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: the 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 and source
char_depth_in = char_rate_in_hr x (exposure_min / 60); effective_char_in = char_depth_in + zero_strength_in; residual_width/depth = nominal - effective x faces (floored at 0 -> consumed); section_modulus_ratio = (rw x rd^2) / (b x d^2).
AWC National Design Specification one-dimensional char model (about 1.5 in/hr nominal) plus a heat-degraded zero-strength layer, by name.
Audience
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