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Performance-Based Seismic Design, Potential for Increased Professional Liability

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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