EN 1993 Moment Frame — Moment-Resisting Frame Design per Eurocode 3 & 8
Complete guide to moment-resisting frame (MRF) design per EN 1993-1-1 and EN 1998-1. Rigid joint classification, panel zone shear verification, drift limits, second-order effects per Clause 5.2.1, ductility classes DCM/DCH, q factors for MRF systems. Worked 3-bay, 4-storey moment frame example with IPE 330 beams and HEB 260 columns.
Quick access: Braced Frame | Framing Systems | Column Design
Frame Classification (EN 1993-1-1 Cl. 5.2.2)
| Joint Type | Stiffness Criterion | Moment Resistance |
|---|---|---|
| Rigid (non-sway) | S_j >= 8 x EI_b / L_b | M_j,Rd >= M_pl,Rd,beam |
| Rigid (sway) | S_j >= 25 x EI_b / L_b | M_j,Rd >= M_pl,Rd,beam |
| Semi-rigid | Between rigid and pinned | Partial or full strength |
| Pinned | S_j <= 0.5 x EI_b / L_b | M_j,Rd <= 0.25 x M_pl,Rd,beam |
Second-Order Effects (EN 1993-1-1 Cl. 5.2.1)
| alpha_cr Range | Method Required |
|---|---|
| alpha_cr >= 10 | First-order analysis sufficient |
| 3 <= alpha_cr < 10 | Amplified first-order (1/(1-1/alpha_cr)) |
| alpha_cr < 3 | Full second-order GMNIA required |
Storey stability: theta = (P_tot x d_r) / (V_tot x h) If theta > 0.10, include P-Delta. If theta > 0.20, frame is unstable.
Worked Example — 3-Bay 4-Storey Moment Frame
4 storeys at 3.6 m, 3 bays (7.2 m, 3.0 m, 7.2 m). IPE 330 beams, HEB 260 columns, S355. DCM, q = 5.0.
Design Base Shear
| Parameter | Value |
|---|---|
| Total mass | 1200 tonnes |
| Period T_1 | 0.85 s |
| S_d(T_1) | 0.25g |
| Base shear F_b | 588 kN |
Drift Check (Wind)
Max deflection: 28 mm at roof. Drift ratio: H/514. Limit H/500 = 28.8 mm. OK.
Panel Zone (Interior Joint)
Column web shear area: 4520 mm2. V_wp,Rd = 834 kN. Demand: 640 kN. OK, no doubler required.
Ductility Requirements (EN 1998-1 Cl. 6.6.2)
| Parameter | DCM | DCH |
|---|---|---|
| Beam section class | 1 or 2 | 1 |
| Column section class | 1 or 2 | 1 |
| q factor | 5.0 | 6.5 |
| Strong column / weak beam ratio | >= 1.3 | >= 1.3 |
| Drift capacity | 0.025 rad | 0.035 rad |
Frequently Asked Questions
What is the strong column / weak beam principle?
Per EN 1998-1 Cl. 6.6.2(2), the sum of column moment resistances at a joint must exceed beam moment resistances by at least 30% (ratio >= 1.3). This ensures plastic hinges form in beams, not columns, maintaining vertical load capacity during seismic events.
When is second-order analysis required?
When alpha_cr < 10 (Clause 5.2.1). For alpha_cr = 3-10, amplified first-order is OK. For alpha_cr < 3, full second-order GMNIA analysis required.
Related Pages
Educational reference only. MRF design per EN 1993-1-1:2005 and EN 1998-1:2004. Verify National Annex. Results are PRELIMINARY - NOT FOR CONSTRUCTION without independent verification.