Wood exposed to fire chars at a roughly constant linear rate: a layer of insulating black char builds up on the surface while the wood just beneath it (the "zero-strength layer") is heat-weakened but still structurally intact-looking. Eurocode 5's reduced cross-section method models this with:
d_char(t) = β_n · t
d_ef(t) = d_char(t) + k0 · d0 (d0 ≈ 7 mm zero-strength layer)
b_res(t) = b0 − 2·d_ef (sides exposed)
h_res(t) = h0 − n_top·d_ef
M_cap(t) / M_cap(0) = [b_res(t)·h_res(t)²] / [b0·h0²]
- Beam width / depth — the starting cross-section. Because capacity scales with b·h², a deeper beam has far more spare cross-section to lose before it fails.
- Charring rate βn — typical notional rates for cross-laminated timber run 0.65–0.9 mm/min, faster than solid glulam because charred laminate layers can fall away at the glue lines.
- Fire-exposed faces — a beam charring on 3 sides (bottom + 2 sides, top protected by a floor) loses cross-section faster than one exposed on a single face only.
- Residual moment capacity — bending capacity of a rectangular section scales with b·h², so char eating into the depth matters far more than into the width.
- Time to failure — the minute the residual capacity ratio drops below the applied load ratio, the member can no longer carry its load; this is exactly the number used to set required fire-resistance ratings (R30/R60/R90/R120) for mass-timber buildings under building codes like the US IBC 2021 (which permits mass timber up to 18 storeys) and Eurocode 5.
This is why exposed mass-timber structure can meet fire codes without extra cladding: the char layer itself is the fire protection, self-limiting the burn rate of the wood underneath — very different from steel, which loses strength rapidly once unprotected steel reaches ~500 °C with no self-insulating layer at all.