A Design Approach to External Fire Spread From Buildings With Exposed Mass Timber

Abstract

Engineered wood products (EWPs) are relatively new, innovative construction materials which, if used correctly, can serve as sustainable alternatives to more common construction types. They provide an opportunity for designers to reduce the embodied carbon of proposals. Given the climate crisis, it is vital that the sector decarbonises. However, lessons must be learnt from previous sustainability-driven design decisions that did not sufficiently consider safety implications. Often there is a desire from building owners, occupiers and/or designers to expose mass timber. Doing so can provide aesthetic, wellbeing, cost and carbon benefits over encapsulation. However, exposing large areas of mass timber introduces an additional fuel load to the compartment -the structural fuel load, and compartment fire experiments to date have shown that this can lead to larger external flaming. This paper aims to investigate potential implications of this phenomenon on: (i) large-scale 'standard' façade tests, and (ii) radiation assessments to neighbouring buildings. First, temperature data recorded outside the opening(s) of seven mass timber compartment fire experiments is compared to temperatures in front of the façade recorded during three 'standard' industry façade fire tests. It is found that BS 8414 and the proposed harmonised European test generally expose the façade to more/as severe temperatures than observed during mass timber compartment fire experiments. Therefore, these tests are considered applicable to exposed mass timber buildings. The NFPA 285 test, which was designed for traditional buildings, is less severe than the mass timber experiments reviewed. Heat fluxes recorded opposite openings in three mass timber compartment fire experimental series are then compared to heat flux predictions made following first principles-based radiation calculations following the enclosing rectangles and configuration factor method (as in BR 187). Using the "low" and "high" fire load assumptions in BR 187 leads to underpredictions of heat flux received. Modelling the external flame as an emitter (of the same temperature as the opening) has little impact on results. Therefore, a higher emitter temperature is recommended when carrying out radiation calculations from exposed mass timber compartments.