Weak Storey vs Soft Storey
What is the difference? Why does it matter in earthquake-resistant design? A complete learning guide with IS 1893 clauses, formulas, and an interactive checker.
Both Soft Storey and Weak Storey are vertical irregularities under IS 1893. But they represent fundamentally different structural vulnerabilities.
“It is one in which the lateral stiffness is less than that in the storey above. The storey lateral stiffness is the total stiffness of all seismic force resisting elements resisting lateral earthquake shaking.”
“It is one in which the lateral strength (cumulative design shear strength of all structural members) is less than that in the storey above. The storey lateral strength is the total strength of all seismic force resisting elements sharing the lateral storey shear in the considered direction.”
| Aspect | 🔵 Soft Storey | 🔴 Weak Storey |
|---|---|---|
| IS 1893 Clause | 4.20.1, Table 6(i) | 4.20.2, Table 6(v) |
| Parameter Measured | Lateral Stiffness (Ki) | Lateral Strength (Si) |
| Irregularity Name | Stiffness Irregularity | Strength Irregularity |
| Trigger Criterion | Ki < Ki+1 (stiffness less than storey above) | Si < 0.8 × Si+1 (strength < 80% of storey above) |
| Primary Concern | Excessive deformation / drift concentration | Brittle collapse / storey mechanism |
| Analysis Required | Dynamic Analysis (RSA or Time History) | Buildings with weak storey not permitted (Zones III–V) |
| Drift Limit (Amendment) | 0.2% inter-storey drift in affected & lower storeys | N/A — structural prohibition |
| Common Building Type | Ground floor parking (piloti buildings), open ground storey | Storeys with significantly fewer or weaker shear walls/columns |
| Physical Analogy | Like a soft spring in a spring-mass system — attracts deformation | Like a weak link in a chain — fails first under load |
| Permitted in Zone II? | Yes, with dynamic analysis | Extreme caution; avoid in all zones |
| Amendment Note | Not applicable to storeys with lower height for utilities, or outrigger frames | If due to URM infills, provisions of 7.10 apply |
🔵 Soft Storey — Stiffness Criterion
A storey is SOFT if:
K_i < K_(i+1)
Where:
K_i = Lateral stiffness of storey i
K_(i+1) = Lateral stiffness of storey above storey i
💡 Note: IS 1893:2016 does not specify a percentage ratio for soft storey classification — any stiffness less than the storey above qualifies. Compare this to earlier codes which used 70% and 80% ratios.
For bare frame buildings considering URM infill stiffness:
K_bare_storey = 0.2 × K_storey_above_with_infill
i.e., if infill walls are present in upper storeys but absent below,
assume stiffness of lower storey = 20% of upper storey stiffness
Inter-storey drift ≤ 0.2% in the soft storey
AND all storeys below
Regular drift limit (Cl. 7.11.1):
Δ_i / h_i ≤ 0.004 (i.e., 0.4%)
⚠️ The soft storey drift limit of 0.2% is more stringent than the general 0.4% limit.
🔴 Weak Storey — Strength Criterion
A storey is WEAK if:
S_i < 0.80 × S_(i+1)
Where:
S_i = Total lateral strength of all elements in storey i (Cl. 4.23)
S_(i+1) = Total lateral strength of storey above storey i
0.80 = 80% threshold
⚠️ Buildings with strength irregularity are NOT PERMITTED in IS 1893 Zones III, IV and V. This is a prohibition, not just a design requirement.
