ACI 318 · Class B tension lap
Class B tension lap lengths for Grade 60 bars at 3,000–5,000 PSI — computed from the ACI 318 simplified method as a field reference. The structural drawings govern.
| Bar size | f′c = 3,000 | f′c = 4,000 | f′c = 5,000 |
|---|---|---|---|
| #3 | 22" | 19" | 17" |
| #4 | 29" | 25" | 23" |
| #5 | 36" | 31" | 28" |
| #6 | 43" | 37" | 34" |
| #7 | 63" | 54" | 49" |
| #8 | 72" | 62" | 56" |
Class B tension lap splice = 1.3 × development length, 12" minimum, computed from the ACI 318 simplified method for Grade 60, uncoated, normal-weight concrete and "other than top" bars with adequate cover and spacing (all modification factors = 1.0). TOP BARS multiply by 1.3; epoxy coating, lightweight concrete, and tight spacing increase the length further. FOR REFERENCE / ESTIMATING ONLY — the structural drawings and the engineer of record govern.
A tension lap splice transfers force between two bars by overlapping them so the concrete bond develops the bar. The length is tied to the development length: a Class B splice is 1.3 × the development length (ACI 318 25.5.2), with a 12-inch minimum.
Development length depends on bar size, steel grade, concrete strength (it shortens as √f′c rises), and several modifiers — top-bar position (×1.3), epoxy coating, lightweight concrete, and clear cover/spacing. This chart assumes the favorable case (Grade 60, uncoated, normal-weight, non-top bars, adequate spacing), so real splices are often longer.
Use this only for estimating. The structural drawings and engineer of record set the required lap. Field PM keeps your rebar inspection records, pour cards, and as-builts in the QA/QC module so splice and placement checks are documented before the pour. See also the rebar size chart.
A Class B tension lap splice is 1.3 times the bar's tension development length (ACI 318 25.5.2), with a 12-inch minimum. Class B is the default for most splices; Class A (1.0 × development length) is allowed only when no more than half the bars are spliced within the lap length and the area provided is at least twice that required.
No — treat it as an estimating reference. Actual lap lengths depend on bar grade, concrete strength, top-bar position, epoxy coating, lightweight concrete, clear cover, and bar spacing. The structural drawings and the engineer of record always govern.
A top bar has more than 12 inches of fresh concrete cast below it during placement. The concrete below can bleed and weaken bond, so ACI applies a 1.3 factor (ψt) — increasing the development and lap length by 30%.
Development length is inversely proportional to the square root of f′c, so higher-strength concrete shortens the lap. That's why the chart shortens from 3,000 to 5,000 PSI for the same bar.
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