Development of Performance-Based Seismic Design Procedures

Sharon L. Wood

Department of Civil Engineering

University of Texas

Austin, Texas

Construction in the United States is regulated by a variety of codes. Although most members of the general public do not know details of the local electrical or plumbing codes, they are secure in the knowledge that compliance provides the required level of safety for typical residential construction. Within structural engineering, building codes define the forces used in design and the procedures for calculating the nominal strength of members.

The intent of building codes (BOCA, 1993; ICBO, 1994; SBCCI, 1993) is clear concerning design of a structure to resist gravity loads. Buildings are expected to support intended loads without structural damage or loss of integrity. With a few notable exceptions, such as the Hyatt Regency walkway in Kansas City, this goal is accomplished easily because the magnitudes of gravity loads are well defined. In addition, the loads applied during construction often exceed those expected during normal use; therefore, most buildings have been subjected to a significant load test before being occupied.

But the intent of building codes is not as straightforward when forces induced by earthquakes are considered. Although the general public may assume that compliance with the seismic provisions of the code provides the same level of structural performance that is achieved for gravity loads, the aim of the code actually is quite different. For a structure with an average occupancy, such as an office or apartment building, the objective of the current building codes is "to safeguard against major failures and loss of life, not to limit damage, maintain functions, or provide for easy repair" (SEAOC, 1990). It is not economical, nor is it architecturally feasible, for buildings other than extremely critical facilities to be designed to resist forces induced during the maximum credible earthquake without damage. Therefore, the



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--> Development of Performance-Based Seismic Design Procedures Sharon L. Wood Department of Civil Engineering University of Texas Austin, Texas Construction in the United States is regulated by a variety of codes. Although most members of the general public do not know details of the local electrical or plumbing codes, they are secure in the knowledge that compliance provides the required level of safety for typical residential construction. Within structural engineering, building codes define the forces used in design and the procedures for calculating the nominal strength of members. The intent of building codes (BOCA, 1993; ICBO, 1994; SBCCI, 1993) is clear concerning design of a structure to resist gravity loads. Buildings are expected to support intended loads without structural damage or loss of integrity. With a few notable exceptions, such as the Hyatt Regency walkway in Kansas City, this goal is accomplished easily because the magnitudes of gravity loads are well defined. In addition, the loads applied during construction often exceed those expected during normal use; therefore, most buildings have been subjected to a significant load test before being occupied. But the intent of building codes is not as straightforward when forces induced by earthquakes are considered. Although the general public may assume that compliance with the seismic provisions of the code provides the same level of structural performance that is achieved for gravity loads, the aim of the code actually is quite different. For a structure with an average occupancy, such as an office or apartment building, the objective of the current building codes is "to safeguard against major failures and loss of life, not to limit damage, maintain functions, or provide for easy repair" (SEAOC, 1990). It is not economical, nor is it architecturally feasible, for buildings other than extremely critical facilities to be designed to resist forces induced during the maximum credible earthquake without damage. Therefore, the

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--> force levels used to design most buildings are consistent with an expectation of structural damage, and buildings in the epicentral area are expected to sustain damage, even during moderate events. Clearly, the differences in the performance expectations of the general public and the structural engineering profession are significant. Experience from recent earthquakes in California demonstrates that U.S. building codes have been successful in meeting the primary objective of limiting the loss of life (Table 1). However, many of the buildings that structural engineers consider to have performed successfully during an earthquake represent a substantial economic loss for the owner. Although it is usually possible to repair a structure damaged by an earthquake, often it is not practical to do so, especially considering the often high replacement cost of nonstructural equipment, finishes, and contents. Consequently, the economic impact of building damage in terms of interruption of business, loss of housing, and disruption to the community can be staggering. It is obvious that successful structural performance during an earthquake can no longer be defined merely in terms of preventing collapse. Funds available for disaster relief are not limitless, and the 1994 Northridge earthquake provided convincing evidence that an economic crisis could develop if a major earthquake occurred near a densely populated area of the United States. The concept of performance-based design was developed in an attempt to narrow the gap between the expectations that society places on building performance during an earthquake and the philosophy that structural engineers use to develop the building codes. In 1995 the Structural Engineers Association of California issued an overview of the objectives of performance-based seismic design. Although actual design procedures have not been developed, target levels of structural response are defined relative to the anticipated condition of the building after earthquakes of varying intensity. TABLE 1 Direct Losses from Recent Earthquakes in California Date Location Magnitude Deaths Damage (million $ 1994) 1983 Coalinga 6.5 0 50 1987 Whittier Narrows 5.9 8 450 1989 Loma Prieta 7.0 63 6,870 1992 Petrolia 6.9 0 70 1992 Landers 7.3 1 100 1994 Northridge 6.7 57 20,000   Source: OTA (1995).

