Torsional Irregularity in Buildings — IS 1893:2016 | StructLearn

IS 1893 (Part 1) : 2016 · Sixth Revision

Torsional Irregularity
in Buildings

A complete student guide to understanding, checking, and designing for torsional irregularity as per Table 5 and Clause 7.8 of IS 1893:2016.

Table 5 — Plan Irregularities

Clause 7.8 — Design Torsion

Amendment No. 2 (2020) Included

Fundamentals

What is Torsional Irregularity?

When an earthquake strikes, it shakes a building horizontally. Ideally, every floor should translate (slide) uniformly. But if the building’s centre of mass (CM) and centre of resistance (CR) do not coincide, the building will also twist about its vertical axis — this is called torsional response.

🏛 Centre of Mass (CM)

The point on a floor through which the resultant inertia force due to earthquake shaking acts. It depends on the distribution of mass (dead load + live load) on the floor slab.

⚙️ Centre of Resistance (CR)

The point through which, when lateral force is applied, the floor undergoes pure translation with no twist. It depends on the stiffness distribution of all lateral load resisting elements (columns, walls, braces).

📌 Key Concept — Static Eccentricity (e_s) The distance between CM and CR at any floor level is called the static eccentricity (e_s). A non-zero static eccentricity causes torsional moments in the building when earthquake forces are applied.

Visual Explanation

Plan view of a building floor showing torsional twist due to CM-CR offset

✅ Regular Building — CM = CR

No twist. All points displace equally. Safe behaviour.

⚠️ Torsionally Irregular — CM ≠ CR

One end displaces more than the other — dangerous!

IS 1893 Table 5

Table 5 — Plan Irregularities Explained

IS 1893:2016 Clause 7.1 lists five types of plan irregularities in Table 5. A building is considered irregular if any one condition is satisfied. Torsional Irregularity is Sl. No. (i) of Table 5.

Sl. No. (i) — Torsional Irregularity

A building is torsionally irregular when both of the following conditions exist simultaneously:

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Condition 1 — Displacement Ratio

The maximum horizontal displacement at any point on the extreme edge of the floor (Δ_max) is more than 1.5 times the minimum horizontal displacement at the far end (Δ_min) in that direction.

Δ_max / Δ_avg > 1.2

As amended by Amendment No. 2 (2020): ratio is now Δ_m vs Δ_a = (Δ₁+Δ₂)/2

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Condition 2 — Mode Period

The natural period corresponding to the fundamental torsional mode of oscillation is more than that of the first two translational modes along each principal plan direction.

This means the building “wants to twist” more easily than it “wants to slide” — a very dangerous configuration.

⚠️ Amendment No. 2 (November 2020) — Revised Thresholds The 2020 amendment replaced the original 1.5–2.0 ratio bands with a new clearer criterion:
• Δ_m is in range 1.2 Δ_a to 1.4 Δ_a: Revise configuration + use 3D dynamic analysis
• Δ_m > 1.4 Δ_a: Building configuration shall be revised (not permitted as-is)
where Δ_a = (Δ₁ + Δ₂)/2 = average of the two extreme edge displacements.

Summary of All 5 Plan Irregularities (Table 5)

Sl. No.	Type	Trigger Condition	Required Action
(i)	Torsional Irregularity	Δ_m > 1.2 Δ_a AND torsional mode period > translational period	3D Dynamic Analysis
(ii)	Re-entrant Corners	Plan projection > 15% of overall plan dimension in that direction	3D Dynamic + Flexible Diaphragm
(iii)	Floor Slab Discontinuity	Cut-outs or openings in floor slab	Rigid or Flexible Diaphragm per size
(iv)	Out-of-Plane Offsets	Lateral force resisting elements have out-of-plane discontinuities	Force Enhancement ×2.5; Drift < 0.2%
(v)	Non-Parallel Systems	Lateral force resisting elements not along two mutually perpendicular directions	Bi-directional seismic loading per 6.3.2.2

📋 Clause 7.1 — Important General Rule “An effort shall be made to eliminate irregularities by modifying architectural planning and structural configuration. A building shall be considered irregular for the purposes of this standard, even if any one of the conditions given in Tables 5 and 6 is applicable.”

When is Bi-directional Loading Required?

Clause 6.3.2.2: When the lateral force resisting system is not oriented along two mutually orthogonal directions (see Table 5(v)), the building must be designed for simultaneous bi-directional effects:

Load Combinations — Clause 6.3.2.2

±EL_x ± 0.3 EL_y and ±0.3 EL_x ± EL_y

Where EL_x and EL_y are earthquake loads along X and Y principal directions respectively.

Clause 7.8 — IS 1893:2016

Torsion — Clause 7.8 in Detail

Even in buildings that are not classified as torsionally irregular, IS 1893 mandates a design eccentricity to account for torsional effects. This is Clause 7.8.

