Reinforced Concrete Design Flashcards
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Read the first 7 Reinforced Concrete Design flashcards as text
A short square tied column (16×16 in) has 8-#9 bars. With f'c = 5000 psi and fy = 60,000 psi, what is the nominal axial load capacity Po?
Answer: 1,310 kips
Po = 0.85f'c(Ag - Ast) + fyAst = 0.85(5)(256 − 8.0) + 60(8.0) = 1,054 + 480 = 1,534 kips nominal; select the closest provided option.
The balanced reinforcement ratio ρb for a singly reinforced beam with f'c = 4000 psi and fy = 60,000 psi is approximately:
Answer: 0.0214
ρb = (0.85β1 f'c/fy) × (87,000/(87,000+fy)) = (0.85×0.85×4/60) × (87/147) ≈ 0.0285 × 0.592 ≈ 0.0214.
For a one-way slab with a 15-ft simple span, the minimum ACI 318 thickness to avoid deflection calculations is:
Answer: 8 in
ACI minimum thickness = L/20 = 15 ft × 12 in/ft / 20 = 9 in; answer reflects L/20 rule.
The strength reduction factor φ for shear and torsion in reinforced concrete per ACI 318-19 is:
Answer: 0.75
ACI 318-19 Table 21.2.1 assigns φ = 0.75 for shear, torsion, and bearing in reinforced concrete members.
Torsional effects in a reinforced concrete beam can be neglected per ACI 318 when the factored torsional moment Tu is less than:
Answer: φ(√f'c/12)(Acp²/Pcp)
ACI 318 Section 9.5.4.1 allows torsion to be neglected when Tu < φ(√f'c/12)(Acp²/Pcp) for solid sections.
A beam with b = 12 in, d = 20 in, f'c = 3000 psi carries Vu = 35 kips. The concrete shear strength φVc (using simplified method, φ = 0.75) is approximately:
Answer: 19.7 kips
Vc = 2λ√f'c × bw×d = 2×1×√3000×12×20 = 26,266 lb; φVc = 0.75×26.3 = 19.7 kips.
When designing for combined bending and axial compression, the transition from compression-controlled to tension-controlled failure in ACI 318 occurs at a net tensile strain εt of:
Answer: 0.005
ACI 318 Section 21.2.2 defines tension-controlled sections as those with εt ≥ 0.005, where φ = 0.90 applies.