Timber beam in shear in the fire situation to SIA 265
Checks a rectangular solid-timber or glulam beam in shear in the fire situation, to SIA 265:2021: the residual section after charring, shear on that section, and its interaction with stress perpendicular to the grain. 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 shear in the fire situation, to SIA 265:2021: the residual section after charring, shear on that section, and its interaction with stress perpendicular to the grain. 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 bfi and hfi.
- Resistance in fire, clause 4.5.2.5, equation (51): fRd,fi = 1.8 · fv,d · ηw.
- Shear in the fire situation: τxz,d,fi = 1.5 · Vz,d,fi / Aef ≤ fRd,fi.
- Shear with stress perpendicular to the grain, clause 4.2.7.1, equation (27), with fc,90,d,fi = 1.8 · fc,90,d · ηw and ft,90,d,fi = 1.8 · ft,90,d · ηw.
- Shear at a notched support: the norm gives no rule for a notched support in fire. The module applies table 16 to the residual section and labels this as an extension of the norm.
- 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 strengths fv,d, fc,90,d and ft,90,d from your copy of SIA 265, whether the piece contains the pith, the transverse stress in fire σ90,d,fi, the notch geometry and the shear at the notch in fire VEd,fi, the design shear forces in fire Vz,d,fi and Vy,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, every intermediate value with its clause and the utilisation of each check 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.
- ηt and ksys do not apply in the fire situation.
- VEd,fi is a separate action from the shear at mid-depth.
- The ultimate limit state is its own module.
Worked example
The beam of the ultimate-limit-state example (GL24h, b × h = 140 × 700 mm), unprotected, after 30 minutes of fire on the bottom face and both sides, with 103 kN of shear in fire and a transverse compression in fire of 1.2 N/mm². No shear at the notch in fire is entered, so the notch check is not evaluated. 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 | 140 mm | |
| Section depth h | 700 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 |
| Transverse stress at the section | Compression perpendicular to the grain | § 4.2.7.1 |
| Design compressive strength ⟂ grain fc,90,d | 1.9 N/mm² | § 3.3.2.1 t.8 / 3.4.2.2 t.9 |
| Design tensile strength ⟂ grain ft,90,d | 0.15 N/mm² | § 3.3.2.1 t.8 / 3.4.2.2 t.9 |
| Loads within h of the support neglected? | No | § 4.2.7.2 |
| Notch at the support | On the tension side | § 5.2.2 |
| Reduced depth hef | 440 mm | § 5.2.2.1 |
| Distance support reaction to notch corner c | 140 mm | § 5.2.2 |
| Notch cut on a slope? | No | § 5.2.2.2 |
| Design shear force in fire, z direction Vz,d,fi | 103 kN | |
| Design shear force in fire, y direction Vy,d,fi | 0 kN | |
| Protected at the start of the fire? | No | |
| Transverse compression in fire, magnitude |σ90,d,fi| | 1.2 N/mm² | § 4.2.7.1 |
| 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 | 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 = 84.0 mm | |
| Residual depthhfi = h − def at each exposed face | hfi = 672.0 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) |
| Step | Value | Ref. |
|---|---|---|
| Shear stress in fire, z directionτxz,d,fi = 1.5 · Vz,d,fi / Aef | τxz,d,fi = 2.737 N/mm² | |
| Shear in the fire situationτd,fi ≤ fRd,fi | η = 0.845 · Pass84 % 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 88.1%. | fRd,fi * = 3.105 N/mm² | EN 1995-1-2 |
| Step | Value | Ref. |
|---|---|---|
| Correction coefficientkt,90 = 4.0 for transverse compression, 1.0 for transverse tension. The sense is the one chosen for the ultimate limit state. | kt,90 = 4.0 | § 4.2.7.1 |
| Transverse stress in fire, signedcompression is introduced as a negative value | σ90,d,fi = -1.200 N/mm² | |
| Compressive resistance ⟂ grain in firefc,90,d,fi = 1.8 · fc,90,d · ηw. ηt and ksys do not apply. | fc,90,d,fi = 3.420 N/mm² | § 4.5.2.5 (51) |
| Tensile resistance ⟂ grain in fireft,90,d,fi = 1.8 · ft,90,d · ηw | ft,90,d,fi = 0.270 N/mm² | § 4.5.2.5 (51) |
| Shear component usedThe interaction of §4.2.7.1 is written for shear in one direction. The z component of the residual section is used, since a reaction applied across the grain acts together with that component. | τxz,d,fi = 2.737 N/mm² | § 4.2.7.1 |
| Shear with transverse compression in the fire situationinteraction ≤ 1 | η = 0.545 · Pass54 % of the capacity used | § 4.2.7.1 (27) |
| Interaction expression[(fc,90,d,fi + σ90,d,fi)/(fc,90,d,fi + kt,90·ft,90,d,fi)]² + (τxz,d,fi/fRd,fi)²·[1 − (fc,90,d,fi/(fc,90,d,fi + kt,90·ft,90,d,fi))²] ≤ 1 | 0.5448 | § 4.2.7.1 (27) |
| Step | Value | Ref. |
|---|---|---|
| Not evaluated | — |
How this module is verified
Verified clause by clause against SIA 265:2021 and against independent calculations used on real projects, before release.
Questions
Can the transverse stress be compression at normal temperature and tension in fire?
No. The sense is inherited from the choice for the ultimate limit state, so the same kt,90 applies and the two verifications cannot contradict each other.
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 shear, ultimate limit state
- Beam in torsion — fire situation
- Beam in bending — fire situation
- SIA 265: modules and clauses
Norm: SIA 265:2021 · Page updated: 21 October 2026