Performance-Based Seismic Engineering:
The Next Generation of Structural Engineering Practice

Three structures, strongly affected by the Northridge Earthquake, illustrate the potential range of building performance.

Introduction

Experience in recent California earthquakes, including the 1989 Loma Prieta and 1994 Northridge events, indicates that modern code provisions in zones of high seismicity are relatively reliable in avoiding life-threatening building damage. Both events, having large magnitudes and epicentral locations close to large population centers, resulted in fewer than 100 fatalities. However, the economic losses associated with these earthquakes, $7 billion for Loma Prieta and more than $15 billion for Northridge, were unacceptably large, given that events of this size can be experienced relatively frequently in zones of high seismicity. As a result, public officials and members of the insurance and financial industries are calling for building construction practices that can better limit damage in future earthquakes.

A number of landmark projects have been initiated to meet this challenge. Performance-based engineering is intended to allow construction with predictable seismic performance and to provide owners and designers with the capability to select alternative performance goals for the design of different buildings. Important projects undertaken to develop these procedures include: Vision 2000 (SEAOC, 1995), a project of the Structural Engineers Association of California (SEAOC); Guidelines and Commentary for Seismic Rehabilitation of Buildings Applied Technology Council (ATC, 1995), a Federal Emergency Management Agency (FEMA) project being jointly executed by ATC, Building Seismic Safety Council (BSSC), and American Society of Civil Engineers (ASCE); and a FEMA-funded project conducted at the Earthquake Engineering Research Center (EERC) of the University of California at Berkeley.

Despite the proliferation of and interest in these projects, structural engineers attempting to implement performance- based design today are faced with many challenges. These include the lack of a set of reference performance standards, the lack of a reliable consensus-backed design standard, general economic and competitive pressures, and a lack of control over much of the design process.

Performance Standards

To implement a performance-based design, one or more performance objectives must be selected. A performance objective is a statement of the desired building behavior, given that it experiences earthquake demands of specified severity. In concept, the building user community is given a choice with regard to the performance objectives for each building. For such a choice to be useful, it is necessary to define the various behavior alternatives in terms meaningful to the layperson.

Important parameters include: the potential for loss of life, the cost of repairing sustained damage, and the amount of time the building is out of service for repair or, in extreme cases, replacement. While these parameters are meaningful to the public, and therefore can serve as a basis for selecting among building performance alternatives, they are not useful as a basis for design. An engineer cannot design for such performance specifications as a business interruption of two weeks or a repair cost that is 20% of replacement value. Therefore, as a prerequisite to practical implementation of performance-based engineering, corresponding relationships must be established among parameters that are meaningful to building users and design professionals. Since a building can experience a wide spectrum of potential behavior states ranging from a complete absence of damage and earthquake effects to complete collapse, establishing corresponding relationships is not a trivial task.

The adoption of a limited series of standard behavior states, from which design performance objectives can be developed, and for which defining parameters meaningful to the building user and technical communities can be established, is clearly required. Both the Guidelines and Commentary for Seismic Rehabilitation and Vision 2000 projects have identified such standard behavior state definitions, as described in the table on page 2. In addition to the definitions indicated in the table, a series of detailed matrices, describing permissible damage levels for the various structural and nonstructural components that compose typical buildings, are also provided in the documents. Although these definitions only qualitatively define parameters useful to the building user community (that is, extent of risk to life safety, potential range of repair costs, and time out of service), they provide a preliminary basis for discussion in terms meaningful to those who must choose the design performance levels.

To complete the specification of performance objectives, particular earthquake levels for which the performance is to be attained must be selected. This can be done on either a probabilistic or deterministic basis. Most people easily relate to the deterministic approach in which a specific magnitude event on a defined fault or source zone is selected as the basis for design. This approach is most useful for determining the "worst case" design performance objective in regions with seismicity controlled by one or more major active faults or source zones. As an example, if told that the worst earthquake ever expected to affect a planned building is an M7.5 event on a fault located 10 miles away, the average person is capable of determining what performance is acceptable should that event occur. However, in regions remotely located from such active sources, this approach is less meaningful. In addition, this approach is not particularly useful for determining secondary performance objectives such as the design earthquake for which the Immediate Occupancy or Operational performance levels should be attained. It is difficult for people to determine what performance should be specified for an M6.0 event on the same fault without knowing the likelihood of such an event. However, if people were told that an M6.0 event has an average return period of 100 years, it would be possible to determine an appropriate performance level for such an event, based on economic analysis.

