Fire resistance duration
in wood construction

When building with wood, there is the question of the fire behaviour of the entire construction. To ensure safety, the fire resistance of the individual components must be taken into account. But what does fire resistance actually mean and how is it calculated?

To assess the fire resistance under fire exposure, the remaining load-bearing capacity (criterion R – Résistance) as well as the room closure (criterion E – Étanchéité) and the obstruction of the temperature passage (criterion I – Isolation) are assessed. Each of these criteria is expressed in minutes with a numerical sequence (30, 60 or 90). The sum of the three numbers gives the fire resistance duration.

In wood construction, the residual load-bearing capacity is calculated by taking into account the burn-up, which depends on the type of wood and the wood material. In case of fire, wood forms a protective charcoal layer that protects the underlying cross-section from the effects of heat. The load-bearing capacity of the remaining cross-section is thus maintained. A burn rate of about 0.65 mm/min can be assumed.

Cross laminated timber and X-LAM are a special case in timber construction. Due to the high temperatures in case of fire, the adhesive joint between the individual board layers can fail. The result: The bottom layer falls off, the protective function for the layer above is lost and there is increased burning. This is taken into account in the design by the so-called staircase model.

The fire resistance duration is a decisive criterion for safety in timber construction. It is made up of the remaining load-bearing capacity, the room closure and the obstruction of the passage of temperature. The calculation must take into account the burn-up, which depends on the type of wood and wood material. For cross laminated timber and X-LAM, a special stair model applies to ensure safety.