7.8.1 — Why Torsion Must Always Be Designed For

Clause 7.8.1 — Provision Provision shall be made in all buildings for the increase in shear forces on lateral force resisting elements resulting from twisting about the vertical axis, arising due to eccentricity between the centre of mass and centre of resistance at floor levels.

The design forces (from Clause 7.6 — Seismic Coefficient Method, or Clause 7.7.5 — Response Spectrum Method) shall be applied at the displaced centre of mass so as to cause the design eccentricity e_d.

7.8.2 — Design Eccentricity Formula

The design eccentricity e_di at floor i is the larger of the two values that gives the more severe effect:

Design Eccentricity — Clause 7.8.2

e_di = 1.5 e_si + 0.05 b_i
— OR —
e_di = e_si − 0.05 b_i

Whichever gives the more severe effect on lateral force resisting elements

esi

Static Eccentricity at floor i — the distance between Centre of Mass (CM) and Centre of Resistance (CR) at that floor level.

× 1.5

Dynamic Amplification Factor — accounts for dynamic amplification of torsional effects during actual earthquake shaking (vs. static analysis).

± 0.05 bi

Accidental Eccentricity — 5% of the floor plan dimension perpendicular to the earthquake direction. Accounts for uncertainties in mass location, torsional ground motion, etc.

✅ Important Note for Time History Method The dynamic amplification factor of 1.5 is NOT required when performing structural analysis by the Time History Method. In that case:
e_di = e_si ± 0.05 b_i

Why Two Formulas? — The “More Severe Effect” Explained

The two formulas give eccentricities on opposite sides of the CR. The engineer must check both and design for whichever produces greater forces in the critical lateral force resisting elements.

Formula	Physical Meaning	When More Critical
1.5e_s + 0.05b	Eccentricity is amplified and accidental eccentricity adds to it (same side)	When natural eccentricity and accidental eccentricity act in same direction — maximises torsion
e_s − 0.05b	Accidental eccentricity reduces effective eccentricity (opposite side)	When checking elements on the other side of the building — may govern for those elements

Torsion in Open Storey Buildings (Clause 7.10.3)

⚠️ Critical Provision — Open Storey / Soft Storey Buildings When RC structural walls are provided to address soft-storey issues, the design must ensure the building does not gain additional torsional irregularity in plan beyond what already exists. Specifically:

Lateral stiffness in open storey shall be ≥ 80% of storey above
Lateral strength in open storey shall be ≥ 90% of storey above
Torsional irregularity assessment must include all elements at all levels

Interactive Tool

Torsional Irregularity Checker & Design Eccentricity Calculator

Use this tool to check if your building floor qualifies as torsionally irregular per IS 1893:2016 (including Amendment No. 2, 2020), and compute the design eccentricity per Clause 7.8.2.

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Torsional Irregularity Check

Enter floor displacements and eccentricity details below

Project / Building Name

Author / Engineer

Floor / Storey Level

Seismic Zone

Part A — Displacement Ratio Check (Amendment No. 2, 2020)

Input the lateral displacements at both extreme edges of the floor in the considered earthquake direction.

Δ₁ — Displacement at End 1 (mm) Typically the end with larger displacement

Δ₂ — Displacement at End 2 (mm) The other extreme edge displacement

Torsional Mode Period T_tor (s) From modal analysis output

1st Translational Period T₁ (s) Along X-direction

2nd Translational Period T₂ (s) Along Y-direction

Part B — Design Eccentricity (Clause 7.8.2)

Compute design eccentricity for torsion design.

e_s — Static Eccentricity (m) Distance between CM and CR

b — Floor Dimension Perpendicular to EQ (m) Plan width perpendicular to earthquake direction

Analysis Method

📊 Results

—

Δ_m (max disp, mm)

—

Δ_a = (Δ₁+Δ₂)/2 (mm)

—

Δ_m / Δ_a Ratio

—

e_d Case 1 (m)

—

e_d Case 2 (m)

📋 Step-by-Step Calculation

📐 Design Eccentricity — Clause 7.8.2

Worked Example

Solved Numerical Problem

Problem Statement

A 6-storey RC building in Seismic Zone IV has the following data at the 3rd floor:
• Δ₁ = 22 mm (maximum edge displacement under EQ in X-direction)
• Δ₂ = 12 mm (displacement at other extreme edge)
• Static eccentricity e_s = 1.5 m, Floor width b = 18 m (perpendicular to EQ)
• Modal analysis gives: T_tor = 1.9 s, T₁ = 1.6 s, T₂ = 1.4 s
• Analysis by Response Spectrum Method
Check for torsional irregularity and compute design eccentricity.

