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🔥 Fire Resistance Explorer
An unprotected steel member in a standard fire. Change the section factor (Am/V — how "chunky" the profile is) and the degree of utilisation (how hard the member is working when the fire starts), and watch how fast steel catches up to the ISO 834 gas temperature — and when it crosses its critical temperature.
Fire
CurveISO 834 standard
Steel Member
Chunkier profile (lower Am/V) heats slower. Typical unprotected I-sections range roughly 50–300 /m.
μ₀ = design fire effect / resistance at t=0. EC3 default in absence of detailed calc: 0.60 → θcr≈554°C.
Live Result
Gas temperature at end of exposure—
Steel temperature at end of exposure—
Critical temperature θcr—
Time to reach θcr—
This uses the exact EN 1993-1-2 incremental unprotected-steel
temperature method (engine/fire-resistance.js →
unprotectedSteelTemperature, Δt=5s per §4.2.5.1) and the exact
§4.2.4 critical-temperature equation — independently re-verified against a Python
cross-check before publishing (ISO 834 curve matched published table values at
t=30/60/90/120 min; θcr(μ₀=0.6)=554.28°C matched the widely-cited EC3 reference
value; steel-temperature time series matched to 3 decimal places across 4 section
factors). A real formula error (a missing "−1" term in the θcr equation) was caught
and fixed during this cross-check — a working example of why every published
coefficient here is independently re-run, not assumed correct on the first try.
Method
ISO 834-1Tg = 20 + 345·log₁₀(8t+1) [°C, t in min] — standard nominal fire curve (EN 1991-1-2 Annex A).
EN 1993-1-2 §4.2.5.1Δθa,t = ksh·(Am/V)/(ca·ρa)·ḣnet,d·Δt — incremental unprotected-steel temperature, Δt≤5s. Standard-fire coefficients: αc=25 W/m²K, εm=0.7, Φ=1, ksh=1 (conservative box value).
EN 1993-1-2 §3.4.1.2ca(θ) — temperature-dependent specific heat of steel, piecewise (eq. 3.2).
EN 1993-1-2 §4.2.4θcr = 39.19·ln(1/(0.9674·μ₀^3.833) − 1) + 482 — critical temperature from degree of utilisation μ₀.
Scope: unprotected steel only (no insulation/protection layer — that requires a separate protected-member method, not modelled here). ksh is fixed at the conservative box value 1.0; the exact I-section shadow factor requires the section's box perimeter and is intentionally not guessed.