A building is classified as Seismic Design Category D. According to CBC, what is the required redundancy factor (ρ) for the seismic force-resisting system if the building has adequate redundancy?
Correct Answer
C) ρ = 1.0
When a building has adequate redundancy in its seismic force-resisting system, the redundancy factor ρ = 1.0. If redundancy is inadequate, ρ = 1.3 is used to increase design forces.
Why This Is the Correct Answer
CBC (ASCE 7 as adopted) defines two values for the redundancy factor ρ: 1.0 when the seismic force-resisting system is deemed to have adequate redundancy (sufficient lateral load paths so that removing any single element does not reduce story strength by more than 33%), and 1.3 when redundancy is inadequate. For SDC D with adequate redundancy, ρ = 1.0.
Why the Other Options Are Wrong
Option A: ρ = 1.5
ρ = 1.5 is not a code-defined value in the CBC/ASCE 7 redundancy factor framework. This value does not appear in the standard; selecting it indicates confusion with other load factors or older code editions.
Option B: ρ = 1.3
ρ = 1.3 applies when the building does NOT have adequate redundancy in SDC D, E, or F — the opposite of the scenario described. The question explicitly states adequate redundancy, so 1.3 is incorrect here.
Option D: ρ = 2.0
ρ = 2.0 is not a standard code value for the redundancy factor. It may be confused with strength reduction factors or overstrength factors (Ω₀) used in seismic design, but it does not apply here.
Memory Technique
Think of redundancy as a safety net: if you have a good net (adequate redundancy), the penalty is zero — multiply by 1.0, which changes nothing. If your net has holes (inadequate redundancy), you pay the 1.3 penalty. 'Adequate = 1.0, Inadequate = 1.3.'
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