Civil Engineering PE Reinforced Concrete Design 2 — Questions and Answers
Question 1: A rectangular beam has b = 14 in, d = 22 in, f'c = 4000 psi, fy = 60,000 psi, and As = 3.0 in². What is the approximate nominal moment capacity Mn?
- 210 ft-kips
- 248 ft-kips (Correct answer)
- 185 ft-kips
- 270 ft-kips
Correct answer: 248 ft-kips
With a = Asfy/(0.85f'cb) = 3.0×60/(0.85×4×14) ≈ 3.78 in, Mn = Asfy(d - a/2) = 3.0×60×(22 - 1.89)/12 ≈ 248 ft-kips.
Question 2: According to ACI 318, the minimum clear cover for reinforcement in a concrete beam exposed to weather (deicing salts) is:
- 1.5 in
- 2.0 in
- 2.5 in (Correct answer)
- 3.0 in
Correct answer: 2.5 in
ACI 318 Table 20.6.1.3 requires 2.5 in clear cover for #6 through #11 bars exposed to weather (deicing salts).
Question 3: For a doubly reinforced concrete beam, the compression steel stress f's is typically determined by:
- Assuming f's = fy always
- Using strain compatibility with εs' = (c - d')/c × 0.003 (Correct answer)
- Setting f's = 0.85f'c
- Using f's = 0.5fy as an ACI simplification
Correct answer: Using strain compatibility with εs' = (c - d')/c × 0.003
Strain compatibility requires εs' = 0.003(c − d')/c, then f's = Es×εs' ≤ fy to verify whether compression steel yields.
Question 4: A T-beam floor system has a 5-in slab, beam web width bw = 12 in, center-to-center spacing of beams = 72 in, and span = 20 ft. The effective flange width per ACI 318 is:
- 48 in
- 60 in (Correct answer)
- 66 in
- 72 in
Correct answer: 60 in
Effective flange width = min(span/4, bw + 8hf each side, center-to-center) = min(60 in, 12+80=92 in, 72 in) = 60 in.
Question 5: The ACI 318 minimum shear reinforcement for a beam is not required when:
- Vu > φVc/2
- Vu ≤ φVc/2 (Correct answer)
- Vu > φVc
- The beam depth exceeds 24 in
Correct answer: Vu ≤ φVc/2
ACI 318 Section 9.6.3.1 allows omission of minimum shear reinforcement when Vu ≤ φVc/2 for most beams.
Question 6: The development length for a #8 top bar (f'c = 4000 psi, fy = 60,000 psi, normal weight concrete, cover = 1.5 in, no transverse reinforcement) is approximately:
- 36 in
- 48 in
- 60 in (Correct answer)
- 72 in
Correct answer: 60 in
Using ld = (3fy/(40λ√f'c)) × (ψt ψe ψs / ((cb+Ktr)/db)) × db with ψt=1.3 (top bar), (cb+Ktr)/db limited to 2.5, gives approximately 60 in.
Question 7: Which failure mode must be prevented to ensure ductile behavior in a flexural member per ACI 318?
- Tension steel yielding before concrete crushes
- Concrete crushing before tension steel yields (Correct answer)
- Diagonal tension failure
- Bond failure at midspan
Correct answer: Concrete crushing before tension steel yields
ACI 318 limits reinforcement ratio to ensure the tension steel yields before concrete reaches εu = 0.003, guaranteeing ductile behavior.
Civil Engineering PE Reinforced Concrete Design 2
A rectangular beam has b = 14 in, d = 22 in, f'c = 4000 psi, fy = 60,000 psi, and As = 3.0 in².
What is the approximate nominal moment capacity Mn?