Lap length is the overlap required between two reinforcing bars to transfer stress from one bar to another. In tension, lap length = 1.5 × Ld (for bars in the same layer). In compression, lap length = Ld. Development length Ld = (ϕ × 0.87 × fy) / (4 × τbd). Design bond stress τbd values are per Table 26 of IS 456.
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Tension Lap Lengths (Fe415)
Lap length in tension = 1.5 × Ld. σs = 0.87 × 415 = 361 MPa. τbd for deformed bars = 1.6 × τbd(plain).
Bar Dia Grade Lap Length Ld Multiplier 8 mm M20 903 mm 602 mm 1.5 Ld 10 mm M20 1128 mm 752 mm 1.5 Ld 12 mm M20 1355 mm 903 mm 1.5 Ld 16 mm M20 1806 mm 1204 mm 1.5 Ld 20 mm M20 2258 mm 1505 mm 1.5 Ld 25 mm M20 2822 mm 1881 mm 1.5 Ld 32 mm M20 3612 mm 2408 mm 1.5 Ld 8 mm M25 774 mm 516 mm 1.5 Ld 10 mm M25 968 mm 645 mm 1.5 Ld 12 mm M25 1161 mm 774 mm 1.5 Ld 16 mm M25 1548 mm 1032 mm 1.5 Ld 20 mm M25 1935 mm 1290 mm 1.5 Ld 25 mm M25 2420 mm 1613 mm 1.5 Ld 32 mm M25 3096 mm 2064 mm 1.5 Ld 8 mm M30 722 mm 481 mm 1.5 Ld 10 mm M30 903 mm 602 mm 1.5 Ld 12 mm M30 1083 mm 722 mm 1.5 Ld 16 mm M30 1445 mm 963 mm 1.5 Ld 20 mm M30 1806 mm 1204 mm 1.5 Ld 25 mm M30 2256 mm 1504 mm 1.5 Ld 32 mm M30 2889 mm 1926 mm 1.5 Ld
Tension Lap Lengths (Fe500)
Lap length in tension = 1.5 × Ld. σs = 0.87 × 500 = 435 MPa. τbd for deformed bars = 1.6 × τbd(plain).
Bar Dia Grade Lap Length Ld Multiplier 8 mm M20 1088 mm 725 mm 1.5 Ld 10 mm M20 1359 mm 906 mm 1.5 Ld 12 mm M20 1632 mm 1088 mm 1.5 Ld 16 mm M20 2175 mm 1450 mm 1.5 Ld 20 mm M20 2720 mm 1813 mm 1.5 Ld 25 mm M20 3399 mm 2266 mm 1.5 Ld 32 mm M20 4350 mm 2900 mm 1.5 Ld 8 mm M25 932 mm 621 mm 1.5 Ld 10 mm M25 1166 mm 777 mm 1.5 Ld 12 mm M25 1398 mm 932 mm 1.5 Ld 16 mm M25 1865 mm 1243 mm 1.5 Ld 20 mm M25 2331 mm 1554 mm 1.5 Ld 25 mm M25 2913 mm 1942 mm 1.5 Ld 32 mm M25 3729 mm 2486 mm 1.5 Ld 8 mm M30 870 mm 580 mm 1.5 Ld 10 mm M30 1088 mm 725 mm 1.5 Ld 12 mm M30 1305 mm 870 mm 1.5 Ld 16 mm M30 1740 mm 1160 mm 1.5 Ld 20 mm M30 2175 mm 1450 mm 1.5 Ld 25 mm M30 2720 mm 1813 mm 1.5 Ld 32 mm M30 3480 mm 2320 mm 1.5 Ld
Compression Lap Lengths
Lap length in compression = Ld (no multiplier needed). Compression bond stress is 25% higher than tension per IS 456 Cl. 26.2.1.1.
Bar Dia Grade Lap Length Ld Multiplier 8 mm M20 482 mm 482 mm 1.0 Ld 10 mm M20 602 mm 602 mm 1.0 Ld 12 mm M20 722 mm 722 mm 1.0 Ld 16 mm M20 963 mm 963 mm 1.0 Ld 20 mm M20 1204 mm 1204 mm 1.0 Ld 25 mm M20 1505 mm 1505 mm 1.0 Ld 32 mm M20 1926 mm 1926 mm 1.0 Ld 8 mm M20 580 mm 580 mm 1.0 Ld 10 mm M20 725 mm 725 mm 1.0 Ld 12 mm M20 870 mm 870 mm 1.0 Ld 16 mm M20 1160 mm 1160 mm 1.0 Ld 20 mm M20 1450 mm 1450 mm 1.0 Ld 25 mm M20 1813 mm 1813 mm 1.0 Ld 32 mm M20 2320 mm 2320 mm 1.0 Ld
Mechanical Splices
Mechanical splices per IS 456 Cl. 26.2.5.2 — permitted as alternative to lap splices, especially for larger diameter bars.
Bar Dia Grade Lap Length Ld Multiplier 16 mm All N/A N/A N/A 20 mm All N/A N/A N/A 25 mm All N/A N/A N/A 32 mm All N/A N/A N/A
Notes
• Lap splices shall be staggered — not more than 50% of bars shall be lapped at any one section (IS 456 Cl. 26.2.5.1).
• Lap splices should not be used in zones of maximum stress (e.g., at mid-span of beams in tension zone or at column faces).
• For bundled bars, lap length shall be increased by 10% for 2-bar bundles and 20% for 3-bar bundles.
• Where lapping is done in flexural tension zone with more than 50% bars lapped, use 2.0 × Ld instead of 1.5 × Ld.
• IS 13920 requires lap splices in columns to be in the central half of the member and special confining reinforcement over the full lap length.
• All Ld values computed using formula: Ld = (ϕ × 0.87 × fy) / (4 × τbd × 1.6), where 1.6 factor is for deformed bars.
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