Thermal Response of Timber Slabs Exposed to Travelling Fires and Traditional Design Fires

Abstract

Engineered timber is an innovative and sustainable construction material, but its uptake has been hindered by concerns about its performance in fire. Current building regulations measure the fire performance of timber using fire resistance tests. In these tests, the charring rate is measured under a series of heat exposures (design fires) and from this the structural performance is deduced. Charring rates are currently only properly understood for the heat exposure of a standard fire, not for other exposures, which restricts the use of performance-based design. This paper studies the charring rates under a range of design fires. We used a multiscale charring model at the microscale (mg-samples), mesoscale (g-samples), and macroscale (kg-samples) for several wood species exposed to different heating regimes and boundary conditions. At the macroscale, the model blindly predicts in-depth temperatures and char depths during standard and parametric fires with an error between 5% and 22%. Comparing simulations of charring under travelling fires, parametric fires, and the standard fire revealed two findings. Firstly, their charring rates significantly differ, with maximum char depths of 42 mm (travelling), 46 mm (parametric), and 59 mm (standard fire), and one (standard fire) to four (travelling fire) charring stages (no charring, slow growth, fast growth, steady-state). Secondly, we observed zero-strength layers (depth between the 200 °C and 300 °C isotherm) of 7 to 12 mm from the exposed surface in travelling fires compared to 5 to 11 mm in parametric fires, and 7 mm in the standard fire. Both traditional design fires and travelling fires, therefore, need to be considered in structural calculations. These results help engineers to move towards performance-based design by allowing the calculation of charring rates for a wide range of design fires. In turn, this will help engineers to build more sustainable and safe structures with timber.

Fire dynamics inside a large and open‐plan compartment with exposed timber ceiling and columns: CodeRed #01

“…These are fires which burn over a localised area and spread along the compartment [12], [13]. In addition, in large compartments fires tend to be fuel controlled rather than ventilation controlled as is often the case for small compartments [14], [15] . No experimental evidence currently exists to investigate the fire dynamics experienced in large compartments (> 100 m 2 ) with exposed timber and how these differ from small compartments and non-combustible large compartments.…”

“…Richter et al 16 has assessed the thermal response (charring rate and zero strength layer) of timber slabs under a range of design fires (travelling fires, Eurocode parametric fires and the standard fire). The authors adopted design fire models used typically for non‐combustible compartments and, similarly to Rackauskaite et al, 10 highlighted the lack of validated design fire tools for open‐plan compartments with exposed timber.…”

OpenSeesdevelopment for modelling timber structural members subjected to realistic fire impact

“…Facilitated by the layer state tag, the thermal properties of each layer of timber consisting of density , specific heat and thermal conductivity k (Figure 3B) are defined based on the EC5 thermal property models 1 and data in, 7,26–28 as well as the consideration of heat generation due to timber combustion, 25 which now is dominated by the material temperature and layer states. In the case that the layer transition does not occur, these thermal properties vary with elevated temperature and are linearly interpolated between the key control input parameters at ambient temperature, dry state (100°C), and char state (300°C).…”

“…A detailed description of the model and validation have been reported in another paper of the authors focusing on the heat transfer model of timber 23 . It is noted that the thermal properties of various timber types are of high uncertainty, and the model parameters can be taken with reference to past experimental studies 1,7,26–28 . More fire tests on a variety of timber sections might be needed for quantifying the thermal property variables and this heat transfer model will be further refined in future studies.…”

A multiphase approach for pyrolysis modelling of polymeric materials

“…On the other hand, mass timber construction is also subject of rising interest globally, based on engineered wood products such as cross-laminated timber (CLT) panels and glued laminated (glulam) framing members. These modes of construction are presented as a potential revolution within the construction industry due to their environmental and aesthetic benefits (Oliver et al, 2014) and fast and cost-efficient assembly on site (Richter et al, 2021). However, these trends have also raised major concerns on the fire safety of these construction materials.…”