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selected based on magnitude and distance, it has been shown that inclusion
of
in a vector IM results in reduced estimates of structural response rela-
tive to the estimates obtained with the scalar IM,
S
a
(
T
1
) (Baker and Cornell,
2006a) This indicates that response predictions obtained using the MR-
based record selection approach are biased when
S
a
(
T
1
) is used as IM.
ε
Epsilon-based record selection
ε
(
T
1
) is an important property to match when selecting ground motions
for analysis, where the ground motion intensity is measured using
S
a
(
T
1
).
The value of
(
T
1
) is in fact a proxy for spectral shape, and it has been
demonstrated that selecting records with similar epsilon values to mean
epsilon values obtained from the seismic hazard deaggregation can result
in a more realistic prediction of seismic responses, especially at ultimate
performance levels including collapse where the spectral shape effects are
more prominent.
ε
CMS-based record selection
The CMS-based record selection strategy is the only method that can take
into account the infl uence of all three ground motion parameters (i.e.,
M
,
R
and
) in record selection. The idea is to predict the expected spectral
shape of the records with magnitude, distance and
ε
values equal to the
target values obtained from the seismic deaggregation. After predicting the
target spectral shape, records with similar spectral shape over a range of
period of interest (usually 0.2
T
1
to 2
T
1
) will be selected, while no strict
criteria on
M
,
R
and
ε
are imposed. In other words, the records with spectral
shapes matching the CMS for a given
M
,
R
and
ε
will be accurate predictors
of structural response, regardless of their actual
M
,
R
and
ε
values (Baker
and Cornell, 2006a). This method will be discussed more in the next section.
A study by Baker and Cornell (2005) suggests that the records selected
based on
ε
or CMS can also be scaled without resulting in biased predic-
tions, unlike other methods such as UHS-based and
M
,
R
-based record
selection. These observations can be explained by the idea that spectral
shape (i.e., spectral acceleration values at other periods, given
S
a
at
T
1
) is
the record property that directly affects seismic response of structures,
whereas magnitude, distance and
ε
ε
are proxies for spectral shape (and
ε
is
a more important proxy) (Baker and Cornell, 2006a).
21.5.3 CMS
The CMS provides the expected response spectrum, conditioned on occur-
rence of a target spectral acceleration value at the period of interest. It has
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