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Performance-Based Seismic Design, Potential for Increased Professional Liability[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org, alescgd(--nospam--at)uwgb.edu
- Subject: Performance-Based Seismic Design, Potential for Increased Professional Liability
- From: <FEMCCLURE(--nospam--at)aol.com>
- Date: Thu, 13 Aug 1998 05:59:37 EDT
A Performance-Based Design Workshop sponsored by FEMA and the Earthquake Engineering Research Institute (EERI) was held in San Diego, CA on July 27 - 28, 1998. One of the important issue papers prepared for this Workshop was prepared by Prof. Daniel J. Alesch, University of Wisconsin-Green Bay, entitled "Education, Initiatives, and Incentives for Adoption of Permance- Based Seismic Design Standards." On page 14 of this paper, it reads: "Predictably, engineers will be sued if buildings do not perform as owners expect them to in the event of an earthquake of the severity for which they were designed." At this Workshop, Vision 2000, "Performance Based Seismic Engineering of Buildings" prepared for California Office of Emergency Service and prepared by Structural Engineers Association of California, Vision 2000 Committee, was discussed. Contact web site: www.seaoc.org for information on how to obtain a copy and its cost. It is important to point out what Vision 2000 says about "Limitations" on page 1-4. "The practice of performance-based seismic engineering presumes the ability of an engineer to predict building given the occurence of a defined earthquake ground motion. The current state of knowledge and available technology is such that our ability to accurately predict the earthquake performance of a specific building subjected to a defined earthquake ground motion is quite limited, and subject to a number of uncertainties. Principal sources of these uncertainties include: definition of the ground motion including, intensity, duration, phasing and frequency content, analysis of the distribution of deformations and stresses produced in the structure in response to the ground motion, knowledge of the actual configuration, strengths, deformations, and energy absorption and dissipation capacities of the structure in its as-constructed and maintained condition, and determination of specific damage to structural and nonstructural components, in response to defined ground motions. Researchers and practicing engineers are actively working to reduce these uncertainties through the development of improved analytical tools for the estimation of ground motion and structural response, laboratory testing of structural assemblies and our continued observations of damage caused to real structures by earthquakes. While our ability to predict performance tody is much better than it was 10 years ago, reliability is still limited. (Emphasis added.) Recommendations contained in this report are intended to provide engineers with guidance on how to better achieve structures with predictable and defined seismic performance. There is, of course, no guarantee that specific structures designed in accordance with these recommendations will achieve the desired performance. Users of this document are cautioned to exercise judgment and care in the application of the recommendations contained herein. This document is not intended to be, and should not be used as, a substitute for currently prevailing building codes. (Emphasis added.) The Structural Engineers Association of California, its membership, and the Vision 2000 committee offer no warranty with regard to the performance of buildings designed in accordance with these recommendations." (End of Quote.) Again, it is important to remember that the"design" or "evaluation" earthquake is usually defined as an earthquake which has a given probability of EXCEEDANCE in a given number of years. (For examples, an earthquake with a 10% probability of exceedance or a 90% probability of non-exceedance in 50 years.) By definition, these "design" or "evaluation" earthquakes can be exceeded. This exceedance can be defined as the "annual probability of exceedance", the "mean annual frequency of exceeding a specified ground motion parameter level" or the "mean return period." In other words, by defining the "design" or "evalution" earthquakes, using probabilitistic definitions, these earthquakes are not the "maximum" size earthquakes possible. These "design" or "evaluation" earthquakes have a certain probability of being exceeded in a given number of years. How can engineers assure their clients - owners, insurance companies, lending agencies, etc.- that their buildings will not be damaged beyond a certain degree of damage when the "design" or "evaluation" earthquake has a probablility of exceedance and is basically "open ended?"? Frank E. McClure FEMCCLURE(--nospam--at)aol.com August 13,1998
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