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he elastic deformations calculated under reduced forces (see Section C12.8.6) are multiplied by the deflection amplification factor, Cd, to estimate the deformations likely to result from the design earthquake ground motion.
The application of the factor R/Ie to the design response spectrum reduces not only design forces but also displacements. To compensate for this, ASCE 7 requires that the deflections at level x at the center of mass, δx, be determined in accordance with where Cd is from Table 11.7, Ie is from Table 1.1, and δxe is the deflection determined by elastic analysis with seismic forces reduced by factor R/Ie. Using a value Cd equal to R would result in the displacement δx being equal to δxe, which would be consistent with the equal displacement rule.
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Redundancy factor, ρ, is automatically obtained horizontal seismic forces according to ASCE 7-16 Section 12.3.4. |
A redundancy factor, ρ, shall be assigned to the seismic force-resisting system in each of two orthogonal directions for all structures in accordance with this section.
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12.3.4.1 Conditions Where Value of ρ is 1.0
The value of ρ is permitted to equal 1.0 for the following:
Structures assigned to Seismic Design Category B or C
Drift calculation and P-delta effects
Design of nonstructural components
Design of nonbuilding structures that are not similar to buildings
Design of collector elements, splices, and their connections for which the seismic load effects, including overstrength of Section 12.4.3, are used
Design of members or connections where the seismic load effects, including overstrength of Section 12.4.3, are required for design
Diaphragm loads determined using Eq. (12.10-1), including the limits imposed by Eqs. (12.10-2) and (12.10-3)
Structures with damping systems designed in accordance with Chapter 18
Design of structural walls for out-of-plane forces, including their anchorage
12.3.4.2 Redundancy Factor, ρ, for Seismic Design Categories D through F
For structures assigned to Seismic Design Category D and having extreme torsional irregularity as defined in Table 12.3-1, Type 1b, ρ shall equal 1.3.
For other structures assigned to Seismic Design Category D and for structures assigned to Seismic Design Categories E or F, ρ shall equal 1.3 unless one of the following two conditions is met, whereby ρ is permitted to be taken as 1.0. A reduction in the value of ρ from 1.3 is not permitted for structures assigned to Seismic Design Category D that have an extreme torsional irregularity (Type 1b). Seismic Design Categories E and F are not also specified because extreme torsional irregularities are prohibited (see Section 12.3.3.1).
a. Each story resisting more than 35% of the base shear in the direction of interest shall comply with Table 12.3-3.
b. Structures are regular in plan at all levels provided that the seismic force-resisting systems consist of at least two bays of seismic force-resisting perimeter framing on each side of the structure in each orthogonal direction at each story resisting more than 35% of the base shear. The number of bays for a shear wall shall be calculated as the length of shear wall divided by the story height or two times the length of shear wall divided by the story height, hsx, for light-frame construction.
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