๐Ÿ“‹ IS 1893 (Part 1): 2016 ยท Clause 7.1 ยท Table 6

Vertical Irregularities
in Seismic Design

Soft Storey ยท Mass Irregularity ยท Weak Storey ยท Floating Columns โ€” A complete student guide with interactive calculations

๐Ÿ” Overview ๐Ÿ›๏ธ Soft Storey โš–๏ธ Mass Irregularity ๐Ÿ’ช Weak Storey ๐ŸŒŠ Floating Columns ๐Ÿ“Š Table 6 ๐Ÿงฎ Calculator โ“ Quiz
๐Ÿ—๏ธ
IS 1893 ยท Clause 7.1

What are Vertical Irregularities?

In earthquake engineering, a regular building is one that is uniform in mass, stiffness, and strength along its height. But real buildings often have abrupt changes in these properties from one floor to another โ€” creating what IS 1893 calls vertical irregularities.

IS 1893 (Part 1): 2016 classifies vertical irregularities in Table 6 under Clause 7.1. These irregularities are important because they cause concentration of seismic forces in specific storeys, leading to premature collapse during earthquakes.

๐Ÿ“Œ
Standard Reference
IS 1893 (Part 1): 2016 โ€” "Criteria for Earthquake Resistant Design of Structures: Part 1 โ€” General Provisions and Buildings." Clause 7.1 and Table 6 define five types of vertical structural irregularities. These are checked in both principal plan directions.

The Five Types of Vertical Irregularities

Table 6 ยท Sl. No. 1a

๐Ÿ›๏ธ Soft Storey

Lateral stiffness of a storey is less than 70% of the storey above, or less than 80% of average stiffness of 3 storeys above.

Ki < 0.70 Ki+1
Table 6 ยท Sl. No. 2

โš–๏ธ Mass Irregularity

Seismic weight of any floor is more than 1.5 times that of adjacent floors. Roof does not count.

mi > 1.5 ร— madj
Table 6 ยท Sl. No. 3a

๐Ÿ’ช Weak Storey

Lateral strength of a storey is less than 80% of that in the storey above.

Si < 0.80 Si+1
Table 6 ยท Sl. No. 4

๐ŸŒŠ Floating / Stub Column

Columns whose lower ends rest on beams โ€” do not reach foundations. Causes concentrated forces and must be avoided.

โš  Prohibited in Zones IIIโ€“V
Table 6 ยท Sl. No. 5

๐Ÿ“ Geometric Irregularity

Horizontal dimension of lateral force resisting system changes by more than 30% between adjacent storeys.

>30% plan setback
๐Ÿ“–
Important Note on Extreme Irregularities
Each irregularity type (Soft Storey, Weak Storey) has two severity levels: Irregular and Extremely Irregular. Extremely irregular buildings trigger additional analysis requirements. In Seismic Zones III, IV, and V, buildings with extreme irregularity require dynamic analysis (Response Spectrum or Time History method).
โš ๏ธ
Engineering Insight

Why Vertical Irregularities Are Dangerous

During an earthquake, seismic energy travels from the ground upward through a building. In a regular building, this energy distributes relatively evenly. But in an irregular building, the abrupt change in stiffness or mass causes energy concentration at the transition zone โ€” leading to dramatic increases in ductility demand, storey drift, and ultimately collapse.

REGULAR BUILDING K = 1000 K = 1050 K = 1100 K = 1150 K = 1200 โœ“ Uniform stiffness Energy distributes evenly โ†’ SOFT STOREY BUILDING K = 1000 K = 1050 K = 1100 K = 1050 K = 350 โ† SOFT! โšก Damage concentrates here! โœ— Abrupt stiffness drop Kโ‚ = 350 < 0.70 ร— Kโ‚‚ = 735
๐Ÿ”ด
Historical Evidence
Many catastrophic building collapses in India โ€” Bhuj 2001, Uttarkashi 1991, Latur 1993 โ€” involved buildings with open ground floors (common in India for parking/shops), which are classic soft storey configurations. The soft ground floor acted as a "fuse" that failed first, triggering pancake collapse of the entire building.
๐Ÿ›๏ธ
Table 6 ยท Sl. No. 1a & 1b

Soft Storey & Extreme Soft Storey

A Soft Storey is defined in Clause 4.20.1 as: "One in which the lateral stiffness is less than that in the storey above." The quantitative criterion is given in Table 6 of IS 1893.

Definitions (IS 1893 Clause 4.24)

The Storey Lateral Translational Stiffness (Ki) is the total lateral translational stiffness of all lateral force resisting elements (columns, walls, braces) in storey i, in a principal plan direction.

