Thermal response of timber connections in the cooling phase

Structures are conventionally designed to maintain load-bearing capacity during the heating phase of a fire. However, in structures with moderate or high thermal inertia, the thermal field which results in the lowest structural resistance is likely to occur after the heating phase. This is of particular interest for timber connections because the strength and elastic modulus of timber reduces until the formation of char while steel plates and fasteners, which transfer forces between elements, conduct heat through the connection. It is unclear how thermal fields develop in timber connections during the cooling phase of fires and what influence different cooling rates have. Experiments on identical timber beam-column subassemblies exposed to the same heating duration but two different cooling phases are presented. The results show that exposed steel components conduct heat into the connection, which propagates a thermal wave through the elements. Although the thermal waves had similar speeds, the specimen absorbed more thermal energy during the longer cooling phase, resulting in higher temperatures. Since the strength and elastic modulus of timber decrease at temperatures below 100°C, these results provide evidence that the structural resistance of a timber connection decreases in the cooling or post-cooling phases and that a longer cooling phase is more severe than a shorter one. Further investigation into thermal exposure during the cooling phase of realistic compartment fires and the response of a wide variety of timber connections is required to quantify the reduced performance and support the development of appropriate design methods.