Under IS 456, tension lap = max(Ld, 30φ) with Ld = φ·0.87fy ÷ 4τbd: a 16 mm Fe500 bar in M25 laps 777 mm (48.5φ). Under ACI 318-19, a Class B tension lap = 1.3 ld: a #5 Grade 60 bar in 4,000 psi concrete with good detailing laps 30.8 in.
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Lap length in tension
777mm
In bar diameters
48.5 × db
Lap length in compression
621 mm
Compression in bar diameters
38.8 × db
Development length (tension)
777 mm
Uses the simplified code equations. Your structural drawings and the engineer of record govern: they may specify longer laps, different splice classes, or seismic detailing.
Stagger laps where possible, and do not lap bars larger than #11 / 36 mm in tension (ACI) or 36 mm (IS): use couplers.
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Compression lap = max(Ld compression, 24φ) = 621 mm
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Formulas
IS 456:2000
Ld = φ × 0.87 fy ÷ (4 τbd) τbd × 1.6 for deformed bars, × 1.25 in compressionTension lap = max(Ld, 30φ) (direct tension: max(2Ld, 30φ))Comp. lap = max(Ld,compression, 24φ)
ACI 318-19 (simplified)
ld (tension) = fy ψt ψe ψg ÷ (C λ √f'c) × db C = 25 (≤ No. 6) or 20 (≥ No. 7) with good spacing/coverClass B lap = 1.3 ld (Class A = 1.0 ld), ≥ 12 inComp. lap = 0.0005 fy db (Grade 60) or (0.0009 fy − 24) db, ≥ 12 in
Worked Examples
IS 456, 16 mm Fe500 deformed bar in M25, flexural tension:
The length two bars overlap so that force passes from one to the other through the concrete. It depends on bar size, steel and concrete strength, and the code. Common results are about 40–60 bar diameters in tension.
What is the lap length for Fe500 in M25 concrete (IS 456)?
For deformed bars, Ld = 0.87 × 500 ÷ (4 × 2.24) = 48.5φ, so the flexural tension lap is 48.5φ: 777 mm for a 16 mm bar. In compression it is 38.8φ.
What is the difference between Class A and Class B splices (ACI)?
Class A (1.0 ld) is allowed only where the steel provided is at least twice that required and no more than half the bars are spliced at one location. Otherwise use Class B (1.3 ld), which is the usual default.
Is 50d a safe lap length?
50d is a common rule of thumb, but code lap lengths can be shorter or longer, for example with high-strength steel, top bars, epoxy coating or lightweight concrete. Use the code equation or the length on your drawings.
References
ACI 318-19 §25.4.2.3 (simplified development length), §25.5.2 (tension lap splices), §25.5.5 (compression lap splices)
IS 456:2000 cl. 26.2.1 (development length), cl. 26.2.5.1 (lap splices)
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