Question 1

What is the Whitney stress block method?

Accepted Answer

The Whitney stress block replaces the actual curved concrete compressive stress distribution with an equivalent rectangular block of depth a = β₁c and uniform stress 0.85f'c. This simplification, adopted in ACI 318, gives the nominal moment capacity as Mn = As fy (d − a/2). β₁ = 0.85 for f'c ≤ 28 MPa and decreases linearly to a minimum of 0.65 for higher strengths.

Question 2

How are minimum and maximum steel areas determined?

Accepted Answer

ACI 318 minimum steel area: As_min = max(0.25√f'c/fy, 1.4/fy) × b×d, to prevent sudden brittle failure at first cracking. Maximum steel area is 75% of the balanced steel area: As_max = 0.75ρ_b × b×d, where ρ_b = (0.85β₁f'c/fy) × 600/(600+fy), ensuring ductile tension-controlled failure.

Question 3

What does the strength reduction factor φ represent?

Accepted Answer

φ accounts for variability in materials and construction quality. For flexure (tension-controlled, εs ≥ 0.005): φ = 0.90. For compression-controlled sections: φ = 0.65. A linear transition applies for 0.002 ≤ εs ≤ 0.005. The design requirement is φMn ≥ Mu, where Mu is the factored moment demand.

Question 4

How does a T-beam differ from a rectangular beam in analysis?

Accepted Answer

When the neutral axis falls within the flange (a ≤ hf), a T-beam is analyzed identically to a rectangular beam of width equal to the effective flange width. When a > hf, the compression zone extends into the web and the section is split into two rectangular blocks for calculation. T-beams generally have higher moment capacity due to the larger compression area.

Reinforced Concrete Section Analysis (Flexure & Shear)

ACI 318 Design Equations