This study uses 3D FE modeling to analyze the thermal-structural response of wood truss assemblies under fire and calibrate a simplified method for predicting the failure time. The FE models are validated against 8 full-scale fire resistance tests. The models reveal that failure of the bottom member, which experiences extensive charring, governed the assembly collapse. Based on a parametric numerical analyses, a simplified method is developed to predict the assembly failure time based on thermal profile in the bottom member for different fires. The simplified method allows prediction of the failure time based on 2D FE thermal analysis of the bottom member combined with an analytical evaluation of its temperature-dependent capacity over time, avoiding the need for 3D FE models of the whole assembly.