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T-beam effective flange width per ACI 318-19 (Table 6.3.2.1)

Technical reference for structural engineers

Why flange width matters

In a T-beam or L-beam, the slab acts as a compression flange. The wider the effective flange, the larger the compression zone and the higher the moment capacity. ACI 318-19 Table 6.3.2.1 limits the effective flange width to prevent credit for slab regions too far from the web, where shear lag reduces the actual stress participation.

ACI 318-19 clause references

ACI 318-19 reference Topic
Table 6.3.2.1Effective flange width limits (overhang per side)
Cl 6.3.2.2Isolated T-beam flange requirements (informational)

Interior T-beam: overhang per side

For an interior T-beam with flanges on both sides, each overhang is the smallest of three limits:

$$\text{overhang} = \min\!\left(8\,h_f,\;\; \frac{s_w}{2},\;\; \frac{l_n}{8}\right)$$

The effective flange width is:

$$b_f = b_w + \text{oh}_{\text{left}} + \text{oh}_{\text{right}}$$

Each limit serves a distinct purpose:

  • 8 hf: prevents credit when the flange is thin relative to the overhang distance
  • sw / 2: half the clear spacing to the adjacent beam, preventing double-counting of the same slab strip
  • ln / 8: fraction of the clear span, reflecting the shear lag that reduces stress in wide flanges

Edge L-beam: one-sided overhang

For an edge L-beam with a flange on one side only, the overhang uses smaller multipliers:

$$\text{overhang} = \min\!\left(6\,h_f,\;\; \frac{s_w}{2},\;\; \frac{l_n}{12}\right)$$
$$b_f = b_w + \text{oh}$$

The reduced factors (6 vs 8 for hf, ln/12 vs ln/8) account for the asymmetric stress distribution when the flange extends in only one direction.

NA in flange vs NA in web

Once bf is determined, the flexural analysis depends on where the neutral axis falls:

  • a ≤ hf (NA in flange): the section behaves as rectangular with width bf. The standard Whitney block formula applies.
  • a > hf (NA in web): the compression zone splits into a flange contribution (Asf) and a web contribution (Asw), each with its own moment arm.

The beam flexural design spoke covers the full T-beam flexure equations for both branches.

318-11 vs 318-19: formula transition

Many online references still cite the older total-width form from ACI 318-11 and earlier: bf ≤ ln/4, bf ≤ bw + 16 hf, bf ≤ center-to-center spacing. ACI 318-19 restated these as overhangs per side in Table 6.3.2.1. For symmetric interior beams the results are numerically identical (e.g., bw + 2 × ln/8 = bw + ln/4). The per-side formulation handles asymmetric spacing and edge beams more directly.

Isolated T-beam (Cl 6.3.2.2)

For precast or isolated T-beams not built monolithically with a slab, ACI 318-19 Cl 6.3.2.2 requires the flange thickness to be at least bw/2 and the total flange width to not exceed 4 bw. This clause applies to isolated members; calcnote targets cast-in-place floor beams and does not currently enforce Cl 6.3.2.2.

What calcnote computes

The beam calculator accepts T-beam or L-beam geometry and computes bf per Table 6.3.2.1:

  • Inputs: beam type (T or L), web width bw, flange thickness hf, clear spacing sw (both sides for T), clear span ln
  • Computes overhang per side using the governing minimum
  • Returns bf = bw + overhangs
  • A direct bf override is available when the flange geometry is known

Sources

  • ACI 318-19: Table 6.3.2.1, Cl 6.3.2.2.
  • Wight, Reinforced Concrete: Mechanics and Design, 7th ed.

Compute effective flange width with your own geometry: run a beam calculation or see a demo result.

What are the three limits on T-beam flange overhang in ACI 318-19?

For an interior T-beam, each side's overhang is the smallest of 8 hf (eight times the flange thickness), sw/2 (half the clear spacing to the adjacent beam), and ln/8 (one-eighth of the clear span). Each limit prevents overestimating the compression zone width by addressing a different physical constraint: flange proportions, slab sharing, and shear lag.

How does L-beam overhang differ from T-beam overhang?

An edge L-beam uses min(6 hf, sw/2, ln/12) because the flange extends on one side only. The multipliers are smaller (6 vs 8, ln/12 vs ln/8) to account for the reduced shear lag transfer in an asymmetric configuration.

When does the flange width affect moment capacity?

When the neutral axis depth a is less than the flange thickness hf, the full bf acts as the compression width, increasing capacity. When a exceeds hf, only the web and the overhang portion within the stress block depth contribute, and the section is analyzed as a two-part compression zone (flange + web).

Did the flange width formula change from ACI 318-14 to ACI 318-19?

ACI 318-19 restated the limits as overhangs per side (8 hf, sw/2, ln/8) in Table 6.3.2.1. Earlier editions used an equivalent total-width form (bf ≤ ln/4, bf ≤ bw + 16 hf, bf ≤ center-to-center spacing). The numerical results match for symmetric interior beams, but the per-side formulation handles asymmetric spacing and edge beams more directly.