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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Topics covered in this section:
Page Tree
Eh = Horizontal seismic load effect
hwall = height of shear wall
hwp = height of wall pier
Lwall = length of shear wall
Lwp = length of wall pier
ρ = Redundancy factor
QE = Effects of horizontal seismic forces from V or Fp
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The Redundancy factor, ρ, is used for the seismic force-resisting system in each of two orthogonal directions for all structures in accordance with Eq. 12.4-3.
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According to 12.3.4.1 - ρ is 1.0 for the following:
Structures assigned to Seismic Design Category B or C
P-delta effects and story drift calculation
Design of collector elements, splices, and their connections using the seismic load effects with the overstrength factor of Section 12.4.3
Design of members or connections where the seismic load effects, including overstrength factor
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 per ASCE Chapter 18
Design of structural walls for out-of-plane forces, including their anchorage
Redundancy Factor, ρ, for Seismic Design Categories D through F
For structures assigned to Seismic Design Category D with extreme torsional irregularity as defined in Table 12.3-1, Type 1b, ρ equals 1.3.
For other structures assigned to Seismic Design Category D and for structures assigned to Seismic Design Categories E or F, ρ is controlled automatically. Moreover, the software uses 1.3 unless one of the following conditions occurs.
Seismic Design Categories E and F are not allowed to use because extreme torsional irregularities are prohibited.
Each story resisting more than 35% of the base shear in the direction of interest according to Table 12.3-3.
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.
Lateral Force-Resisting Element | Requirement |
|---|---|
Braced frames | Removal of an individual brace, or connection thereto, would not result in more than a 33% reduction in story strength, nor does the resulting system have an extreme torsional irregularity (horizontal structural irregularity Type 1b). |
Moment frames | Loss of moment resistance at the beam-to-column connections at both ends of a single beam would not result in more than a 33% reduction in story strength; nor does the resulting system have an extreme torsional irregularity (horizontal structural irregularity Type 1b). |
Shear walls or wall piers with a height-to-length ratio greater than 1.0 | Removal of a shear wall or wall pier with a height-to-length ratio greater than 1.0 within any story, or collector connections thereto, would not result in more than a 33% reduction in story strength; nor does the resulting system have an extreme torsional irregularity (horizontal structural irregularity Type 1b). The shear wall and wall pier height-to-length ratios are determined as shown in Figure. |
Cantilever columns | Loss of moment resistance at the base connections of any single cantilever column would not result in more than a 33% reduction in story strength; nor does the resulting system have an extreme torsional irregularity (horizontal structural irregularity Type 1b). |
Other | No requirements. |
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