ABSTRACT
Timber construction is undergoing resurgence due to its indubitable environmental credentials and the speed with which wooden structures can be erected. The complexity of scheme for which it is adopted and the scale is rapidly shifting. Traditionally a material for low-rise construction, timber towers are now frequently conceived. Unlike many materials commonly adopted for tall or complex buildings, timber is combustible. In some circumstances, the building fabric may contribute to the fire load which introduces new considerations that challenge the conventional ways of achieving ‘fire-safe’ buildings. If the widespread adoption of timber for tall or complex buildings is to become a sustainable reality, new design approaches are required which shift away from the practices and principles that have become convention.
The structural fire engineering status quo is to define a fire scenario, appraise what this scenario means for heat transfer to structural elements and, subsequently, undertake an analysis to assess mechanical response. This ‘in-series’ approach is valid and relevant for noncombustible structures, where the elements do not directly affect the fire dynamics and, thus, the design fire condition. The same approach is less relevant for timber structures, which may either be exposed by design, or may have the potential to become exposed before the fire has burnt out. In these instances, the heat transfer to the timber structural element is a fundamental aspect of defining the rate at which it may combust and, thus, contribute to the fire condition and its severity. This coupled behaviour requires an alternative approach and greater coordination between structural engineers and fire safety engineers.
This paper presents a pragmatic zone model based tool for appraising exposed or partially exposed timber structures subject to fire. The model makes use of effective thermal properties for the purposes of informing mass burning rate and, therefore, contribution to the heat release rate of the enclosure fire condition. The approach is shown to yield acceptable results when benchmarked against ‘real’ fire experiments involving partially or fully exposed CLT structures.
