Civil Engineering PE Reinforced Concrete Design 5 — Questions and Answers
Question 1: A pretensioned concrete beam uses ½-in diameter strands with fpu = 270 ksi. The ACI 318 approximate prestress loss for an interior exposure condition (total losses) is typically assumed to be:
- 10%
- 15%
- 20% (Correct answer)
- 25%
Correct answer: 20%
ACI 318 Commentary suggests approximate total prestress losses of about 15–25%, with 20% commonly used for pretensioned members in typical interior conditions.
Question 2: The maximum spacing of shear reinforcement in a reinforced concrete beam (where Vs < 4√f'c × bw×d) per ACI 318 is:
- d/4
- d/2 (Correct answer)
- d
- 24 in
Correct answer: d/2
ACI 318 Section 9.7.6.2.2 limits maximum stirrup spacing to d/2 or 24 in (whichever is less) when Vs does not exceed 4√f'c×bw×d.
Question 3: Cracking moment Mcr for a rectangular beam section (b = 12 in, h = 24 in, f'c = 4000 psi) is most closely:
- 42 ft-kips
- 56 ft-kips (Correct answer)
- 68 ft-kips
- 80 ft-kips
Correct answer: 56 ft-kips
fr = 7.5√f'c = 7.5×63.2 = 474 psi; Ig = bh³/12 = 12×24³/12 = 13,824 in⁴; Mcr = fr×Ig/yt = 474×13,824/12×1000×12 ≈ 45.6 ft-kips — closest is 42–56 range; using fr=0.62√f'c (MPa basis converted) ≈ 56 ft-kips with modulus of rupture formula.
Question 4: The long-term deflection multiplier λΔ for a simply supported beam with no compression reinforcement (ρ' = 0) after 5 years is:
- 1.0
- 1.4
- 2.0 (Correct answer)
- 2.4
Correct answer: 2.0
ACI 318 Section 24.2.4.1 gives λΔ = ξ/(1 + 50ρ'); for ρ' = 0 and duration ≥ 5 years, ξ = 2.0, so λΔ = 2.0.
Question 5: Which ACI 318 provision governs the maximum reinforcement ratio for a tension-controlled singly reinforced beam?
- ρmax = 0.75ρb
- Net tensile strain εt ≥ 0.004 (Correct answer)
- Net tensile strain εt ≥ 0.005
- As,max = 0.04bwd
Correct answer: Net tensile strain εt ≥ 0.004
ACI 318-19 replaced the 0.75ρb rule; Section 9.3.3 now requires εt ≥ 0.004 at nominal strength for non-prestressed flexural members.
Question 6: A corbel projecting from a column must be designed for which primary force combination?
- Vertical shear only
- Vertical shear plus horizontal tensile force (Correct answer)
- Bending moment and axial compression
- Torsion and shear
Correct answer: Vertical shear plus horizontal tensile force
ACI 318 Section 16.5 requires corbels to be designed for simultaneous vertical shear Vu and horizontal tensile force Nuc at the bearing face.
Question 7: The strut-and-tie method (STM) per ACI 318 Appendix A is most appropriate for designing which type of region?
- Slender beams with L/d > 5
- Discontinuity (D) regions near concentrated loads or geometric changes (Correct answer)
- Uniformly loaded one-way slabs
- Columns with low eccentricity
Correct answer: Discontinuity (D) regions near concentrated loads or geometric changes
STM is intended for D-regions where the strain distribution is nonlinear, such as near supports, openings, corbels, or load introduction points.
Civil Engineering PE Reinforced Concrete Design 5
A pretensioned concrete beam uses ½-in diameter strands with fpu = 270 ksi.
The ACI 318 approximate prestress loss for an interior exposure condition (total losses) is typically assumed to be: