ACI 318-19
ACI 318-19 Section 25.4/25.5 | Development Length
Rebar needs to be embedded deep enough into concrete so that it can reach its yield strength without slipping or pulling out of the concrete. This length is called development length.
Governing Formula
Variables
Development length in tension
in or mm
Casting position factor (1.3 for top bars)
unitless
Epoxy coating factor
unitless
Size factor (0.8 for #6 and smaller)
unitless
Spacing or cover dimension
in or mm
Quick Example
1
Identify factors
For a bottom uncoated #5 bar, ψt = 1.0, ψe = 1.0, ψs = 0.8.
2
Check confinement
Assuming typical cover and spacing, the confinement term (cb + Ktr)/db is usually limited to 2.5.
3
Calculate length
Plug all values into the formula.
When does this govern?
Development length is critical at beam-column joints, footing dowels, and points of bar cutoffs where stresses are highest.
Common Mistake
Forgetting the 1.3 top bar factor for horizontal reinforcement that has more than 12 inches of fresh concrete cast below it (like top steel in deep beams).