Rebar Tension Development Length (ACI 318-19)
The straight-bar tension development length from the ACI 318-19 general equation: ld = (3/40) x fy x psi_t x psi_e x psi_s x psi_g / (lambda x sqrt(f'c) x (cb + Ktr)/db) x db, with the confinement term capped at 2.5, the psi_t x psi_e product capped at 1.7, and the 12 in floor. A #8 Grade 60 bottom bar in 4,000 psi concrete, well confined, develops in 28.5 in; cast as a top bar the 1.3 casting-position factor stretches it to 37 in -- the trap the 40-bar-diameter rule of thumb papers over. Straight-bar tension only; hooks, compression bars, and lap splices are separate checks. A design aid, not a substitute for a licensed engineer's detailing.
Formula and source
ld = (3/40) x fy x psi_t x psi_e x psi_s x psi_g / (lambda x sqrt(f'c) x (cb + Ktr)/db) x db; (cb + Ktr)/db capped at 2.5; psi_t x psi_e capped at 1.7; ld >= 12 in.
ACI 318-19 (Building Code Requirements for Structural Concrete), by name: §25.4.2.3 (general tension development equation), §25.4.2.4 (confinement cap), Table 25.4.2.5 (modification factors), §25.4.2.1 (12 in minimum).
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