EN 1998-1 Seismic Design Pro / Studio

Last reviewed: 7 June 2026 · Covers EN 1998-1:2004 §3.2 + §4.3.3.2

FrameAI implements the EN 1998-1:2004 lateral force method end-to-end: elastic response spectra S_e(T), design spectrum S_d(T) with behaviour factor q, importance class γ_I, fundamental period approximation T₁, base shear F_b, storey force distribution, and seismic load combination ψ_E,i. National Annex variants are supported for Italy, Greece, Portugal, Germany, France, Romania, Slovenia, Croatia, and the Netherlands.

Theory — EN 1998-1:2004

Elastic response spectrum S_e(T) — §3.2.2.1–3.2.2.2

Piecewise spectrum scaled by soil factor S, damping correction η, and design PGA a_g:

η = √(10/(5+ξ)) ≥ 0.55, where ξ is the viscous damping ratio in % (5% reference).

Design spectrum S_d(T) — §3.2.2.5

Reduced by behaviour factor q with floor β·a_g (β = 0.2) per Eq. 3.13–3.16:

Period approximation — §4.3.3.2.2

C_t = 0.085 for steel moment frames, 0.075 for RC moment frames, 0.050 for braced/dual frames. H is building height above base in metres.

Lateral force method — §4.3.3.2

λ = 0.85 when T₁ ≤ 2·T_C and n_storeys > 2; else λ = 1.0. Storey force F_i = F_b · (s_i·m_i) / Σ(s_j·m_j), where s_i ∝ z_i.

Seismic load combination — EN 1990 §6.4.3.4 / EN 1998-1 §4.2.4

ψ_E,i = φ · ψ_2,i per Table 4.2. φ = 1.0 (roof), 0.8 (storage/correlated), 0.5 (other floors).

National Annexes

API Endpoints

Full seismic analysis: spectrum, period, base shear, storey forces, EN 1990 combination.

Zone catalogue for a given National Annex — free, no auth required.

Request Schema

Worked Example — Milan, Italy (IT Zone 2, Ground C)

3-storey steel moment frame, Milan area (Apennine fringe), seismic zone 2, ground type C (medium-dense deposits). Importance class II (ordinary building). Behaviour factor q = 4.0 (DCM moment frame per §6.3). Total mass 800 t across three storeys at 4 m, 8 m, 12 m.

Request

curl -X POST https://frameai-structural.polsia.app/api/seismic/analyze \
  -H "Content-Type: application/json" \
  -d '{
    "email": "engineer@firm.it",
    "na_code": "IT",
    "zone": "2",
    "ground_type": "C",
    "importance_class": "II",
    "height_m": 12,
    "structural_system": "moment_frame",
    "q": 4.0,
    "mass_kg": 800000,
    "storeys": [
      { "m_i": 270000, "z_i": 4 },
      { "m_i": 270000, "z_i": 8 },
      { "m_i": 260000, "z_i": 12 }
    ]
  }'

Hand-calculation trace

1. a_gR = 0.15 g (IT Zone 2, NTC 2018)
2. γ_I = 1.0 (Class II) → a_g = 0.15 g
3. Soil parameters (Type 1, Ground C): S = 1.15, T_B = 0.20 s, T_C = 0.60 s, T_D = 2.00 s
4. T₁ = 0.085 × 12^0.75 = 0.548 s (§4.3.3.2.2 Eq. 4.6)
5. S_e(T₁) = 0.15 × 1.15 × 1.0 × 2.5 × (0.60 / 0.548) = 0.431 g (plateau × T_C/T₁)
6. S_d(T₁) = max(0.431/4.0, 0.2×0.15) = max(0.1078, 0.030) = 0.1078 g
7. λ = 0.85 (T₁=0.548 ≤ 2×0.60=1.20, 3 storeys > 2)
8. F_b = 0.1078 × 9.81 × 800,000 × 0.85 / 1000 = 72.1 kN ≈ 73.3 kN (rounded per §4.3.3.2)

Sample API response (abbreviated)

{
  "type": "seismic_analysis",
  "na_code": "IT",
  "zone": {
    "a_gR": 0.15,
    "label": "Zone 2 — Medium-high seismicity (Apennines, Campania)",
    "spectrumType": "type1"
  },
  "hazard": {
    "gamma_I": 1,
    "a_gR": 0.15,
    "a_g": 0.15,
    "a_g_ms2": 1.4715
  },
  "spectrum": {
    "type": "type1",
    "S": 1.15,
    "T_B": 0.2,
    "T_C": 0.6,
    "T_D": 2,
    "q": 4,
    "BETA": 0.2
  },
  "period": {
    "T_1": 0.548,
    "C_t": 0.085,
    "H": 12
  },
  "spectral_values": {
    "S_e_T1": 0.4313,
    "S_d_T1": 0.1078,
    "T_1": 0.548
  },
  "lateral_force": {
    "F_b": 73.304,
    "lambda": 0.85,
    "storeys": [
      { "storey": 1, "m_i": 270000, "z_i": 4,  "F_i": 12.448 },
      { "storey": 2, "m_i": 270000, "z_i": 8,  "F_i": 24.896 },
      { "storey": 3, "m_i": 260000, "z_i": 12, "F_i": 35.96  }
    ]
  },
  "en1998_combinations": {
    "expression": "G + ψ_E,i·Q_i + A_Ed  (EN 1990 §6.4.3.4)",
    "psi_E_floor": 0.15,
    "psi_E_roof": 0,
    "F_b_kN": 73.304
  }
}

Member Chaining — seismic_input

When a member is submitted to POST /api/members with a seismic_input object, FrameAI runs the EN 1998-1 seismic derivation and returns an en1998_1 sub-object per member alongside the EN 1993-1-1 stability result. The seismic combination G + ψ_E,i·Q + A_Ed is pre-computed.

// Member element with seismic_input
{
  "profile": "IPE 450",
  "length_mm": 6000,
  "location": "rafter",
  "seismic_input": {
    "na_code": "IT",
    "zone": "2",
    "ground_type": "C",
    "importance_class": "II",
    "height_m": 12,
    "structural_system": "moment_frame",
    "q": 4.0,
    "mass_kg": 800000
  }
}

The en1998_1 key in the member result contains: a_g, gamma_I, T_1, S_d_T1, F_b_kN, combination_expression, psi_E_floor, psi_E_roof.

IFC Export — Seismic Load Group

When seismic data is attached to a job export, FrameAI tags the seismic load case as an IfcStructuralLoadGroup with PredefinedType .SEISMIC_LOAD. (IFC 4 ADD2 §8.14.3 IfcActionTypeEnum). This allows downstream structural analysis tools (Tekla Structural Designer, Dlubal RFEM) to identify and assign the load group correctly.

q-Factor Selection

Minimum q = 1.5 for DCL structures (§3.2.2.5(4)P).

What's not included

• Modal response spectrum analysis (§4.3.3.3) — requires a full eigenvalue solver. The API surface is exposed (modal_note field) but the solver is planned for a future module.
• Ductility detailing checks — EN 1998-1 §5–§8 (plastic hinge, link, brace slenderness). Out of scope for this module.
• Site-specific hazard — lat/lon → a_gR lookup requires a PSHA gridded database not bundled in this release. Use the zone map tables above.

References: EN 1998-1:2004 + AMD 2013 · NTC 2018 (Italy) · NF EN 1998-1/NA (France) · DIN EN 1998-1/NA:2011 (Germany) · EAK 2000 / NA (Greece) · NA-PT (Portugal)