12.7.2 Effective Seismic Weight
The effective seismic weight,
W
, of a structure
shall include the dead load, as defined in Section 3.1,
above the base and other loads above the base as
listed below:
1. In areas used for storage, a minimum of 25 percent
of the floor live load shall be included.
Table 12.6-1 Permitted Analytical Procedures
Seismic
Design
Category
Structural Characteristics
Equivalent Lateral
Force Analysis,
Section 12.8
a
Modal Response
Spectrum Analysis,
Section 12.9
a
Seismic Response
History Procedures,
Chapter 16
a
B, C
All structures
P
P
P
D, E, F
Risk Category I or II buildings not exceeding 2
stories above the base
P
P
P
Structures of light frame construction
P
P
P
Structures with no structural irregularities and not
exceeding 160 ft in structural height
P
P
P
Structures exceeding 160 ft in structural height
with no structural irregularities and with
T <
3
.
5
T
s
P
P
P
Structures not exceeding 160 ft in structural
height and having only horizontal irregularities of
Type 2, 3, 4, or 5 in Table 12.3-1 or vertical
irregularities of Type 4, 5a, or 5b in Table 12.3-2
P
P
P
All other structures
NP
P
P
a
P: Permitted; NP: Not Permitted;
T
s
=
S
D
1
/
S
DS
.

MINIMUM DESIGN LOADS
89
EXCEPTIONS:
a. Where the inclusion of storage loads adds no
more than 5% to the effective seismic weight at
that level, it need not be included in the
effective seismic weight.
b. Floor live load in public garages and open
parking structures need not be included.
2. Where provision for partitions is required by
Section 4.2.2 in the floor load design, the actual
partition weight or a minimum weight of 10 psf
(0.48 kN/m
2
) of floor area, whichever is greater.
3. Total operating weight of permanent equipment.
4. Where the flat roof snow load,
P
f
, exceeds 30 psf
(1.44 kN/m
2
), 20 percent of the uniform design
snow load, regardless of actual roof slope.
5. Weight of landscaping and other materials at roof
gardens and similar areas.
12.7.3 Structural Modeling
A mathematical model of the structure shall be
constructed for the purpose of determining member
forces and structure displacements resulting from
applied loads and any imposed displacements or
P-delta effects. The model shall include the stiffness
and strength of elements that are significant to the
distribution of forces and deformations in the structure
and represent the spatial distribution of mass and
stiffness throughout the structure.
In addition, the model shall comply with the
following:
a. Stiffness properties of concrete and masonry
elements shall consider the effects of cracked
sections.
b. For steel moment frame systems, the contribution
of panel zone deformations to overall story drift
shall be included.
Structures that have horizontal structural irregular-
ity Type 1a, 1b, 4, or 5 of Table 12.3-1 shall be
analyzed using a 3-D representation. Where a 3-D
model is used, a minimum of three dynamic degrees of
freedom consisting of translation in two orthogonal
plan directions and rotation about the vertical axis shall
be included at each level of the structure. Where the
diaphragms have not been classified as rigid or flexible
in accordance with Section 12.3.1, the model shall

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