4-3.
Seismic Design Categories.
taken as the maximum of the sum of the shears in any
two adjacent columns, in the plane of a moment
All buildings shall be assigned a Seismic Design
frame, divided by the story shear.
For columns
Category based on their assigned Seismic Use Group
common
to
two
bays
with
moment-resisting
and their applicable spectral acceleration coefficients,
connections on opposite sides at the level under
SDS and SDI for Ground Motion A. Each building or
consideration, 70 percent of the shear in that column
structure shall be assigned to the more severe Seismic
may be used in the column shear summation. For
Design Category in accordance with Table 4-2a or
shear walls, rmax x shall be taken as equal to the shear
Table 4-2b. The category designations are used in
in the most heavily loaded wall or wall pier
FEMA 302 to determine permissible structural
multiplied by 10/lw (the metric equivalent is 3.3 (lw),
systems, limitations on height and irregularity, and
where lw is the wall or wall pier length in feet (m),
coefficients related to overstrength and drift.
divided by the story shear. For dual systems, rmax x
4-4.
Redundancy.
shall be taken as the maximum value as defined
FEMA 302 prescribes a reliability factor, D, to be
above considering all lateral-load-resisting elements
in the story. The lateral loads shall be distributed to
assigned to all buildings based on the extent of
elements based on relative rigidities considering the
structural redundancy inherent in the lateral-force-
interaction of the dual system. For dual systems, the
resisting system. The value of D may be taken as 1.0
value of D need not exceed 80 percent of the value
for buildings in Seismic Design Categories A, B, C.
calculated above.
For buildings in Seismic Design Categories D, E, and
F, D shall be taken as the largest of the values of
Ax = the floor area in square feet of the
?x calculated at each story of the building, x, in
diaphragm level immediately above the story.
accordance with the following equation:
The metric equivalent of Equation 4-1 is:
6.1
20
ρx = 2 -
?x = 2 -
(4-1)
rmax x Ax
rmax x Ax
where Ax is in square meters.
where:
The value of D need not exceed 1.5, which is
permitted to be used for any structure. The value of D
rmax x = the ratio of the design story shear
shall not be taken as less than 1.0.
resisted by the single element carrying the most shear
force in the story to the total story shear, for a given
Exception: For structures with lateral-force-resisting
direction of loading. For braced frames, the value of
systems in any direction comprised solely of special
rmax x is equal to the lateral-force component in the
moment frames, the lateral-force-resisting system
most heavily loaded brace element divided by the
shall be configured such that the value of D calculated
For moment frames, rmax x shall be
story shear.
in accordance with this section does not exceed 1.25.
4-5
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