A reinforcing bar only works where the surrounding concrete can grip it, so a bar must extend a minimum distance — its development length ld — beyond the point where it is fully stressed. ACI 318-19 §25.4 offers two routes to that length in tension: the general equation 25.4.2.4a, which rewards good detailing through a confinement term (cb + Ktr)/db capped at 2.5, and the simplified Table 25.4.2.3 expressions, conservative shortcuts that assume fixed confinement and typically return longer lengths. For a 16 mm bottom bar in 28 MPa concrete the general route gives 369 mm where the simplified one demands 605 mm.
Modification factors adjust the base length for conditions that weaken the bond: top-cast bars take ψt = 1.3, epoxy-coated bars ψe = 1.5 (with the ψt·ψe product capped at 1.7), small bars up to 19 mm earn a 0.8 reduction, and lightweight concrete carries λ = 0.75. Code floors apply throughout — never less than 300 mm in tension, 200 mm in compression.
Lap splices are priced in the same currency: a Class A tension lap is 1.0 ld, while the default Class B lap is 1.3 ld, required unless twice the needed steel is provided over the splice and no more than half the bars are spliced within one lap length. Compression development ldc uses its own §25.4.9 expressions and comes out much shorter, since bearing helps in compression.