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SPECIFIC ISSUES AND CHALLENGES IN PERFORMANCE ASSESSMENT
Each step summarized in the previous section has many issues that need much discussion and further
research. The following sections address a few of these issues and point out some of the challenges
that have to be confronted in order to permit a consistent implementation of the framework equation.
Because of space limitations, only a few specific issues concerned with IMs and EDPs are addressed.
Intensity Measures, IM
Intensity measures are quantities that describe the magnitude (M) and distance (R) dependence (other
parameters such as fault mechanism also could be considered) of ground motions characteristics that
significantly affect the upstream variables of the performance assessment approach. In the context of
Eq. (1), this implies evaluation of the MAP of IMs through seismic hazard analysis. Of course,
simplicity favors scalar measures, and in particular, measures for which hazard analysis results are
available. Choosing, for example, PGA for the IM may be initially appealing, but it implies that the
distribution G(EDP\IM) may have a very broad variability. This in turn means that it will require a
large sample of records and nonlinear analyses to estimate G(EDP\IM) with sufficient confidence. A
well selected spectral ordinate (e.g., S a at the fundamental period T ; ) is an improvement over PGA and
has been the popular choice in the recent past. It is a matter of debate whether S a (Tj) is indeed the
"best" choice ("best" implies a balance between simplicity and accuracy).
S a (Tj") does not account for the frequency content at T y T 17 which dominates higher mode effects (T <
Tj) and period elongation effects for inelastic systems (T > T t ). Again, this may lead to G(EDP\IM)
distributions with a very broad variability. This is illustrated in Fig. 2, which shows normalized
incremental dynamic analyses (IDAs), together with statistical values, for a 6-story frame structure
subjected to 40 ground motions that are scaled so that the S a at the fundamental period (T, = 0.6 sec.) is
the same. Scaling records to a common S a at T, (which is implied by using S a (T|) as the IM) results in
a median spectrum that resembles a typical spectrum for ordinary ground motions, but results in large
variability in spectral ordinates at periods even very close to T,, see Fig. 3.
NORMALIZED MAXIMUM STORY DRIFT
N-6, T,=0 6, £=0 05, Peak-oriented model 9=0.010, BH, K,,S,, LMSR-N
ELASTIC STRENGTH DEMAND SPECTRA
Scaled Records (T=OJ s), LMSR, § = 0.05
1 2 3 4 5
Norm. Max. Story Drift Over Height, 9ull)l)