Frequently asked questions
What is the critical temperature method in EN 1993-1-2?
EN 1993-1-2 §4.2.4 defines the critical steel temperature θ_a,cr as the temperature at which a steel member loses its load-bearing capacity under fire conditions. It is computed from Eq 4.22: θ_a,cr = 39.19 · ln[1/(0.9674·μ₀³·⁸³³) − 1] + 482, where μ₀ is the degree of utilisation at the fire limit state (ratio of applied action to resistance). For μ₀ = 0.5 (typical), θ_a,cr ≈ 585°C.
What is the section factor A_m/V and why does it matter?
The section factor A_m/V (m⁻¹) is the ratio of the heated perimeter to the cross-section area of a steel member. A high section factor means the member heats up quickly relative to its thermal mass — thin-walled sections (IPE, UB) have high A_m/V and heat up faster than stocky columns (HEB, HEM). For 3-sided exposure (beam against slab), only three faces are exposed to fire, giving a lower A_m/V than a fully-exposed column (4-sided).
What fire rating do I need for my building?
Required fire ratings (R30 to R120) are set by national building regulations, not EN 1993-1-2. In the Netherlands: Bouwbesluit 2012 sets R30 to R120 depending on occupancy class and storey height. In Germany: Landesbauordnungen typically require F30 to F120 (equivalent to R30–R120). The structural engineer confirms compliance against the requirements set by the building code and fire engineer.
How accurate is the unprotected steel calculation?
The incremental heat transfer model per EN 1993-1-2 §4.2.5.1 Eq 4.25 uses the ISO 834 standard fire curve, radiation + convection terms per EN 1991-1-2, and temperature-dependent specific heat c_a per EN 1993-1-2 §3.4.1.2. The shadow factor k_sh (≈0.9 for open sections) accounts for re-radiation between flanges. The model is consistent with reference calculations in EN 1993-1-2 design guides (e.g. SCI P080). Accuracy: ±5–15°C at 30–120 min compared to full numerical FEM.
When does intumescent paint become the right choice?
Intumescent paint (thin-film or thick-film) is ideal when exposed steelwork is architecturally visible, spray finishes are aesthetically unacceptable, or tight construction tolerances preclude board systems. For R30–R60 it is typically cost-competitive; for R90–R120 it may require thick-film systems or hybrid solutions. Product thermal performance must be established by EN 13381 Part 8 fire testing — the λ_p value is not constant with temperature, so parametric modelling per §4.3 is used. This tool does not model intumescent in the heat transfer calculation; use the Pro pipeline for that.