Storey Drift — What 0.004h Really Means
A complete student guide to understanding, applying, and checking the seismic storey drift limit specified by the Indian Standard IS 1893 (Part 1) : 2016.
🤔 What is Storey Drift?
Storey drift is the relative horizontal displacement between two consecutive floors of a building during an earthquake. Think of it as how much one floor “slides” sideways compared to the floor directly below it.
If Floor 3 displaces 18 mm to the right and Floor 2 displaces 10 mm to the right, the storey drift of Storey 3 is 18 − 10 = 8 mm.
Fig. 1 — Storey Drift = Relative horizontal displacement between consecutive floors during seismic loading
🏗️ Why Is Drift Control Important?
Protects Non-Structural Elements
Excessive drift damages partition walls, facades, glass cladding, plumbing, and electrical conduits — even when the structure itself survives.
Prevents P-Δ Instability
Large lateral displacements combined with gravity loads create destabilizing second-order moments (P-Δ effect) that can cause collapse.
Ensures Structural Integrity
Controlled drift keeps structural members within elastic or low-inelastic range, ensuring the building can be repaired after design-level earthquakes.
Life Safety
Collapse of cladding and partitions can block escape routes, rupture fire-fighting water mains, and injure occupants even without structural failure.
📖 Clause 7.11.1.1 — IS 1893 (Part 1) : 2016
“The storey drift in any storey due to the minimum specified design lateral force, with partial load factor of 1.0, shall not exceed 0.004 times the storey height.”
For the purposes of displacement requirements only (Clauses 7.11.1, 7.11.2 and 7.11.3), it is permissible to use seismic force obtained from the computed fundamental period T of the building without the lower bound limit on design seismic force specified in Clause 7.2.2.
There shall be no drift limit for single storey buildings which have been specially designed to accommodate storey drift.
🔍 Clause-by-Clause Breakdown
| Term / Phrase | What it Means | Practical Implication |
|---|---|---|
| “in any storey” | The check must be done at every single storey, not just the worst one | Run the drift check for all n storeys of the building |
| “minimum specified design lateral force” | The seismic base shear VB from IS 1893 | Use the equivalent static or response spectrum forces |
| “partial load factor of 1.0” | Do NOT apply the 1.5× or 1.2× factored load combinations for this check | Unfactored seismic loads (1.0 × EQ) are used for drift checking |
| “shall not exceed 0.004h” | Maximum permissible drift = 0.4% of storey height | For 3 m storey: limit = 12 mm; for 4.5 m storey: limit = 18 mm |
| “without lower bound” | For drift only, T can be the computed period (not the empirical lower bound) | Allows a more realistic (smaller) seismic force for displacement calculation |
| “no drift limit — single storey” | Exception for single storey buildings designed for drift | Industrial sheds, warehouses with flexible connections can be exempt |
Computing Δᵢ (Storey Drift)
Where:
δᵢ = absolute lateral displacement of floor i
δᵢ₋₁ = absolute lateral displacement of floor below (i−1)
Both obtained from structural analysis under unfactored seismic loads.
Drift Ratio vs. Drift
θᵢ = Drift ratio (dimensionless)
The IS 1893 limit is: θᵢ ≤ 0.004
Equivalent to: Δᵢ ≤ 0.004 × hᵢ
✅ Same limit regardless of the units used (if consistent).
📊 Quick Reference — Drift Limits by Storey Height
| Storey Height (h) | Drift Limit = 0.004 × h | Soft Storey Limit = 0.002 × h | Typical Use |
|---|---|---|---|
| 2.8 m (2800 mm) | 11.2 mm | 5.6 mm | Residential apartment |
| 3.0 m (3000 mm) | 12.0 mm | 6.0 mm | Standard residential |
| 3.5 m (3500 mm) | 14.0 mm | 7.0 mm | Commercial office |
| 4.0 m (4000 mm) | 16.0 mm | 8.0 mm | Commercial/retail |
| 4.5 m (4500 mm) | 18.0 mm | 9.0 mm | Institutional/hotel |
| 5.0 m (5000 mm) | 20.0 mm | 10.0 mm | Industrial/warehouse |
| 6.0 m (6000 mm) | 24.0 mm | 12.0 mm | Industrial/atrium |
Compute Seismic Base Shear VB
Calculate design base shear using Vₐ = Aₕ × W where Aₕ = (Z/2) × (I/R) × (Sₐ/g) per IS 1893 Clause 6.4.2.
For drift check only: compute Aₕ using the computed fundamental period T (not the lower-bound empirical value).
Distribute Lateral Forces to Floors
Per IS 1893 Clause 7.6.3, distribute VB to floors using: Qᵢ = VB × (Wᵢ × hᵢ²) / Σ(Wⱼ × hⱼ²)
Apply Unfactored Load (Load Factor = 1.0)
Critically, apply seismic loads with partial load factor = 1.0 for drift check. Do not use 1.2(DL+IL+EQ) or 1.5(DL+EQ).
Run Structural Analysis → Get Floor Displacements
Perform equivalent static, response spectrum, or time-history analysis. Use cracked section properties: 0.70 Ig for columns, 0.35 Ig for beams (IS 1893 Clause 6.4.3.1).
Extract absolute lateral displacements: δ₁, δ₂, δ₃ … δₙ at each floor level.
Compute Storey Drift for Each Storey
Δᵢ = δᵢ − δᵢ₋₁ for each storey i = 1 to n. (For ground storey: δ₀ = 0)
Check Against Limit
For each storey: verify Δᵢ ≤ 0.004 × hᵢ
Also check: Drift Ratio θᵢ = Δᵢ / hᵢ ≤ 0.004
For soft storeys: check Δᵢ / hᵢ ≤ 0.002
If Limit Exceeds — Redesign
Increase member stiffness (larger column/beam sections, add shear walls, add bracings), or adjust the structural layout. Repeat from Step 4.
