Civil Engineering PE Steel Structure Design Methods 2 — Questions and Answers
Question 1: In LRFD design, which load combination governs for a steel beam subjected to dead load D = 20 kips and live load L = 40 kips?
- 1.2D + 1.6L = 88 kips (Correct answer)
- 1.4D = 28 kips
- 1.2D + 1.0L = 64 kips
- 1.0D + 1.0L = 60 kips
Correct answer: 1.2D + 1.6L = 88 kips
AISC LRFD load combination 1.2D + 1.6L = 1.2(20) + 1.6(40) = 88 kips governs over 1.4D = 28 kips.
Question 2: What does the resistance factor φ = 0.90 represent in LRFD steel design for tension yielding?
- A safety margin accounting for variability in material strength and fabrication (Correct answer)
- The ratio of live load to dead load
- A reduction for long-term creep effects
- The inverse of the factor of safety used in ASD
Correct answer: A safety margin accounting for variability in material strength and fabrication
φ = 0.90 for tension yielding represents the probability-based reduction accounting for material and fabrication variability to ensure adequate reliability.
Question 3: A W-shape beam has a plastic section modulus Zx = 120 in³ and Fy = 50 ksi. What is the plastic moment Mp?
- 6,000 kip-in (Correct answer)
- 5,400 kip-in
- 4,800 kip-in
- 7,200 kip-in
Correct answer: 6,000 kip-in
Mp = Fy × Zx = 50 ksi × 120 in³ = 6,000 kip-in.
Question 4: In ASD for steel, the allowable bending stress Fb for a compact section in a braced frame with Fy = 36 ksi is typically:
- 0.66Fy = 23.76 ksi (Correct answer)
- 0.60Fy = 21.6 ksi
- 0.75Fy = 27 ksi
- 0.50Fy = 18 ksi
Correct answer: 0.66Fy = 23.76 ksi
ASD allowable bending stress for compact laterally braced beams is Fb = 0.66Fy per AISC 9th edition.
Question 5: Which steel design method explicitly accounts for the probability of failure through reliability index β?
- LRFD (Correct answer)
- ASD
- Both ASD and LRFD equally
- Allowable stress design only for fatigue
Correct answer: LRFD
LRFD was calibrated using reliability theory to achieve a target reliability index β ≈ 3.0 for typical members.
Question 6: For a tension member, AISC LRFD requires checking two limit states. Which pair is correct?
- Yielding on gross area and fracture on net area (Correct answer)
- Yielding on net area and fracture on gross area
- Bearing and shear rupture only
- Block shear and bearing only
Correct answer: Yielding on gross area and fracture on net area
AISC requires φPn for both gross-section yielding (φ=0.90, Pn=FyAg) and net-section fracture (φ=0.75, Pn=FuAeU).
Question 7: In steel design, the shear factor of safety in ASD is typically set at approximately:
- 1.67 (Fv = 0.40Fy) (Correct answer)
- 2.0 (Fv = 0.50Fy)
- 1.50 (Fv = 0.45Fy)
- 2.5 (Fv = 0.33Fy)
Correct answer: 1.67 (Fv = 0.40Fy)
ASD allowable shear stress is Fv = 0.40Fy, corresponding to a safety factor of about 1.67 against shear yielding.
Civil Engineering PE Steel Structure Design Methods 2
In LRFD design, which load combination governs for a steel beam subjected to dead load D = 20 kips and live load L = 40 kips?