100 percent, but not less than 80 percent of VS,
T =
elastic, fixed-base period of the
provided time-history analysis is used for design of
structure above the isolation system as determined by
the structure.
Section 5.3.3 of FEMA 302.
TD = effective period, in seconds, of the
(4) Ground motion.
seismically
isolated
structure
at
the
design
(a) Design spectra. A design spectrum shall
displacement in the direction under consideration as
be constructed for the design earthquake. This design
prescribed by Equation 8-2.
spectrum shall be taken as not less than the design
TM = effective period, in seconds, of the
earthquake response spectrum given in Figure 3-2.
Properly
substantiated
site-specific
spectra
are
seismically isolated structure at the maximum
required for the design of all structures located on a
displacement in the direction under consideration as
Class E or F site, or located at a site with S1 greater
prescribed by Equation 8-4.
than 0.60g.
Structures that do not require site-
specific spectra and for which sitespecific spectra
(3)
Structural elements above the isolation
have not been calculated shall be designed using the
system.
The design lateral shear force on the
response spectrum shape given in Figure 3-2.
structure above the isolation system, if regular in
configuration, shall be taken as not less than 80
Exception:
percent of VS, as prescribed by Equation 8-8 and the
If a site-specific spectrum is
calculated for the design earthquake, the design
limits specified by Section 13.3.4.3 of FEMA 302.
spectrum is permitted to be taken as less than 100
Exception: The design lateral shear force on
percent, but not less than 80 percent, of the design
earthquake response spectrum given in Figure 3-2.
the structure above the isolation system, if regular in
configuration, is permitted to be taken as less than 80
percent, but not less than 60 percent of VS, provided
A design spectrum shall be constructed for the
maximum considered earthquake.
This design
time-history analysis is used for design of the
spectrum shall be taken as not less than 1.5 times the
structure.
design earthquake response spectrum given in Figure
3-2. This design spectrum shall be used to determine
The design lateral shear force on the structure above
the total maximum displacement and overturning
the isolation system, if irregular in configuration,
shall be taken as not less than VS, as prescribed by
forces for design and testing of the isolation system.
Equation 8-8 and the limits specified by section
Exception:
If a site-specific spectrum is
13.3.4.3 of FEMA 302.
calculated for the maximum considered earthquake,
Exception: The design lateral shear force on
the design spectrum is permitted to be taken as less
than 100 percent, but not less than 80 percent of 1.5
the structure above the isolation system, if irregular
in configuration, is permitted to be taken as less than
8 - 18
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