ACI 318 and CSA A23.3 use the same framework for lap splices but produce different lengths. Both codes make a tension lap either Class A (1.0 × development length) or Class B (1.3 × development length), with Class B as the normal case, and both have separate compression laps. The difference is in the development length itself: the equations, factors and units differ, and for comparable bars and concrete, ACI's simplified method gives noticeably longer laps than CSA's. A US lap chart cannot be converted to millimetres and used on a Canadian job.
This post compares the two codes clause by clause and works one example through both. For the Canadian lengths, see our lap splice length chart for metric bars. Values below are from ACI 318-19 and CSA A23.3 (we checked the Canadian wording against the RSIC Manual of Standard Practice, which follows A23.3-19); confirm the edition named in your project specifications.
Where each code applies
- CSA A23.3 is the Canadian concrete design standard. The 2024 Ontario Building Code references it for engineered concrete (Part 4). It uses metric bars (10M to 55M) to CSA G30.18 and grades such as 400W and 500W.
- ACI 318 is the US building code for structural concrete. The International Building Code adopts it, so on IBC projects the lap schedule is based on ACI 318. It uses inch-pound bar numbers (No. 3 to No. 18) and grades such as Grade 60.
Neither code prints a single lap schedule for every job. Under ACI 318, the engineer shows lap locations and lengths on the structural drawings; under CSA, the same is true, with industry tables such as RSIC's used as aids. A "lap chart" on a drawing is the engineer's calculation for that project's concrete, bar grade and conditions.
For the wider differences between US and Canadian detailing, see ACI vs CSA rebar detailing.
Side by side: the lap splice rules
| Requirement | CSA A23.3 | ACI 318-19 |
|---|---|---|
| Tension lap classes | Class A = 1.0 ld, Class B = 1.3 ld | Class A = 1.0 ld, Class B = 1.3 ld |
| Minimum tension lap | 300 mm | 12 in. (305 mm) |
| When Class A is allowed | Steel provided at least twice that required, and less than half the bars spliced within the lap length | A similar two-part test on surplus steel and the share of bars spliced |
| Default | Class B | Class B |
| Largest bar that can be lapped | 35M (45M and 55M only lapped to smaller bars in compression) | No. 11 (No. 14 and No. 18 only lapped to smaller bars in compression) |
| Non-contact (spaced) laps | No farther apart than 1/5 of the lap length or 150 mm | No farther apart than 1/5 of the lap length or 6 in. |
| Bundled bars | Lap increased 10% (2 bars), 20% (3 bars), 33% (4 bars) | Lap increased 20% (3 bars), 33% (4 bars) |
| Compression lap, normal-strength bar | 0.073 × fy × db for fy up to 400 MPa, min 300 mm | 0.0005 × fy × db for fy up to 60,000 psi, min 12 in. |
| Columns | Special splice rules tied to bar stress | Special splice rules tied to bar stress |
The structure is close enough that a Canadian detailer can read a US lap schedule and understand it. The numbers inside it are another matter.
Compression laps line up closely. CSA's 0.073 × 400 MPa gives about 29 bar diameters; ACI's 0.0005 × 60,000 psi gives 30 bar diameters. ACI also lengthens compression laps by one third where the concrete is weaker than 3,000 psi.
How development length differs
Lap length is a multiple of development length (ld), so this is where the codes split.
CSA A23.3 simplified method: ld = 0.45 × k1 × k2 × k3 × k4 × (fy ÷ √f'c) × db, in MPa and mm, for members with minimum stirrups or ties (or slabs and walls with wide bar spacing) where the clear cover is at least one bar diameter and the clear spacing at least 1.4 bar diameters. The k factors cover top bars, epoxy coating, low-density concrete and bar size (0.8 for 20M and smaller). A23.3 also caps √f'c at 8 MPa and sets a 300 mm minimum.
ACI 318-19 simplified method: ld = (fy × ψt × ψe × ψg) ÷ (25 × λ × √f'c) × db for No. 6 and smaller bars, and the same with 20 in place of 25 for No. 7 and larger, in psi and inches, when the cover, spacing and stirrup conditions are met. The ψ factors cover top bars, coating and bar grade, and λ covers lightweight concrete. The minimum is 12 in.
Three differences that matter in practice:
- The coefficients. Converted to the same units, ACI's simplified expressions give longer development lengths than CSA's for comparable bars and conditions. ACI also has a detailed equation that credits cover and confining reinforcement, which can bring its lengths down.
- Bar grade. ACI 318-19 added a grade factor that lengthens development for high-strength Grade 80 and Grade 100 bar. CSA handles higher grades through fy in the equation, and Grade 500 bar simply gets proportionally longer laps.
- Bar diameter used. The RSIC tables, following A23.3-19, use the bar designation number (15 for 15M) as the diameter. ACI uses the nominal diameter of the bar number (0.625 in. for No. 5).
