Standard Hooks, Bends & Minimum Bend (Mandrel) Diameters
IS 2502 fixes the standard hook (typically 180° with a straight extension) and bend (90°, 135°, 45°) shapes for reinforcement and the minimum internal bend (mandrel/pin) diameters as multiples of bar diameter. Bending tighter than the specified minimum over-strains and cracks the bar — which is why the IS 2502 pin diameter must respect the IS 1786 / IS 1599 bend acceptance.
Key Requirements
•Standard hook = semicircular (180°) bend plus a straight extension of a specified number of bar diameters
•Minimum internal bend diameter specified as a multiple of bar diameter (larger for higher grades / larger bars)
•Site bar-bender pin must be ≥ the specified minimum mandrel diameter — never bend tighter
•Stirrups/links use the specified hook bend (often 135° for seismic, per IS 13920)
•Re-bending of already-bent bars is not permitted (cracks/embrittles — see IS 1599 re-bend)
Reference Tables
Standard bend shapes (indicative — verify against current edition)
Detail
Typical form
Standard hook
180° semicircular + straight extension (≈ 4 bar dia, min ~65 mm)
Standard bend
90° / 135° / 45° with specified internal radius
Stirrup/link hook
135° hook with extension (seismic per IS 13920)
Indicative — confirm hook extensions, bend angles and minimum mandrel diameters against the current BIS edition and IS 1786/IS 13920.
Practical Notes
✓The commonest site abuse is bending around an undersized pin — it reproduces the exact crack the IS 1786/IS 1599 bend test screens against, at hooks where anchorage matters most.
✓Seismic detailing (IS 13920) needs 135° stirrup hooks with adequate extension — 90° hooks can open under reversed loading.
Common Mistakes
⚠Bending tighter than the minimum mandrel diameter (cracks the bar at the bend).
⚠Re-bending previously bent bars (strain-age embrittlement / cracking).
⚠Using 90° instead of 135° stirrup hooks in seismic detailing.