Timber compression and biaxial bending in the fire situation to SIA 265
Checks a rectangular solid-timber or glulam member under axial compression and bending about both axes in the fire situation, to SIA 265:2021: the residual section after charring, the cross-section where the member is held against buckling, and buckling with bending where it is not. 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 member under axial compression and bending about both axes in the fire situation, to SIA 265:2021: the residual section after charring, the cross-section where the member is held against buckling, and buckling with bending where it is not. Every step shows its formula and the clause it comes from.
What it checks
- Charring and residual section: βn (clause 4.5.2.3, table 13), dchar,n (clause 4.5.2.2, equation (50)), def (equation (49)), residual width and depth, Aef, Wy,fi and Wz,fi.
- Resistances in fire, clause 4.5.2.5, equation (51): 1.8 · fc,0,d · ηw in compression and 1.8 · fm,d · ηw · km,fi in bending; kh and ksys do not apply.
- Stabilised zone in fire, cross-section, clause 4.2.4.2, equation (22).
- Unstabilised zone in fire, buckling about each axis, clause 4.2.8.5, equation (37), with kc on the residual section (buckling lengths unchanged by fire).
- For comparison only, the interaction 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 from your copy of SIA 265, the lateral support spacings, the buckling lengths, the design actions in fire Nd,fi, My,d,fi and Mz,d,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, the three checks with their utilisations, every intermediate value with its clause, 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.
- The same fire actions are applied to both zones. Where they differ along the member, run the module a second time with the other set.
- With fire on several faces, narrow sections are flagged (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 member of the ultimate-limit-state example (GL24h, b × h = 680 × 280 mm, service class 2), unprotected, after 60 minutes of fire on all four faces, with N_d,fi = 44 kN, M_y,d,fi = 94 kN·m and M_z,d,fi = 1 kN·m. In this example the cross-section check and the buckling check about y-y do not pass. 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 | 2 | § 3.2.1.3 t.4 |
| Section width b | 680 mm | |
| Section depth h | 280 mm | |
| Design compressive strength ∥ grain fc,0,d | 16 N/mm² | § 3.4.2.2 t.9 |
| Characteristic compressive strength ∥ grain fc,0,k | 24 N/mm² | § Annexe B |
| Design bending strength fm,d | 16 N/mm² | § 3.4.2.2 t.9 |
| Characteristic bending strength fm,k | 24 N/mm² | § Annexe B |
| Modulus of elasticity, 5% fractile E0,05 | 9600 N/mm² | § Annexe B |
| Lateral support spacing, bending about y-y ay | 6820 mm | § 4.2.9.3 |
| Lateral support spacing, bending about z-z az | 6820 mm | § 4.2.9.3 |
| Buckling length given by | Entered directly | |
| Buckling length about y-y lk,y | 6820 mm | |
| Buckling length about z-z lk,z | 6820 mm | |
| Design compressive force in fire Nd,fi | 44 kN | |
| Design moment about y-y in fire My,d,fi | 94 kN·m | |
| Design moment about z-z in fire Mz,d,fi | 1 kN·m | |
| 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 | 60 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 = 42.00 mm | § 4.5.2.2 (50) |
| Effective loss of thicknessdef = dchar,n + dred | def = 49.00 mm | § 4.5.2.2 (49) |
| Residual width | bfi = 582.0 mm | |
| Residual depth | hfi = 182.0 mm | |
| Notional residual sectionAef = bfi · hfi | Aef = 105,924 mm² | § 4.5.2.2 f.13 |
| Section modulus about y-y | Wy,fi = 3,213,028 mm³ | |
| Section modulus about z-z | Wz,fi = 10,274,628 mm³ | |
| Compressive resistance in firefRd,C,fi = 1.8 · fc,0,d · ηw. kh and ksys do not apply. | fRd,C,fi = 23.040 N/mm² | § 4.5.2.5 (51) |
| Bending resistance in fire, y-yfRd,m,y,fi = 1.8 · fm,d · ηw · km,y,fi | fRd,m,y,fi = 23.040 N/mm² | § 4.5.2.5 (51) |
| Bending resistance in fire, z-z | fRd,m,z,fi = 23.040 N/mm² | § 4.5.2.5 (51) |
| Compressive stress in fireσc,0,d,fi = Nd,fi / Aef | σc,0,d,fi = 0.4154 N/mm² | |
| Bending stress in fire, y-y | σm,y,d,fi = 29.2559 N/mm² | |
| Bending stress in fire, z-z | σm,z,d,fi = 0.0973 N/mm² | |
| Note on the fire actionsThe same fire actions are applied to both zones. Where they differ along the member, run the module a second time with the other set. | — |
| Step | Value | Ref. |
|---|---|---|
| Stabilised zone in fire, cross-section(σc/fc)² + σm,y/fm,y + σm,z/fm,z ≤ 1 | η = 1.274 · Fail127 % of the capacity used | § 4.2.4.2 (22) |
| Alternative including kfi, cross-section checkeach resistance taken as 1.5 · f · ηw · kfi with kfi = 1.15Not part of SIA 265. Shown for comparison only. | interaction * = 1.3298 | EN 1995-1-2 |
| Step | Value | Ref. |
|---|---|---|
| Slenderness in fire, y-ybuckling lengths are unchanged by fire | λy,fi = 129.663 | |
| Relative slenderness in fire, y-y | λrel,y,fi = 2.0636 | § 4.2.8.4 (34) |
| Auxiliary factor in fire, y-yk = 0.5 · [1 + βc·(λrel − 0.3) + λrel²] | ky,fi = 2.7175 | § 4.2.8.2 (31) |
| Buckling coefficient in fire, y-ykc = 1 / (k + √(k² − λrel²)) | kc,y,fi = 0.2229 | § 4.2.8.2 (30) |
| Unstabilised zone in fire, buckling about y-yσc/(kc,y·fc) + σm,y/(km,y·fm) ≤ 1 | η = 1.351 · Fail135 % of the capacity used | § 4.2.8.5 (37) |
| Step | Value | Ref. |
|---|---|---|
| Slenderness in fire, z-zbuckling lengths are unchanged by fire | λz,fi = 40.547 | |
| Relative slenderness in fire, z-z | λrel,z,fi = 0.6453 | § 4.2.8.4 (34) |
| Auxiliary factor in fire, z-zk = 0.5 · [1 + βc·(λrel − 0.3) + λrel²] | kz,fi = 0.7255 | § 4.2.8.2 (31) |
| Buckling coefficient in fire, z-zkc = 1 / (k + √(k² − λrel²)) | kc,z,fi = 0.9461 | § 4.2.8.2 (30) |
| Unstabilised zone in fire, buckling about z-zσc/(kc,z·fc) + σm,z/(km,z·fm) ≤ 1 | η = 0.023 · Pass2 % of the capacity used | § 4.2.8.5 (37) |
How this module is verified
Verified clause by clause against SIA 265:2021 and against independent calculations used on real projects, before release.
Questions
Which actions do I enter for the fire situation?
The actions of the accidental combination for fire from your own analysis. For floors in categories A and B, 0.6 times the actions at normal temperature may be assumed.
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 and biaxial bending, ultimate limit state
- Beam in biaxial bending — fire situation
- Column in compression — fire situation
- SIA 265: modules and clauses
Norm: SIA 265:2021 · Page updated: 20 October 2026