CLT floor panel in the fire situation
Checks a cross-laminated timber (CLT) floor panel spanning one way in the fire situation: the charred depth, the layers that remain, the residual section and bending on what is left, with the fire values of EN 1995-1-2. Every step shows its formula and where it comes from.
Pro EN 1995-1-2
Part of the Pro plan
This module is included in the Pro plan and above, with its calculation note.
About this check
clauses, worked example and questions
Checks a cross-laminated timber (CLT) floor panel spanning one way in the fire situation: the charred depth, the layers that remain, the residual section and bending on what is left, with the fire values of EN 1995-1-2. Every step shows its formula and where it comes from.
What it checks
- Charring: dchar = β0 · T from the exposed face, and the effective loss of thickness def = dchar + dred.
- Residual lay-up: the remaining thickness, the neutral axis of the remaining layers along the span, the residual second moment of area I0,net,f and the distance to the extreme fibre. A cross layer left on the burnt face is ignored, since it carries no bending; the residual section is no longer symmetrical, so the larger distance governs.
- Bending strength in fire: fm,d,f = kmod,f · kf · fm,k / γM,f (EN 1995-1-2).
- Bending in the fire situation: σm,d,f = Md,f / W0,net,f ≤ fm,d,f.
What you enter and what you get
You enter the lay-up and the width considered, the characteristic bending strength fm,k, the design moment in fire, the charring rate, the fire resistance period, the zero-strength layer, and the factors in fire kmod,f, kf and γM,f.
You get the residual lay-up, the section properties, the check with its utilisation and every intermediate value. With the Pro plan, it prints as an A4 calculation note.
Assumptions and limits
- Panel spanning one way, exposed on one face; charring from that face only.
- The charring rate and the factors in fire are entered by you for the product used.
- SIA 265 has no provisions for CLT; the fire values follow EN 1995-1-2.
- The strength at normal temperature (its own module) and the deflection (its own module) are separate.
Worked example
The five-layer panel 40-20-20-20-40 mm, one metre wide, with f_m,k = 24 N/mm², a design moment in fire of 5.2 kN·m, a charring rate of 0.65 mm/min over 60 minutes, d_red = 7 mm, k_mod,f = 1.0, k_f = 1.15 and γ_M,f = 1.0. These are the module's own results, computed when this page was built.
| Input | Value | Ref. |
|---|---|---|
| Number of layers | 5 | |
| Layer 1, along the span d1 | 40 mm | |
| Layer 1–2, across the span d12 | 20 mm | |
| Layer 2, along the span d2 | 20 mm | |
| Layer 2–3, across the span d23 | 20 mm | |
| Layer 3, along the span d3 | 40 mm | |
| Width considered b | 1000 mm | |
| Characteristic bending strength fm,k | 24 N/mm² | |
| Design bending moment in fire Md,f | 5.2 kN·m | |
| Charring rate β0 | 0.65 mm/min | |
| Fire resistance period T | 60 min | |
| Zero-strength layer dred | 7 mm | |
| Modification factor in fire kmod,f | 1.0 | |
| Conversion to the 20% fractile kf | 1.15 | |
| Partial factor in fire γM,f | 1.0 |
| Step | Value | Ref. |
|---|---|---|
| Charring depthdchar = β0 · T, the first layer is still in place | dchar = 39.00 mm | |
| Effective loss of thicknessdef = dchar + dred | def = 46.00 mm | |
| Remaining thicknesshef = h − def | hef = 94.00 mm | |
| Neutral axis of the residual sectionfirst moment of the remaining parallel layers, measured from the unburnt face | zs,f = 57.333 mm | |
| Residual second moment of area | I0,net,f = 39,333,333 mm⁴ | |
| Distance to the extreme fibremeasured to the furthest face of a layer that still runs along the span. A surviving cross layer on the burnt face is ignored, since it carries no bending. | cmax = 43.333 mm | |
| Residual section modulusW = I0,net,f / cmax. The residual section is no longer symmetrical, so the larger of the two distances governs. | W0,net,f = 907,692 mm³ | |
| Design bending strength in firefm,d,f = kmod,f · kf · fm,k / γM,f | fm,d,f = 27.600 N/mm² | EN 1995-1-2 |
| Bending stress in fireσm,d,f = Md,f / W0,net,f | σm,d,f = 5.7288 N/mm² | |
| Bending in the fire situationσm,d,f ≤ fm,d,f | η = 0.208 · Pass21 % of the capacity used |
How this module is verified
Verified step by step against the values of EN 1995-1-2 and against independent calculations used on real projects, before release.
Questions
What happens when a layer burns through?
The layers that remain are recomputed from the unburnt face: only the layers that still run along the span count for bending, and the neutral axis moves.
Which plan includes this module?
The Pro plan, with its calculation note.
Related
- CLT floor panel: bending and rolling shear
- CLT floor panel — deflection and vibration
- EN 1995: modules and checks
Norm: EN 1995-1-2 · Page updated: 20 October 2026