ACI 318-19

ACI 318-19 Section 8.10 | Two-Way Slab Direct Design Method (DDM)

Two-way slabs transfer floor loads in two perpendicular directions simultaneously. The Direct Design Method (DDM) first computes the total static moment Mo across the clear span, splits it into negative support moments and positive midspan moments, and then apportions those moments laterally between the stiffer Column Strip and softer Middle Strip.

Code Equation

Governing Formula

M_o = \frac{q_u \ell_2 \ell_n^2}{8}, \qquad M_{\text{ neg,int }} = 0.65 M_o, \qquad M_{\text{ pos,int }} = 0.35 M_o

Standard: ACI 318-19Dual US Customary & SI Metric

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Notation & Variables

M_o

Total factored static moment in a two-way slab span

kip-ft or kN·m

q_u

Factored uniform area load per unit area (1.2D + 1.6L)

psf or kPa

\ell_2

Transverse span width center-to-center of supports

ft or m

\ell_n

Clear span in the direction of moments (face-to-face of columns, ≥ 0.65 l1)

ft or m

Worked Example

Compute clear span ln

For a 24 ft × 24 ft flat plate bay with 20" square columns (1.67 ft): ln = 24 - 1.67 = 22.33 ft.

Calculate total static moment Mo

Given factored pressure qu = 0.220 ksf: Mo = (0.220 × 24 × 22.33²) / 8 = 329.1 kip-ft.

M_o = \frac{0.220 \times 24 \times 22.33^2}{8} = 329.1 \text{ kip-ft }

Distribute to interior span sections

Interior negative moment = 0.65(329.1) = 213.9 kip-ft; positive midspan moment = 0.35(329.1) = 115.2 kip-ft.

Governing Criteria

When does this govern?

Governs regular flat plate and flat slab residential/commercial floor systems with at least 3 continuous spans in each direction.

Common Pitfall

Common Mistake

Using center-to-center span l1 instead of face-to-face clear span ln in the squared term of Mo, which overestimates design moments by 15% to 20%.

Related Resources

ACI 318-19 Section 8.10 | Two-Way Slab Direct Design Method (DDM)

Notation & Variables

M_o

Total factored static moment in a two-way slab span

kip-ft or kN·m

q_u

Factored uniform area load per unit area (1.2D + 1.6L)

psf or kPa

\ell_2

Transverse span width center-to-center of supports

ft or m

\ell_n

Clear span in the direction of moments (face-to-face of columns, ≥ 0.65 l1)

ft or m

Worked Example

Compute clear span ln

For a 24 ft × 24 ft flat plate bay with 20" square columns (1.67 ft): ln = 24 - 1.67 = 22.33 ft.

Calculate total static moment Mo

Given factored pressure qu = 0.220 ksf: Mo = (0.220 × 24 × 22.33²) / 8 = 329.1 kip-ft.

M_o = \frac{0.220 \times 24 \times 22.33^2}{8} = 329.1 \text{ kip-ft }

Distribute to interior span sections

Interior negative moment = 0.65(329.1) = 213.9 kip-ft; positive midspan moment = 0.35(329.1) = 115.2 kip-ft.