Fire design of exposed wood
The structural fire resistance of an exposed wood member by NDS 2024 Chapter 16: the char depth for the rating, the residual section, and the member checked on it with the fire member strengths of Table 16.3.3. Sawn timbers, glulam, SCL and CLT panels, up to 2 hours.
Inputs of the fire design #
- Fire exposure: the material, the required fire resistance in hours (NDS 16.3 calculations stop at 2 hours), the faces the fire reaches, and the nominal char rate βn. NDS 16.2.1 permits 1.5 in./hr for sawn lumber, glulam, SCL and CLT; a lower rate gets a warning because it needs published char data for the species.
- Exposure: four sides; three sides with the top protected (a beam under a deck or slab); or three sides with one side protected (a column against a wall). The protected face's own protection is yours: it must last the rating. A CLT panel is exposed on its bottom face.
- Member: sawn timbers by species, grade and size (NDS Supplement Table 4D); glulam by stress class, width and depth; SCL with the manufacturer's values and its volume factor exponent; CLT by layup, plies and lamination thickness (equal laminations: unequal ones need the FDS).
- Span and loads: a simple span (or a column height) with a uniform dead and live load and an axial dead and live load. For CLT the uniform load is per foot of panel width (psf × 1 ft).
- Loads in the fire case: this is your choice, so it is an input. AWC's method (Technical Report 10) compares the full allowable stress design loads, D + L, with the fire member strength, and that is prefilled. ASCE 7-22 2.5.2 gives 1.2D + 0.5L for extraordinary events. The building code and you decide.
- Bracing and bearing: the unbraced length of the compression edge (0 when a deck braces it, as long as the deck lasts the rating), the column effective lengths about the strong and the weak axis, and a bearing length with the number of exposed edges at the bearing (0 skips the bearing check).
The LRFD / ASD switch is hidden: the fire check has one format.
Auto-design (editor toolbar) varies the sawn timber size, or the glulam or SCL width and depth, or the CLT plies. Objective: the least wood. The rating, exposure, grade and loads stay yours.
Checks performed #
| Check | Basis |
|---|---|
| Char depth | βt from βn at one hour (Eq. 16.2-1), achar = βt t^0.813 (Eq. 16.2-2); CLT by Eq. 16.2-3, laminations falling off at each glue line |
| Effective char depth and residual section | aeff = 1.2 achar on each exposed face (Eq. 16.3-1), section properties of the reduced section (16.3.1.2, Table C16.3-1) |
| Bending | fb = M / S on the residual section against 2.85 Fb with CF or CV on the initial section and CL on the residual one, FbE from 2.03 × 1.20 Emin (Table 16.3.3); glulam takes the lesser of CV and CL |
| Shear | 1.5 V / A against 2.75 Fv |
| Compression | P / A against 2.58 Fc CP, about both axes, FcE from 2.03 × 0.822 Emin; le/d ≤ 50 |
| Bending and compression | NDS Eq. 3.9-3 with the fire values |
| Bearing | the bearing width reduced by 1.5 achar at each exposed edge (Eq. 16.3-2, new in NDS 2024), against 1.67 Fc⊥ |
| CLT bending | the commentary's conservative approach (C16.3.1.3): the panel of the full strong-axis laminations that remain, (FbS)eff by PRG 320 Annex A, times 2.85 |
| Glulam laminations | a note: one outer tension lamination substituted for a core lamination up to 1 hour, two up to 2 hours (16.3.4) |
No load duration, wet service or temperature factor applies in the fire (Table 16.3.3 footnote 2). The drawing is the section after the fire: the initial outline, the char layer and the residual section, or the CLT panel with its char layer.
Limitations #
- The member at normal temperature is not designed here. Design it with the beam, column or CLT type.
- Protected members and membrane protection (FDS, TR 10 Part II), connections and their protection, char at intersections and abutting edges (16.2.3), lumber decking (16.3.5), thermal separation and burn-through are not covered.
- One simple span with uniform load and an axial load. Other load patterns need their moments entered as an equivalent uniform load.
- CLT rolling shear in the fire is not calculated: check it with the manufacturer's values. The PRG 320 values behind the CLT check are still to be confirmed against the standard.
- Glulam stress class values were read against Supplement Table 5A on 2026-11-12 (p. 80). The 28F and 30F SP classes' Ex,min was corrected to 1.09 x 10⁶ psi.
Verification examples #
Verification status: 19 values compared in 10 examples (8 published, 2 hand calculations from the cited equations): 19 within tolerance, 0 recorded deviations. Code basis: not yet recorded for this module.
The char depths are NDS 2024's own tables, printed to 0.1 in.: Table 16.3.1 at 1, 1-1/2 and 2 hours (1.5 / 1.8, 2.1 / 2.5, 2.6 / 3.2 in.) and six entries of Table 16.3.2 for CLT. One CLT entry (1-1/2 hours, 1-1/4 in. laminations) prints 2.9 in. where Eq. 16.2-3 gives 2.85 in.; it is not used.
The member checks are hand calculations. No worked example of NDS Chapter 16 is on disk. AWC's Technical Report 10 and its Design for Code Acceptance 2 have them and are listed as needed. An 8x14 beam exposed on three sides for an hour and an 8x8 post exposed on four are checked by hand on the Verification page.