Timber bearing in the fire situation to SIA 265
Checks the bearing of a solid-timber or glulam member on a support in the fire situation, to SIA 265:2021: the contact surface reduced by charring, and compression perpendicular to the grain on what remains. 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 the bearing of a solid-timber or glulam member on a support in the fire situation, to SIA 265:2021: the contact surface reduced by charring, and compression perpendicular to the grain on what remains. 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 contact surface: how far the charring reaches into the bearing, from the side faces and the end of the member, depending on where the bearing sits, which faces are exposed, and whether the support is non-combustible or of timber. The time until the contact surface starts to lose area is given.
- Resistance in fire, clause 4.5.2.5, equation (51): fRd,fi = 1.8 · fc,90,d · ηw.
- Bearing in the fire situation: σc,90,d,fi = Rd,fi / Afi ≤ 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 (wood type, strength class, service class, width b), the type of support and the contact area, the strength fc,90,d from your copy of SIA 265, the design reaction in fire Rd,fi, whether the member was protected at the start of the fire, the material for charring, the fire exposure time, the zero-strength layer, the position of the bearing (at the end of the member or within the span), the overhang past the bearing, the exposed side faces, the gap to the bearing edge, and what the member bears on.
You get every step of the charring, the residual bearing length, width and area, the resistance and the utilisation with a pass or fail. With the Basic plan, it prints as an A4 calculation note.
Assumptions and limits
- Rectangular members of solid timber or glulam; the charring model of SIA 265 clause 4.5.2.
- No table of the norm is reproduced: you enter the strength from your own copy of SIA 265.
- ηt, kh and ksys do not apply in the fire situation.
- The ultimate limit state of the same bearing at normal temperature is its own module.
Worked example
The bearing of the ultimate-limit-state example (GL24h, 280 mm wide, bearing 280 × 280 mm at the end of the member, on concrete), unprotected, after 30 minutes of fire on both side faces, with a design reaction in fire of 22 kN. 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 | |
| Type of support | General | § 3.3.2.1 t.8 / 3.4.2.2 t.9 |
| Contact area given by | Bearing length and width | |
| Bearing length lA | 280 mm | |
| Bearing width bA | 280 mm | |
| Design compressive strength ⟂ grain fc,90,d | 1.9 N/mm² | § 3.3.2.1 t.8 / 3.4.2.2 t.9 |
| Design support reaction in fire Rd,fi | 22 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 |
| Position of the bearing | At the end of the member | |
| Overhang of the member past the bearing ee | 0 mm | |
| Side faces of the member exposed | Both | |
| Gap from the side face to the bearing edge | Bearing centred on the width — computed | |
| The member bears on | Concrete, masonry or steel |
| 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) |
| Nearest charring face to a bearing edgemeasured to a side face of the member. The contact surface keeps its full size while def ≤ emin. Here it is zero, so the contact surface starts to lose area as soon as the fire begins, the bearing covering the whole width of the member. | emin = 0.0 mm | |
| Contact surface intact untildef(t0) = emin, the zero-strength layer included. The exposure of 30 min is past this, so the contact surface has started to lose area. | t0 = 0.0 min | |
| Residual bearing widthbA − 28.0 mm on both sides | bA,fi = 224.0 mm | § 4.5.2.2 |
| Residual bearing lengthlA − 28.0 mm at the near edge − 0.0 mm at the far edge | lA,fi = 252.0 mm | § 4.5.2.2 |
| Residual contact areaAfi = bA,fi · lA,fi | Afi = 56,448 mm² | § 4.5.2.2 |
| Design resistance in firefRd,fi = 1.8 · fc,90,d · ηw | fRd,fi = 3.420 N/mm² | § 4.5.2.5 (51) |
| Design stress in fireσc,90,d,fi = Rd,fi / Afi | σc,90,d,fi = 0.390 N/mm² | |
| Bearing in the fire situationσc,90,d,fi ≤ fRd,fi | η = 0.114 · Pass11 % of the capacity used | § 4.5.2.5 (51) |
| Alternative including kfifRd,fi* = 1.5 · fc,90,d · ηw · kfi, kfi = 1.15Not part of SIA 265. Utilisation would be 11.9%. | fRd,fi * = 3.277 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 does the contact surface shrink when the support is concrete?
The member chars from its exposed faces. Where the bearing reaches the side faces or the end of the member, the charred depth there no longer carries load, so the contact surface loses that strip. A timber support also chars from its own faces.
Which design reaction do I enter?
The reaction from the accidental combination for fire, Rd,fi, from your own analysis.
Is this module free?
The fire situation is part of the Basic plan, with the calculation note of every module. The ultimate limit state of the bearing is free.
Related
Norm: SIA 265:2021 · Page updated: 20 October 2026