Unfortunately, most building users cannot perform economic analyses based on probabilistically defined events or make rational choices between performance objectives. People tend to believe that any event with a return period significantly exceeding their expected life, or even the period of time during which they expect to own or occupy a building is improbable, and given a choice, would often select performance objectives that provide for lower levels of performance than provided by current building code provisions. In such an environment, the structural engineer will often be faced with the requirement to select the design performance objectives on behalf of the building user. This is highly undesirable, as the engineer may inherit significant liability if, after the occurrence of an event, the user is dissatisfied with the performance achieved. Consequently, a series of standard performance objectives, appropriate for the design of different classes of buildings, are urgently needed. Such standards would relieve unsophisticated users of the need to make a difficult selection for which they are unprepared.

Such a series of standard performance objectives, recommended by Vision 2000, are indicated in the figure above. Each diagonal line in the figure indicates design performance levels and earthquakes recommended for the design of buildings of different occupancies and uses. Individual, informed building users, of course, could select more stringent performance objectives, if desired. Adoption of such a standard would relieve the design engineer and building user from having to select such a basis. Note that the performance standards recommended by Vision 2000 are a refinement of the objectives perceived to be fulfilled by current codes. No detailed cost-benefit study of these standards has been conducted.

However, such studies should be performed prior to wide-scale adoption of such standards. Note also that these cost-benefit studies should be performed on the basis of the entire building stock within a region. Since damaging earthquakes are relatively infrequent events, the optimal economic selection for an individual building owner may result in unacceptable regional economic losses when a large earthquake actually occurs, if all owners in the region have selected an apparent optimum. Design standards should be set sufficiently high so that initial capital outlays for building design and construction exceed the optimal choice for individual buildings, but prevent unacceptably large regional losses.

VISION 2000 PERFORMANCE OBJECTIVES

Design Standards

The engineer attempting a performance-based design today must do so in the absence of a standard for such work. Consequently, specific design procedures must be developed for each project. Two basic approaches are commonly used. The first approach is to adopt criteria based on modification of the current building code provisions. The second is to attempt to predict building behavior.

The principal problem with developing performance-based criteria from the current building codes is that the performance provided by the building code itself is not well defined or understood. Therefore, it becomes very difficult to judge the probable benefits likely to be attained by designing for an arbitrary modification of the basic code criteria. Nevertheless, many engineers currently adopt this approach because it is simple and readily acceptable to building officials.

The second approach is more likely to provide the desired performance in design events. In this approach, structural analysis techniques are used to predict levels of damage in the various building elements, based on the inelastic demands predicted for these elements. Despite the more rational basis for performance-based design using this approach, problems with its implementation still persist. These include the limited accuracy of analytical techniques for the prediction of demands on the elements of complex structures, the limited availability of data to calibrate inelastic element demands against building performance levels, and an unwillingness on the part of some building officials to accept designs developed using this technique.

Economic and Competitive Pressures

Some of the most significant barriers to the implementation of performance-based design are the economic and competitive pressures faced both by the building designer and the building developer. Most building developers in today's market are not interested either in personal occupancy or long-term ownership of the property. As a result, they believe earthquakes of sufficient size to cause severe damage are unlikely to occur during the time the developer controls the building. Such developers are unlikely to invest in the construction of a building with enhanced earthquake performance characteristics unless they believe the added cost will increase either the rental or sales value of the property. Unfortunately, neither is the case.

Despite statements to the contrary following damaging earthquakes, the public generally believes that building codes provide adequate protection against earthquake-induced losses. The public is generally unaware of the performance standards inherent in the building codes or the likely behavior of buildings in earthquakes. There fore, they are unlikely to discriminate in their selection of a building based on potential earthquake performance. Thus, there is limited market value added to a building designed and constructed for such enhanced performance.

The exception are those buildings developed specifically for the occupancy of a business or institution knowledgeable in earthquake risk management. Most large corporations and institutions today have an in-house risk management group, responsible for protecting the organization against large catastrophic losses. In recent years, many such organizations with properties in regions of high seismicity have attempted to implement performance-based design concepts in the development of new properties. In some cases, however, more conventional design approaches were adopted because the additional cost inherent in the development of buildings with enhanced earthquake performance characteristics could not be justified by management.

