where:
where:
W = total seismic dead load weight of the
DD = design displacement, in inches (mm),
structure above the isolation interface as defined in
at the center of rigidity of the isolation system in the
Sections 5.3.2 and 5.5.3 of FEMA 302.
direction under consideration as prescribed by
Equation 8-1.
kMmin =
minimum effective stiffness, in
DM =
kips/inch (kN/mm), of the isolation system at the
maximum displacement, in inches
maximum displacement in the horizontal direction
(mm) at the center of rigidity of the isolation system
under consideration.
in the direction under consideration as prescribed in
Equation 8-3.
units of the acceleration of gravity, g, are in./sec2
y = the distance, in feet (mm), between the
(mm/sec2) of the units of the design displacement,
center of rigidity of the isolation system rigidity and
DD, are inches (mm).
the element of interest measured perpendicular to the
direction of seismic loading under consideration.
(e) Total displacement. The total design
e = the actual eccentricity, in feet (mm),
displacement,
DTD,
and
the
total
maximum
displacement, DTM, of elements of the isolation
measured in plan between the center of mass of the
system shall include additional displacement due to
structure above the isolation interface and the center
actual and accidental torsion calculated considering
of rigidity of the isolation system, plus accidental
the spatial distribution of the lateral stiffness of the
eccentricity, in feet (mm), taken as 5 percent of the
isolation system, and the most disadvantageous
longest plan dimension of the structure perpendicular
location of mass eccentricity.
to the direction of force under consideration.
The total design displacement, DTD,
b =
1.
the shortest plan dimension of the
and the total maximum displacement, DTM, of
structure, in feet (mm), measured perpendicular to d.
elements of an isolation system with uniform spatial
d =
distribution of lateral stiffness shall not be taken as
the longest plan dimension of the
less than that prescribed by the following equations:
structure, in feet (mm).
(3) Minimum lateral force.
12e
= DD + y 2
2
DTD
1
(8-5)
b + d
(a) Isolation system structural elements at
or below the isolation system. The isolation system,
12e
DTM = DM + y 2
the foundation, and all structural elements below the
2
1
(8-6)
b + d
8 - 15
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