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How does ideCAD define the redundancy factor, ρ, according to ASCE 7-16?
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SYMBOLS
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
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C12.3.4 Redundancy
The standard introduces a revised redundancy factor, ρ, for structures assigned to Seismic Design Category D, E, or F to quantify redundancy. The value of this factor is either 1.0 or 1.3.This factor has the effect of reducing the response modification coefficient, R, for less redundant structures, thereby increasing the seismic demand. The factor is specified irecognition of the need to address the issue of redundancy in the design.
The desirability of redundancy, or multiple lateral force-resisting load paths, has long been recognized. The redundancy provisions of this section reflect the belief that an excessive loss of story shear strength or development of an extreme torsional irregularity (Type 1b) may lead to structural failure. The value of ρ determined for each direction may differ.
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This section provides a convenient list of conditions where ρ is 1.0. C12.3.4.2
QE = Effects of horizontal seismic forces from V or Fp
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Redundancy is an important property for structures designed with the expectation that damage will occur. Redundant structures have alternative load paths so that if some elements are severely damaged and lose load-carrying capacity, other elements will be able to continue to provide a safe load path. Adequate redundancy is ensured when a large number of plastic hinges must form throughout the structure progressively before the formation of a mechanism and when no element is required to provide the full seismic resistance of the structure. The redundancy factor, ρ, is automatically obtained horizontal seismic forces according to ASCE 7-16 Section 12.3.4.
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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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The Redundancy factor, 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. There are two approaches to establishing a redundancy factor, ρ, of 1.0. Where neither condition is satisfied, ρ is taken as equal to 1.3. It is permitted to take ρ equal to 1.3 without checking either condition. A reduction in the value of ρ from 1.3 is not permitted for
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 that have an extreme torsional irregularity (Type 1b). 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 also specified allowed to use because extreme torsional irregularities are prohibited (see Section 12.3.3.1).
The first approach is a check of the elements outlined in
Each story resisting more than 35% of the base shear in the direction of interest according to Table 12.3-3
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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 |
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Braced frames | Removal of an individual brace, or connection thereto, would not result in more than a 33% reduction in story strength |
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, nor does the resulting system have an extreme torsional |
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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 |
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ratio greater than 1.0 within any story |
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, 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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