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ACI 318-19 Development Length — Straight Bars in Tension

Eq. 25.4.2.4a with every Table 25.4.2.5 factor, including the reinforcement grade factor ψ_g introduced in ACI 318-19 (1.0 / 1.15 / 1.3 for Grade 60 / 80 / 100) · √f′c ≤ 100 psi · (c_b+K_tr)/d_b ≤ 2.5 · ℓ_d ≥ 12 in · Table 25.4.2.3 shown for comparison.

Bar & Concrete

Bar size (ASTM A615)

Grade

f′c psi

Concrete

Casting position (ψ_t)

Coating (ψ_e)

Confinement

c_b — smaller of cover to bar center and half the center-to-center spacing in

A_tr — transverse steel within s in²

s — transverse spacing in

n — bars developed

Bar spacing, center-to-center in

Results

ℓ_d Eq. 25.4.2.4a in—

ℓ_d ft-in—

d_b in—

ψ_t·ψ_e · ψ_s · ψ_g—

K_tr in—

(c_b+K_tr)/d_b used—

Table 25.4.2.3 — favorable / other in—

Eq. 25.4.2.4aℓd = (3/40)·(fy/(λ√f′c))·(ψtψeψsψg/((cb+Ktr)/db))·db

Eq. 25.4.2.4bKtr = 40·Atr/(s·n); Ktr = 0 permitted as a design simplification

Table 25.4.2.5ψg = 1.0 (Gr 40/60), 1.15 (Gr 80), 1.3 (Gr 100) — new in ACI 318-19; ψt·ψe ≤ 1.7; ψs = 0.8 for No. 6 and smaller

§25.4.1.4√f′c used for development length ≤ 100 psi

§25.4.2.1ℓd ≥ 12 in

VerifyGrade 80, f′c = 4000 psi, #8, favorable conditions (Table 25.4.2.3): ℓd = 72.7 in. Eq. 25.4.2.4a with (cb+Ktr)/db = 1.5 and 1.0 reproduces Table 25.4.2.3 exactly (regression test).

Step-by-Step Solution

Preliminary calculation tool. Final design requires the full text of ACI 318 and review by a licensed PE. This tool follows ACI 318-19, the edition referenced by the 2021 and 2024 IBC; ACI CODE-318-25 was published in January 2025 and is expected in the 2027 IBC (changes include development length equations and seismic provisions).