Engineered timber products such as cross-laminated timber (CLT) are gaining popularity with designers due to attractive aesthetic, sustainability, and constructability credentials. The fire behaviour of such materials is a key requirement for buildings formed predominantly of exposed, structural timber elements. Whilst design guidance focuses on the residual structural capacity of timber elements exposed to a ‘standard fire’, the fundamental characteristics of CLT’s performance in fire, such as ignition, flame spread, delamination, and extinction are not currently considered. This paper focuses on the issues relating to increased fuel load due to a combustible building material itself. Whilst an increasingly common protection solution to this conundrum is to fully encapsulate the timber elements, there is limited supporting test data on this approach. Through understanding these concepts from a fundamental, scientific perspective, the behaviour can be properly understood, and, rather than limiting design, can be incorporated into design to satisfy suitable performance criteria.
In this paper therefore, the concept of auto-extinction – a phenomenon by which a timber sample will cease flaming when the net heat flux to the sample drops below a critical value – is explored experimentally and related to firepoint theory. A series of c.100 small scale tests in a Fire Propagation Apparatus (FPA) have been carried out to quantify the conditions under which flaming extinction occurs. Critical mass loss rate at extinction is shown to occur at a mass flux of 3.5g/m2s or a temperature gradient of 28K/mm at the charline. External heat flux and airflow were not found to affect the critical mass loss rate at the range tested. This approach is then compared with a compartment fire with multiple exposed timber surfaces. With further testing and refinement, this method may be applied in design, enabling architects’ visions of exposed, structural timber to be safely realised.
A series of compartment fire experiments has been undertaken to evaluate the impact of combustible cross laminated timber linings on the compartment fire behaviour. Compartment heat release rates and temperatures are reported for three configuration of exposed timber surfaces. Auto-extinction of the compartment was observed in one case but this was not observed when the experiment was repeated under identical condition. This highlights the strong interaction between the exposed combustible material and the resulting fire dynamics. For large areas of exposed timber linings heat transfer within the compartment dominates and prevents auto-extinction. A framework is presented based on the relative durations of the thermal penetration time of a timber layer and compartment fire duration to account for the observed differences in fire dynamics. This analysis shows that fall-off of the charred timber layers is a key contributor to whether auto-extinction can be achieved.
Fire safety is widely perceived as a barrier to implementation of tall timber buildings, particularly for engineered mass timber buildings with significant areas of exposed timber and timber structural framing. This negative perception is exacerbated by a lack of scientific data or experimental evidence on a range of potentially important issues that must be properly understood to undertake rational, performance-based engineering design of such structures. With the goal of delivering fully engineered structural fire designs, this paper presents and discusses a framework for using scientific knowledge, along with fire engineering tools and methods, to enable the design of timber buildings such that, when subject to real fire loads, their performance is quantified. The steps in this framework are discussed with reference to the available literature, in an effort to highlight areas where additional knowledge and tools are needed.