Engineering Eurocode 8 – Seismic Design Helper (EC8)

Quickly estimate design ground acceleration, elastic/design spectra and storey seismic forces according to Eurocode 8 concepts. Intended as an educational pre‑design tool, not a substitute for the full standard.

Eurocode 8 Seismic Design Calculator

Step 1 – Site & Seismic Action EC8 §3.2–3.3 (conceptual)

Reference peak ground acceleration a_g,R (g)

National Annex value (e.g. 0.16–0.30 g).

Importance factor γ_I

Ground type (soil class)

Used to select S, T_B, T_C, T_D for the design spectrum.

Structural damping ξ (% of critical)

EC8 reference spectrum is for 5% damping. Other values use η = √(10 / (5 + ξ)).

Step 2 – Structural System & Behaviour Factor q EC8 §5 (conceptual)

Lateral‑force resisting system

Ductility class

Behaviour factor q

You can override the suggested q from the dropdowns above.

Fundamental period T₁ (s)

From modal analysis or empirical formulas.

Step 3 – Storey Mass & Height (for Lateral Force Distribution) EC8 §4.3.3.2 (simplified)

Enter storey masses (including a portion of live load) and heights above foundation. The tool distributes the base shear V_b as F_i ∝ m_i h_i.

Number of storeys

Storey	Height h_i (m)	Mass m_i (tonnes)

Global Seismic Parameters

Design ground acceleration a_g: g

Soil factor S:

Damping correction η:

Behaviour factor q:

Fundamental period T₁: s

Elastic spectral acceleration S_e(T₁): g

Base Shear & Storey Shear Forces

The base shear is approximated as V_b = S_d(T₁) · M_tot / q, where M_tot is the total seismic mass. It is then distributed to storeys as F_i ∝ m_i h_i.

Total seismic mass M_tot: tonnes

Base shear V_b: kN (assuming g = 9.81 m/s²)

Storey	h_i (m)	m_i (t)	w_i = m_i h_i	F_i (kN)	ΣF_i above (kN)

Design Spectrum Sample Points (Horizontal)

Values are given in terms of S_d(T)/g for key periods. For detailed design, refer to the full EC8 spectrum definitions and National Annex parameters.

T (s)	S_d(T)/g

How this Engineering Eurocode 8 helper works

This tool implements the main conceptual steps of Eurocode 8 (EN 1998‑1) for regular buildings: definition of the design ground acceleration, construction of the elastic response spectrum, reduction to a design spectrum using the behaviour factor q, and distribution of the base shear to storeys using a simple lateral force method.

Important: This calculator is for education and preliminary sizing only. It does not replace a full EC8 design, National Annex provisions, or professional judgement.

1. Design ground acceleration a_g

Design ground acceleration on type A ground:

\[ a_g = \gamma_I \cdot a_{g,R} \]

\(a_{g,R}\): reference peak ground acceleration on rock (from National Annex).
\(\gamma_I\): importance factor (1.0 for ordinary buildings, >1.0 for essential facilities).

2. Soil factor S and spectrum corner periods

Eurocode 8 defines different soil factors and corner periods for each ground type. This tool uses typical reference values (you should check your National Annex for exact numbers):

Type A: S ≈ 1.0, T_B ≈ 0.15 s, T_C ≈ 0.4 s, T_D ≈ 2.0 s
Type B: S ≈ 1.2, T_B ≈ 0.15 s, T_C ≈ 0.5 s, T_D ≈ 2.0 s
Type C: S ≈ 1.15, T_B ≈ 0.20 s, T_C ≈ 0.6 s, T_D ≈ 2.0 s
Type D: S ≈ 1.35, T_B ≈ 0.20 s, T_C ≈ 0.8 s, T_D ≈ 2.0 s
Type E: S ≈ 1.4, T_B ≈ 0.15 s, T_C ≈ 0.5 s, T_D ≈ 2.0 s

3. Damping correction factor η

For damping ratio \(\xi\) (% of critical), EC8 uses:

\[ \eta = \sqrt{\frac{10}{5 + \xi}} \]

For the reference 5% damping, η = 1.0.

4. Elastic and design response spectra

The horizontal elastic response spectrum S_e(T) is defined piecewise in EC8. This tool implements a simplified version for the short‑period branch, plateau, and descending branch:

For period T:

0 ≤ T ≤ T_B: \[ S_e(T) = a_g \cdot S \cdot \left(1 + \frac{T}{T_B}(\beta - 1)\right) \]
T_B < T ≤ T_C: \[ S_e(T) = a_g \cdot S \cdot \beta \]
T_C < T ≤ T_D: \[ S_e(T) = a_g \cdot S \cdot \beta \cdot \frac{T_C}{T} \]

where β is the amplification factor on the plateau (typically 2.5 for 5% damping).

The design spectrum S_d(T) is obtained by dividing the elastic spectrum by the behaviour factor q:

\[ S_d(T) = \frac{S_e(T)}{q} \]

5. Base shear and lateral force distribution

For regular buildings where a lateral force method is permitted, EC8 allows the base shear to be computed from the design spectrum at the fundamental period T₁:

\[ V_b = S_d(T_1) \cdot M_{tot} \]

\(M_{tot}\): total seismic mass of the building (sum of storey masses).
In this tool, V_b is reported in kN using \(V_b = S_d(T_1)\,g\,M_{tot}\).

The base shear is then distributed to storeys according to:

\[ F_i = V_b \cdot \frac{m_i h_i}{\sum_j m_j h_j} \]

\(m_i\): mass at storey i.
\(h_i\): height of storey i above the foundation.

6. Limitations and good practice

Always check the applicability of the lateral force method (regularity in plan and elevation, height limits, etc.).
Use the exact parameters from your country’s National Annex (a_g,R, S, T_B, T_C, T_D, behaviour factors, etc.).
For irregular or important structures, a full modal response spectrum or time‑history analysis is usually required.

FAQ – Eurocode 8 seismic design

What is Eurocode 8?

Eurocode 8 (EN 1998) is the European standard for the design of structures for earthquake resistance. Part 1 (EN 1998‑1) covers general rules, seismic actions and rules for buildings.

What is the behaviour factor q?

The behaviour factor q represents the ability of a structure to dissipate energy through inelastic behaviour. A higher q reduces the design forces but requires higher ductility and detailing.

Can I use this tool for final design?

No. The calculator is intended for preliminary checks, teaching and quick sensitivity studies. Final design must follow the full text of EN 1998 and the relevant National Annex, and should be carried out by a qualified engineer.

How should I choose the soil type?

Soil type (A–E) is based on geotechnical parameters such as shear wave velocity, standard penetration test (SPT) results and undrained shear strength. Use your site investigation report and the definitions in EC8 §3.1 to select the correct class.