Simulation of timber material and structural members in fires with OpenSees framework

Abstract

Engineered timber offers significant sustainability benefits and architectural advantages, but its combustibility can lead to rapid fire spread in large open-plan compartments, limiting its wider adoption. This thesis addresses this challenge by developing advanced simulation tools and design strategies to improve the fire performance of timber structural systems. The related work is presented in 7 chapters, with brief introductions to each chapter as follows: Chapter 1 introduces the research background, motivation, and the workflow of this thesis. Chapter 2 developed an enhanced heat transfer model for timber within the open-source OpenSees framework, incorporating temperature-dependent thermal properties and the heat generated by timber combustion. Validated against experimental data, this model accurately predicts temperature evolution in timber sections exposed to realistic (non-standard) fire scenarios. Chapter 3 established a coupled fire-structure analysis framework for modelling structural timber members subjected to different fire-exposure conditions with appropriate thermal actions based on the developed heat transfer model, demonstrating good agreement against selected fire test results. In this approach, thermal outputs from CFD compartment fire simulations were applied to OpenSees models of a timber-concrete composite slab for preliminary studies. This integrated simulation demonstrated the platform's capability to predict structural responses under realistic fire conditions. Chapter 4 proposed a novel timber-insulation mixed ceiling design to mitigate fast-growing fire in a large compartment with timber ceiling. High-fidelity CFD simulations calibrated against large-scale fire tests showed that alternating non-combustible insulation and timber strips in lower layer of the ceiling can mitigate slow flame spread, achieving a similar fire behavior to compartment with non-combustible within 30 min. Chapter 5 considered more realistic compartment and fuel configurations, which includes ceiling beam effect, non-uniform fuel load distribution, window height, and more challenging ignition locations. Results indicate that conventional uniform fuel distribution may underestimate fire risks. Beam arrangements influence local fire development, while compartment opening heights affect overall fire development. These factors must be considered in performance-based fire safety design. Optimized mixed ceilings can handle more challenging fire scenarios. Chapter 6 further developed an orthotropic damage-plasticity material model for two-way timber slabs and implemented in OpenSees to simulate load-bearing slab behavior in fire. Validation against a full-scale two-way CLT slab fire test showed that this model reliably captures structural deflections and failure. Applying the model to composite slabs with the mixed ceiling design demonstrated enhanced fire resistance (exhibiting only 40% to 70% of the deflection) relative to conventional full-timber slabs. Chapter 7 summarizes the overall outcomes from the works of this thesis, followed by description of further research directions to be addressed for achieving fire safe timber-integrated slabs.