Behavior of fire-retardant treated bolted timber–steel composites (TSCs) and effective charring depth based on experiment

The bolted joints make timber–steel composites (TSCs) easily to be assembled; however, seams exist between the timber components for bolted TSC components resulting in the risk of fire spreading into the joint seam when the component is under fire. The efficient charring depth provided by timber needs to be clarified; furthermore, the raising temperature potentially affects the strength of steel component. In this study, TSCs were tested in fire for 1 hour, and the following issues were examined, including the experimental charring depth of timber components, the influence of fire-retardant finish in order to provide the efficient charring depth, and additionally the suggested charring depth are proposed for the design of TSCs. In conclusion, the charring depth in the seams was greater than the regulation values, and evaluation methods for the charring depth at seams should be reconsidered. Although the fire-retardant finish reduced the formation rate of the char layer, the flames still breached the seams. The results reveal that Douglas fir TSCs with fire retardant are the most efficient specimen, with effective charring depth 52 mm. While the effective charring depth of Douglas fir TSCs without fire retardant and Japanese Cedar TSCs with fire retardant are increased, at least 64 mm and 65 mm needed, respectively.