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Re: Performance-Based Seismic Design, Potential for Increased Professional Liability

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Excellent evaluation Frank.  As the project manager for OES for the Vision 2000
work it is important to add to your evaluation that the work of the committee was
only the beginning.  Significant work still needs to be done by engineers and
seismologists.  An example of the continuting work on the seimology work is a
project known as TriNet.

TriNet has also been funded (initially) by FEMA through a Northridge Hazard
Mitigation Grant.  This project will place more sohpisticated equipment throughout
southern California to obtain better information on the characteristics of
earthquakes.  This project will also provide the engineering community with "real
time" information about the ground motion.  This information will be available
within minutes in the form of maps which will appear on the web page.  This a
first major step in being able to provide the "design" earthquake information that
engineers will for performance based engineering.

We have to remember that the recording and measurement of earthquakes by
seismologist is also a relatively new field.  For California engineers, seismic
design prinicpals began to appear after the Long Beach earthquake in 1933.
Seismologists began collecting and maintaining data on earthquakes just a couple
of years before then.  We have made tremendous progress in the last 65 years in
better understanding building performance and earthquake ground motion.

Performance based engineering is the way of the future, but it will take some more
time to fully develop and be able to implement with any degree of comfort.  In the
mean time, it is important for engineers to concentrate on making sure the load
path is well connected, that walls are well anchored, and that there is some level
of redundancy in the lateral force system.  This is the first big step to
performance based engineering.

Rick Ranous

FEMCCLURE(--nospam--at) wrote:

> 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: 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)    August 13,1998