According to CBC Chapter 18, what is the minimum reinforcement ratio for concrete beams in special moment frames?
Correct Answer
D) 0.0025
The CBC requires a minimum reinforcement ratio of 0.0025 for concrete beams in special moment frames to ensure minimum ductility and prevent brittle failure.
Why This Is the Correct Answer
CBC Chapter 18 (Concrete), referencing ACI 318 seismic provisions, requires a minimum longitudinal reinforcement ratio (ρ_min) of 0.0025 for beams in special moment frames. This lower bound ensures the beam has enough ductility to avoid brittle flexural failure during seismic events without being over-reinforced.
Why the Other Options Are Wrong
Option A: 0.0050
0.0050 is double the required minimum. While higher reinforcement improves capacity, the code-specified minimum is 0.0025 — using 0.0050 as the minimum would be overly conservative and does not match ACI 318 provisions.
Option B: 0.0033
0.0033 is the standard minimum flexural reinforcement ratio (1.4/fy for Grade 60 steel) used for non-seismic beams. Special moment frames have their own lower minimum of 0.0025 under ACI 318 Chapter 18 provisions.
Option C: 0.0075
0.0075 is associated with maximum reinforcement limits or specific detailing zones, not the minimum reinforcement ratio for special moment frame beams.
Memory Technique
Special Moment Frame minimum = 0.0025. Remember: 'Special = 25' — 25 ten-thousandths. It is a lower minimum because ductility detailing (hoops, confinement) compensates for lighter longitudinal steel.
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According to CBC seismic provisions, what is the concept behind the 'strong column, weak beam' design philosophy?
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