What is the behaviour factor q in Eurocode 8?
The behaviour factor q (also written as R in some codes) reduces the elastic design spectrum to account for the ductility and overstrength of the structure. A higher q means more seismic energy is dissipated through inelastic behaviour — reducing design forces but demanding more ductile detailing. q ranges from 1.5 (no ductility) to 6.5 (high ductility moment frames) per EN 1998-1:2004 §6.3.2.
What is the α_u/α_1 refinement?
α_u/α_1 is the ratio of the ultimate base shear to the yield base shear — an overstrength indicator. EN 1998-1 §6.3.2 allows q to be multiplied by this factor when it is determined by a nonlinear analysis (pushover). Default values: 1.10 for single-storey structures, 1.20 for regular multi-storey frames, 1.30 for irregular structures. For standard engineering, the table value without refinement (q_base) is often used.
How do I choose between DCL, DCM and DCH?
DCL (Low ductility) is for structures designed for gravity only with minimal seismic detailing — q ≤ 2. DCM (Medium) is the most common for standard buildings — Class 1 or 2 sections in dissipative zones, q up to 4.0. DCH (High) is for essential or large buildings requiring capacity design (strong column–weak beam), Class 1 sections only, and full ductility detailing — q up to 6.5.
How do National Annexes change the q-factor?
Some countries impose caps on the base q values in EN 1998-1 Table 6.2. For example: NL caps MRF DCM/DCH at 4.0 (standard allows up to 4.0 refined / 6.5). DE caps DCH MRF at 5.0 (standard allows 6.5). FR caps EBF DCM at 4.0. Belgium follows EN 1998-1 closely with no additional caps. Always check the relevant National Annex for the project location.
What section class is required for each ductility class?
EN 1998-1 §6.3.2 Table 6.3 defines cross-section class requirements: DCL accepts Class 1–3 in dissipative zones (no ductility demand). DCM requires Class 1 or 2 in dissipative zones; Class 3 allowed in non-dissipative. DCH requires Class 1 only in dissipative zones. Class 1 sections can develop full plastic hinges without buckling; Class 2 tolerates local buckling before plastic capacity is reached.
Does irregularity reduce the q-factor?
Yes — EN 1998-1 §4.2.3.3 applies a 0.8 factor to q for structures irregular in elevation (soft storeys, mass irregularities, setbacks). This accounts for the difficulty of achieving uniform energy dissipation in non-uniform buildings. Irregularity in plan also triggers torsional analysis requirements.
What is the difference between EBF and ordinary braced frames?
In concentrically braced frames (CBF), diagonals carry the lateral load — but they buckle in compression, limiting q to 2.5–4.0. In eccentrically braced frames (EBF), a deliberate "link" beam segment shear/Y-bends before braces buckle, allowing much higher energy dissipation — q up to 5.0–6.0 for DCH. The link length e determines whether the link dissipates in shear (short, preferred) or bending (long).
DCM moment frame with NL NA — why is q = 4.8?
For a DCM moment frame in NL: base q = 4.0 (Table 6.2). With α_u/α_1 = 1.20 (multi-storey regular), refined q = 4.0 × 1.20 = 4.8. But the NL NA caps DCM MRF at q = 4.0 — so the effective q = 4.0. This is one of the verification cases in the calculator.