHRP Guidelines for the Seismic Rehabilitation of Buildings defined performance in terms of discretely defined performance levels with names intended to connote the expected level of damage: Collapse Prevention, Life Safety, and Immediate Occupancy. These first-generation procedures also introduced the concept of performance related to damage of both structural and nonstructural components. Performance objectives were developed by linking one of these performance levels to a specific level of earthquake hazard. Although intended for existing buildings, these procedures are also being extrapolated for use in performance-based seismic design of new buildings.

The need for next-generation procedures

As the state of knowledge and experience base advances, limitations in present-generation procedures are being identified by researchers and practitioners. These include questions regarding the accuracy of analytical procedures in predicting actual building response, questions regarding the level of conservatism present in acceptance criteria, the inability to reliably and economically apply performance-based procedures to the design of new buildings, and the need for alternative ways of communicating performance to stakeholders that is more meaningful and useful for decision-making purposes.

Next-generation, performance-based design procedures are needed for the following reasons:
• to revise the discrete performance levels defined in first-generation procedures to create new performance measures (for example, repair costs, casualties, and time of occupancy interruption) that better relate to the decision-making needs of stakeholders, and that communicate these losses in a way that is more meaningful to stakeholders;
• to create procedures for estimating probable repair costs, casualties, and time of occupancy interruption, for both new and existing buildings; and
• to develop a framework for performance assessment that properly accounts for, and adequately communicates to stakeholders, limitations in our ability to accurately predict response, and uncertainty in the level of earthquake hazard.

Framework for next-generation procedures

The next-generation, performance-based seismic design procedures being developed under the FEMA-funded ATC-58 Project will express performance directly in terms of quantified risks that a building owner or decision maker will be able to understand. Stakeholders prefer to define these risks in terms of the potential for casualties, repair costs, and occupancy interruption. Stakeholder guidance will be developed to assist decision makers in selecting appropriate levels of risk as the basis of design and upgrade projects. Engineering guidelines will be prepared to assist design professionals in developing building designs that are reliable and capable of meeting the selected risk criteria.

FEMA 445 Program Plan

The recently published FEMA 445 Program Plan provides a step-by-step, task-oriented program to develop next-generation, performance-based seismic design procedures and guidelines for structural and nonstructural components in new and existing buildings (in two phases). The plan offers background on current code design procedures, introduces performance-based seismic design concepts, identifies improvements needed in current seismic design practice, and outlines the tasks and projected costs to develop next-generation, performance-based seismic design procedures and guidelines. This plan is a refinement and extension of two earlier FEMA plans: FEMA 283 Performance-Based Seismic Design of Buildings—an Action Plan, which was prepared by the Earthquake Engineering Research Center, University of California at Berkeley in 1996l; and FEMA 349 Action Plan for Performance-Based Seismic Design, which was prepared by the Earthquake Engineering Research Institute in 2000.

Work is organized around six broad categories of work: planning and management program; structural performance products; nonstructural performance products; risk management products; guidelines products; and stakeholders guide products. Work in each technical area is being performed by one of three product development teams, consisting of a structural performance products team, a nonstructural performance products team, and a risk management products team.

Planning and management program tasks are being carried out within a project management structure consisting of a project management committee, a project technical committee, and a project steering committee. Collectively, these committees provide management, technical oversight, and control of the work performed by the three product development teams.

Additional information
Additional information about the ATC-58 Project, including a list of project participants and downloadable copies of project reports prepared to date, including the FEMA 445 Program Plan, is available on the ATC website, www.atcouncil.org.

Acknowledgments
ATC gratefully acknowledges the financial support for this project provided by the Federal Emergency Management Agency.

Christopher Rojahn is the executive director of the Applied Technology Council in
Redwood City, Calif. He can be reached at crojahn@atcouncil.org.
The Applied Technology Council (ATC) is a non-profit, tax-exempt corporation established in 1973 through the efforts of the Structural Engineers Association of California. ATC’s mission is to develop and promote state-of-the-art, user-friendly engineering resources and applications for use in mitigating the effects of natural and other hazards on the built environment. ATC also identifies and encourages needed research and develops consensus opinions on structural engineering issues in a nonproprietary format. ATC thereby fulfills a unique role in funded information transfer.