Threshold Criteria โ€” Table 6, Sl. No. 1

Soft Storey โ€” Criterion A (Sl. No. 1a)
Ki < 0.70 ร— Ki+1
Stiffness of storey i is less than 70% of storey i+1 (the storey directly above)
Soft Storey โ€” Criterion B (also Sl. No. 1a)
Ki < 0.80 ร— Average(Ki+1, Ki+2, Ki+3)
Stiffness of storey i is less than 80% of the average stiffness of the 3 storeys above
โ†’ A storey is soft if EITHER Criterion A OR Criterion B is satisfied
Extreme Soft Storey (Sl. No. 1b)
Ki < 0.60 ร— Ki+1
OR: Ki < 0.70 ร— Average(Ki+1, Ki+2, Ki+3)
Extreme soft storey โ†’ Requires dynamic analysis in Zones III, IV, V

How to Compute Ki for Columns

For a storey with columns in a moment frame, the lateral stiffness of each column is:

Column Lateral Stiffness (Fixed-Fixed)
kcol = 12 EI / hยณ
E = Elastic modulus of concrete | I = Moment of inertia of column section
h = Clear height of storey (in metres)
Total storey stiffness: Ki = ฮฃ(12EI/hยณ) for all columns in storey
โš ๏ธ
Open Ground Floor โ€” The Most Common Soft Storey in India
Buildings with open ground floors for parking (stilt floor) are extremely common in India. The ground floor has only bare columns (no infill walls), while upper floors have brick infill walls. This makes the ground storey 5โ€“10 times more flexible than upper storeys โ€” a severe soft storey condition.

Design Requirement for Soft Storey Buildings

  • Buildings with soft storey shall have the soft storey columns/walls designed for 2.5 times the storey shears and moments calculated under seismic design forces.
  • For extreme soft storey, dynamic analysis is mandatory in Zones III, IV, and V.
  • Alternatively, the soft storey must be eliminated by adding stiffening elements (RC walls, bracings).
โœ…
Remedial Measures for Soft Storey
1. Add RC shear walls in the soft storey. 2. Provide bracings. 3. Infill the soft storey (reduce height difference). 4. Design for amplified forces (2.5ร— shear). 5. Use a podium structure with a robust transfer system.
โš–๏ธ
Table 6 ยท Sl. No. 2

Mass Irregularity

Mass irregularity occurs when the seismic weight of a floor differs significantly from adjacent floors. Since earthquake inertia force = mass ร— acceleration, a sudden jump in mass creates a "heavy" floor that attracts disproportionate seismic forces.

Mass Irregularity Criterion (Table 6 ยท Sl. No. 2)
mi > 1.5 ร— mi-1 OR mi > 1.5 ร— mi+1
mi = Seismic weight of floor i
A floor is mass-irregular if its weight is MORE THAN 1.5 times any adjacent floor
EXCEPTION: Roof floor is not included in this check.

What constitutes "Seismic Weight of a Floor"?

As per Clause 7.7.1 and 7.7.2 of IS 1893:

  • Dead Load (DL) of the floor slab, beams, columns, walls above and below.
  • Imposed Load (IL): 25% of IL for floors with IL โ‰ค 3 kN/mยฒ; 50% of IL for floors with IL > 3 kN/mยฒ.
  • Weight of permanent equipment and fixtures.
๐Ÿ’ก
Practical Examples of Mass Irregularity
โ€ข A swimming pool on the 3rd floor โ€” massive dead + live load concentrated on one floor.
โ€ข A heavy mechanical plant room on an intermediate floor.
โ€ข A transfer slab or outrigger floor that carries significantly more mass.
โ€ข A floor with thick concrete walls (e.g., safe room or server room).

Design Consequence

For buildings with mass irregularity, IS 1893 recommends dynamic analysis (Modal Response Spectrum Method as per Clause 7.7) rather than the simple Equivalent Static Method, because the static method assumes linear mass distribution and cannot capture the effect of concentrated mass accurately.

๐Ÿ’ช
Table 6 ยท Sl. No. 3a & 3b

Weak Storey & Extreme Weak Storey

A Weak Storey is defined in Clause 4.20.2 as: "One in which the lateral strength (cumulative design shear strength of all structural members) is less than that in the storey above."

