Timber bar in tension in the fire situation to SIA 265
Checks a rectangular solid-timber or glulam bar in axial tension parallel to the grain in the fire situation, to SIA 265:2021: the residual section after charring and the tensile 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 bar in axial tension parallel to the grain in the fire situation, to SIA 265:2021: the residual section after charring and the tensile stress on that section. Every step shows its formula and the clause it comes from.
What it checks
- Charring: notional charring rate βn (clause 4.5.2.3, table 13), charring depth dchar,n = βn · t (clause 4.5.2.2, equation (50)) and effective loss of thickness def = dchar,n + dred (equation (49)).
- Residual section: bfi and hfi, each reduced by def at every exposed face, and the notional residual section Aef = bfi · hfi (clause 4.5.2.2, figure 13).
- Resistance in fire, clause 4.5.2.5, equation (51): fRd,fi = 1.8 · ft,0,d · ηw. The load duration factor ηt does not apply in the fire situation.
- Tension in the fire situation, clause 4.5.2.5, equation (51): σt,0,d,fi = Nd,fi / Aef ≤ fRd,fi.
- For comparison only, the resistance with the factor kfi of EN 1995-1-2: 1.5 · ft,0,d · ηw · kfi, with kfi = 1.15. It is 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 tensile strength ft,0,d from your copy of SIA 265 (clause 3.4.2.2, table 9), the design tensile force in fire Nd,fi, whether the member was protected at the start of the fire, the material for charring, the fire exposure time t, the zero-strength layer dred, and the exposed faces.
You get the charring depth, the residual section, the resistance and stress in fire, and the utilisation with a pass or fail, each with its clause. 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 exposure time is the required resistance time minus the time the protection stays effective. A protected member chars at twice the notional rate for the first 15 minutes after the protection fails, then at the normal rate.
- dred allows for the loss of stiffness and strength near the charred surface; it is at least 7 mm for fire resistance up to 90 minutes (clause 4.5.2.2).
- For category A and B floor areas, Nd,fi = 0.6 · Nd may be assumed.
- With fire on several faces, the module warns when the width before the fire is below the minimum for R60 (140 mm, clause 4.5.2.4); heat accumulation must then be considered. It is flagged, not calculated.
- Only the fire situation is verified here. The ultimate limit state of the same member is its own module.
Worked example
The bar of the ultimate-limit-state example (GL24h, b × h = 120 × 160 mm, service class 1), unprotected, charring as softwood glulam, after 30 minutes of fire on all four faces, with dred = 7 mm and a design tensile force in fire of 90 kN. 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 | 160 mm | |
| Design tensile strength parallel to grain ft,0,d | 12.0 N/mm² | § 3.4.2.2 t.9 |
| Design tensile force in fire Nd,fi | 90 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 = 104.0 mm | |
| Notional residual sectionAef = bfi · hfi | Aef = 6,656 mm² | § 4.5.2.2 f.13 |
| Design resistance in firefRd,fi = 1.8 · ft,0,d · ηw | fRd,fi = 21.600 N/mm² | § 4.5.2.5 (51) |
| Design stress in fireσt,0,d,fi = Nd,fi / Aef | σt,0,d,fi = 13.522 N/mm² | |
| Tension in the fire situationσt,0,d,fi ≤ fRd,fi | η = 0.626 · Pass63 % of the capacity used | § 4.5.2.5 (51) |
| Alternative including kfifRd,fi* = 1.5 · ft,0,d · ηw · kfi, with kfi = 1.15Not part of the norm. Utilisation would be 65.3%. | fRd,fi * = 20.700 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
Why is the resistance 1.8 · f_t,0,d in fire?
SIA 265 equation (51) converts the design strength to the fire situation with this factor; the residual section carries the load. ηt is not applied.
Is the Eurocode value used?
No. The EN 1995-1-2 resistance with kfi is shown beside the result for comparison only; the verification follows SIA 265.
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 tension, ultimate limit state
- Tension and biaxial bending — fire situation
- Beam in bending — fire situation
- SIA 265: modules and clauses
Norm: SIA 265:2021 · Page updated: 21 October 2026