Finite element modeling for fire resistance of timber composite panels

PI :  Yann Rogaume (Research Unit for the Study and Research of Wood Materials — LERMAB)

Co-applicants : M Khelifa (LERMAB)

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Context — Even though timber is a combustible material, timber structures are remarkably resistant to fire. Timber behavior is particularly favorable when the sections are sufficiently dimensioned to allow for the formation of a charred layer with insulating properties which act to protect the central sections. Timber combustion occurs in three main stages: heating and the subsequent increase in temperatures which cause drying, followed by pyrolysis giving off combustible gases which burn around the timber and cause the formation of charcoal, and finally the slow burning of charcoal. The ability of timber to resist the spread of fire is due mainly to the charcoal layer, which limits the influx of heat. From a structural point of view, the charcoal layer has no resistance. The loss of the load-carrying capacity of a burning timber element is mainly explained by the loss of the timber section in the presence of temperatures below 300 ° C.

The flammability and fire resistance of timber structures depends on timber species, density, shape, technological type material (solid timber, glulam, LVL) as well as humidity, which also determines the burning rate. However, the recommendations of Eurocode 5, essentially validated on solid timber, fail to accurately reproduce the actual mechanisms and observed phenomena, particularly in the case of engineered timber products such as Glulam, LVL and CLT.

Objectives — The main aim of this project is the development of a model for the combustion of timber, which takes into account the observed physical behavior to accurately simulate how timber structures respond to fire. The overarching goal is to better predict the behavior of engineered timber products such as Glulam, LVL and CLT. For this purpose, a prototype thermal model was developed and validated at LERMAB. As part of this project, we propose developing an extension of the thermal model to include hygro-thermal effects and the effects of the charcoal layer, which are known to delay the influx of heat.

Approach — The approach proposed in this study is organized into three stages. The first step is a literature review on the stat of the art on the fire performance of wood-based panels used in construction, including the study of the hygro-thermal behavior of the wood material. The second stage focuses on choosing the appropriate thermal model for timber under high temperature conditions. The selected model integrates the three modes of heat transfer , ie: conduction, radiation and convection during exposure to fire. Theoretical and numerical aspects associated with the model will be then presented and discussed. Validation of the numerical procedure will be carried out using the Abaqus suite of finite element software, with applications to simulate actual fire tests available in literature. The model will be used to conduct a study on the heat transfer and the formation of the charcoal layer to better understand the evolution of hygro-thermal behavior of multi-layered engineered timber panels in fire.

Expected results and impacts — The next step will be to perform the same type of work on multi-layered timber panels with different materials and thermal properties, and to investigate their effects on the fire response. The greater aim is that this digital model will be used to assist building professionals in the design phase of fire resistant timber structures.