What Is ACI 318?

ACI 318, "Building Code Requirements for Structural Concrete," is the primary design standard for reinforced concrete buildings in the United States. Published by the American Concrete Institute and incorporated by reference into the IBC, it governs the design of beams, columns, slabs, walls, footings, and connections in reinforced and prestressed concrete construction. The current edition is ACI 318-19.

The Strength Design Method (LRFD for Concrete)

ACI 318 uses strength design (equivalent to LRFD): the required strength (factored demand) must not exceed the design strength (nominal strength × resistance factor φ):

φMn ≥ Mu (flexure) | φVn ≥ Vu (shear) | φPn ≥ Pu (axial)

Key resistance factors (φ) in ACI 318-19: 0.90 for tension-controlled flexure, 0.75 for shear and torsion, 0.65 for compression-controlled columns, 0.65 for bearing.

Beam Flexural Design

For a simply supported beam with known moment Mu, the design procedure:

  1. Select concrete strength f'c (typically 4,000–6,000 psi) and steel yield strength fy (60,000 psi for Grade 60)
  2. Estimate beam depth h ≈ L/12 to L/8 for deflection control (ACI Table 9.3.1.1)
  3. Calculate the required steel ratio ρ from: Mu = φ × As × fy × (d − a/2) where a = As×fy / (0.85×f'c×b)
  4. Verify ρ_min ≤ ρ ≤ ρ_max (ACI requires both minimum steel for crack control and maximum steel to ensure ductile failure)
  5. Select rebar: As_required from the calculation, then choose bar size and count from standard rebar tables

Beam Shear Design

Concrete beams resist shear through a combination of concrete shear capacity (Vc) and stirrup shear capacity (Vs):

φVn = φ(Vc + Vs) ≥ Vu

Vc = 2λ√f'c × bw × d (simplified method, ACI 318-14 and earlier)
Or the more detailed Table 22.5.5.1 in ACI 318-19 which accounts for longitudinal reinforcement ratio and axial load.

When Vu > φVc/2, stirrups are required. Stirrup spacing s = Avfytd/Vs, but must not exceed ACI maximum spacing limits (d/2 or 24 inches, reduced to d/4 when Vs > 4λ√f'c bw d).

Development Length

Rebar must be extended a minimum development length (ld) past the point of peak stress to transfer force between the bar and the concrete. ACI 318 Section 25.5 provides development length equations. For a No. 8 (1-inch diameter) Grade 60 bar in 4,000 psi concrete with normal cover and spacing, ld ≈ 47 bar diameters ≈ 47 inches. Hooks and mechanical anchorage can reduce development length.

Column Design and Interaction Diagrams

Columns carry combined axial load (Pu) and bending moment (Mu). The design capacity is not a single value but a curve — the P-M interaction diagram — that shows all combinations of axial load and moment that the column can resist. The design is adequate when the point (Pu, Mu) falls inside the interaction diagram.

ACI 318 limits maximum column reinforcement ratio to 8% (to avoid constructability issues) and minimum to 1% (to prevent buckling of the column under load). Most columns are designed with 2–4% steel ratio for economy.