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, including the definition of design response spectra and base shear.

Which parts of Eurocode 8 does this calculator implement?

This tool focuses on EN 1998‑1:2004, Section 3 (Design seismic action) and Section 4 (Buildings). It implements the elastic and design response spectra of 3.2.2.2 and 3.2.2.5, the behaviour factor q concept, and the base shear calculation for regular buildings in plan and elevation.

Can I use this calculator as a replacement for a full Eurocode 8 design?

No. The calculator is an educational and preliminary‑design aid. A complete Eurocode 8 design requires detailed modelling, load combinations, ductility checks, capacity design and verification of detailing rules. Always verify results with the full text of EN 1998 and the relevant National Annex and use professional engineering judgement.

Where do I get the values of agR, S, TB, TC and TD?

The reference peak ground acceleration agR, soil factor S and the corner periods TB, TC, TD are defined in the National Annex to Eurocode 8 for each country and seismic zone. Many national seismic maps and design guides provide these values. This calculator lets you input them directly so it can be used with any National Annex.

What is the behaviour factor q in Eurocode 8?

The behaviour factor q accounts for the energy dissipation capacity of the structure through inelastic behaviour. Higher q values correspond to more ductile systems and lead to lower design forces. Eurocode 8 provides recommended q values for different structural systems and ductility classes (DCL, DCM, DCH).

Eurocode 8 Seismic Design Calculator (Response Spectrum & Base Shear)

Compute elastic and design response spectra, design ground acceleration, behaviour factor and base shear according to EN 1998‑1 (Eurocode 8) for buildings.

EN 1998‑1:2004 Response spectrum Base shear

This tool is for educational and preliminary design use only. Always verify with the full text of Eurocode 8, the relevant National Annex and professional engineering judgement.

Seismic action & site parameters

Reference peak ground acceleration a_gR [g]

From National Annex seismic map (e.g. 0.25 g).

Importance factor γ_I

Ground type

Used to pre‑fill S, T_B, T_C, T_D. Adjust to match National Annex.

Soil factor S [–]

Override if your National Annex specifies different S.

T_B [s]

T_C [s]

T_D [s]

Damping correction factor η

η = 1 for 5% damping. For other damping, η = √(10 / (5 + ξ)).

Structural system & behaviour factor

Ductility class

Structural system (approximate q)

Behaviour factor q [–]

Adjust according to EN 1998‑1 Table 5.1 and National Annex.

Correction factor λ [–]

λ = 0.85 for buildings with ≥3 storeys and T1 ≤ 2T_C, otherwise 1.0.

Global building parameters (direction considered)

Fundamental period T₁ [s]

From modal analysis or empirical formula of EN 1998‑1.

Total seismic mass m [tonnes]

Includes dead load + relevant portion of live load.

Gravity acceleration g [m/s²]

Key results

Design seismic action

a_g = – g
S · a_g = – g
T_B = – s, T_C = – s, T_D = – s
η = –, q = –

Base shear (direction considered)

T₁ = – s
S_d(T₁) = – g
Base shear F_b = – kN
Minimum base shear check: – kN (0.8·S_d(T_C))

Elastic & design response spectra (selected periods)

T [s]	S_e(T) [g]	S_d(T) [g]

Full spectrum values (0.0–4.0 s) are listed in the “Spectrum Table” tab and can be copied to spreadsheets or structural analysis software.

Response spectrum plot

Approximate plot of elastic and design spectra. For detailed plotting, export the table and use your preferred graphing tool.

Detailed spectrum table (0.0–4.0 s)

Values are computed every 0.05 s. Click “Copy table” to copy as tab‑separated values for Excel or other tools.

T [s]	S_e(T) [g]	S_d(T) [g]

Eurocode 8 seismic design: what this calculator does

This Eurocode 8 seismic design calculator implements the elastic and design response spectra of EN 1998‑1:2004, Section 3, and the base shear calculation for buildings in Section 4. It is intended as a fast, transparent tool for preliminary design, checking hand calculations, and teaching.

