Timber column in compression in the fire situation to SIA 265
Checks a rectangular solid-timber or glulam column in axial compression in the fire situation, to SIA 265:2021: the residual section after charring and buckling on that slenderer section. Every step shows its formula and the clause it comes from.
Basic SIA 265:2021
Part of the Basic plan
This module is included in the Basic plan and above, with its calculation note.
About this check
clauses, worked example and questions
Checks a rectangular solid-timber or glulam column in axial compression in the fire situation, to SIA 265:2021: the residual section after charring and buckling on that slenderer section. Every step shows its formula and the clause it comes from.
What it checks
- Charring: βn (clause 4.5.2.3, table 13), dchar,n (clause 4.5.2.2, equation (50)) and def = dchar,n + dred (equation (49)) on each exposed face, giving the residual section Aef.
- Buckling on the residual section: the buckling lengths are unchanged by fire, but the section is slenderer; λrel (clause 4.2.8.4, equation (34)) and kc (clause 4.2.8.2, equations (30) and (31)).
- Resistance in fire, clause 4.5.2.5, equation (51): fRd,fi = 1.8 · fc,0,d · ηw · kc,fi.
- Buckling in the fire situation: σc,0,d,fi = Nd,fi / Aef ≤ fRd,fi.
- For comparison only, the resistance with the factor kfi of EN 1995-1-2 (not part of SIA 265).
What you enter and what you get
You enter the member, the strengths fc,0,d, fc,0,k and E0,05 from your copy of SIA 265, the role of the member, the buckling lengths, the design compressive force in fire Nd,fi, whether the member was protected at the start of the fire, the material for charring, the exposure time, the zero-strength layer and the exposed faces.
You get the residual section, every intermediate value with its clause, the utilisation with a pass or fail, and figures. With the Basic plan, it prints as an A4 calculation note.
Assumptions and limits
- Rectangular sections; the charring model of SIA 265 clause 4.5.2.
- For floors in categories A and B, the module notes that Nd,fi = 0.6 · Nd may be assumed.
- With fire on several faces, narrow sections are flagged (minimum width before the fire for R30 and R60, clause 4.5.2.4); heat accumulation is flagged, not calculated.
- The ultimate limit state at normal temperature is its own module.
Worked example
The column of the ultimate-limit-state example (GL24h, b × h = 120 × 240 mm), unprotected, after 30 minutes of fire on all four faces, with a design compressive force in fire of 147 kN. In this example the check does not pass, and the module warns that the section is narrower than the minimum width for R60 with fire on several faces. These are the module's own results, computed when this page was built.
| Input | Value | Ref. |
|---|---|---|
| Wood type | Glulam | |
| Strength class | GL24h | |
| Service class ηw | 1 | § 3.2.1.3 t.4 |
| Section width b | 120 mm | |
| Section depth h | 240 mm | |
| Design compressive strength ∥ grain fc,0,d | 14.5 N/mm² | § 3.4.2.2 t.9 |
| Characteristic compressive strength ∥ grain fc,0,k | 24 N/mm² | § Annexe B |
| Modulus of elasticity, 5% fractile E0,05 | 9400 N/mm² | § Annexe B |
| Role of the member λlim | Main load-bearing element | § 4.2.8.4 |
| Buckling length given by | Entered directly | |
| Buckling length about y-y lk,y | 5200 mm | |
| Buckling length about z-z lk,z | 2600 mm | |
| Design compressive force in fire Nd,fi | 147 kN | |
| Protected at the start of the fire? | No | |
| Material for charring βn | Softwood or beech — glulam | § 4.5.2.3 t.13 |
| Fire exposure time t | 30 min | |
| Zero-strength layer dred | 7 mm | § 4.5.2.2 |
| Exposed faces | top, bottom, left, right |
| Step | Value | Ref. |
|---|---|---|
| Notional charring rate | βn = 0.70 mm/min | § 4.5.2.3 t.13 |
| Charring depthdchar,n = βn · t | dchar,n = 21.00 mm | § 4.5.2.2 (50) |
| Effective loss of thicknessdef = dchar,n + dred | def = 28.00 mm | § 4.5.2.2 (49) |
| Residual widthbfi = b − def at each exposed face | bfi = 64.0 mm | |
| Residual depthhfi = h − def at each exposed face | hfi = 184.0 mm | |
| Notional residual sectionAef = bfi · hfi | Aef = 11,776 mm² | § 4.5.2.2 f.13 |
| Radius of gyration about y-yiy,fi = 0.289 · hfi | iy,fi = 53.18 mm | |
| Radius of gyration about z-ziz,fi = 0.289 · bfi | iz,fi = 18.50 mm | |
| Governing slenderness (z-z)buckling lengths are unchanged by fire | λfi = 140.57 | § 4.2.8.4 |
| Relative slendernessλrel = (λfi/π)·√(fc,0,k / E0,05) | λrel,fi = 2.2609 | § 4.2.8.4 (34) |
| Auxiliary factork = 0.5 · [1 + βc·(λrel − 0.3) + λrel²]. Not to be confused with the Eurocode conversion factor kfi shown further down. | kfi † = 3.1540 | § 4.2.8.2 (31) |
| Buckling coefficientkc = 1 / (k + √(k² − λrel²)), with k from equation (31) above | kc,fi = 0.1868 | § 4.2.8.2 (30) |
| Design resistance in firefRd,fi = 1.8 · fc,0,d · ηw · kc,fi | fRd,fi = 4.876 N/mm² | § 4.5.2.5 (51) |
| Design stress in fireσc,0,d,fi = Nd,fi / Aef | σc,0,d,fi = 12.483 N/mm² | |
| Buckling in the fire situationσc,0,d,fi ≤ fRd,fi | η = 2.560 · Fail256 % of the capacity used | § 4.5.2.5 (51) |
| Alternative including kfifRd,fi* = 1.5 · fc,0,d · ηw · kfi · kc,fi, kfi = 1.15Not part of SIA 265. Utilisation would be 267.2%. | fRd,fi * = 4.673 N/mm² | EN 1995-1-2 |
How this module is verified
Verified clause by clause against SIA 265:2021 and against independent calculations used on real projects, before release.
Questions
Do the buckling lengths change in fire?
No. They stay as at normal temperature; the residual section is smaller, so the slenderness rises and kc falls.
Is this module free?
The fire situation is part of the Basic plan, with the calculation note of every module. The ultimate limit state is free.
Related
- Bar in compression, ultimate limit state
- Compression and biaxial bending — fire situation
- SIA 265: modules and clauses
Norm: SIA 265:2021 · Page updated: 20 October 2026