Mandatory ductile detailing rules for RCC frames in seismic zones III, IV, and V (and zone II for important structures). Ensures the structure dissipates energy through plastic hinging at predetermined locations rather than brittle shear failure. Covers beams, columns, joints, walls, and shear walls.
General Material Requirements (Cl 5)
| Rule | Value | Clause | Note |
|---|
| Concrete grade | M20 minimum (M25 for ≥ 4 storeys) | 5.2 | M20 floor; M25 if total height > 15 m |
| Reinforcement grade | Fe 415 / 500 / 550 (with elongation ≥ 14.5%) | 5.3 | D-grade bars (Fe415D, 500D, 550D) preferred |
| fu / fy ratio | ≥ 1.15 | 5.3 | Ensures strain hardening before failure |
| Elongation | ≥ 14.5% | 5.3 | Minimum for ductility |
| Member size | ≥ 200 mm (frame member) | 5.4 | Both width and depth |
Beam Detailing (Cl 6)
| Rule | Value | Clause | Note |
|---|
| Min top steel (% of bd) | ρmin = 0.24 √fck / fy | 6.2.1(b) | More than IS 456 minimum |
| Max steel (% of bd) | ≤ 2.5% | 6.2.2 | Tension steel |
| Min top steel at any section | ≥ ¼ of max top steel | 6.2.3 | Continuity through bottom |
| Min bottom steel at any section | ≥ ½ of top steel at that section | 6.2.4 | Reverse moment capacity |
| Min bottom steel at supports | ≥ ⅓ of top steel at the support | 6.2.4 | — |
| Hoop spacing — 2d from face | ≤ d/4 OR 8 × min bar dia OR 100 mm | 6.3.5(a) | Plastic hinge zone |
| Hoop spacing — rest of beam | ≤ d/2 | 6.3.5(b) | Outside hinge zone |
| First hoop from joint face | ≤ 50 mm | 6.3.5(c) | Critical for joint shear |
| Hoop diameter | ≥ 8 mm (≤ M20) / ≥ 10 mm (M25+) | 6.3.5 | — |
| Hoop hook | 135° hook with ≥ 10d extension | 6.3.4 | Standard 90° hook NOT allowed |
Column Detailing (Cl 7)
| Rule | Value | Clause | Note |
|---|
| Min cross-section | ≥ 300 × 300 mm (typical) OR 20 × max bar dia | 7.1.1 | 20 × largest column bar diameter |
| b / D ratio | ≥ 0.4 | 7.1.2 | Avoid very flat sections |
| Length / least dim | ≤ 18 | 7.1.3 | Slenderness limit |
| Longitudinal steel | 0.8% to 6% of cross section | 7.2.1 | Same as IS 456 |
| Splice location | Middle 1/2 of column height only | 7.2.2 | Not in plastic hinge zone |
| Lap length | Full Ld in tension | 7.2.2 | With special hoops over splice |
| Confining hoop length lo | max(D, h/6, 450 mm) | 7.4.1 | Length at top & bottom of column |
| Confining hoop spacing | ≤ B/4 OR 6 × min long bar dia OR 100 mm | 7.4.6(a) | But ≥ 75 mm |
| Cross-tie 90° spacing along long bars | ≤ 300 mm | 7.4.4 | Each long bar engaged by hoop or cross-tie |
| Hoop hook in column | 135° with ≥ 6d extension | 7.4.5 | — |
Beam-Column Joint (Cl 8)
| Rule | Value | Clause | Note |
|---|
| Joint shear capacity | Vj ≤ τc × Aj | 8.1 | Aj = effective horizontal joint area |
| τc (joint, confined 4 sides) | 1.5 √fck | 8.1 | All 4 beams in |
| τc (confined 3 sides) | 1.2 √fck | 8.1 | — |
| τc (other cases) | 1.0 √fck | 8.1 | Edge / corner joints |
| Hoops in joint | Same as column hinge zone | 8.2 | Continue confining hoops through joint |
| Special confinement (no beams) | Half spacing of column lo zone | 8.2 | Reduce hoop spacing further |
Strong Column — Weak Beam (Cl 7.2.1)
| Rule | Value | Clause | Note |
|---|
| ΣMc ≥ 1.4 × ΣMb | At each joint, both directions | 7.2.1.1 | Sum of column moment capacities ≥ 1.4 × sum of beam capacities |
| Purpose | Force plastic hinges into beams (not columns) | — | Beams fail ductile, columns brittle — design beams to fail first |
Notes
• IS 13920 is MANDATORY for buildings ≥ 4 storeys in zone III, all buildings in zones IV & V
• Ductile detailing is more steel-intensive — typical 15–25% more shear reinforcement than IS 456
• Hooks must be 135°, NOT 90° — 90° hooks straighten under cyclic loading and fail
• Crossties (open hoops with 135° hook one end, 90° + 6d the other) are allowed
• Splice in middle half only — never splice all column bars at the same elevation
• Strong column weak beam philosophy: column capacity ≥ 1.4 × beam capacity at every joint
• All seismic projects need an IS 13920 review during design and at first concrete pour
• IS 13920:2016 supersedes IS 13920:1993 — the new version is significantly stricter
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