๐Ÿง 
Soft Storey vs. Weak Storey โ€” Key Difference
Soft Storey = Stiffness problem (how much the storey deforms under load). Weak Storey = Strength problem (how much lateral force the storey can resist before yielding/failing). A storey can be soft without being weak, and vice versa โ€” but often both occur together.
Weak Storey Criterion (Table 6 ยท Sl. No. 3a)
Si < 0.80 ร— Si+1
Si = Lateral shear strength of storey i (Clause 4.23)
Strength of storey i < 80% of storey above
Extreme Weak Storey Criterion (Table 6 ยท Sl. No. 3b)
Si < 0.65 ร— Si+1
Extreme weak storey: Dynamic analysis mandatory in Seismic Zones III, IV & V

Computing Storey Lateral Strength (Si)

The lateral shear strength of a storey is the sum of lateral strengths of all lateral force resisting elements. For a reinforced concrete moment frame:

Lateral Strength of a Column (simplified)
Scol = min(Vflexure, Vshear capacity)
Vflexure = 2 ร— Mp / h (plastic moment / storey height)
Total storey strength: Si = ฮฃ Scol for all columns
๐Ÿšจ
Critical Design Rule
Buildings with an extreme weak storey shall not be built in Seismic Zones III, IV, and V without performing a full dynamic analysis and providing special detailing as per IS 13920. The weak storey shall be designed for amplified seismic forces.
๐ŸŒŠ
Table 6 ยท Sl. No. 4

Floating Columns (Stub Columns)

A floating column (also called a stub column or hanging column) is a column whose lower end rests on a beam and does not extend down to the foundation. It appears to "hang" in mid-air or "float" in the middle of the structure.

FOUNDATION Transfer Beam (heavily loaded) FLOATING COLUMN ! ! Normal column Floating column Transfer beam

Why is it Dangerous?

Floating columns transfer their entire vertical load (and seismic shear) to the supporting beam. During an earthquake:

  • The transfer beam experiences large shear forces and bending moments it wasn't fully designed for.
  • Vertical seismic effects can cause the floating column to punch through the beam.
  • The discontinuity in the load path creates torsional eccentricity and unpredictable failure modes.
  • A sudden concentration of forces at the beam-column junction can cause brittle shear failure.
๐Ÿšซ
IS 1893 Prohibition โ€” Table 6, Sl. No. 4
Floating columns are classified as a vertical irregularity. In Seismic Zones III, IV, and V, buildings with floating columns should NOT be used. If unavoidable, special dynamic analysis and detailed structural design of the transfer system is mandatory. The transfer beam must be designed for amplified forces as per IS 13920.

Common Architectural Scenarios with Floating Columns

  • Buildings with an overhanging upper floor that adds columns not present at the ground level.
  • Ballroom or showroom concept where the ground floor is wide open and upper floors have more columns.
  • Mixed-use buildings where commercial ground floor needs open plan but residential upper floors have dense column grids.
๐Ÿ“
Table 6 ยท Sl. No. 5

Vertical Geometric Irregularity

Geometric irregularity in the vertical direction occurs when the plan dimension (footprint) of the lateral force resisting system changes significantly between adjacent storeys.

Vertical Geometric Irregularity Criterion (Table 6 ยท Sl. No. 5)
Li > 1.30 ร— Li+1 OR Li < Li+1 / 1.30
Li = Horizontal dimension of the LFRS at level i
If any storey's horizontal dimension differs by more than 30% from adjacent storey โ†’ Geometric irregularity
Common examples: Step-back buildings, Podium structures, Setback towers

Examples

  • Step-back buildings on hillsides where each storey steps back along the slope.
  • Podium-tower buildings where a wide base (podium) supports a narrow tower.
  • Inverted pyramid shape buildings where upper floors are wider than lower ones.
๐Ÿ“
Analysis Requirement
Buildings with vertical geometric irregularity require 3D dynamic analysis. The step-back/setback creates torsional response and non-uniform force distribution that cannot be captured by simple 2D static analysis.
๐Ÿ“Š
IS 1893 (Part 1): 2016

Table 6 โ€” Vertical Irregularities (Complete)

The table below reproduces Table 6 of IS 1893 (Part 1): 2016, which lists all vertical irregularities with their quantitative thresholds.

Sl. No. Type Definition / Criterion Category Design Implication
1a Soft Storey Ki < 0.70 Ki+1
OR
Ki < 0.80 ร— avg(Ki+1, Ki+2, Ki+3)		 Irregular Design columns/walls for 2.5ร— storey shear; Dynamic analysis preferred
1b Extreme Soft Storey Ki < 0.60 Ki+1
OR
Ki < 0.70 ร— avg(Ki+1, Ki+2, Ki+3)		 Extreme Dynamic analysis MANDATORY in Zones III, IV, V; Special detailing required
2 Mass Irregularity mi > 1.5 ร— mi-1 OR mi > 1.5 ร— mi+1
Roof not checked		 Irregular Dynamic analysis (RSM) required; static method insufficient
3a Weak Storey Si < 0.80 ร— Si+1 Irregular Design for amplified forces; special detailing as per IS 13920
3b Extreme Weak Storey Si < 0.65 ร— Si+1 Extreme Dynamic analysis MANDATORY in Zones III, IV, V; Prohibited without special analysis
4 Floating / Stub Column Columns whose lower ends rest on beams instead of foundations Critical AVOID in Zones III, IV, V; If used, dynamic analysis + amplified force design mandatory
5 Vertical Geometric Irregularity Horizontal dimension of LFRS > 1.30 ร— adjacent storey dimension Irregular 3D dynamic analysis; attention to torsion and step-back effects
๐Ÿ“Œ
How to Use Table 6
For each storey in both principal plan directions (X and Y), compute the relevant parameter (K, S, or m). Compare against the thresholds. If ANY storey triggers ANY criterion, the building is classified as vertically irregular and must follow the associated design requirements. Multiple irregularities can coexist.

