Research on the multi-scale combustion performance of timber structures considering fungal decay: From material pyrolysis to building fire dynamics

Fungal decay significantly threatens the fire safety of timber structures, yet its cross-scale impact from material intrinsic properties to building-scale fire consequences remains poorly understood, leading to frequent underestimation of fire risk in existing timber buildings and heritage. This study systematically investigates the effects of accelerated fungal decay on the combustion characteristics of Douglas-fir across material and building scales, via multi-type material characterization tests, cone calorimetry, impedance-based non-destructive testing, and Fire Dynamics Simulator (FDS) numerical simulation. Results reveal that decay preferentially degrades cellulose and hemicellulose, causing microstructural deformation and significantly impaired thermal stability of wood. After 90 days of decay, the maximum pyrolysis temperature decreased by 16.9 °C, while the ignition temperature dropped by 15.14 °C as measured by the thermogravimetry-derivative thermogravimetry (TG-DTG) method (a consistent 14.65 °C decrease was obtained from the cone calorimetry test, with minor numerical differences due to different temperature calculation principles), and toxic CO emissions during combustion increased significantly. Based on the characterization results, a pyrolysis model was established which is used to conduct full-scale FDS simulations. Simulation results confirm that decay accelerates fire development, flame spread and flashover occurrence in timber buildings, with the peak heat release rate increasing from 11.6 MW to 16.2 MW. Additionally, A novel material-building dual-scale Comprehensive Fire Risk Index (CFRI) is proposed, which reveals that fungal decay can amplify the comprehensive fire risk by up to 148.8%. This study provides critical theoretical and technical support for fire safety evaluation of timber structures and conservation of timber architectural heritage.