The maximum considered earthquake shall be used to
times the design earthquake response spectrum given
calculate the total maximum displacement of the
in Figure 3-2.
isolation system.
(b)
Time histories.
Pairs of appropriate
(c) Response-spectrum analysis. Response-
horizontal ground-motion time-history components
spectrum analysis shall be performed using a modal
shall be selected and scaled from not less than three
damping value for the fundamental mode in the
recorded events. Appropriate time histories shall be
direction of interest not greater than the effective
based on recorded events with magnitudes, fault
damping of the isolation system or 30 percent of
distances, and source mechanisms that are consistent
critical, whichever is less. Modal damping values for
with those that control the design earthquake (or
higher modes shall be selected consistent with those
maximum considered earthquake).
Where three
appropriate for response spectrum analysis of the
appropriate recorded ground-motion time-history
structure above the isolation system with a fixed
pairs are not available, appropriate simulated ground-
base. Response-spectrum analysis used to determine
motion time-history pairs are permitted to be used to
the total design displacement and the total maximum
make up the total number required. For each pair of
displacement shall include simultaneous excitation of
horizontal ground-motion components, the square
the model by 100 percent of the most critical
root sum of the squares of the 5 percent damped
direction of ground motion, and 30 percent of the
spectrum of the scaled, horizontal components shall
ground motion on the orthogonal axis.
The
be constructed. The motions shall be scaled such that
maximum displacement of the isolation system shall
the average value of the square-root-sum-of-the
be calculated as the vectorial sum of the two
squares spectra does not fall below 1.3 times the 5
orthogonal displacements. The design shear at any
percent damped spectrum of the design earthquake
story shall not be less than the story shear obtained
(or maximum considered earthquake) by more than
using Equation 8-9 and a value of VS taken as that
10 percent for periods from 0.5TD seconds to 1.25 TM
equal to the base shear obtained from the response-
seconds.
spectrum analysis in the direction of interest.
(5) Analytical procedure.
(d)
Time-history analysis.
Time-history
analysis shall be performed with at least three
(a) General. Response-spectrum and time-
appropriate
pairs
of
horizontal
time-history
history analyses shall be performed in accordance
components as defined in Paragraph 8-3f(4)(b). Each
with Section 5.4 of FEMA 302, and the requirements
pair of time histories shall be applied simultaneously
of the following Paragraphs.
to the model considering the most disadvantageous
location of mass eccentricity.
The maximum
(b)
Input earthquake.
The design
displacement of the isolation system shall be
earthquake shall be used to calculate the total design
calculated from the vectorial sum of the two
displacement of the isolation system and the lateral
orthogonal components at each time step.
The
forces and displacements of the isolated structure.
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