Developing Performance-Based Seismic Engineering for Use in Professional Practice

Chris Poland
President, Degenkolb Engineers
San Francisco

The new performance-based seismic engineering procedures are intended for the design and rehabilitation of structures to ensure performance at appropriate levels for all earthquakes. Defining various performance levels allows control and minimization of property and business interruption losses after a seismic event; it also serves to specify the state of the practice, which aids in liability protection. While performance-based seismic engineering techniques have been successfully applied in practice, often with considerable savings for clients in construction costs, there are at least three areas where more focus is needed. Educating clients with accurate information is one. Establishing performance-level criteria and developing a consistent vocabulary are also important, as well as keeping design professionals involved in continuing research efforts to verify techniques.

Communicating with Clients

The foundation of performance-based seismic engineering consists of multiple-tier performance-level criteria that allow use beyond code provisions. This field of engineering incorporates many new complex tools and techniques. As a result, it has become increasingly difficult to communicate ideas such as probable maximum loss (PMLs) and static pushover analyses to clients in a straightforward manner. For this reason, engineers must have a firm grasp of the underlying concepts and be able to communicate them effectively to architects, contractors, and clients alike. When equipped with consistent background information, clients will gain a basic understanding of the concepts involved and begin to see how performance-based engineering can benefit their projects.

In some instances, however, performance-based engineering may not be a viable approach in the decision-making process. Owners of small residential and commercial buildings, especially those who were not involved in the original building design or construction, may find these new concepts too complex to understand, and too difficult to incorporate into long-range plans. For these types of construction, performance-based engineering needs to be incorporated, when appropriate, as a matter of public policy into the general standards for construction.

Achieving Consensus in Performance-Level Criteria

Procedures for Postearthquake Safety Evaluation of Buildings (ATC-1989) is one of the most commonly used references for postearthquake building evaluation. It outlines three standard tags that are posted according to the severity of damage: green, yellow, or red. Performance-based seismic engineering defines as many as ten states classifying earthquake damage. Currently, there is no published correlation between these damage states and possible tag classification, and interpretations of actual earthquake performance may vary. As a result, there is a need for performance-level criteria to be more clearly defined and correlated to the current postearthquake tagging program.

Lack of a common vocabulary to communicate performance-based engineering concepts also creates confusion. In ATC-20 at least three sets of terminology are used to describe performance levels. Furthermore, the Vision 2000 report and the FEMA 273 Guidelines (FEMA 1997) generated different vocabularies for describing essentially similar performance levels, hazard levels, and performance objectives. Achieving consensus in definition and communication will greatly benefit the field of performance-based seismic engineering.

Ongoing Research

Current seismic provisions of building codes are based on the performance of structures during actual earthquakes. Many of the new tools associated with performance-based seismic engineering are still being developed today. The different evaluation techniques available may generate radically different conclusions given the same building, the same hazard, and the same performance objective. Many times these techniques cannot predict the level of damage a building will experience in an actual earthquake given a strong motion record taken in the base of the building. Until sufficient data are gathered from new seismic events and in-depth calibration studies are completed, conflicting results will persist.

Significant research is needed and should be ongoing. As the process continues, it is imperative that design professionals collaborate to ensure that performance-based seismic engineering methodologies are applicable to professional practice. Keeping everyone involved, including building owners, will help to add definition to the tools and techniques used, and to improve their use in practice.

References

Applied Technology Council. 1989. ATC 20 Procedures for Postearthquake Safety Evaluation of Buildings. Redwood City, Calif.: Applied Technology Council.

FEMA. 1997. NEHRP Guidelines for the Seismic Rehabilitation of Buildings. Prepared by the Applied Technology Council. [Washington, D.C.: Federal Emergency Management Agency]. FEMA 273.

Structural Engineers Assn. of California (SEAOC), Vision 2000 Committee. April 3, 1995. Performance Based Seismic Engineering of Buildings. J. Soulages, ed. 2 vols. [Sacramento, Calif.]