BCE Reinforced Concrete Design 1 — Questions and Answers
Question 1: In the ACI 318 strength design method, which load combination is used for dead load (D) and live load (L)?
- U = 1.0D + 1.0L
- U = 1.2D + 1.6L (Correct answer)
- U = 1.4D + 1.7L
- U = 1.5D + 1.5L
Correct answer: U = 1.2D + 1.6L
ACI 318-19 specifies U = 1.2D + 1.6L as the governing load combination for dead and live loads in the strength design (LRFD) method.
Question 2: The strength reduction factor φ for tension-controlled beams in ACI 318 is:
- 0.65
- 0.75
- 0.90 (Correct answer)
- 1.00
Correct answer: 0.90
ACI 318 assigns φ = 0.90 for tension-controlled sections (net tensile strain ≥ 0.005), reflecting high confidence in flexural strength calculations when ductile failure governs.
Question 3: What does the balanced reinforcement ratio (ρb) represent in beam design?
- The minimum steel ratio to prevent shrinkage cracking
- The steel ratio at which concrete and steel simultaneously reach their limiting strains (Correct answer)
- The maximum steel ratio allowed by ACI
- The ratio at which the beam has equal top and bottom steel
Correct answer: The steel ratio at which concrete and steel simultaneously reach their limiting strains
The balanced reinforcement ratio ρb is the theoretical steel ratio at which concrete reaches εcu = 0.003 and tensile steel reaches yield strain simultaneously at the same load.
Question 4: For a simply supported rectangular beam, the location of maximum bending moment is at:
- The supports
- Midspan (for uniform load) (Correct answer)
- The quarter points
- The third points
Correct answer: Midspan (for uniform load)
For a simply supported beam with a uniformly distributed load, the maximum bending moment occurs at midspan and equals wL²/8.
Question 5: Which mode of shear failure in a reinforced concrete beam is prevented by stirrups?
- Flexural compression failure
- Diagonal tension cracking leading to shear failure (Correct answer)
- Bond splitting failure
- Punching shear failure
Correct answer: Diagonal tension cracking leading to shear failure
Stirrups (transverse reinforcement) cross diagonal tension cracks, preventing sudden shear failure by transferring tensile forces across cracks and confining the concrete.
Question 6: The minimum flexural reinforcement (ρmin) requirement in ACI 318 beams is intended to:
- Prevent excessive deflection under service loads
- Ensure the beam does not fail below the cracking moment (Correct answer)
- Limit crack widths at service loads
- Provide fire resistance to the section
Correct answer: Ensure the beam does not fail below the cracking moment
Minimum reinforcement ensures that the flexural strength of the cracked section (φMn) exceeds the cracking moment (Mcr), preventing sudden brittle failure upon first cracking.
BCE Reinforced Concrete Design 1
In the ACI 318 strength design method, which load combination is used for dead load (D) and live load (L)?