but each having different harmonic periods over the
previously low or moderate seismicity, the new
range O, T1, T2, etc., is subjected to a given ground
seismic design forces may be lower than previously
motion accelerogram.
The entire time history of
prescribed.
acceleration response is found for each system, and
the corresponding maximum value, Sa, is plotted on
(2)
Time histories of ground motion are
the period axis for each system period. The curve
required for nonlinear inelastic dynamic analyses,
and can also be used for general dynamic analyses.
response spectrum for the given accelerogram and
Time histories may be actual or modified ground
damping ratio. The peak response of the oscillator
motion records, or may be synthetic time histories
(Sa) is a point on the response spectra for the period
developed to match a target spectrum. Since a single
of the oscillator.
The ground motion in this
time history cannot be completely representative of
document is defined by two spectral ordinates, as
all possible ground motions at the site, a suite (e.g.,
described in Chapter 3. The two ordinates represent
usually at least three) of time histories is generally
spectral response accelerations as a percentage of the
required. Time histories are not prescribed by this
acceleration due to gravity, g. Equations are also
document, and their use requires authorization from
provided for the development of response spectra
the cognizant design authority.
from the two ordinates, and for the modification of
the spectral response for various soil conditions at
2-3.
Site Hazards Other than Ground Motion.
the site. For firm sites, the design spectral ordinate
at 0.2 second, SDS, is roughly equivalent to two and
a.
General.
The analysis and design
one-half times the Z coefficient in the Uniform
procedures of this document are primarily aimed at
Building Code (UBC), or the Aa coefficient in the
improving the performance of structures under the
prior NEHRP provisions. The spectral ordinate at
loads and resulting deformations imposed by seismic
1.0 second for firm sites is approximately equivalent
shaking.
Other seismic hazards could, however,
to 1.2 times the Av coefficient in the prior NEHRP
exist at the building site that could damage the
provisions. The response spectra are prescribed for
structure, regardless of its ability to resist ground
dynamic analyses, and the equations also define
shaking.
These hazards include fault rupture,
equivalent lateral forces for static analysis. These
liquefaction or other shaking-induced soil failures,
changes in the representation of ground motions
landslides, and inundation from offsite effects such
were instigated by USGS as a result of an extensive
as dam failure or tsunami.
national program to develop spectral parameters that
better represent actual site response, and to
b.
incorporate the most current knowledge regarding
possible extent of damage from such site hazards
regional seismicity.
The new design values will
should be considered in the site selection process. In
result in higher seismic design forces, as compared
some situations, it may be feasible to mitigate the
to the 1997 UBC or FEMA 222A in sites near major
site hazards. In many cases, the likelihood of the
faults and in areas of very low or negligible
site hazard occurring will be sufficiently small that
seismicity (e.g., UBC Zone 0). In some areas of
2-3
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