Contents

• Concept
• Charred panel calculation
• Fire protection
• Design calculation
• Results

Concept

Fire design of CLT panels can be found at CLT panel, fire design under Timber design tab.

As the relevant Eurocode (EN 1995-1-2) does not distinguish cross-laminated timber panels from design point of view, the standard advised “Reduced cross-section method” is used for verification under fire conditions (prescriptive rules / member analysis / simple calculation models). For reduction of fire effect, panels can be initially protected from fire exposure by structural elements or fire protective claddings (EN 1995-1-2 3.4.3).

Charred panel calculation

The basic concept of the standard’s “Reduced cross-section method” is to calculate the charring depth of a shell panel, reduce the thickness of the initial panel by the effective value of the charred region, and verify the resistance of the obtained CLT panel against the relevant accidental combinations, based on the material properties at room temperature.

There are several parameters affecting the charring process and the calculation of the reduced panel, and they can be set at Calculation parameters of CLT panel, Fire design. The applied fire protection is also one of the affecting parameters.

One-dimensional charring rate

In case of shells, one-dimensional charring can be assumed (EN 1995-1-2 3.4.2 (1)). By default, FEM-Design is using the value 0.65 mm/min for the design charring rate (β₀) that corresponds to Glued laminated timber with a characteristic density of >= 290 kg/m³ at Table 3.1. This value can be overwritten.

Consider delamination of charred layers

According to some experimental tests, the glue between the layers softens the connection, and the delamination of the charred layers can be observed during fire. For the consideration of this effect, a charring rate multiplier can be defined for each side. It is working exactly in the same way as the standard describes the increased charring rate (k₃) after the protection has fallen from an initially protected member: after the delamination of a layer, the charring rate is increased until the char depth exceeds 25 mm in the next layer.

To see the effect, check the unprotected member:

In the figure on the left, the charring rate (red line) is increased (β₀ is multiplied by the top/bottom multiplier, respectively) on the first 25 mm of every layer, starting from the second one; while in the figure on the right, the charring rate remains constant during the duration of fire.

At section exposure, ambient or fire exposed boundary conditions can be set on each side.

After the calculation of the effective Cross-section (d_ef), there might be thin layers remaining at the top and/or bottom side. Due to the nonlinear characteristic of the char line, these layers can be neglected, which can also lead to more favorable results from mechanical point of view.

Fire protection

The final parameter that affects the charring rate is the application of a fire protection. In FEM-Design, this can be done in two ways:

• Structural protection (thanks to connected structures)
• Fire protection by cladding

Structural protections

Structural protections are assumed to function during the whole fire duration and its application excludes (overwrites) any other kind of fire protection, if exists. It delays the start time of charring, and as it does not get destroyed, neither fall off, the delamination of charred layers setting has no effect in case of this protection type.

When the charring starts (t ≥ t_ch ), being on the safe side, the program applies the original value of the one-dimensional charring rate that is set in the Calculation parameter.

Fire protection by cladding

Fire protection by cladding can be assigned to CLT panels by either manually (Manual design) or automatically (Auto design), if necessary.

The material type of fire protection can be wood, one/two-layer gypsum board, rock fibre butt, or user defined one. In case of consideration of delamination of layers, the reduced rate (k₂) when the charring starts, and the increased rate (k₃) after the failure of protection ends earlier in case of the first layer is completely charring before t_a time. After the first layer has fallen off, the charring process continues as for an unprotected member.

Design calculation

In order to run fire calculations of a CLT panel, one Ua (accidental limit state) - type Load combination (or Load group) with at least one +Fire - type load case must be defined. The safety factor for fire design (γ_Mfi) can be set at Application data of the selected timber panels.

Design Check first obtains the reduced panel, considering the applied fire protection, if exists. Then, it verifies it against the relevant accidental combinations by the room temperature design procedure, using the fire values of modification factors appearing in the calculations (γ_M,fi, k_fi, k_mod,fi).

Auto design is aiming to find the thinnest suitable protection for the panel, if fire protection is necessary at all. Upon setting the desired fire protection type, minimum/maximum thickness and thickness increment for the iteration, the program starts to check without any protection. Then, it continuously increasing the thickness until the panel’s utilization becomes lower than the Limit utilization.

Results

In the model space, utilization results for fire combinations are available at Results > CLT panel, fire design > Utilization (Timber design tab). In order to have the possibility to check the developing stress distribution in the reduced panel, Utilization is divided into two detailed results:

Design calculation

The standard CLT panel utilization results are supplemented with two additional data sets: Fire design data and Reduced panel (section):

Stress distribution

This detailed result is “result point”- based, similarly to the analysis-type detailed result of laminated shells.

The results can be selected using following options:

Maximum criteria items can be grouped by:

All components  
For example: All components+ finds combinations component by component that gives the largest positive/negative value of stress component along all the layers in the selected result point. The result of the combinations belonging to the components is displayed in all stress figures (note: there can be five different combinations displayed in the detailed result).

All layer… (by stress type)
For example: All layer, Sigma x+ finds the combination that gives the largest positive x-directional normal stress along all the layers in the selected result point. The result of this combination is displayed on all stress figures.

Layer... (by stress type)
For example: 2. layer, Sigma x+ finds the combination that gives the largest positive x-directional normal stress in the second layer in the selected result point. The result of this combination is displayed in all stress figures.