Worked example: a No. 5 bar vs a 15M bar
Compare similar bars in similar concrete: bottom bars (not top bars), uncoated, normal-weight concrete, in a beam with minimum stirrups, using each code's simplified method.
ACI 318-19, No. 5 Grade 60 bar, 4,000 psi concrete:
- ld = 60,000 ÷ (25 × √4,000) × 0.625 = 60,000 ÷ 1,581 × 0.625 = about 23.7 in.
- Class B lap = 1.3 × 23.7 = about 31 in. (roughly 790 mm).
CSA A23.3, 15M Grade 400 bar: from the RSIC Class B tables, 570 mm in 25 MPa concrete and 520 mm in 30 MPa concrete (4,000 psi is about 27.6 MPa, between the two).
Here is the same comparison for three common sizes:
| Bar pair | ACI Class B, Grade 60, 4,000 psi | CSA Class B, Grade 400, 25 MPa | CSA Class B, Grade 400, 30 MPa |
|---|---|---|---|
| No. 5 / 15M | about 31 in. (790 mm) | 570 mm | 520 mm |
| No. 6 / 20M | about 37 in. (940 mm) | 750 mm | 690 mm |
| No. 8 / 25M | about 62 in. (1,570 mm) | 1,170 mm | 1,070 mm |
The ACI bar is slightly stronger (Grade 60 is 60,000 psi, about 414 MPa, against 400 MPa) and the bars are not identical in size, but those account for only a small part of the gap. Most of it comes from the different simplified expressions. The point is not that one code is "safer". Each is calibrated as a whole, with its own load factors, resistance factors and detailing rules. The point is that the lengths do not transfer.
Converting a US design for a Canadian job
When drawings or a lap schedule come from a US designer, or a US product's literature quotes ACI laps:
- Do not convert the ACI lap table to millimetres. Ask for laps calculated to CSA A23.3, or have a Canadian engineer review the reinforcement.
- Convert bar sizes by area. Some pairs are close (No. 5 and 15M), but others have no near match: No. 4 falls between 10M and 15M, and No. 7 between 20M and 25M, so a substitution can change the steel area. See Canadian vs US rebar sizes.
- Check grades. Grade 60 and 400W are close but not the same, and the W designation carries weldability and ductility requirements.
- Recheck spacing. CSA A23.1's minimum clear spacing (1.4 bar diameters or 30 mm) is stricter than ACI's, and lapped pairs need that gap too. See minimum clear spacing between rebar in beams.
- Watch large bars. ACI's No. 11 limit sits close to CSA's 35M, but a US design using lapped No. 11 bars converted to a larger metric bar may lose the option to lap.
Going the other way, a Canadian lap schedule used on a US job would generally be too short under ACI's simplified method. Either way, the engineer of record for the jurisdiction sets the laps.
What stays the same in both codes
Some practical rules hold under either code:
- Splice where stress is low and stagger laps where the drawings call for it. Our post on lap length and splice zones for beams and columns covers the Canadian approach.
- Class B unless shown otherwise. Class A needs information only the engineer has. See Class A vs Class B tension lap splices.
- Top bars and epoxy coating lengthen laps in both codes.
- Mechanical couplers replace laps where bars are too large or too congested. See rebar couplers vs lap splices.
Laps detailed to the right code
We detail, cut and bend rebar from 10M to 55M to Canadian standards, with lap lengths taken from your engineer's drawings. If your drawings came from a US design, we can flag the laps and bar sizes that need a Canadian review before anything is cut.
Send us your drawings for a free take-off and quote, or see our rebar products.
Frequently asked questions
What is the ACI lap splice length for rebar?
ACI 318 sets a tension lap as Class A (1.0 times the development length) or Class B (1.3 times), never less than 12 in. The development length depends on bar size, grade, concrete strength, location and coating, so there is no single number. For a No. 5 Grade 60 bottom bar in 4,000 psi concrete, the simplified method gives a Class B lap of about 31 in.
Are ACI and CSA lap splices the same?
The framework is the same: Class A and Class B tension laps at 1.0 and 1.3 times the development length, separate compression laps, and similar limits on spaced laps. The development length equations are different, so the resulting lengths are not interchangeable.
Can I use an ACI lap splice chart on a Canadian job?
No. Canadian work is designed to CSA A23.3 with metric bars and Canadian grades. Lap lengths should come from the engineer's drawings or a chart calculated to CSA A23.3, not a converted ACI table.
Does the IBC have its own rebar lap splice schedule?
The International Building Code adopts ACI 318 for structural concrete, so lap splices on IBC projects follow ACI 318. The schedule on the drawings is prepared by the engineer from the ACI provisions.
What is the maximum bar size that can be lap spliced?
Under ACI 318, No. 11 bars, with No. 14 and No. 18 bars lapped only in limited compression cases. Under CSA A23.3, 35M, with 45M and 55M lapped only to smaller bars in compression. Larger bars are joined with mechanical or welded splices.