The adoption of a performance-based engineering approach can result in a significant increase in building development costs. Design procedures conforming to current code requirements employ simple analytical techniques and evaluate building response to only a single earthquake demand level. Performance-based earthquake designs typically address multiple-performance goals. To reliably provide buildings capable of meeting these multiple-performance objectives, it is not only necessary to employ more complex and time-consuming analytical techniques, but also to evaluate the building's probable performance for several earthquake demand levels. This translates to higher engineering fees. As a result, structural engineers are unlikely to provide performance-based design services in a competitive market unless the building developer specifically requests them, or it is required in the building code. Since developers are unlikely to request performance-based design, adoption of performance-based requirements by the building codes will be necessary for their widespread use. Adoption of performance-based approaches in the building codes will be difficult to accomplish, as significant resistance will occur based on economic grounds.

Design Process Control

Perhaps the most significant barrier to the adoption of performance-based earthquake resistive design is the lack of control exercised by the structural engineer over the building design process. The structural engineer's role is typically limited to designing and detailing a building's structural components. Design criteria development and discussions with the building developer are typically handled by, or through, the architect, who has limited understanding of earthquake performance issues and consequently, is not an effective advocate for performance-based design issues. The basic site selection, building configuration, and even the framing system used for a building, all highly important factors in determining the building's performance, are typically decided by the architect and developer, often with minimal consultation by the structural engineer. Many structural engineers find it difficult to persuade the architect that a selected system is less than appropriate for a building, for fear that the architect will find another structural engineer who will support the original design concept, regardless of whether or not it will perform adequately.

Even if the necessary design procedures were available today and the engineer could design structures with predictable and acceptable seismic performance characteristics, this would not assure that the buildings themselves will meet the intended seismic performance objectives. Buildings are a complex collection of systems-structural, mechanical, fire protection, electrical, architectural, and others. The performance of the individual components of the nonstructural systems can be as important to the overall earthquake performance of a building as is the performance of the structure, particularly for enhanced performance objectives that address building operability. Yet the structural engineer has little participation in the design of these systems for seismic resistance and is often unaware of any details for installing these systems. Consequently, poor earthquake performance of these systems is common, even in buildings that have been designed to very stringent structural standards.

An example of such behavior is the performance of the Olive View Hospital, in Sylmar, California, during the 1994 Northridge Earthquake (see front and back covers). The Olive View Hospital replaced a previous facility, destroyed by the 1971 San Fernando Earthquake. It was designed to criteria that were substantially in excess even of those enforced by the State of California for construction of hospitals, since the 1971 event. This building was located within a few kilometers of the epicenter for the Northridge Earthquake, with recorded accelerations at the roof of the structure exceeding 2g. The building structure behaved very well and could be said to meet the performance requirements for the Operational performance level. However, utility piping within the building failed, resulting in extensive water damage to the facility and forcing its closure. As a result, the building's performance, as opposed to the structure's performance, was below the Immediate Occupancy level. While this performance was still excellent, it did not meet the intent of the design-to provide immediate post-earthquake health-care service.

Conclusion

The adoption of performance-based seismic design will require significant change in current structural engineering and building development practice. It will require the adoption of standard performance objectives for different classes of construction, development of substantially more complex and time-consuming analytical procedures as well as direct communication between the engineer and building users. Finally, structural engineers will require greater involvement in the overall site selection, building layout, and conceptual design process as well as substantially increased oversight of the design and installation of all systems necessary for adequate seismic resistance.

References

Applied Technology Council. 1995. Guidelines and Commentary for Seismic Rehabilitation of Buildings Report No. ATC-33.03; Redwood City, California.

Federal Emergency Management Agency. 1995. NEHRP Recommended Provisions for Seismic Regulation for New Buildings,1994 Edition. Building Seismic Safety Council , Federal Emergency Management Agency Report Nos. FEMA-222A (Provisions) and FEMA-223A (Commentary). Washington, D.C.

SAC. 1995. Program to Reduce Hazards In Steel Moment Frame Structures, Topical Reports on Case Study Buildings, SAC-96-02. The SAC Joint Venture. Sacramento, California.

SEAOC. 1991. Recommended Lateral Force Requirements for Buildings. Structural Engineers Association of California, Seismology Committee. Sacramento, California.

SEAOC. 1995. Vision 2000 - A Framework for Performance Based Design, Volumes I, II, III. Structural Engineers Association of California, Vision 2000 Committee. Sacramento, California.