Eurocode Design Guide · Part 4 of 5

Seismic Design per EN 1998 (EC8)

Performance objectives, seismic hazard and elastic response spectrum, ductility classes (DCL/DCM/DCH), behaviour factor q, equivalent lateral force method, modal response spectrum analysis, and structural irregularity criteria.

Performance Requirements
Seismic Hazard & Design Spectrum
Ductility Classes
Behaviour Factor q
Analysis Methods
Irregularities & Torsion

1. Performance Requirements

EN 1998-1 §2.1 defines two fundamental performance requirements:

No-collapse (ultimate): The structure shall withstand the design seismic action without local or global collapse, retaining structural integrity and residual load capacity. Design earthquake = 10% probability of exceedance in 50 years (return period T_R = 475 years).
Damage limitation (serviceability): The structure shall withstand a more frequent seismic action without damage and associated limitations of use. Service earthquake = 10% probability of exceedance in 10 years (T_R = 95 years).

Interstorey Drift Limits — EC8 §4.4.3.2

The damage limitation check is expressed as d_r · ν / h ≤ limit, where d_r = design interstorey drift = q_d · d_s (q_d = displacement behaviour factor, §4.3.4), h = interstorey height, and ν = reduction factor for the damage-limitation earthquake (recommended: 0.4 for importance classes I–II, 0.5 for III–IV).

Non-structural infill type	Limit (d_r·ν / h)	Notes
Brittle (rigid) partitions attached to the structure	0.005	Most common — masonry, plasterboard on rigid frame
Ductile partitions	0.0075	Flexible infill with ductile connectors
No infill / partitions isolated from structure	0.010	Bare frame or fully isolated cladding

EC8 vs ASCE 7 drift limits: ASCE 7 Table 12.12-1 uses Δ/h_sx limits referenced to the design story drift under the design-level earthquake — 0.010 (Risk Cat. IV, stiff), 0.020 (Risk Cat. II, moment frames). EC8 limits apply to the reduced (damage-limitation) earthquake via the ν factor, so a direct numerical comparison requires accounting for the return-period ratio between design and service earthquakes.

Performance Level	Return Period	Exceedance prob.	ASCE 7 Analogue
No-collapse (design)	475 years	10% in 50 yr	DBE (Design Basis EQ)
Damage limitation	95 years	10% in 10 yr	SLE (Service Level EQ)
Near-collapse (rare)	2475 years	2% in 50 yr	MCE_R

2. Seismic Hazard & Design Spectrum

The EC8 elastic design spectrum is defined by the peak ground acceleration on type A ground (a_gR), a soil factor S, and spectral shape parameters T_B, T_C, T_D which define the plateau and descent periods.

EC8 §3.2.2 — Elastic response spectrum Se(T)

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

T_B ≤ T ≤ T_C S_e(T) = a_g · S · η · 2.5 g (plateau)

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

Where:

a_g = γ_I · a_gR — design PGA (importance factor × reference PGA from NA map)
S = soil factor (1.0 for type A; 1.2–1.4 for types C–E)
η = damping correction factor = √(10/(5+ξ)) ≥ 0.55 (ξ = damping % = 5 for RC)
γ_I = importance factor: 0.8 (γI–), 1.0 (γII), 1.2 (γIII), 1.4 (γIV hospital)

Ground Types

Type	Description	v_s,30 (m/s)	S (Type 1)	T_B	T_C	T_D
A	Rock / very dense soil	> 800	1.0	0.15s	0.4s	2.0s
B	Dense sand / gravel	360–800	1.2	0.15s	0.5s	2.0s
C	Medium-dense sand / gravel	180–360	1.15	0.20s	0.6s	2.0s
D	Loose cohesionless soil	< 180	1.35	0.20s	0.8s	2.0s
E	Alluvium over rock (<20m)	—	1.4	0.15s	0.5s	2.0s
S1/S2	Special sites (liquefiable, peat)	< 100	Site-specific study required

EC8 vs ASCE 7 spectrum: ASCE 7 defines the spectrum by S_DS and S_D1 (2/3 × MCE_R spectral values). EC8 defines it by a_gR × S and spectral shape parameters. The plateau acceleration in EC8 is 2.5·a_g·S; in ASCE 7 it is S_DS. These are not directly comparable — the return period basis and soil amplification models differ.

3. Ductility Classes

EC8 allows three ductility classes. The chosen class determines required detailing, minimum material ductility, and the achievable behaviour factor q.

DCL

Low Ductility

Design to strength only. Follows EC2/EC3 with limited seismic provisions. q ≤ 1.5. Only for low-seismicity zones (a_g·S ≤ 0.1g). No special detailing required.

DCM

Medium Ductility

Moderate ductile behaviour. q = 1.5–4.0 (RC frames). Special detailing in critical regions. B500B (or higher) rebar required. Shear design capacity-based in plastic hinge zones.

DCH

High Ductility

Fully ductile. q up to 6.75–8 (frames). Strict capacity design throughout. B500C rebar in critical regions. Most stringent detailing. Required for high-seismicity zones or tall structures.

Capacity design principle: In DCM and DCH, EC8 requires that plastic hinges form in beams (not columns) and that shear failure is prevented by designing shear capacity from the actual flexural overstrength at hinges. This is the basis of the "strong column – weak beam" concept, identical in philosophy to AISC 341.

