Lap Length of Steel Bars as per IS 456: Rules, Table and Example

As per IS 456:2000, the lap length of a bar in flexural tension is L_d or 30φ, whichever is greater; in direct tension it is 2L_d or 30φ, whichever is greater; and in compression it is the compression development length but not less than 24φ. For Fe500 TMT bars this works out to about 57φ in M20 and 49φ in M25 for tension laps, so a 16 mm bar in an M20 beam needs roughly 910 mm of lap, not the "50d" that many sites still use.

This article explains where these numbers come from, gives a ready table for 8–25 mm bars, and covers the staggering and location rules that matter as much as the length itself.

What a lap splice does

Bars come in 12 m lengths, so in long beams, slabs and columns one bar has to be continued by another. In a lap splice the two bars are overlapped and the force passes from one bar into the concrete by bond, and from the concrete into the second bar. The overlap must be long enough for the full bar force to be transferred, which is exactly what development length L_d measures. That is why IS 456 states lap length in terms of L_d. If you need the background, read our guide on development length in RCC.

Development length formula and design bond stress

IS 456 (clause 26.2.1) gives:

Ld = φ σs / (4 τbd)

• φ = nominal bar diameter
• σ_s = stress in the bar at design load = 0.87 f_y for a fully stressed bar (435 N/mm² for Fe500)
• τ_bd = design bond stress

IS 456 gives τ_bd for plain bars in tension. For deformed bars (all TMT bars to IS 1786) these values are increased by 60%, and for bars in compression they are further increased by 25%.

Putting σ_s = 0.87 f_y into the formula gives L_d as a multiple of φ:

• Fe500, M20: tension 56.6φ, compression 45.3φ
• Fe500, M25: tension 48.5φ, compression 38.8φ
• Fe415, M20: tension 47.0φ, compression 37.6φ
• Fe415, M25: tension 40.3φ, compression 32.2φ

Notice that the familiar "47d" and "50d" figures belong to Fe415 in M20. With Fe500, now the most common grade on Indian sites, the tension value rises to about 57d in M20.

IS 456 lap length rules

1. Flexural tension (beam and slab bars): lap length = L_d or 30φ, whichever is greater. The straight part of the lap must be at least 15φ or 200 mm.
2. Direct tension (ties, tension members, hanger bars): lap length = 2L_d or 30φ, whichever is greater. Direct-tension members are better spliced by welding or mechanical couplers.
3. Compression (column bars): lap length = L_d in compression, but not less than 24φ.
4. Different diameters: lap length is calculated on the smaller bar.
5. Large bars: lap splices are not to be used for bars larger than 36 mm. Use welding or couplers instead.

Extra length for top bars and corner bars

IS 456 asks for the tension lap to be multiplied by 1.4 when the bar is a top bar as cast and the minimum cover is less than twice the bar diameter. The same factor of 1.4 applies when the bar sits at a corner with cover less than 2φ on either face, or when the clear distance between adjacent laps is less than 75 mm or 6φ, whichever is greater. If both conditions apply, the factor becomes 2.0. These cases come up often in shallow slabs and in beam corners.

Lap length table: Fe500 bars in M20 and M25

All values below were computed from the formula above (σ_s = 435 N/mm²) and rounded up to the next 10 mm. They do not include the 1.4 or 2.0 factors.

All lengths are in mm. In every case the L_d term governs over the 30φ and 24φ minimums.

Worked example: 16 mm bar in an M20 beam

Bottom bar of a beam, 16 mm Fe500, concrete M20.

1. τ_bd for M20 plain bar = 1.2 N/mm². For a deformed bar: 1.2 × 1.6 = 1.92 N/mm².
2. σ_s = 0.87 × 500 = 435 N/mm².
3. L_d = 16 × 435 / (4 × 1.92) = 6960 / 7.68 = 906.25 mm.
4. 30φ = 30 × 16 = 480 mm, which is less than L_d.
5. Lap length = 906 mm. Provide 910 mm (many engineers write 950 mm for easy marking).
6. Staggering: adjacent laps are treated as staggered only if their centres are at least 1.3 × 910 = 1183 mm apart. Keep them about 1200 mm apart.

Same bar as a top bar with 25 mm cover: 25 mm is less than 2φ = 32 mm, so multiply by 1.4. That gives 906.25 × 1.4 = 1268.75 mm, so provide about 1270 mm.

Column bar in compression, 16 mm, M25: τ_bd = 1.4 × 1.6 × 1.25 = 2.8 N/mm². L_d = 16 × 435 / (4 × 2.8) = 621.4 mm, and 24φ = 384 mm, so provide 630 mm. Many designers still lap column bars with the tension value (780 mm here), because wind or earthquake moments can put column bars into tension.

Staggering laps

IS 456 recommends that splices be kept away from sections of maximum stress and staggered. In flexural members, splices should not be at sections where the bending moment exceeds 50% of the moment of resistance, and not more than half the bars should be spliced at any one section. In practice:

• Lap alternate bars, shifting the next set by at least 1.3 × lap length.
• In slabs, stagger laps of adjacent bars so that a line of laps does not form a weak band across the slab.
• In columns, if all bars must be lapped at one level, confine the lap zone with closely spaced ties as per the drawing.

Where not to lap

• Beam bottom bars at mid-span, where sagging moment is highest. Lap them nearer the supports.
• Beam top bars over supports, where hogging moment is highest. Lap them in the middle third of the span.
• Inside beam–column joints. In earthquake-resistant frames to IS 13920, beam laps are also not allowed within twice the effective depth from the joint face, and column laps must be in the central half of the column height.
• At the root of a cantilever. Main cantilever bars should run unbroken well past the support.
• Bars larger than 36 mm. Use couplers or welding.
• Congested zones. Where a lap would leave less than the required clear spacing for aggregate, use couplers.

When preparing a bar bending schedule, add every lap length to the bar's cut length. Our house estimator includes laps in its BBS output.

Common mistakes

• Using the old Fe415 "50d" figure for Fe500 bars in M20.
• Using compression lap lengths for beam bars, or for column bars in frames that see moment reversal.
• Lapping every bar at the same section, usually just above floor level in columns.
• Forgetting the 1.4 factor for top bars in thin slabs with small cover.
• Lapping two different diameters on the larger bar's length (this wastes steel) instead of the smaller bar's.
• Leaving a gap between lapped bars more than the drawing allows. Lapped bars should be in contact or close together and tied with binding wire.