Section 25.4.2/Development Length of Deformed Bars
ACI 318 Section 25.4.2 covers the development length required for deformed reinforcing bars in tension.
The development length (ld) is the minimum length of reinforcing bar that must be embedded in concrete to develop the full tensile strength of the bar. The calculation per 25.4.2.3 considers bar size, concrete strength, bar coating (epoxy), bar spacing and cover, transverse reinforcement, and lightweight concrete factors. For a #8 bar in normal weight 4000 psi concrete with adequate cover and spacing, the development length is approximately 47 inches (about 4 feet).
Why this section exists
Reinforcing bars resist tension forces in concrete members. The bars transfer force to the concrete through bond along their length. If a bar is not embedded long enough, it pulls out of the concrete before reaching its yield strength, causing a sudden, brittle failure. The development length ensures the bond between the bar and concrete is sufficient to develop the full bar capacity before the bar terminates or is lapped with another bar.
What plan reviewers look for
Plan reviewers check structural details for bar development lengths at supports, lap splices, and bar cutoff points. They verify that the reinforcing schedule or general notes state the development lengths for each bar size and condition. They check that adequate embedment is available in the concrete member for the required development length.
Common violations
Related ACI 318 requirements
Section 25.5 covers lap splice lengths (based on development length). Section 25.4.3 covers development of hooked bars. Section 9.7.6covers minimum reinforcement development at supports. Section 25.4.9 covers development of headed deformed bars.