What lap length should I use for a 16 mm Fe500 bar in an M25 column?

Question

The drawing says "lap as per IS 456" but the contractor wants to use 40d everywhere. This is a column (bars mainly in compression), M25 concrete, Fe500 bars, seismic zone III.
Is 40d too much or too little here? And where should the laps be placed?

TheCivilStudies Team · Accepted Answer

For bars in compression, IS 456 Cl. 26.2.5.1 asks for the development length in compression, but not less than 24φ. Design bond stress is increased by 25% for compression:
τbd (M25, plain)        = 1.4 N/mm²
× 1.6 for deformed bars = 2.24 N/mm²
× 1.25 for compression  = 2.80 N/mm²

Ld = 0.87 × fy × φ / (4 × τbd)
   = 0.87 × 500 × 16 / (4 × 2.80)
   = 621 mm
24φ = 384 mm  →  use 625 mm (≈ 39φ)
So 40φ = 640 mm is fine and slightly conservative.
In seismic zone III also follow IS 13920:
Lap only in the middle half of the column height, never near the beam-column joint.Provide closed hoops at ≤ 150 mm spacing over the full lap length.Don't lap more than 50% of the bars at one section.
You can check other bar sizes with the RCC Lap Length Calculator on this site.

What lap length should I use for a 16 mm Fe500 bar in an M25 column?

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