What is inter-storey drift in Eurocode 8?

Inter-storey drift is the relative horizontal displacement between two consecutive floors of a building divided by the storey height. Eurocode 8 limits this drift to control damage to structural and non-structural elements during earthquakes.

How is inter-storey drift calculated?

For storey i, the inter-storey drift is calculated as the difference between the lateral displacements at the top and bottom of the storey, divided by the storey height. In formula form: drift_i = (u_i - u_{i-1}) / h_i, where u is the horizontal displacement and h_i is the storey height.

What are typical Eurocode 8 drift limits?

Eurocode 8 suggests different drift limits depending on the performance requirement and the type of non-structural elements. Typical values are around 0.5% for brittle elements like unreinforced masonry infills, about 0.75% for more ductile non-structural elements, and up to 1.0% to 1.5% for primary structural elements in damage limitation or near-collapse checks. Always verify the exact values in the current version of EN 1998-1 and any National Annex.

Does this calculator replace a full Eurocode 8 design?

No. This calculator is a quick-check tool to compute inter-storey drift ratios and compare them with typical Eurocode 8 limits. It does not perform a full seismic analysis or capacity design. A qualified structural engineer must interpret the results and ensure full compliance with EN 1998 and the relevant National Annex.

Eurocode 8 Inter-Storey Drift Calculator

Compute inter-storey drift ratios from seismic analysis results and check them against typical Eurocode 8 drift limits for structural and non-structural elements.

Inter-Storey Drift Check (EN 1998-1)

Number of storeys

Total number of storey levels (excluding ground if fixed at zero displacement).

Displacement & height units

Use the same unit for both storey heights and lateral displacements.

Eurocode 8 drift limit type

Then enter storey heights and lateral displacements.

Storey data

Heights are measured between floor levels. Displacements are absolute lateral displacements at each level from seismic analysis.

Storey	Height h_i	Disp. u_i-1	Disp. u_i

Storey 1 uses u₀ at ground level (often 0.0).

Results

Storey	h_i (m)	Δ_i (m)	Drift ratio Δ_i/h_i	Drift (%)	Check

Eurocode 8 inter-storey drift: definition and formula

Inter-storey drift is a key serviceability and damage-control parameter in seismic design. It measures how much one floor moves laterally relative to the floor below during an earthquake.

For storey i (between levels i−1 and i):
$$\Delta_i = u_i - u_{i-1}$$ $$\theta_i = \frac{\Delta_i}{h_i}$$

where:

u_i = lateral displacement at level i
u_i-1 = lateral displacement at level i−1
h_i = storey height between levels i−1 and i
θ_i = inter-storey drift ratio for storey i

Typical Eurocode 8 drift limits

Eurocode 8 (EN 1998-1) specifies drift limits to control damage to non-structural and structural elements. Exact values depend on the limit state, importance class, and National Annex, but commonly used reference values are:

≈ 0.5% (θ ≤ 0.005) for brittle non-structural elements (e.g. unreinforced masonry infills).
≈ 0.75% (θ ≤ 0.0075) for more ductile non-structural elements.
≈ 1.0% (θ ≤ 0.01) for primary structural elements in damage limitation checks.
≈ 1.5% (θ ≤ 0.015) for near-collapse / global stability checks.

Always confirm the applicable drift limits in the current version of EN 1998-1 and your National Annex, as some countries adopt stricter or more permissive values.

How to use this Eurocode 8 drift calculator

Set the number of storeys and choose the unit (m or mm) you use in your analysis model.
Generate the storey table and enter the height of each storey and the lateral displacements at each level.
Select the drift limit type that matches the element you are checking (brittle infills, ductile partitions, structural frame, etc.).
Run the calculation to obtain Δ_i, drift ratios, and a pass/fail check for each storey.
Review the summary to quickly identify the governing storey and maximum drift ratio.

Worked example

Consider a 5-storey RC frame with uniform storey height h = 3.0 m. From a seismic analysis you obtain the following lateral displacements (top of each storey, in metres):

u₀ = 0.000 m (base)
u₁ = 0.010 m
u₂ = 0.022 m
u₃ = 0.036 m
u₄ = 0.050 m
u₅ = 0.062 m

The inter-storey drifts are:

Δ₁ = 0.010 − 0.000 = 0.010 m → θ₁ = 0.010 / 3.0 = 0.0033 (0.33%)
Δ₂ = 0.022 − 0.010 = 0.012 m → θ₂ = 0.0040 (0.40%)
Δ₃ = 0.036 − 0.022 = 0.014 m → θ₃ = 0.0047 (0.47%)
Δ₄ = 0.050 − 0.036 = 0.014 m → θ₄ = 0.0047 (0.47%)
Δ₅ = 0.062 − 0.050 = 0.012 m → θ₅ = 0.0040 (0.40%)

If you adopt a Eurocode 8 drift limit of 0.5% for brittle infills (θ_lim = 0.005), all storeys satisfy the requirement because the maximum drift is about 0.47%.

Engineering notes and limitations

This tool assumes linear interpolation of displacements between storeys and does not account for P–Δ effects.
Use design displacements from the appropriate seismic load combination (e.g. including behaviour factor q).
For irregular structures or torsionally sensitive buildings, check drifts at multiple points along the plan, not only at the centre of mass.
Results should always be reviewed by a qualified structural engineer familiar with Eurocode 8 and local regulations.

Frequently asked questions

Should I use design or elastic displacements?

Eurocode 8 distinguishes between different limit states. For damage limitation checks on non-structural elements, you typically use design displacements from the seismic combination that includes the behaviour factor q. For near-collapse or global stability checks, some National Annexes may require amplified (quasi-elastic) displacements. Always follow the guidance in EN 1998-1 and your National Annex.

What if storey heights are not uniform?

The calculator allows you to enter a different height for each storey. This is important for podium structures, stepped buildings, or cases where the ground floor is significantly taller than the upper floors.

Can I export the drift table?

You can copy the results table directly from the browser into your report or spreadsheet. A future version of this tool may include one-click CSV export, but the numerical values shown are already suitable for documentation.