Enter the number of storeys, storey heights (mm), and absolute displacements (mm) from your analysis. Drift ratio limit = 0.004 per IS 1893.
| Storey | h (mm) | δᵢ (mm) | δᵢ₋₁ (mm) | Drift Δᵢ (mm) | Limit 0.004h (mm) | Drift Ratio θ | Status |
|---|
IS 1893 Clause 7.11.1.1 states: “There shall be no drift limit for single storey building which has been designed to accommodate storey drift.”
This applies to industrial sheds, warehouses, factories, and similar single-storey structures where the designer has explicitly accounted for drift in the connection design, secondary element detailing, and serviceability checks. However, the designer must still ensure safety against P-Δ effects and non-structural damage.
Per IS 1893 Amendment No. 2 (2020), Table 6, for buildings with stiffness irregularity (soft storey condition), the inter-storey drift shall be limited to 0.2% of storey height (i.e., 0.002h) in the irregular storey and all storeys below it.
A soft storey exists when the lateral stiffness of a storey is less than that of the storey above. This commonly occurs at ground level with open parking (pilotis), or large open spaces below solid floors above.
For flat slab buildings, IS 1893 (Amendment No. 2) specifies that lateral drift shall be estimated:
- Considering total lateral displacement including torsional effects
- Using three-dimensional models
- Scaling need not be done for displacement response quantities
For buildings with in-plane discontinuity located in Seismic Zone II, the lateral drift under the design lateral force shall be limited to 0.2% of the building height — an especially tight constraint applicable to irregularly shaped structures.
IS 1893 normally requires that the design seismic force uses the lower bound (empirical) period Tₐ unless the computed period T is smaller. However, for drift checking only, the code permits using the computed fundamental period T (which is typically longer) without the lower bound limit.
This is significant because a longer period → smaller Sₐ/g → smaller base shear → smaller computed displacements. Using T (computed) for drift means you can obtain a more realistic — and often more favourable — displacement check.
Note the distinction between storey drift (relative inter-storey displacement) and overall sway (absolute top displacement):
| Standard | Parameter | Limit |
|---|---|---|
| IS 1893 : 2016 | Storey Drift (seismic) | 0.004 × h per storey |
| IS 1893 : 2016 | Overall building sway | H/250 (implicit) |
| IS 456 : 2000 | Lateral sway (wind) | H/500 (total height) |
Both checks must be satisfied for a code-compliant design. Wind drift (IS 456) is generally more stringent than seismic drift (IS 1893) for low-rise buildings.
Load Factor: 1.0 (unfactored)
Period: Computed T allowed
Exception: Single storey — no limit
Soft Storey: 0.002 (0.2%)
Note: Elastic (design-level) drift
Load Factor: Uses Cd amplification
Standard: Risk Category II, typical = 0.020h
Note: Inelastic (amplified) drift checked
Is 1893 vs ASCE: IS 1893 generally more conservative
ν: Reduction factor (0.4 for RC Class II)
Note: Damage limitation check at reduced seismic action
Effective elastic drift: ~0.001h to 0.003h
Comparison: Similar to IS 1893 in practical terms
The 0.004 (or 1/250) limit is based on empirical research on the behaviour of non-structural elements during earthquakes. A drift ratio beyond ~0.4% causes significant damage to brittle partition walls, window frames, and cladding. This limit was established in IS 1893 : 2002 and retained in the 2016 edition as a balance between structural performance and non-structural protection without excessive cost impact on the structural system.
IS 1893 uses elastic displacements directly from analysis — no amplification by Cd or μ factor (unlike ASCE 7). You apply unfactored seismic loads (load factor = 1.0), run the structural analysis, and extract the displacements directly. This is sometimes called “serviceability drift” or “elastic drift.”
IS 1893 Clause 6.4.3.1 requires cracked section properties for seismic analysis: 70% of Ig for columns, 35% of Ig for beams. Using cracked sections gives larger displacements, which is more realistic and conservative for drift checks. Using gross sections would underestimate drift and potentially give a false compliance.
Drift and strength are separate checks. A building can satisfy drift limits (because it is stiff) but still fail in strength if members are undersized. Both checks must independently pass. In practice, for taller buildings, drift often governs the design of lateral members (shear walls, frames), while for shorter buildings, strength tends to govern.
Storey drift (Δᵢ, in mm) is the absolute relative displacement. Inter-storey drift ratio (IDR = θᵢ = Δᵢ/hᵢ) is the dimensionless ratio. The code limit 0.004 is expressed as a ratio, equivalent to saying “Δᵢ shall not exceed 0.004 × hᵢ mm.” Both expressions are equivalent — just different representations of the same limit.
Yes. IS 1893 requires checking earthquake effects in both principal plan directions. You must check storey drift in X-direction under ELx loading AND in Y-direction under ELy loading. Both directions must independently satisfy the 0.004h limit. When torsional effects are significant, the maximum displacement (at the most displaced corner) must be used.
- Add RC Shear Walls: Most effective — dramatically increases lateral stiffness
- Increase Column Sizes: Increases frame stiffness
- Add Steel Bracing: Especially for steel/composite structures
- Increase Beam Depth: Increases frame rigidity
- Add Outrigger System: For tall buildings — connects core walls to perimeter columns
- Reduce Bay Width: Shorter beams increase overall stiffness
- Base Isolation: Reduces seismic demand, hence drift
