Effects of the cooling phase on the fire performance of encapsulated timber elements

Timber structures are vulnerable during the cooling phase of a fire, especially in compression, a factor not currently addressed by design methods based on standard fire exposure. This paper presents an experimental investigation on the thermal and mechanical performance of encapsulated cross-laminated timber (CLT) elements during and after standard fire heating. Data from two reduced-scale furnace tests on CLT specimens, encapsulated with either one or two layers of fire-rated plasterboard, were used to evaluate the reduction in compression and bending capacity. The effective cross-section and bending capacities were determined using the Reduced Cross-Section Method (RCSM) and an Advanced Calculation Method (ACM), both provided in the current and proposed versions of Eurocode 5. The most significant capacity reductions were predicted for the cooling phase when using the ACM, where the bending capacity dropped below 45% of the original capacity for both cases. In the RCSM, the capacity fell below 85% for both samples when using the current Eurocode 5 and below 67% for both samples when using the revised Eurocode 5. These results show the inadequacy of simplified methods based on standard fire exposure to predict the capacity of timber elements in the cooling phase of a fire.