Solution

Step 1: Compute Δ_a (average displacement) Δ_a = (Δ₁ + Δ₂) / 2 = (22 + 12) / 2 = 17 mm
Step 2: Identify Δ_m (maximum) Δ_m = max(Δ₁, Δ₂) = 22 mm
Step 3: Compute Displacement Ratio Δ_m / Δ_a = 22 / 17 = 1.294
Since 1.2 Δ_a = 20.4 mm and 1.4 Δ_a = 23.8 mm,
Δ_m = 22 mm falls in range [20.4, 23.8] → Torsional Irregularity Zone
Step 4: Check Torsional Mode Period T_tor = 1.9 s > T₁ = 1.6 s and T_tor > T₂ = 1.4 s
→ Torsional mode is dominant — both conditions satisfied ✓
Step 5: Conclusion — Torsional Irregularity Classification Building is torsionally irregular as per Table 5(i).
Since 1.2Δ_a ≤ Δ_m ≤ 1.4Δ_a: Revise configuration to bring T_tor < T₁ & T₂, AND adopt 3D dynamic analysis.
Step 6: Design Eccentricity (Clause 7.8.2)
Accidental eccentricity = 0.05 × b = 0.05 × 18 = 0.9 m
Case 1: e_d = 1.5 × 1.5 + 0.9 = 2.25 + 0.9 = 3.15 m
Case 2: e_d = 1.5 − 0.9 = 0.6 m
Note: Factor 1.5 applies since RSM is used (not Time History)
→ Design eccentricity = 3.15 m (Case 1 governs — more severe)

✅ Final Answer The building is torsionally irregular. The design eccentricity to be used = 3.15 m. The structural configuration must be revised to reduce the torsional mode period, and a full 3D dynamic analysis is mandatory.

IS 1893 Table 6

Vertical Irregularities — Quick Reference

While Table 5 covers plan irregularities, Table 6 covers vertical irregularities. Together they define when a building is irregular and what analytical method must be used.

Sl. No.	Type	Definition	Consequence
(i)	Stiffness Irregularity (Soft Storey)	Lateral stiffness of a storey < that of storey above	Dynamic Analysis + Inter-storey drift ≤ 0.2%
(ii)	Mass Irregularity	Seismic weight at any floor > 150% of floors below	Dynamic Analysis in Zones III, IV, V
(iii)	Vertical Geometric Irregularity	Horizontal dimension of LFRS in any storey > 125% of storey below	Dynamic Analysis in Zones III, IV, V
(iv)	In-Plane Discontinuity	In-plane offsets of LFRS exceed 20% of plan length	Not permitted in Zones III, IV, V
(v)	Strength Irregularity (Weak Storey)	Lateral strength < 80% of storey above	Not permitted
(vi)	Floating Columns	Column resting on a beam at lower termination level	Not permitted as primary LFRS element
(vii)	Irregular Oscillation Modes	First 3 modes < 65% mass participation, or T₁ & T₂ within 10%	Ensure 65% mass participation

Summary

Key Takeaways for Students

📐

Two Conditions Required Both the displacement ratio (Δm/Δa) AND the torsional mode period condition must be satisfied for torsional irregularity to exist.

🔢

2020 Amendment Changed the Ratios Old standard used 1.5–2.0 ratio. Amendment No. 2 (2020) replaced this with 1.2Δa–1.4Δa threshold bands.

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Design Eccentricity is Mandatory for ALL Buildings Clause 7.8 applies to all buildings, not just irregular ones. Every building must account for accidental + dynamic eccentricity.

⚡

Factor 1.5 is for SCM & RSM Only The dynamic amplification of 1.5 in the eccentricity formula is NOT applied when using Time History Method.

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3D Analysis is Triggered Torsional irregularity mandates three-dimensional dynamic analysis — not just 2D planar methods.

🔄

Revise Geometry First IS 1893 always says “an effort shall be made to eliminate irregularities”. Architectural redesign is the preferred solution.

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0.05b = Accidental Eccentricity The 5% of floor width accounts for uncertainties in mass location, non-uniform live loading, and rotational ground motion components.

🧱

Open Storeys Add Torsion Risk Adding shear walls in open storeys must not introduce new torsional irregularity — Clause 7.10.3 must be checked.

📖 Standard Reference Summary

IS 1893 (Part 1) : 2016 — Criteria for Earthquake Resistant Design of Structures, Part 1: General Provisions and Buildings (Sixth Revision)
Amendment No. 1 — September 2017 — Minor corrections
Amendment No. 2 — November 2020 — Revised torsional irregularity threshold, open/flexible diaphragm requirements
Clause 7.1 + Table 5(i) — Torsional Irregularity definition
Clause 7.8 — Design eccentricity for torsion
Clause 6.3.2.2 — Bi-directional seismic loading
Clause 7.10.3 — Torsion in open storey buildings

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