hx = height above the base Level x.
system shall be taken as not less than 80 percent of
DTM, as specified by Paragraph 8-3e(2)(e).
The
wi =
portion of w that is located at or
design lateral shear force on the isolation system and
assigned to Level I, respectively.
structural elements below the isolation system shall
be taken as not less than 90 percent of Vb as
hi = height above the base Level I.
prescribed by Equation 8-7. The limits of Paragraphs
8-3e(3)(a) and (b) shall be evaluated using values of
DTD and DTM determined in accordance with
At each level designated as x, the force, Fx, shall be
′
applied over the area of the structure in accordance
Paragraphs 8-3e(2)(a) and (c), except that DD is
with the mass distribution at the level. Stresses in
′
permitted to be used on lieu of DD and DM is
each structural element shall be calculated as the
′
permitted to be used in lieu of DM where D D and
effect of force, Fx, applied at the appropriate levels
′
above the base.
DM are prescribed by the following equations:
(5) Drift limits. The maximum interstory drift
DD
′
DD =
(8-10)
of the structure above the isolation system shall not
2
T
exceed 0.015hsx.
The drift shall be calculated by
1+
T
D
Equation 5.3.7-1 of FEMA 302, with the Cd factor of
the isolated structure equal to the RI factor defined in
Paragraph 8-3e(3)(b).
DM
′
DM =
(8-11)
2
T
1+
f.
Dynamic Lateral Response Procedure.
T
M
(1) General. Except as required by Paragraph
where:
8-3d, every seismically isolated structure or portion
thereof may be designed and constructed to resist
DD = design displacement, in inches (mm),
earthquake displacements and forces as specified in
at the center of rigidity of the isolation system in the
this Paragraph and the applicable requirements of
direction under consideration as prescribed by
Section 5.4 of FEMA 302.
Equation 8-1.
(2) Isolation system and structural elements
DM =
maximum displacement in inches
below the isolation system.
(mm), at the center of rigidity of the isolation system
in the direction under consideration as prescribed by
(a)
The total design displacement of the
Equation 8-3.
isolation system shall be taken as not less than 90
percent of DTD as specified by Paragraph 8-3e(2)(e).
The total maximum displacement of the isolation
8 - 17
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