Plan Irregularities
in Seismic Design
Torsion · Re-entrant Corners · Floor Cut-outs · Out-of-Plane Offsets · Non-Parallel Systems — a complete student learning guide with interactive calculators.
What is Plan Irregularity?
Clause 7.1 — IS 1893 (Part 1):2016
Why Irregular Buildings Fail
Buildings with asymmetric mass, stiffness, or geometry develop unexpected torsion, stress concentrations, and irregular vibration modes during earthquakes — leading to disproportionate damage.
Table 5 — Plan Irregularities
Defines 5 types of plan-level irregularity: Torsional, Re-entrant Corners, Floor Cut-outs, Out-of-Plane Offsets, and Non-Parallel Systems.
Table 6 — Vertical Irregularities
Covers 7 types of vertical irregularity (Soft Storey, Mass, Geometric, In-plane Discontinuity, Weak Storey, Floating Columns, Irregular Modes). See companion resource.
Design Consequence
Plan irregularities escalate the required analysis from simple Seismic Coefficient Method (SCM) to 3D Dynamic Analysis (RSM or Time History), and demand enhanced load combinations.
How to Use This Resource
Read each irregularity type with triggers, thresholds, and code requirements.
Torsion design eccentricity, displacement ratio, and load combination formulas.
Visual schematics for each irregularity type with dimension labels.
Check torsional ratio, re-entrant corner projection, cut-out area, and design eccentricity.
Table 5 — Plan Irregularities
Five types, fully explained with amendments (2017 & 2020)
📖 Concept
A well-designed building translates horizontally without twisting when lateral loads are applied. This happens when lateral stiffness is balanced in plan relative to mass distribution, and the floor slab is stiff in-plane (plan aspect ratio < 3). When stiffness is offset from the mass centroid, the building twists — torsional irregularity.
⚡ Trigger Conditions (Both must be satisfied)
- Condition 1: Maximum horizontal displacement at any extreme edge > 1.5 × minimum horizontal displacement at the far end of the same floor in that direction (original 2016 text).
- Amendment No. 2 (2020) — Updated Condition: Maximum total displacement Δm (at one end) is in the range 1.2Δa to 1.4Δa, where Δa = (Δ₁ + Δ₂)/2 (average displacement).
- Condition 2: Natural period of the fundamental torsional mode > first two translational modes along each principal plan direction.
✅ Code Requirements
When Δm is in range 1.2Δa to 1.4Δa:
- Revise building configuration so fundamental torsional period < first two translational periods.
- Use 3D dynamic analysis method (RSM or Time History).
When Δm > 1.4Δa: The structural configuration must be revised — analysis alone is insufficient.
Original 2016 limit was 2.0; this was revised to 1.4 by Amendment No. 2 (2020) for more stringent control.
📖 Concept
L-shaped, T-shaped, U-shaped, or + (plus) shaped buildings have corners that “re-enter” (point inward). These corners act as stress concentration points. During an earthquake, different wings of the building vibrate at different amplitudes and phases, causing large forces to accumulate at the re-entrant corner junction in the floor diaphragm.
⚡ Trigger Condition
- The structural configuration in plan has a projection (wing) > 15% of its overall plan dimension in that direction.
- Checked independently in both X and Y plan directions.
- Example: If overall plan length = 30 m, a projection > 4.5 m triggers this irregularity.
✅ Code Requirements (As amended in 2020)
Use 3D dynamic analysis with flexible floor diaphragm to capture force concentrations at re-entrant corners, particularly in the floor diaphragm and special elements adjoining the corner.
This is in addition to rigid diaphragm analysis if applicable — the worst effect governs the design.
📖 Concept
Floor slabs act as rigid diaphragms — they transfer lateral loads to vertical elements (columns, shear walls) in proportion to their stiffness. Large openings (for atria, staircases, service shafts) reduce the in-plane stiffness of the slab, making it flexible. A flexible diaphragm cannot distribute loads uniformly, causing some elements to be under-loaded and others over-loaded — a dangerous condition.
⚡ Trigger Condition
- The area of geometric cut-out in a floor slab is significant enough to cause in-plane flexibility (qualitative trigger; quantitative rules apply for treatment).
✅ Code Requirements (As amended in 2020)
Cut-out area ≤ 50%: Floor slab shall be modelled as rigid or flexible depending on the location and size of the openings. Engineering judgment needed.
Cut-out area > 50%: Floor slab shall be taken as flexible in-plane in structural analysis.
📖 Concept
When a shear wall or column shifts laterally (out-of-plane) from one storey to the next, the load path is disrupted. The floor slab at the offset level must transfer the lateral force horizontally — an action it was not primarily designed for. This creates large shear and bending in the slab and supporting beams, and is known to cause brittle failure.
⚡ Trigger Condition
- Any vertical element (wall, column, brace) resisting lateral loads is laterally offset (not in-plane) from its position in the storey above or below.
- This creates a discontinuity in the lateral load path.
✅ Code Requirements (As amended in 2020)
- Analyse in 3D; review all vertical walls/frames out-of-plane at every storey.
