Scale modeling of thermo-structural fire tests of wood members

Abstract

Standard fire resistance testing is essential to qualifying wood-based materials to be approved for use in building assemblies. In a previous study, we developed a theoretically based, experimentally validated scaling methodology for reducing the size of the test article while maintaining the same thermal and structural response exhibited in the large-scale test. This paper is focused on demonstrating these scaling laws for wood with combined thermo-structural loading. Due to load capacity limits, dimensional lumber boards at ½-scale and ¼-scale were subjected to combined bending and thermal loading. Samples were placed in static three-point bending with the loading scaled to have structural similitude, while simultaneously, the bottom surface was exposed to a scaled fire exposure. Analytical modeling of wood pyrolysis demonstrated that, due to char kinetics as the heating rate is increased in the tests, equivalently less char is formed in the reduced-scale tests. Therefore, we developed a char timescale correction factor, calculated from both model predictions and measured charring rates, which modified the previous Fourier number time scaling laws. Scaling exposure time by our updated thermo-structural test scaling laws, the smaller ¼-scale test deflection results predict the behavior of the equivalent larger ½-scale tests by between 11.0% to 16.1%.