WoodST: An Advanced Modelling Tool for Fire Safety Analysis of Timber Structures

The urge to develop green and sustainable building structures has seen an increase in the use of wood products in the building and construction industry. Being classified as a combustible material, stringent fire safety measures are typically prescribed for timber structures. The analysis of timber connections under fire conditions remains however a significant challenge to engineers and researchers. Among the available constitutive models for wood materials simulation, finite element models are unsuitable for accurate prediction of thermomechanical behaviors and failure modes, that play a critical role in the fire safety of timber structures.

Recently, Dr. Zhiyong Chen, Dr. Christian Dagenais, and Dr. Chun Ni from FPInnovations which is a Canadian not-for-profit R&D private organization which spans the pulp and paper industry, forest operations, wood products, and bio-sourced products developed a temperature-dependent constitutive model, Wood^ST, for numerical simulations of wood-based materials and connections exposed to fire. The model is a combination of several mechanics sub-models: orthotropic linear elasticity, extended Yamada-Sun strength criteria, thermomechanical relationships, strain-based damage and hardening evolution, plastic flow and hardening law. The simulation objectives included the prediction of failure modes, load-displacement and time-displacement relationships. The research was conducted in collaboration with Dr. Steven Kuan from the British Columbia Institute of Technology and is currently published in Journal of Structural Engineering.

In their work, the research team modelled the structural performance of a laminated veneer lumber (LVL) beam, and a glulam bolted connection under forces and fire to validate the feasibility of the developed model. In comparison with the results of the experimental tests, the model was observed to reasonably simulate the thermal and mechanical behaviors of the LVL beam and glulam connection exposed to fire. The predicted time to failure lied within 10% of the actual failure time, an indication that the model could be accurately used to predict failure time given the nature of the fire event. For the LVL beam and glulam bolted connection, the heat transfer models provided a right prediction of the residual wood dimensions comparable to the experiment results.

The sub-models, in particular, play a significant role in the success of the developed model. For instance, the extended Yamada-Sun strength criteria are useful in determining whether the materials yield or fail in any directions, a strained-based damage evolution describes the softening of the brittle failure due to tension or shear. In contrast, multilinear reduction models describe the influence of fire on the mechanical behaviors of the materials. Further validation of the model, e.g. I-joists and wood-frame floors, has been made since its development, but more validation on timber connections with various types of fasteners and wood products would be necessary.

In a nutshell, the study presented a temperature-dependent plastic-damage constitutive model for simulating the structural performance of timber materials and connections under forces and fire. The obtained results compared well with the results of the experimental tests. The model was, notably, demonstrated to be capable of simulating the thermomechanical responses of LVL beam and glulam connection with only 10% difference. In a statement to Advances in Engineering, the authors highlighted their findings will help researchers and engineers to study in further details, the thermomechanical behavior and fire resistance of wood-based materials and connections.