Codes referenced
Code of practice. SCR, strong-column weak-beam, joint hoops, lap splice restrictions.
Seismic zone map of India (II to V), zone factor Z, importance factor I, response reduction factor R.
Base code for non-seismic detailing. IS 13920 supplements IS 456, does not replace it.
Material spec; IS 13920 mandates Fe500 or higher for seismic detailing.
What "ductile" means — and why it matters
Under seismic shaking, a structure is pushed and pulled many times through its yield point. A ductile member deforms visibly without losing strength — the building tilts, cracks appear, but it stays standing and gives occupants time to leave. A brittle member fractures suddenly at peak load — the building collapses without warning.
IS 456 detailing alone does not ensure ductility — it ensures strength under static gravity load. IS 13920 layers additional rules on top: extra confinement where concrete is expected to crush, 135° seismic hooks that survive cover spalling, strong-column weak-beam hierarchy so beams yield before columns, and lap splice placement that avoids the most-stressed zones.
Five requirements that differ from IS 456
At each beam end (over 2d from the face) and each column end (over l/6 or b or 450 mm — whichever is largest), provide additional closed stirrups/hoops at spacing ≤ h/4 (h/4 of the smaller dimension). SCR cross-section area per IS 13920 §6.1.6. Critical for confining the core concrete when cover loss exposes longitudinal bars.
All shear reinforcement (stirrups, ties, column hoops) in seismic zones III, IV and V must use 135° hooks with 10 bar-diameter straight extension — not 12d as IS 1786 specifies for general cases. Once cover spalls in a quake, a 90° hook opens and the stirrup fails.
Sum of column moment capacities at a joint must exceed sum of beam moment capacities by ≥ 1.4 (ΣMc ≥ 1.4 ΣMb). This forces the beams to yield before the columns — beam damage is repairable; column damage can collapse the building.
Lap splices of longitudinal bars are not permitted within the joint region or within l_d (development length) of the joint face. All column splices happen in the mid-storey zone, away from maximum moment.
At a beam-column joint face, the negative moment capacity at the face must exceed the positive moment capacity in the same span by ≥ 30%. Prevents moment reversal (uplift) leaving the connection without capacity in either direction.
Web width b_w must be ≥ 200 mm; if not, provide separate cross-ties in the web for confinement. Design shear in beams uses capacity-design moments (not factored gravity moments) to ensure stirrups are sized for the worst-case cyclic demand.
When does IS 13920 apply?
| IS 1893 zone | Zone factor Z | IS 13920 mandatory? | Common projects |
|---|---|---|---|
| II | 0.10 | Recommended (mandatory for I = 1.5 / 1.2 buildings) | Most of peninsular India |
| III | 0.16 | Yes (mandatory) | Delhi NCR, parts of UP, Bihar, Gujarat, Maharashtra |
| IV | 0.24 | Yes (mandatory) | Jammu & Kashmir, parts of HP, NE India, Bihar-Nepal border |
| V | 0.36 | Yes (mandatory) | Kutch, parts of HP (highest seismic risk in India) |
Reference: IS 1893:2016 Table 3 (zone factor); IS 13920:2016 §3 (applicability).
Worked example — exterior beam-column joint, zone IV
Scenario. Exterior joint in a moment frame, beam 300 × 600, column 400 × 400, M30 concrete, Fe500 reinforcement.
| Element | Requirement | Provided |
|---|---|---|
| Beam SCR length | 2d = 1200 mm each end | 2d = 1200 mm each end |
| Beam SCR stirrup spacing | ≤ h/4 = 150 mm | 100 mm c/c |
| Column SCR length | max(l/6, b, 450) = max(667, 400, 450) = 667 mm | 700 mm each end |
| Stirrup hook angle | 135° with 10d extension | 135° × 10 × 8 = 80 mm extension |
| Lap splices in joint | Not permitted | All column splices at mid-storey |
| ΣMc / ΣMb | ≥ 1.4 | 1.6 (column section over-provisioned) |
Outcome. The joint passes IS 13920 — column SCR is conservatively 700 mm (slightly above the 667 mm minimum), beam SCR is at 100 mm c/c (below the 150 mm maximum), all hooks are 135° × 10d, and no splice falls inside the joint or within l_d of the joint face. The detailing drawing is buildable.
Common deviations seen on Indian sites
Stirrups bent 90° in zone IV. Once cover spalls in a quake, the hook opens and the stirrup fails. IS 13920: 135° × 10d.
Column bars spliced at floor level — in the joint. Reverse cyclic loading opens the splice. IS 13920: only at mid-storey.
Beam section larger than column (architectural reason). ΣMb > ΣMc — columns yield first, collapse follows. Resize columns or strengthen beams.
Beam stirrups at 150 mm c/c everywhere. IS 13920 requires SCR at < h/4 (75 mm for 300 mm beam) over 2d from the face.
Stirrup hooks use 12d as per IS 1786 / IS 2502 default. In seismic zones, IS 13920 overrides to 10d — the difference is small but the requirement is seismic-specific.
Beam width < 200 mm or two-leg stirrup insufficient to confine the core. IS 13920 requires cross-ties (3-leg or 4-leg stirrup) in narrow webs.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how a ductile detailing detail turns into a beam that can yield or a joint that cannot.
