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IMPLEMENTING AND REALIZING PBD
Several important issues were discussed related to the implementation and realization of PBD in
practice. These include: (i) the challenges of estimating actual performance, maintaining the
required design level precision and accounting for the various sources of uncertainty, (ii)
potential legal impediments, and (iii) the education and knowledge dissemination required.
Estimation of Actual Performance, Design Level Precision and Uncertainty
Performance is associated with the behavior of a structural system, however, not directly as
calculated with present methods. As a result, issues, which may be paramount to performance,
such as building occupants relating performance to human perception of vibration, or damage to
nonstructural components and contents, are not incorporated in current procedures. Furthermore,
the strong economic promise of performance-based design makes the consideration of
nonstructural and asset (direct and indkect) losses of foremost importance in the implementation
of PBD. A well-designed structure may reach other performance limit states solely due to
indirect losses from downtime or other nonstructural damage and this must be incorporated in the
evaluation process. These issues largely instigated PBD, for example, the nearly $30Billion U.S.
in losses from the 1994 Northridge earthquake were primarily attributed to nonstructural damage.
Maintaining the level of precision in the design of structural systems that is required by
performance-based design provisions is also of concern. In performance-based provisions, design
will no longer be based on one limit state, but rather multiple limit states, requiring further level
of detail and precision to be attainable. To compound this issue, previous and current design
codes provide procedures for proportioning strength, not estimating actual behavior.
Estimation of actual performance to a reasonable level of precision requires a framework
accounting for (i) the intensity of the ground motion, (ii) the engineering demand imposed on the
system, (iii) the corresponding damage generated, (iv) a decision variable based on knowledge of
this demand and (v) realistic performance targets. Perhaps the most critical of these items are the
intensity of the ground motion and the performance targets. Neither of these items is directly
related to the engineers' most common tasks and arguably these may introduce the greatest
amount of uncertainty in the process.
This raises important questions regarding our ability (and confidence) in predicting earthquake
hazard, as well as our ability to discretely characterize the structural system. Various sources of
uncertainty exist in the estimation of earthquake hazard, including: (i) the occurrence of
earthquakes in space and time, (ii) the earthquake magnitude, (iii) the attenuation from the source
to the site, and (iv) the inherent randomness of ground motion time histories. Uncertainties in
characterizing the structural system include: (i) geometric and material properties and (ii) soilfoundation-structure
interaction (SFSI), and (iii) modeling uncertainties at both the component
and system levels. Compounded with these are the errors and randomness of the general
construction of structures. At the decision state, there are uncertainties associated with (i) the
consequences of exceeding the limit state (life safety, economic losses, downtime), (ii) economic
assumptions (e.g. current economic status, discount rate, (iii) the recovery rates (availability of
finances, state of economy in region).