4. Behaviour Factor q

The behaviour factor q reduces the elastic forces to account for inelastic dissipation. It is the EC8 equivalent of the ASCE 7 Response Modification Factor R (with differences in definition).

EC8 §6.3 — Design spectrum from elastic spectrum

SI S_d(T) = S_e(T) / q (but ≥ β·a_g)

β = 0.2 (lower bound fraction of a_g)
q applies only to the no-collapse limit state; damage limitation uses the elastic spectrum

Structural System	Ductility	q (EC8)	R (ASCE 7 equiv.)
Moment-resisting frame (RC)	DCH	4.5 · α_u/α₁ ≤ 6.75	8.0 (SMRF)
Moment-resisting frame (RC)	DCM	3.9	5.0 (IMRF)
RC dual system (wall-dominant)	DCH	4.4 · α_u/α₁ ≤ 6.6	6.0 (SMRF+walls)
RC ductile walls	DCH	4.4 · α_u/α₁	5.0–6.0
RC walls (DCM)	DCM	3.0	4.0–5.0
Steel moment frame	DCH	6.5	8.0 (SMRF)
Steel concentrically braced	DCH	4.0	6.0 (SCBF)
Steel eccentrically braced	DCH	6.0	8.0 (EBF)
Non-dissipative (DCL)	DCL	1.5	3.0

α_u/α₁ = ratio of base shear at full plastic mechanism to first yield. Default value 1.0; for regular frames with multiple bays ≥ 3: use 1.3 (frames) or 1.2 (dual systems).

5. Analysis Methods

Equivalent Lateral Force (ELF) — EC8 §4.3.3.2

Applicable when the structure is regular in plan and elevation, and T₁ ≤ 4T_C and T₁ ≤ 2.0s. Base shear:

EC8 — Seismic base shear

SI F_b = S_d(T₁) · m · λ kN

T₁ = fundamental period — EC8 allows: T₁ = C_t · H^¾ (C_t = 0.075 RC MRF, 0.085 steel MRF, 0.05 walls/other)
m = total seismic mass = Σ(G_k + ψ_E,i · Q_k,i); ψ_E,i = φ · ψ_2,i
λ = 0.85 (T₁ ≤ 2T_C & ≥ 2 stories); 1.0 otherwise
Forces distributed proportional to height and mass: F_i = F_b · (z_im_i) / Σ(z_jm_j)

Modal Response Spectrum Analysis (MRSA) — EC8 §4.3.3.3

Required for structures irregular in elevation, or T₁ > 4T_C. All modes contributing more than 5% of the seismic mass must be included. Combination by SRSS (well-separated modes) or CQC (closely-spaced modes). The EC8 base shear from MRSA must be ≥ 85% of the ELF base shear; if not, all results are scaled up accordingly.

Non-linear Methods

EC8 permits pushover analysis (non-linear static) and time-history analysis (non-linear dynamic) for assessment and special structures. These methods require more sophisticated material models and careful treatment of record selection.

6. Irregularities & Torsion

Plan Regularity Criteria (EC8 §4.2.3)

Roughly symmetric in plan with respect to two orthogonal horizontal axes
Floor slabs behave as rigid diaphragms
Slenderness ratio of plan dimensions λ = L_max/L_min ≤ 4
No large recesses or re-entrant corners (setback ≤ 5% of each plan dimension)
Torsional stiffness: r_x,y ≥ l_s (radius of gyration of floor mass); r_x,y ≥ e_0x,0y (eccentricity)

Elevation Regularity

Lateral stiffness and mass continuous to the top without abrupt change
Setbacks ≤ 20% (lower floors); ≤ 30% (progressive)
No pilotis (soft storey) without redistribution: K_i/K_i+1 ≥ 0.7 (stiffness); Q_i/Q_i+1 ≥ 0.8 (strength)

Accidental Eccentricity

EC8 requires considering accidental eccentricity e_ai = ±0.05 · L_i in each plan direction (L_i = floor dimension perpendicular to seismic direction). This is applied by shifting the floor mass or amplifying torsional moments with a factor δ.

EC8 vs ASCE 7 on torsion: ASCE 7 §12.8.4.2 uses 5% accidental eccentricity and amplification factor A_x (up to 3.0 for extreme torsion). EC8 uses a similar 5% rule but without A_x amplification — instead, torsional-sensitive structures must use 3D MRSA.

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Educational use only. Seismic hazard data (a_gR) and ground type definitions are country-specific and must be taken from the applicable National Annex. Always use the current version of EN 1998-1 and the project-specific NA. CivilStrCalc accepts no liability for design decisions based on this content.

Seismic Design per EN 1998 (EC8)

Contents

1. Performance Requirements

Interstorey Drift Limits — EC8 §4.4.3.2

2. Seismic Hazard & Design Spectrum

Ground Types

3. Ductility Classes

4. Behaviour Factor q

5. Analysis Methods

Equivalent Lateral Force (ELF) — EC8 §4.3.3.2

Modal Response Spectrum Analysis (MRSA) — EC8 §4.3.3.3

Non-linear Methods

6. Irregularities & Torsion

Plan Regularity Criteria (EC8 §4.2.3)

Elevation Regularity

Accidental Eccentricity

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