EP 1110-2-12
30 Sep 95
Chapter 5
Provisions for the Development of Seismic Regula-
tions for Buildings" (Applied Technology Council
Design Response Spectra and
1984) for further information on developing design
Acceleration Time Histories
response spectra to define the design earthquakes.
5-1.
Defining the Design Earthquake
5-3.
Developing Acceleration Time
Histories
In a linear-elastic response spectrum analysis,
response spectra define the free field ground motion
a. Matching design response spectrum. The
for the design earthquake. A response spectrum gives
more refined methods of analysis discussed in para-
the maximum damped response (expressed as dis-
graph 2-2d are of the time-history type. Time histo-
placement, velocity, or acceleration) of all possible
ries usually express the ground motion as a record of
linear single degree-of-freedom systems using the
acceleration with respect to time. Acceleration time
natural frequency (or period) to describe the system.
histories should be developed so their response spec-
Viscous damping expressed as a percentage of critical
trum is consistent with the previously established site-
damping is used to develop a response spectra. A
specific design response spectrum described in
design earthquake is often defined by a set of
paragraph 5-5c. The time histories should also have a
response spectra for various damping ratios. The
strong motion duration appropriate to the particular
response spectra produced by recorded earthquake
design earthquake.
events are characterized by a jagged shape made up
of peaks and valleys of varying magnitude; however,
b. Procedures. There are two basic procedures
design response spectra are smoothed so that they are
for developing acceleration time histories: (1) select-
not frequency sensitive.
ing a suite of past recorded earthquake ground
motions, and (2) synthetically developing or modify-
ing one or more ground motions.
5-2.
Developing Design Response Spectra
(1) When selecting a suite of time-history
a. Deterministic and probabilistic approaches.
records for the first procedure, the intent is to cover
Design response spectra are developed by using either
the valleys of the spectrum produced by one record,
a "deterministic approach" or a "probabilistic
which fall significantly below the site-specific design
approach." The probabilistic approach is based on
response spectrum, with better matching spectral
probabilistic seismic hazard analysis methodology
values at these frequencies as produced by the other
which in essence uses the same elements as the deter-
records in the suite. It is also necessary that the
ministic approach, but adds an assessment of the
spectra produced by the suite of records not signifi-
likelihood that ground motion will occur during a
cantly exceed the site-specific design response spec-
specified time period.
trum. Primary advantage of this procedure is that the
structure is analyzed by real, natural ground motions
b. Procedures. There are two basic procedures
that are representative of what the structure could
for developing design response spectra using either
experience.
the deterministic or probabilistic approach. They are:
(1) anchoring the spectral shape to the peak ground
(2) When using the second procedure, it is possi-
acceleration; and (2) estimating the spectrum directly.
ble to either completely synthesize an accelerogram,
Although procedure (1) is more often used, the use of
or modify an actual recorded earthquake accelero-
procedure (2) is increasing, and for some situations is
gram so that the response spectrum of the resultant
preferred because it incorporates factors besides just
accelerogram closely fits or matches the site-specific
the local site conditions.
design response spectrum. The primary advantage of
this procedure is that a good fit to the design
c. Obtaining design response spectra. It is
response spectrum can be achieved with a single
beyond the scope of this EP to present the detailed
accelerogram, thus only a single dynamic analysis is
procedures for developing design response spectra, or
required.
for forecasting PGA's for design earthquakes. Refer
to ETL 1110-2-301, ETL 1110-2-303, and "Tentative
5-1
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