- For Seismic Zones III, IV, and V:
a) Forces and moments in connecting elements and supporting vertical elements shall be enhanced by factor ≥ 2.5.
b) Lateral drift shall be < 0.2% in the offset storey and storeys below.
📖 Concept
A standard building has its lateral resisting elements (shear walls, frames, braces) aligned with two orthogonal (perpendicular) axes. When these elements are skewed at various angles, the building responds to earthquake shaking in a complex 3D manner. Forces applied along one axis generate responses in both axes — inherent bidirectional coupling. Standard 2D analysis seriously underestimates the design forces.
⚡ Trigger Condition
- The vertical structural systems resisting lateral forces are not parallel to the two principal orthogonal plan directions.
- Even a single diagonal shear wall triggers this irregularity.
✅ Code Requirements (As amended in 2020)
Design for bidirectional load combinations per Clause 6.3.2.2 or 6.3.4.1:
This means the full earthquake load in one direction combined with 30% in the perpendicular direction — producing 8 possible load combination envelopes.
Key Formulas
Torsion, Eccentricity, Load Combinations, Displacement Checks
🔄 Torsional Irregularity Check (Amdt. No. 2, 2020)
If Δm > 1.4Δa → Structural configuration must be revised.
⚖ Design Eccentricity (Clause 7.8.2)
🔀 Re-entrant Corner Check
🕳 Floor Cut-out Area Check
>50%: Treat floor as flexible in-plane.
↔ Non-Parallel Load Combinations (Cl. 6.3.2.2)
📐 Out-of-Plane Offset Force Enhancement
Visual Schematics
Plan view diagrams for each type of irregularity
Type i — Torsional Irregularity
When CR is offset from CM, applied lateral force produces both translation and rotation. One edge displaces more (Δ₂) than the other (Δ₁).
Type ii — Re-entrant Corner (L-shape)
In an L-shaped building, the “missing” corner creates a re-entrant corner (highlighted in orange). The projection P must be >15% of overall dimension L to trigger the irregularity.
Type iii — Floor Cut-Out
Large openings (atria, lift shafts) reduce in-plane slab stiffness. When the opening ratio exceeds 50%, the slab must be modelled as flexible.
Type iv — Out-of-Plane Offset (Elevation)
The lateral load path is disrupted at the floor where the wall shifts. The transfer slab carries loads horizontally — forces must be enhanced by ×2.5.
Type v — Non-Parallel System
When any lateral resisting element is not aligned with X or Y axis, bidirectional load combinations (30% rule) must be used to capture coupled response.
Interactive Plan Irregularity Checker
Based on IS 1893 (Part 1):2016 Table 5 including Amendments 1 & 2
Plan Irregularity Calculator
Select the check type and enter your building parameters
📄 Generate Full Report
Export a complete printable HTML report with all inputs, calculations, step-by-step working, and IS 1893 references.
Quick Reference — Table 5 Summary
At-a-glance comparison of all five plan irregularity types
| Sl. | Type | Threshold / Trigger | Analysis Required | Special Action | Amendment |
|---|---|---|---|---|---|
| i | Torsional | Δm > 1.2Δa AND torsional mode period > translational modes | 3D Dynamic (RSM / TH) | Revise config if Δm > 1.4Δa | Amdt. 2, 2020 |
| ii | Re-entrant | Projection > 15% of overall plan dimension (either direction) | 3D Dynamic + Flexible Diaphragm | Capture force concentrations at corners; worst of rigid+flexible governs | Amdt. 2, 2020 |
| iii | Floor Cut-outs | Significant openings reducing in-plane slab stiffness | 3D analysis with appropriate diaphragm model | ≤50%: rigid or flex; >50%: flexible in-plane model mandatory | Amdt. 2, 2020 |
| iv | OOP Offset | Any lateral element laterally shifted between storeys | 3D analysis; all elements reviewed out-of-plane | Zones III–V: enhance forces ×2.5; drift <0.2% at offset storey | Amdt. 2, 2020 |
| v | Non-Parallel | Any lateral resisting system not parallel to orthogonal axes | Bidirectional load combinations required | Use ±ELx±0.3·ELy and ±0.3·ELx±ELy (8 combos) | Amdt. 1, 2017 |
🎯 Key Takeaways for Students
Any One = Irregular
Even a single irregularity from Table 5 (or Table 6) makes the building “irregular” and triggers enhanced requirements.
Torsion Requires Two Conditions
Both the displacement ratio AND the period comparison must be checked. One alone is insufficient.
15% Projection Rule
Re-entrant corners are checked in BOTH plan directions independently. Even a small wing in one direction can trigger it.
Amendments Override 2016 Text
Amendments 1 (2017) and 2 (2020) modified several Table 5 clauses. Always use the amended version in practice.
1.5 Factor in ed
The 1.5 dynamic amplification factor in design eccentricity applies to SCM and RSM only — NOT Time History Method.
Zone-Dependent Consequences
Higher seismic zones (III, IV, V) attract stricter consequences: drift limits, force enhancement, mandatory dynamic analysis.