1. Design ground acceleration a_g

The design ground acceleration on type A ground is obtained from the reference peak ground acceleration a_gR and the importance factor γ_I:

a_g = γ_I · a_gR

a_gR is defined in the National Annex for each seismic zone. γ_I depends on the importance class of the structure (I to IV).

2. Elastic response spectrum S_e(T)

For a given soil factor S and corner periods T_B, T_C, T_D, the elastic response spectrum (5% damping) is defined piecewise in EN 1998‑1 §3.2.2.2. For Type 1 spectrum:

For 0 ≤ T ≤ T_B:
  S_e(T) = a_g · S · [1 + (T / T_B) · (2.5 · η − 1)]

For T_B < T ≤ T_C:
  S_e(T) = a_g · S · 2.5 · η

For T_C < T ≤ T_D:
  S_e(T) = a_g · S · 2.5 · η · (T_C / T)

For T > T_D:
  S_e(T) = a_g · S · 2.5 · η · (T_C · T_D / T²)

The calculator uses this formulation and lets you choose η (damping correction factor). For 5% damping, η = 1. For other damping ratios ξ, Eurocode 8 suggests:

η = √(10 / (5 + ξ))

3. Design spectrum S_d(T)

The design spectrum is obtained by dividing the elastic spectrum by the behaviour factor q:

S_d(T) = S_e(T) / q

q represents the ductility and energy dissipation capacity of the structural system. Eurocode 8 provides recommended q values for different systems and ductility classes (DCL, DCM, DCH). The calculator allows you to select a system and then fine‑tune q.

4. Base shear F_b

For a regular building in plan and elevation, the design base shear in the considered direction is:

F_b = S_d(T₁) · m · g

where:

T₁ is the fundamental period in the direction considered,
m is the total seismic mass of the structure,
g is the acceleration of gravity.

The calculator also reports the minimum base shear according to EN 1998‑1 (0.8·S_d(T_C)), which is often used as a lower bound for design.

5. Typical workflow with this tool

Obtain a_gR, S, T_B, T_C, T_D from the National Annex.
Select the importance class to set γ_I.
Choose the ground type and adjust S and corner periods if needed.
Select the structural system and ductility class, then set an appropriate q.
Determine T₁ from modal analysis or empirical formulas.
Estimate the total seismic mass m (dead load + relevant live load).
Run the calculator to obtain S_e(T), S_d(T), and F_b.
Distribute F_b along the height of the building according to EN 1998‑1 §4.3.3.2.

Limitations and good practice

This tool assumes Type 1 spectrum and 5% reference damping (η adjustable).
It does not perform modal combination (SRSS, CQC) or torsional effects.
It does not check detailing rules, capacity design or irregularities.
Always cross‑check with the full Eurocode 8 text and your National Annex.

Eurocode 8 seismic design – FAQ

Does this calculator support both Type 1 and Type 2 spectra? ▶

The current version implements the Type 1 spectrum, which is typically used for regions with higher seismicity. Many National Annexes specify whether Type 1 or Type 2 should be used. You can still use the tool with Type 2 by inputting equivalent S, T_B, T_C, T_D values, but always verify against the National Annex. A future update may add an explicit Type 1 / Type 2 toggle.

How accurate is the base shear compared to full structural software? ▶

For regular buildings where the fundamental mode dominates the response, the base shear from this calculator should be close to that from commercial software using the same spectrum and q value. Differences arise when higher modes, torsion, soil–structure interaction or 3D effects are important.

Can I use this for bridges or non‑building structures? ▶

The response spectrum formulation is general, but Eurocode 8 has specific parts and rules for bridges, tanks, silos and other structures. This calculator is tailored to buildings (EN 1998‑1) and should not be used as‑is for other structure types without appropriate adaptations.