Rebar Lap Splice and Development
ACI 318 development length for your bar size and concrete strength, the Class A and Class B lap splice lengths, the 25 percent splice limit, and the standard hook.
A #4 at 4,000 psi uncoated with the 3 in cover rule develops in 13 in, and a Class A splice is 14 in, which is the 12 in floor plus a quarter of the development length. A #8 at 4,000 psi develops 43 in, and a Class B splice is 56 in.
—Development length ld
—Class A splice (max 14 in, 50 percent)
—Class B splice (1.3 x ld)
—Splice zones and the 25 percent limit
—Standard 90 degree hook development
—Extra steel the laps take
How the math works
- Development length (ACI 318-19 25.4.2.1): ld = (3 × fy × ψt × ψe × ψs) ÷ (40 × 1.0 × λ × √f′c) × db for #3 through #11, with a 12 in minimum, and fy = 60,000 psi. #3 and #4 use 1.0 in place of ψs in the base formula, which is why a #4 develops so short.
- ψt (spacing and cover): 0.4 for #3 and #4 with 3 in side cover and stirrups crossing the splice, 0.7 for #3 and #4 otherwise, 0.8 for #6 and larger with 3 in side cover and transverse steel, and 1.2 for #6 and larger in the other case.
- ψe (coating) and ψs (size): ψe is 1.0 for uncoated, 1.5 for an epoxy-coated bar in an uncoated lap splice, and 1.2 for an uncoated bar in an epoxy-coated splice. ψs is 0.7 for #3 and #4 uncoated, 1.0 for #5, 0.8 for #6 through #11, and 1.2 for a coated bar #3 through #11.
- Class A and Class B (25.5.2): Class A is 0.25 × ld with a 12 in minimum and a 14 in cap, where not more than half the bars are spliced and the transverse steel rule is met, and Class B is 1.3 × ld where 100 percent of the bars are spliced at a point of tension stress. Lap splice lengths are staggered at least 1/4 of the lap length.
- Top bars and hooks (25.4.2.3, 25.3.2): a top bar with more than 12 in of fresh concrete below it multiplies the development length by 1.3, and a standard 90 degree hook develops at 12db with 2.5db side cover and 3.5db top cover on #3 through #8, with the hook legs extended past the bend.
Rebar detailing for planning on Grade 60 bar in normal and lightweight concrete. Bars #11 and larger, a splice in a seismic moment frame, a lap splice in a coupling-bar zone, a splice at a point of maximum stress, a #14 and #18 bar, welded and mechanical splicers, the development of a welded wire mesh lap, and a bar in a joint are design items in ACI 318 and the structural drawings, and a splice is a structural connection, not a material takeoff.
How long does rebar need to lap splice? Class A is 14 in on #3 through #5 at 4,000 psi, and Class B is 1.3 times the development length: 56 in on a #8 at 4,000 psi uncoated. Lap splice lengths are the bar size, the concrete strength, the coating, the cover and spacing, and whether the bar is a top bar, all read against ACI 318 Chapter 25.
The 12 in minimum is the floor: a #3 at 8,000 psi computes under 12 in, and the splice is 12 in.
What is the 25 percent splice rule? In a tension lap splice, ACI 318 caps the spliced bars at 50 percent of the bars at a section for a Class A splice, and where the splices cannot meet that, the splice length goes to 1.3 × development length as a Class B splice, which is the 25 percent case at the section. Stagger the splices at least a quarter of the lap length so no section carries more than half the bars spliced.
A seismic frame, a lap at a point of maximum stress, and a splice on a bar over a support are the cases that push you to Class B, welded, or mechanical.
Is a hook needed on rebar? A standard 90 degree hook develops at 12db (4.5 in on a #4, 12 in on a #8) with 2.5db of side cover and 3.5db of top cover past the bend, and hooks on #8 through #11 use a 4db bend pin. The hook is the end anchor where the straight development length will not fit in the concrete, and a #9 and larger in a joint is a headed bar, not a hook.
A 135 degree seismic hook with a 6db extension is a different animal, and it is a seismic detailing item.