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Re: Performance-Based Seismic Design

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Roger,

As I understand it the UBC seismic design criteria are similar the life
safety objective in performance based design, and only minimum performance
related criteria are checked such as drift and ensuring strong column/weak
beam action. I would like to think we can improve on our knowledge of what
happens during an earthquake.

Performance related design does allow a client to set his own design
objectives, but these are not allowed to fall below the current UBC code
requirements. It is on the other hand a powerful tool for two reasons: 1) it
allows the owner some say over how much damage is acceptable to him/ her
over the lifespan of his structure and 2) it allows us as engineers to
quantify the inelastic demand at various elements in a structure.

Granted, we need to impress umpon the client the uncertainties in predicting
the earthquake magnitude as well as the describe the importance of quality
construction and supervision. At the same time, we should recognize that
improvements in computed modeling, and an increased understanding of steel
and concrete post yield behaviour can lead to better designs.

A structure with redundancy and a direct load path with good connections is
still the goal. Performance related design simply allows the designer to
check how redundant a structure is, by modeling the stiffnesses and
strengths of the elements.  While the load path could be argued to be
intuitive, it is also valuable to have your intuition checked by modeling
what you believe the behavior to be. It quickly becomes clear which parts of
your structure are attracting the most load, and the strengths and
stiffnesses can be modified to more evenly distribute where the structure
fails.  At the same time, rotations can be measured against laboratory results. 

Yes, we can still look at drift ratios, but modeling post yield behaviour
can tell us which elements in plan are working the hardest. We can then
begin to control the amount of damage.


Ted Weicker

(in Denmark)


********************
(As Frank McClure said on Vision 2000,)
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.)

**************************
Theodore Weicker

Dept.     241
Initials  THW
telephone (45)45 97 22 11 ext. 1225