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structural capacity information (fragility curves), the PSDA results can be
used to compute the annual frequency of exceeding a specifi ed damage
state. Several methodologies for generating seismic fragility curves have
been developed over the years, including empirical, elastic-spectral,
nonlinear static, and nonlinear dynamic approaches (Nielson and Des-
Roches, 2007). Development of these component-specifi c and region-spe-
cifi c fragility functions is a topic of current and future research.
20.3
Probabilistic seismic demand analysis (PSDA)
PSDA (Luco, 2002) involves a series of steps including: the selection of
ground motion dataset for nonlinear analyses; modeling a class of structures
by varying important structural parameters considered in design; selection
of proper EDPs and IMs; and formulating the relationship between EDP
and IM (i.e. PSDMs). Each one of these steps has been examined by various
researchers in the past decade and substantial progress has been made on
vital parts of the process, such as nonlinear dynamic analysis of structural
models and selection of suitable IMs and EDPs. Some controversial issues
like selection and scaling of ground motions for nonlinear dynamic analyses
and functional shape of PSDMs are topics of ongoing discussions and
require further research efforts.
The selection of the time histories in nonlinear dynamic analyses is criti-
cal, since the spectral shape of the ground motion has a substantial effect
on the structural demand due to nonlinear response of the soil and struc-
ture. It is common practice to select the ground motion time histories based
on seismological properties, such as magnitude and distance to the fault.
Similar site conditions, style of faulting, and directivity effects may also be
considered in the selection process. Additionally, the scale factor required
to scale the time series to the design ground motion level may be consid-
ered. In general, scale factors closer to unity are preferred and many ground
motion experts recommend a limit on the amount of scaling applied
(Watson-Lamprey and Abrahamson, 2006). The issues on ground motion
selection and scaling for PSDA are closely related with the IM selection
process. Selection of the proper IM is also critical in terms of reducing the
uncertainty in prediction of the structural response, decreasing the number
of required nonlinear dynamic analyses, and decreasing the effect of ground
motion selection and scaling on the PSDM. An IM containing information
on the ground motion characteristics (e.g. spectral shape) and the structure
(e.g. fundamental period) is preferable, since it will refl ect the main char-
acteristics of seismological and structural parameters on the structural
response effectively.
Many researchers focused on the selection process and proposed quan-
titative measures for choosing the most appropriate IM for the structure
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