Fire-Exposed Wood Char Depth and Residual Capacity
Screens whether a charred wood beam or post kept enough sound section to carry load after a fire.
Example
You enter
- Fire exposure (minutes) 30
- Member width b (in, actual) 5.5
- Member depth d (in, actual) 9.5
- Charred faces across the width 2
- Charred faces across the depth 1
- Nominal char rate (in/hr) 1.5
- Zero-strength layer (in) 0
You get
- Effective char depth 1.0245
- Residual width (in) 3.451
- Section modulus ratio 0.4994
Checked against published worked examples from: AWC National Design Specification char model.
Details, formula, and sources
ScopeThe AWC/NDS one-dimensional char depth (about 1.5 in/hr) plus the heat-degraded zero-strength layer, the residual cross-section, and the bending-capacity (section-modulus) fraction that drives the keep-or-replace call. Heavy timber survives fire because the residual section is large. Screens the bending section only; a structural engineer governs connections, splitting, and the load path.
Formula or tablechar_depth_in = char_rate_in_hr x (exposure_min / 60)^0.813 (NDS 16.2.1, nonlinear); effective_char_in = 1.2 x char_depth_in + any additional zero_strength_in (default 0); residual_width/depth = nominal - effective x faces (floored at 0 -> consumed); section_modulus_ratio = (rw x rd^2) / (b x d^2).
Source and editionAWC National Design Specification one-dimensional char model (about 1.5 in/hr nominal) plus a heat-degraded zero-strength layer, by name.
Free accessThe one-dimensional char rate and the zero-strength-layer concept are public AWC/NDS structural fire-design provisions; the section-modulus ratio is geometry.
What governsEstimate. AHJ and licensed professional govern.
API field namesexposure_min, nominal_width_in, nominal_depth_in, faces_across_width, faces_across_depth, char_rate_in_hr, zero_strength_in, effective_char_in, residual_width_in, section_modulus_ratio
- Nominal char rate 1.5 in/hr one-dimensional nominal; species and density shift it, so it is editableAWC NDS
- Effective char depth a_eff = 1.2 a_char, the 20% NDS adds for the heat-degraded zone (1.8 / 2.5 / 3.2 in at 1 / 1.5 / 2 hr, Table 16.2.1A); an extra layer can be enteredAWC NDS 16.2.1
- Bending only the section-modulus ratio screens bending capacity; shear, connections, and stability are out of scopestructural engineering