Development length (Ld) is the minimum length of bar required to develop its full design stress. Lap length is the overlap needed to transfer stress between two bars. Per IS 456, Ld = (ϕ × σs) / (4 × τbd).
Formula (IS 456 Clause 26.2.1)
Ld = (ϕ × σs) / (4 × τbd)
Where ϕ = bar diameter (mm), σs = stress in bar (0.87 × fy for limit state), τbd = design bond stress (Table 26 of IS 456)
Quick Calculator
Steel Grade
Fe 415 Fe 500 Fe 550
Bar Diameter
8 10 12 16 20 25 28 32
Concrete Grade
M20 M25 M30 M35 M40
Dev. Length (Ld)
1032
mm (tension)
16mm ϕ · Fe 415 · M25 · σs = 361 MPa
Fe 415 — Ld in mm (tension)
Bar Dia (mm) M20 M25 M30 M35 M40 8 mm 602 516 481 425 380 10 mm 752 645 602 531 475 12 mm 903 774 722 637 570 16 mm 1204 1032 963 849 760 20 mm 1505 1290 1204 1062 950 25 mm 1881 1613 1504 1327 1187 28 mm 2107 1806 1685 1486 1330 32 mm 2408 2064 1926 1699 1520
Design Bond Stress τbd (IS 456 Table 26)
Grade τbd (N/mm²) Note M20 1.2 For plain bars, multiply by 0.625 M25 1.4 M30 1.5 M35 1.7 M40 1.9 M45 2 M50 2.1
Lap Length Rules
Tension — bars in contact
Ld or 30ϕ (whichever is greater)
Tension — bars NOT in contact
Ld or 30ϕ + gap
Compression
Ld or 24ϕ (whichever is greater)
>50% bars lapped at one section
1.3 × Ld
Flexural tension zone, >50% lapped
2.0 × Ld
Practical Notes
• Compression Ld = 80% of tension Ld (Clause 26.2.1)
• Deformed bars: Ld values above apply. For plain bars: multiply by 1.6
• Increase Ld by 50% at top of pour (top bars) — reduced bond due to bleed water
• Stagger laps: no more than 50% bars lapped at same section
• Laps not permitted at sections of maximum stress
• Minimum lap = 200mm or 15ϕ, whichever is greater
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