What is rebar development length?

The embedment needed for a deformed bar to develop its yield strength in the concrete through bond — the length past the critical section before the bar can be counted on for its full fy. ACI 318-19 §25.4.2.3 gives the simplified equations: ld = fy·Ψt·Ψe/(25·λ·√f'c)·db for #6 and smaller bars, and fy·Ψt·Ψe/(20·λ·√f'c)·db for #7 and larger, with a 12-inch minimum, valid when spacing/cover/stirrup conditions are met.

What is the development length for a #5 Grade 60 bar in 4000 psi concrete?

About 24 inches for a bottom bar (37.9 bar diameters × 0.625 in), and about 31 inches for a top bar (×1.3). At f'c = 3000 psi the bottom-bar value grows to about 28 inches. All values assume normalweight concrete, uncoated bars, and the simplified-method spacing and cover conditions.

What is the top-bar factor?

Ψt = 1.3 applies to horizontal bars with more than 12 inches of fresh concrete cast below them. Bleed water and plastic settlement collect under such bars, weakening bond on the underside — hence 30% more embedment. Slab top mats and beam top bars over deep webs almost always take it.

When do the simplified equations NOT apply?

The favorable-case simplified equations require either (a) clear spacing ≥ db, clear cover ≥ db, and code-minimum stirrups/ties along ld, or (b) clear spacing ≥ 2db and clear cover ≥ db. Otherwise the simplified method multiplies by 1.5, or you use the detailed equation of §25.4.2.4 with the confinement term (cb + Ktr)/db. Epoxy coating adds Ψe up to 1.5; lightweight concrete adds λ = 0.75.

How long is a lap splice?

Tension lap splices are Class A (1.0·ld) only where the area of steel provided is at least twice that required over the splice length and no more than half the steel is spliced at one location; otherwise — which is the common case — Class B at 1.3·ld. A #5 Grade 60 bottom bar in 4000 psi concrete needs roughly a 31-inch Class B lap.

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Rebar Development Length — Reference

By Michael Flynn, PE · water resources & dam safety engineer

Straight tension development, ACI 318-19 simplified method (§25.4.2.3). Grade 60, normalweight concrete, uncoated bars, favorable spacing/cover case. Minimum l_d = 12 in.

l_d = f_y·Ψ_t·Ψ_e / (25·λ·√f′_c) · d_b (≤ #6) | f_y·Ψ_t·Ψ_e / (20·λ·√f′_c) · d_b (≥ #7)

Bottom Bars, Grade 60 — l_d in inches (rounded up)

Bar	d_b (in)	f′_c = 3,000	f′_c = 4,000	f′_c = 5,000
#3	0.375	17	15	13
#4	0.500	22	19	17
#5	0.625	28	24	22
#6	0.750	33	29	26
#7	0.875	48	42	38
#8	1.000	55	48	43
#9	1.128	62	54	48
#10	1.270	70	61	54
#11	1.410	78	67	60

In bar diameters: 43.8/37.9/33.9·d_b (≤ #6) and 54.8/47.4/42.4·d_b (≥ #7) at 3,000/4,000/5,000 psi respectively. Note the jump from #6 to #7 — the code's two-tier equation, not a typo.

Modifiers

Condition	Factor
Top bar (> 12″ fresh concrete below), Ψ_t	× 1.3
Epoxy-coated, cover < 3d_b or spacing < 6d_b, Ψ_e	× 1.5
Epoxy-coated, other, Ψ_e	× 1.2
Lightweight concrete, λ = 0.75	× 1.33
Simplified-method unfavorable spacing/cover case	× 1.5
Class B tension lap splice	1.3 × l_d

Ψ_t·Ψ_e need not exceed 1.7. Favorable case requires: clear spacing ≥ d_b, clear cover ≥ d_b, and code-minimum stirrups over l_d — or clear spacing ≥ 2d_b with cover ≥ d_b.

Sharper answers exist. The detailed equation of ACI 318-19 §25.4.2.4 with the confinement index (c_b + K_tr)/d_b can cut these lengths meaningfully when cover and transverse steel are generous, and hooked or headed bars (§25.4.3–25.4.4) develop in far less length where straight embedment doesn't fit. This card is the simplified upper-bound method used for quick detailing checks.

Sources: ACI 318-19, §25.4.2 (development), §25.5 (splices), Table 25.4.2.5 (modification factors). Wight & MacGregor, Reinforced Concrete: Mechanics and Design. Values computed for f_y = 60,000 psi; Grade 80 bars scale by 80/60 plus additional ACI provisions.

Sizing the flexural steel first? RC beam flexure (ACI 318) → · Section properties? Section tool.

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Rebar Development Length — Reference

Bottom Bars, Grade 60 — ld in inches (rounded up)

Modifiers

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Bottom Bars, Grade 60 — l_d in inches (rounded up)