Timber beam in torsion in the fire situation to SIA 265
Checks a rectangular solid-timber or glulam beam in torsion, alone or with shear, in the fire situation, to SIA 265:2021: the residual section after charring and the torsional shear stress on that 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 beam in torsion, alone or with shear, in the fire situation, to SIA 265:2021: the residual section after charring and the torsional shear stress on that 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 = βn · t (clause 4.5.2.2, equation (50)) and def = dchar,n + dred (equation (49)) on each exposed face, giving the residual width bfi and depth hfi.
- Resistance in fire, clause 4.5.2.5, equation (51): fRd,fi = 1.8 · fv,d · ηw. ksys does not apply.
- Torsional shear stress in fire τtor,d,fi: the same expression as at normal temperature (the elasticity solution for a rectangle, since SIA 265 gives none), on the notional residual section.
- Torsion in the fire situation: τtor,d,fi ≤ fRd,fi.
- Torsion combined with shear in fire, clause 4.2.5.3, equation (25), when you choose to combine them.
- 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 (wood type, strength class, service class, b, h), the design shear strength fv,d from your copy of SIA 265, whether the piece contains the pith, the design torsional moment in fire Td,fi, whether to combine it with shear, whether the member was protected at the start of the fire, the material for charring, the fire exposure time, the zero-strength layer and the exposed faces.
You get the residual section, every intermediate value with its clause, and the utilisation with a pass or fail. With the Basic plan, it prints as an A4 calculation note.
Assumptions and limits
- Rectangular sections of solid timber or glulam; the charring model of SIA 265 clause 4.5.2.
- Torsion is verified against the shear strength; the norm gives no separate torsional strength.
- No table of the norm is reproduced: you enter the strength from your own copy of SIA 265. For pieces containing the pith the design strength is halved (clause 3.3.2.1, table 8).
- ηt and ksys do not apply in the fire situation.
- The ultimate limit state of the same member is its own module.
Worked example
The beam of the ultimate-limit-state example (GL24h, b × h = 280 × 520 mm), unprotected, after 30 minutes of fire on all four faces, with a design torsional moment in fire of 0.06 kN·m and no shear combined. 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 | 280 mm | |
| Section depth h | 520 mm | |
| Design shear strength fv,d | 1.8 N/mm² | § 3.4.2.2 t.9 |
| Piece contains the pith? | No | § 3.3.2.1 t.8 n.6 |
| Design torsional moment in fire Td,fi | 0.06 kN·m | |
| Combine with shear in fire? | No | § 4.2.5.3 |
| 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 = 224.0 mm | |
| Residual depthhfi = h − def at each exposed face | hfi = 464.0 mm | |
| Design shear resistance in firefRd,fi = 1.8 · fv,d · ηw. ksys does not apply. | fRd,fi = 3.240 N/mm² | § 4.5.2.5 (51) |
| Torsional shear stress in firesame expression as at normal temperature, on the notional residual section | τtor,d,fi = 0.0104 N/mm² | |
| Torsion in the fire situationτtor,d,fi ≤ fRd,fi | η = 0.003 · Pass0 % of the capacity used | § 4.5.2.5 (51) |
| Alternative including kfifRd,fi* = 1.5 · fv,d · ηw · kfi, kfi = 1.15Not part of the norm. Utilisation would be 0.3%. | fRd,fi * = 3.105 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
Must the combination with shear match the ultimate limit state?
No. It is chosen independently, since a member may carry shear together with torsion at normal temperature and torsion alone in the fire situation, or the reverse. The choice starts by matching the ultimate limit state.
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
- Beam in torsion, ultimate limit state
- Beam in shear — fire situation
- Timber design to SIA 265
- SIA 265: modules and clauses
Norm: SIA 265:2021 · Page updated: 21 October 2026