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--> TABLE 2 Performance Objectives for Buildings Earthquake Designation Return Period (years) Probability of Exceedance Condition of Standard Occupancy Building Frequent 43 50% in 30 years Fully operational Occasional 72 50% in 50 years Operational Rare 475 10% in 50 years Life safety Very rare 970 10% in 100 years Near collapse   Source: SEAOC (1995). Again considering an average building, four states of damage have been related to four earthquake intensities (Table 2). Expected levels of damage to structural members, architectural elements, mechanical systems, and the building contents also have been defined for each damage condition. During the design process, the engineer would consider each earthquake level, and check that the calculated structural response is consistent with the expected performance. Although the objectives of the performance-based design procedures are well defined, implementation is not a simple matter of considering a few more load cases during design. The structural engineering community must address a large number of technical issues for which consensus opinions have not emerged. Of primary importance is the development of a clear understanding of what aspects of structural response trigger damage. Building codes have traditionally defined force levels, but damage levels more often are defined related to displacements, and the influence of earthquake duration cannot be ignored. Therefore, reliable analytical tools are needed to calculate the distortion of the structure when it is subjected to various levels of earthquake excitation. Specialized nonlinear analytical models have been used in research for more than 20 years, but they are not sufficient to accomplish the objectives of performance-based design. The results of these analyses are extremely sensitive to the choice of input parameters, most algorithms are limited to modeling two-dimensional response, the influence of nonstructural elements typically is ignored, and the nonlinear analysis models are computationally intensive. Even if the structural engineering community were able to develop comprehensive and efficient modeling tools, determination of the earthquake risk at a given location would remain a major concern. Seismologists continue to develop new theories about source mechanisms (Allen, 1995), earthquakes occur along previously unidentified faults, and large-amplitude velocity and displacement pulses have been identified in near-field ground motions (Iwan, 1995). In addition, the 1985 Mexico and 1989 Loma Prieta earthquakes have highlighted the influence of local soil conditions on building response.

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--> In addition to the technical challenges, performance-based design represents a new relationship between the structural engineering community and society. Structural engineers are attempting to predict the performance of complex structural and nonstructural systems that may be subjected to highly variable earthquake forces years after construction is completed. Building owners are being given the opportunity to select appropriate performance levels for a facility and the option to design the structure to stricter performance levels, with the belief that disruption to normal operations will be less immediately after an earthquake. The ambiguity of the design process is thereby reduced, and expectations are stated explicitly, but the complexity of the process is increased exponentially. References Allen, C. R. 1995. Earthquake hazard assessment: Has our approach been modified in the light of recent earthquakes? Earthquake Spectra 11(3):357-366. BOCA (Building Officials & Code Administrators International). 1993. The BOCA National Building Code. Country Club Hills, Ill.: BOCA. ICBO (International Conference of Building Officials). 1994. Uniform Building Code. Whittier, Calif.: ICBO. Iwan, W. D. 1995. Near-field considerations in specification of seismic design motions for structures. Proceedings, 10th European Conference on Earthquake Engineering, Vienna, Austria. Aldershot, U.K.: Ashgate Publishing Company. OTA (Office of Technology Assessment), Congress of the United States. 1995. Reducing Earthquake Losses. Washington, D.C.: U.S. Government Printing Office. SBCCI (Southern Building Code Congress International). 1993. Standard Building Code. Birmingham, Ala.: SBCCI. SEAOC (Structural Engineers Association of California), Seismology Committee. 1990. Recommended Lateral Force Requirements--Commentary. Sacramento, Calif.: SEAOC. SEAOC (Structural Engineers Association of California). 1995. Vision 2000—Performance-Based Seismic Engineering of Buildings. Sacramento, Calif.: SEAOC.