About this calculator
This tool computes ACI 318-style tension and compression development lengths, standard hook anchorage length, and Class A / Class B tension lap-splice lengths (plus a compression lap length) for a single deformed reinforcing bar. It's meant for quick bar-anchorage and splice checks by engineers and detailers working out rebar layouts.
Method & formulas
Tension development length follows the ACI 318 confinement-based expression, using a fixed, conservative confinement term in place of a case-by-case (cb+Ktr)/db calculation:
ld = [fy·ψt·ψe·ψs / (1.1·λ·√f'c · (cb+Ktr)/db)] · db , with (cb+Ktr)/db = 1.5 assumed
Compression development and standard hook anchorage:
ldc = max(0.24·fy·db/(λ√f'c), 0.043·fy·db), not less than 200 mm
ldh = max(0.24·ψe·fy·db/(λ√f'c), 8db, 150 mm)
Lap splice lengths, and the modifier factors used above:
Tension splice: Class A = 1.0·ld ; Class B = 1.3·ld ; minimum 300 mm
Compression splice ≈ ldc , minimum 300 mm
ψt = top-bar factor (1.3 when >300 mm fresh concrete is cast below the bar)
ψe = coating factor (1.0 uncoated, up to 1.5 epoxy-coated with tight cover/spacing)
ψs = bar-size factor (0.8 for db ≤ 20 mm, 1.0 otherwise) ; λ = 0.75 lightweight, 1.0 normal-weight
Worked example
Tool defaults: db=16 mm, f'c=28 MPa, fy=420 MPa, ψt=ψe=ψs=λ=1.0.
√f'c = 5.29
ld = (420×1×1×1) / (1.1×1×5.29×1.5) × 16 ≈ 770 mm
ldc = max(0.24×420×16/5.29, 0.043×420×16, 200) = max(305, 289, 200) ≈ 305 mm
ldh = max(0.24×420×16/5.29, 128, 150) ≈ 305 mm
Class A splice = max(770, 300) ≈ 770 mm
Class B splice = 1.3 × 770 ≈ 1001 mm
Compression splice = max(305, 300) ≈ 305 mm
Assumptions & limitations
- Uses a fixed, conservative confinement term instead of computing (cb+Ktr)/db from actual cover and transverse-steel details — cross-check against the full ACI 318 §25.4 procedure for congested or lightly-confined sections.
- Defaults assume normal-weight, cast-in-place, uncoated bar; set the ψ-factors to match your actual bar position, coating, and concrete type.
- Does not check available embedment length, hook-bend clearances, or bar congestion at the anchorage zone.
- Splice class (A vs. B) depends on the ratio of steel provided to required and the percentage of bars spliced at one location — confirm the correct class from your actual design rather than assuming.
- Verify results against the governing code and a licensed engineer's judgment before finalizing rebar detailing.
FAQ
Q: When is Class A allowed instead of Class B?
A: Class A applies only when the steel area provided is at least twice what analysis requires over the full splice length, and no more than half the bars are spliced at that location; otherwise Class B (1.3·ld) governs.
Q: Why doesn't the compression splice use a class factor?
A: Compression lap splices aren't classified A/B like tension splices — the length is based directly on the compression development length ldc, subject to a 300 mm absolute minimum.
Q: Does this account for confining transverse reinforcement (Ktr)?
A: No — it uses a fixed conservative value for (cb+Ktr)/db rather than computing Ktr from your actual stirrup/tie layout, so results correspond to the simplified-table equivalent rather than the full expression.