After providing RC walls / braced frames in open storeys:
K_open ≥ 0.80 × K_above (stiffness must be ≥ 80% of storey above)
S_open ≥ 0.90 × S_above (strength must be ≥ 90% of storey above)
RC Wall Plan Density ρ_w ≥ 2% per principal direction
(for Seismic Zones III, IV and V)
K_i = Σ (12 E I / h³) for all columns in storey i
(for fixed-fixed columns)
K_i = Σ (3 E I / h³) for fixed-free columns
Where:
E = Modulus of Elasticity of concrete
I = Moment of inertia of column section
h = Clear height of the storey
For Soft Storey (Stiffness Irregularity)
For Weak Storey (Strength Irregularity)
Ground Floor Shop / Parking (Open Storey)
The most common soft+weak storey scenario in India. Upper floors have brick infill walls adding stiffness and strength, but the ground floor is open for commercial use. This storey becomes both soft (low stiffness) and weak (low shear strength) simultaneously, making it extremely vulnerable.
Set-Back Buildings
When upper floors are set back and have more shear walls than a lower floor, the lower floor can become weak. Conversely, taller storeys (like a mezzanine level) can create stiffness irregularity due to more flexible tall columns.
Tall Storey (Ballroom / Lobby)
A hotel lobby or ballroom that spans 2 normal floor heights results in columns that are twice as tall. Since stiffness ∝ 1/h³, doubling the height reduces column stiffness by 8 times. This is a textbook soft storey condition, even if strength may not be critical.
Transfer Plate/Beam Structures
When structural walls above are not continued to the ground but are supported on transfer beams/plates, the storey below becomes both soft (all lateral load must be carried by few columns) and potentially weak. This is directly addressed by the “In-Plane Discontinuity” category in Table 6.
Infill Irregularity (URM)
RC buildings where upper floors have full brick infill but the ground floor has none (for shops) are the dominant cause of earthquake casualties in India. IS 1893:2016 devotes an entire clause (7.9) to this, requiring equivalent diagonal strut modelling of infill panels.
Historical Earthquake Evidence
Bhuj earthquake (2001), Nepal earthquake (2015), and numerous other seismic events showed that a vast majority of building collapses occurred due to open ground storey failure — a combination of soft and weak storey behaviour. This evidence directly shaped IS 1893:2016 provisions.
All seven types of vertical irregularities from Table 6, with the two storey-type irregularities highlighted.
| # | Type of Irregularity | Trigger Criterion | Consequence | Zones III–V |
|---|---|---|---|---|
| i | 🔵 Stiffness Irregularity (Soft Storey) | Ki < Ki+1 | Dynamic analysis; drift ≤ 0.2% | Dynamic Analysis |
| ii | Mass Irregularity | Wi > 1.5 × Wi±1 | Dynamic Analysis Method | Dynamic Analysis |
| iii | Vertical Geometric Irregularity | Lateral dimension > 125% of storey below | Dynamic Analysis Method | Dynamic Analysis |
| iv | In-Plane Discontinuity (Vertical Elements) | In-plane offset > 20% of plan length | Not permitted in Zones III, IV, V (amended) | Not Permitted |
| v | 🔴 Strength Irregularity (Weak Storey) | Si < 0.8 × Si+1 | Not permitted; or Cl. 7.10 if URM infills | Not Permitted |
| vi | Floating / Stub Columns | Columns not extending to foundations | Undesirable; if unavoidable — special measures | Avoid |
| vii | Irregular Modes (Two Plan Directions) | First 3 modes < 65% MPF; periods within 10% | Ensure 65% MPF; period separation ≥ 10% | Modal Check |
Fig: Ground floor acts as soft storey — less stiff than all floors above, attracting concentrated deformation (δ) under earthquake loading.
Fig: Ground floor lacks shear walls, acting as weak storey — S₁ < 0.8×S₂. Under earthquake, plastic hinges form here causing storey collapse mechanism.
Enter storey stiffness and strength values to check for soft storey and weak storey conditions per IS 1893:2016.
🧮 IS 1893:2016 — Soft Storey & Weak Storey Checker
Based on Table 6 — Vertical Irregularities. Enter values for up to 6 storeys.
4.20 Storey: Space between two adjacent floors.
4.20.1 Soft Storey: “It is one in which the lateral stiffness is less than that in the storey above. The storey lateral stiffness is the total stiffness of all seismic force resisting elements resisting lateral earthquake shaking.”
