Experimental study on the bending resistance of cross-laminated timber exposed to ISO 834-1 standard fire

In a fire, the load-bearing capacity of a timber structure is influenced not only by charring but also by the elevated temperatures behind the char layer. The Eurocode accounts for this increased temperature in solid timber by subtracting a compensating zero strength layer (ZSL) from the cross-section in addition to the char depth. However, this approach does not directly apply to cross-laminated timber (CLT) structures, which have transverse layers that do not carry load in the direction of the span and feature glue lines. If the glue fails to keep the lamellae in place during a fire, the charring performance and temperature distribution will differ from solid timber. This paper assesses the load-bearing capacity of CLT panels exposed to standard fire conditions, including the falling off of the charred layers. The assessment is based on fire tests conducted on two different CLT structures and the measured temperatures within the CLT panels. The structural capacity is compared to the value calculated using the ZSL method. The findings indicate that glue failure significantly increases the charring rate, impacting the equivalent ZSL differently when glue lines do not hold the layers. The fire test revealed that glue lines led to falling lamellae, increased charring rates, and affected load-bearing capacity and ZSL height.