Civil Engineering Reference
In-Depth Information
Uniform hazard spectrum (UHS)-based approach
The uniform hazard spectrum (UHS) is defi ned as a set of spectral accelera-
tion amplitudes over a range of vibration periods with equal probability of
exceedance in a period of time and is widely used in current seismic design
codes. The simplest method for record selection is to choose the records
such that their response spectra match a target UHS over a range of periods.
However, the UHS is a composite of predicted responses at different
periods that correspond to a specifi c hazard level, and may not be repre-
sentative of individual ground motion spectra. The seed records used in this
method can be either artifi cial, natural or manipulated natural records.
Artifi cial records can be generated using some available software to match
a UHS. Alternatively, natural records can be manipulated by changing the
frequency content, so that the response spectra of the records match a
target UHS. Scaled natural records that are matched to a target UHS over
a range of periods are often recommended by some codes. This range must
include the important modes of the structure as well as the effects of period
elongation due to inelastic deformation of the structure. A period range of
0.2
T
1
to 2
T
1
(where
T
1
is the fundamental vibration period of a structure)
is usually recommended and used for this purpose. This range is similar to
the 0.2
T
1
to 1.5
T
1
range specifi ed by ASCE 7-05 (ASCE, 2005), but statisti-
cal studies suggest that nonlinear structures are often sensitive to response
spectra at periods longer than 1.5
T
1
(Baker and Cornell, 2008; Haselton and
Baker, 2006; Vamvatsikos and Cornell, 2005).
Although conceptually simple, the UHS-based methods are deemed to
be conservative, particularly when ultimate performance limits of a struc-
ture (such as bar buckling, fracture, collapse, etc.) are the primary objective
of the analysis. This is because the response spectra of the records are forced
to match a target UHS, such as that specifi ed in the National Building Code
of Canada (NRCC, 2010). Furthermore, the UHS tends to be dominated at
any individual periods by motions above the median, whereas individual
spectra are unlikely to be equally above-median at all periods. That is, no
natural record can be found to match the UHS over a wide period range
and hence the analysis results using UHS-matched records may be conser-
vative and biased. To solve these problems, the use of the CMS in lieu of
the UHS has been recommended in recent years (Baker, 2011).
MR-based record selection
In this method, records with the closest
M
and
R
to the mean magnitude
and mean distance obtained from the seismic deaggregation are chosen. A
difference of one unit in magnitude may be treated as equivalent to a dif-
ference of 40 km in distance (Baker and Cornell, 2006a). For the records
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