Max One-Way Slab / Beam Span for a Given Depth (ACI 318-19)
With the slab or beam depth fixed, the longest span that still waives a deflection calculation.
Example
You enter
- Available thickness h (in) 10
- Support condition both-ends
- Steel yield fy (psi) 60000
- Concrete unit weight (pcf) 145
You get
- Max span (no deflection check) 23.33 ft
- Grade (yield) modifier 1
Checked against published worked examples from: StructurePoint.
Details, formula, and sources
Scopemax_span = h x denom / (12 kfy klw). A 10 in both-ends-continuous Grade 60 slab spans up to 23.3 ft without a deflection check (a beam 17.5 ft); a longer span needs a deeper member or an explicit calculation. A design aid, not the engineer of record's stamped design.
Formula or tablemax_span = available_thickness x denom / (12 x kfy x klw), the ACI 318-19 minimum-thickness relation hmin = 12 l / denom x kfy x klw solved for the span; denom = {simply:20, one-end:24, both-ends:28, cantilever:10}.
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, solved for the span, 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 namesavailable_thickness_in, support, fy_psi, wc_pcf, max_span_ft, kfy
- Span from depth max_span = h x denom / (12 kfy klw), the minimum-thickness table invertedACI 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