Questionnaire — ask before the reinforcement drawing is released for fabrication
- What is the seismic zone of the site per IS 1893:2016 Table 3, and does that make IS 13920:2016 mandatory for this building (zone III and above) or only recommended (zone II)?
- Is special confining reinforcement provided over 2d from each beam end and over max(l/6, b, 450 mm) at each column end, with the SCR cross-section area meeting IS 13920 §6.1.6?
- Is the stirrup spacing inside the SCR zone at or below h/4 of the smaller dimension?
- Are all shear reinforcement hooks 135° with a 10 bar-diameter extension, rather than the 90° hooks the general IS 1786 / IS 2502 practice produces, or the 12d extension the general case calls for?
- Has the strong-column weak-beam check been done at every joint, with ΣMc at or above 1.4 ΣMb?
- Are the column bar lap splices located in the mid-storey zone, clear of the joint region and of l_d from the joint face?
- Does the negative moment capacity at the joint face exceed the positive capacity in the span by the required 30% (a ratio of at least 1.3), so moment reversal cannot leave the connection without capacity?
- Where the beam web is narrower than 200 mm, are separate cross-ties provided for confinement of the core?
Inspector's checklist — what to verify before the reinforcement is covered
- SCR extent marked and measured on the reinforcement over 2d from the beam face and over max(l/6, b, 450 mm) from the column end, not sketched vaguely on the drawing.
- Stirrup spacing inside the SCR zone measured with a tape, confirmed at or below h/4.
- Hook angle and extension gauged on site at 135° with a 10d extension (135° × 10 × 8 = 80 mm on an Ø8 stirrup), rejecting any 90° hook.
- Lap splice positions checked against the bar placing drawing and confirmed outside the joint region and beyond l_d from the joint face, at mid-storey.
- Joint face reinforcement checked for the 1.3 minimum flexural capacity ratio over the span capacity.
- Beam web width b_w confirmed at or above 200 mm, with cross-ties (3-leg or 4-leg) present in narrow webs.
- Bar grade confirmed as Fe500 or higher, which IS 13920 mandates for seismic detailing, and the concrete grade checked against the design.
- Joint hoops present and the SCR zone free of bars displaced during fixing and concreting.
What happens if… three worked examples
What happens. On the zone IV exterior joint as detailed, ΣMc / ΣMb = 1.6. Raise the beam capacity to 1.15 times the original and the ratio becomes 1.6 / 1.15 = 1.39, below the 1.4 required by IS 13920. The beam can absorb 1.6 / 1.4 = 1.14 times its original capacity before the hierarchy inverts, so a 15% increase is already past the limit.
Likely outcome. Columns now yield before the beams. The building loses the strong-column weak-beam mechanism, and damage concentrates in the columns, which is the failure IS 13920 exists to prevent. The joint fails the check and has to be reworked: resize the columns or strengthen the beams to restore ΣMc ≥ 1.4 ΣMb, and re-run the joint-face 1.3 capacity ratio check with the new beam capacity.
What happens. IS 13920 wants 135° hooks with a 10 bar-diameter extension on all shear reinforcement in seismic zones, which on an Ø8 stirrup is 135° × 10 × 8 = 80 mm. The yard bends to its own default: a 90° hook, and the general IS 1786 12d extension, which on Ø8 is 96 mm. A straight-edge check on the extension length passes, because 96 mm is more than 80 mm.
Likely outcome. Under reversed cyclic loading the cover concrete spalls, and a 90° hook opens out and lets go of the stirrup, which is exactly when the confinement is needed. A 96 mm extension on a 90° hook does not rescue it. Reject the batch at receipt, or accept it only after the hooks have been re-bent to 135° with the 10d extension.
What happens. The column SCR length is max(l/6, b, 450 mm) = max(667, 400, 450) = 667 mm, and the drawing provides 700 mm at each end. With l/6 = 667 mm the storey height is about 4,000 mm (667 × 6), which leaves a band of roughly 4,000 − 700 − 700 = 2,600 mm at mid-storey where a splice is legitimate. A splice at floor level sits inside the joint region and within l_d of the joint face, where IS 13920 does not permit it.
Likely outcome. Reverse cyclic loading opens the splice while the joint is taking its worst demand, and the column loses the bars that tie the joint together. The splice has to be moved into the mid-storey zone and the drawing reissued before the bars are fixed; a splice already cast in is a retrofit conversation, not a detailing fix.
Frequently Asked Questions
What is the minimum zone for which IS 13920 applies?
What is special confining reinforcement (SCR) under IS 13920?
Why does IS 13920 require 135° hooks at stirrup ends rather than 90°?
Related articles
IS 456:2000 — Annotated Walkthrough
The base code IS 13920 supplements. Cl 26 detailing, cover, laps.
BBS & IS 1786 Hooks
Standard 90°/135°/45° hooks per IS 1786 — the seismic 135° overrides this in IS 13920 zones.
Reinforcement Steel — IS 1786
Fe500 / Fe550 properties — IS 13920 requires Fe500 minimum for seismic detailing.
Cube Testing — IS 516
M30 and higher grades per IS 13920 are the typical seismic concrete — testing must clear higher targets.
Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, RMC plant operations, consulting and technical training across India. For a seismic-detailing audit or capacity-design verification, contact amit@concreteinfo.in.
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