One-Way Slab / Beam Minimum Thickness for Deflection (ACI 318-19)
The depth that lets a designer skip a deflection calculation entirely.
Example
You enter
- Span l (ft, clear span) 12
- Support condition simply
- Steel yield fy (psi) 60000
- Concrete unit weight (pcf) 145
You get
- Base thickness (l / denominator) 7.20 in
- Grade (yield) modifier 1
- Minimum thickness hmin 7.20 in
Checked against published worked examples from: StructurePoint.
Details, formula, and sources
ScopeACI 318-19 Table 7.3.1.1 / 9.3.1.1 gives l/20 simply supported, l/24 one end continuous, l/28 both continuous, l/10 cantilever for a one-way slab (l/16, l/18.5, l/21, l/8 for a beam), times (0.4 + fy/100,000) for non-Grade-60 steel and a lightweight factor. A simply supported 12 ft one-way slab needs 7.2 in; a both-ends-continuous Grade 40 slab only 4.1 in. The serviceability depth a strength check assumes, for members not supporting damageable partitions. A design aid, not the engineer of record's stamped design.
Formula or tabledenom = slab {simply:20, one-end:24, both-ends:28, cantilever:10} or beam {simply:16, one-end:18.5, both-ends:21, cantilever:8}; base = 12 l / denom; kfy = (fy == 60,000) ? 1 : 0.4 + fy/100,000; klw = (wc >= 145) ? 1 : max(1.65 - 0.005 wc, 1.09); hmin = base kfy klw.
Source and editionThe ACI 318-19 Table 7.3.1.1 (one-way slabs) and Table 9.3.1.1 (beams) minimum thickness for deflection control, with the (0.4 + fy/100,000) non-Grade-60 modifier and the lightweight-concrete factor, by name.
Free accessACI 318 is readable free through the ACI online reading room at concrete.org; the minimum-thickness tables are in the published code.
What governsEstimate. AHJ and licensed professional govern.
API field namesl_ft, support, fy_psi, wc_pcf, base_in, kfy, hmin_in
- Base thickness slab l/20, l/24, l/28, or l/10 and beam l/16, l/18.5, l/21, or l/8 by support condition, on the clear spanACI 318-19 Table 7.3.1.1 / 9.3.1.1
- Grade modifier (0.4 + fy/100,000) for fy other than 60,000 psiACI 318-19 Table footnote
- Scope members not supporting partitions likely to be damaged by deflection; not a strength designACI 318-19 7.3.1.1