Comparison: Soft Storey vs Weak Storey Thresholds

Parameter Irregular Threshold Extreme Threshold IS Reference
Stiffness Ratio Ki/Ki+1 < 0.70 (or < 0.80 of 3-storey avg) < 0.60 (or < 0.70 of 3-storey avg) Table 6, Sl. 1a, 1b
Strength Ratio Si/Si+1 < 0.80 < 0.65 Table 6, Sl. 3a, 3b
Mass Ratio mi/madj > 1.50 โ€” (only one level) Table 6, Sl. 2
Geometric Ratio Li/Li+1 > 1.30 โ€” (only one level) Table 6, Sl. 5
โš™๏ธ
Design Requirements

Design Consequences & Code Requirements

Analysis Method Requirements

Condition Zone II Zone III Zone IV & V
Regular building, โ‰ค12m height ESM* ESM ESM
Regular building, >12m height ESM RSMโ€  RSM
Irregular (any Table 6 type) RSM preferred RSM RSM/THMโ€ก
Extreme Soft/Weak Storey RSM RSM Mandatory RSM/THM Mandatory

* ESM = Equivalent Static Method | โ€  RSM = Response Spectrum Method | โ€ก THM = Time History Method

Force Amplification Requirements

โšก
Soft Storey โ€” 2.5ร— Design Force
Clause 7.1 of IS 1893 states: columns and beams of soft storey shall be designed for 2.5 times the storey shears and moments calculated as per seismic design forces. This amplification factor accounts for the inelastic deformation demand concentrated in the soft storey.

Key Takeaways for Students

Stiffness โ‰  Strength A storey can be stiff but weak in shear (brittle concrete) or flexible but strong (ductile steel). Check both independently.
Two Criteria for Soft Storey Both the direct comparison (Ki/Ki+1) AND the 3-storey average criterion must be checked. Failing either one qualifies as soft storey.
Roof Exemption The mass irregularity check explicitly exempts the roof floor โ€” only intermediate floors are compared.
Floating Columns = Discontinuous Load Path The primary rule in seismic design is continuous load path. Floating columns violate this, making the structure unpredictable under earthquake loading.
Direction-Dependent Irregularity checks must be performed in BOTH principal plan directions (X and Y). A building may be regular in one direction but irregular in the other.
Compounding Effect A building can have multiple vertical irregularities simultaneously (e.g., a floor that is both soft AND has mass irregularity). Each must be addressed.

๐Ÿงฎ Vertical Irregularity Calculator

IS 1893 (Part 1): 2016 โ€” Table 6 Compliance Checker | Enter storey data and check against code criteria

Enter Lateral Translational Stiffness (K) in kN/m for each storey โ€” Storey 1 = Ground Floor
Storey i Ki (kN/m) Ki+1 (kN/m) Ki/Ki+1 Crit. A (โ‰ฅ0.70?) 3-Storey Avg Ki/Avg (โ‰ฅ0.80?) Status
Enter Seismic Weight (Wi) in kN for each floor. Roof floor is excluded from mass irregularity checks per IS 1893.
Floor i Wi (kN) Wi-1 (kN) Wi+1 (kN) Ratio vs Below Ratio vs Above Status
Enter Lateral Shear Strength (Si) in kN for each storey. This is the total shear strength of all LFRS elements in the storey.
Storey i Si (kN) Si+1 (kN) Ratio Si/Si+1 Weak (โ‰ฅ0.80?) Extreme (โ‰ฅ0.65?) Status
Enter the plan dimension (L) in metres of the Lateral Force Resisting System at each storey level.
Storey i Li (m) Li+1 (m) Ratio Li/Li+1 Ratio Li+1/Li Status
๐ŸŽ“
Self Assessment

Practice Quiz โ€” Vertical Irregularities

Test your understanding of IS 1893 Table 6 criteria. Score: 0 / 6