4.20.2 Weak Storey: “It is one in which the lateral strength (cumulative design shear strength of all structural members) is less than that in the storey above. The storey lateral strength is the total strength of all seismic force resisting elements sharing the lateral storey shear in the considered direction.”
Student Note: These are pure definitions. The specific criteria (80% for weak storey; and any reduction for soft storey) come from Table 6.
4.23 Storey Lateral Shear Strength (Si): Total lateral strength of all lateral force resisting elements in the storey considered in a principal plan direction.
4.24 Storey Lateral Translational Stiffness (Ki): Total lateral translational stiffness of all lateral force resisting elements in the storey considered in a principal plan direction.
Student Note: Both parameters must be assessed in each principal plan direction (X and Y). A building may be soft/weak in one direction but regular in the other.
7.10.1: RC MRF buildings with open storey(s) at any level (due to URM infill discontinuation or structural wall discontinuation) are known to have flexible and weak storeys. Measures must increase both stiffness and strength in open storey and storeys below, along both plan directions.
Permitted Measures: (a) RC structural walls, or (b) Braced frames in select bays.
7.10.2 — Requirements for RC walls: Founded on proper foundations; continuous over full height preferably; connected to the MRF.
7.10.3 — Post-remediation checks (must all pass):
- (a) No additional torsional irregularity created
- (b) Lateral stiffness in open storey ≥ 80% of storey above
- (c) Lateral strength in open storey ≥ 90% of storey above
7.10.4: RC wall plan density ρw ≥ 2% per principal direction in Seismic Zones III, IV and V. Walls to be well distributed in plan.
7.10.5: All RC structural walls in Zones III, IV and V to be designed per IS 13920 (ductile detailing).
7.9.1: Variation of storey stiffness and storey strength must be examined along building height considering in-plane stiffness and strength of URM infill walls. If this renders it irregular per Table 6, treat as irregular especially in Zones III, IV and V.
7.9.2.1 — Modulus of Elasticity of Masonry (Em):
E_m = 550 × f_m (MPa)
f_m = 0.433 × f_b^0.64 × f_mo^0.36
where f_b = brick compressive strength (MPa)
f_mo = mortar compressive strength (MPa)
7.9.2.2 — Equivalent Diagonal Strut (for URM infill modelling):
w = 0.175 × α_h^(-0.4) × L
α_h = h × [E_m × t × sin(2θ) / (4 × E_c × I_c × h)]^(1/4)
Thickness of strut = t (URM infill thickness)
Valid when: h/l < 12 AND l/t < 12
General Limit (Cl. 7.11.1.1): Storey drift in any storey shall not exceed 0.004 times the storey height (i.e., 0.4%) under design base shear VB with partial safety factor = 1.0.
Soft Storey Limit (Amendment to Table 6, Item i): Inter-storey drift ≤ 0.2% in storeys with stiffness irregularity and all storeys below.
Example: For a storey height of 3.0 m:
- General limit: 3000 × 0.004 = 12 mm
- Soft storey limit: 3000 × 0.002 = 6 mm (more stringent)
The official amendment to IS 1893 (Part 1): 2016 made several key changes:
- Table 6(i) Soft Storey: Added inter-storey drift limit of 0.2%; added exception for storeys with lower height for utilities or outrigger frames; clarified URM infill treatment (SPD > 20% → explicit modelling).
- Table 6(v) Weak Storey: Strengthened to “Buildings with strength irregularity shall not be permitted” — from earlier language that merely required special measures.
- Table 6(iv) In-Plane Discontinuity: In Zone II, drift limit ≤ 0.2%; in Zones III, IV, V — not permitted at all.
- Table 6(vi) Floating Columns: Renamed and sharpened: if floating columns are part of primary lateral load resisting system — not permitted.
Download a standalone printable report summarising all key concepts, formulas, and your calculator results.
