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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