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REBAR & STEEL · REBAR LAP LENGTH CALCULATOR

Lap Length of 12mm Bar

The tension lap of a 12 mm bar equals its development length, Ld. In M20 concrete with Fe500 steel, Ld = 56.6φ, so the lap is 680 mm, and 544 mm in compression. With Fe415 steel it is 564 mm. IS 456 never allows less than 30φ (360 mm) in tension.

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Lap spliceLap in tension 680 mm680 mmcompression 544 mm · 56.6 × dbmyconstructioncalculator.comNot to scale
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Lap length in tension
680mm
In bar diameters56.6 × db
Lap length in compression544 mm
Compression in bar diameters45.3 × db
Development length (tension)680 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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Working
  1. τbd (M20) = 1.20 N/mm² × 1.6 (deformed bars) = 1.92 N/mm²
  2. Ld = φ × 0.87 fy ÷ (4 τbd) = 12.0 × 0.87 × 500 ÷ (4 × 1.92) = 680 mm (56.6 φ)
  3. Compression: τbd × 1.25 → Ld = 544 mm (45.3 φ)
  4. Tension lap = max(Ld, 30φ) = 680 mm
  5. Compression lap = max(Ld compression, 24φ) = 544 mm
Results are estimates for planning and material ordering only. They are not a substitute for structural design or approval by a qualified engineer. Read the full disclaimer.
Formula


σs            = 0.87 × fy                      (435 MPa for Fe500)
τbd           = table value × 1.6 for deformed bars   (M20: 1.2 × 1.6 = 1.92 MPa)
Ld (tension)  = φ × σs ÷ (4 × τbd)
Lap, tension  = Ld, at least 30φ   (2 Ld in direct tension)
Lap, compr.   = Ld ÷ 1.25, at least 24φ


Worked Example


A 12 mm bar in M20 concrete with Fe500 steel:


    

  1. σs = 0.87 × 500 = 435 MPa; τbd = 1.2 × 1.6 = 1.92 MPa
    2. 

  2. Ld = 12 × 435 ÷ (4 × 1.92) = 680 mm (56.6φ); 30φ = 360 mm, so Ld governs
    3. 

  3. Tension lap = 680 mm
    4. 

  4. Compression: 12 × 435 ÷ (4 × 2.40) = 544 mm (45.3φ); 24φ = 288 mm → lap = 544 mm
    5. 



Lap Length of 12mm Bar by Concrete Grade


Deformed (TMT) bars, flexural tension.






Concrete
Fe415 tension
Fe415 compression
Fe500 tension
Fe500 compression






M15
677 mm
542 mm
816 mm
653 mm




M20
564 mm
451 mm
680 mm
544 mm




M25
484 mm
387 mm
583 mm
466 mm




M30
451 mm
361 mm
544 mm
435 mm




M35
398 mm
319 mm
480 mm
384 mm




M40
360 mm
288 mm
429 mm
343 mm






More Lap Lengths by Bar Size
Frequently Asked Questions
What is the lap length of a 12 mm bar in M20 concrete?
680 mm in tension and 544 mm in compression with Fe500 steel, or 564 mm and 451 mm with Fe415. These are for deformed (TMT) bars in flexural members; laps in direct tension are twice as long.
Is the 50d lap rule correct?
It is close, not exact. By IS 456 the tension lap for Fe500 bars is 56.6d in M20 and 48.5d in M25, so 50d is slightly short in M20. With Fe415 it is 47.0d in M20. The lap shown on the structural drawings always governs.
Where should laps be placed?
Away from the points of highest stress, and staggered so that not all bars are lapped at the same section (IS 456 §26.2.5.1). In beams, bottom bars are usually lapped near the supports and top bars near mid-span.
Why is the compression lap shorter?
IS 456 allows 25% more bond stress in compression, so the compression development length is Ld ÷ 1.25. The lap must still be at least 24 bar diameters.
References
  • IS 456:2000 §26.2.1: development length Ld = φσs ÷ 4τbd, with σs = 0.87fy
  • IS 456:2000 §26.2.1.1: design bond stress τbd, increased by 60% for deformed bars and by 25% in compression
  • IS 456:2000 §26.2.5.1: flexural tension lap = Ld, at least 30φ; compression lap = Ld in compression, at least 24φ