# Fire Performance of Self-Tapping Screws in Tall Mass-Timber Buildings
## 3 Results and Discussion
### Effect of the Withdrawal Resistance on Self-Tapping Screws  
During the mechanical tests, all pull-out tests of STS failed due to withdrawal of the screw from the glulam and characterized as a shear failure of the wood fibers along the fastener shank. This was expected because the axial tensile strength of the STS was higher than the anticipated withdrawal capacity of the glulam. Given that the load capacity of the fastener majorly depends on the mean relative density of the timber and the yield point of the steel [24], the key factor in the failure is the capacity of wood fibers to resist shear forces, which are transferred through the threaded length of the screws [12]. The average withdrawal resistance for the STS under ambient and fire conditions are presented in Table 4.  
As expected, and reported by [PERSON] [24], the withdrawal strength increased with screw diameter with a higher tensile strength of the fastener. For instance, the average withdrawal strengths of 0.176 kN/mm and 0.192 kN/mm were calculated in ambient conditions for 08 mm and \(\mathcal{O}\)10 mm full-thread screws, respectively, and using a length of penetration of 120 mm. This relationship was not linear due of the variability in the wood properties (i.e., density, moisture content, and the grain orientation). With the variability on the wood fibers' strength, the withdrawal strength can be affected and can modify the global behavior of the material with higher variability [24]. However, the average withdrawal strengths obtained in this study were consistent with those reported in gulam or softwood elements by [PERSON] [24], [PERSON] [43], and technical values given by MTC Solutions tests [44]. The latter publishes an average value of 0.183 kN/mm and 0.206 kN/mm for \(\mathcal{O}\)8 mm and \(\mathcal{O}\)10 mm STS, respectively. In fire conditions, the average withdrawal load resistance decreased with increasing fire duration. For instance, the decrement was 22% and 34% between ambient conditions and one hour of fire exposure for \(\mathcal{O}\)8 mm and \(\mathcal{O}\)10 mm diameters with exposed heads, respectively. The decrement was 51% and 62% between ambient conditions and two hours of fire exposure for \(\mathcal{O}\)8 mm and \(\mathcal{O}\)10 mm diameters with exposed heads, respectively. The high heat-conduction of steel-conducted temperature along the fastener shank induced a local loss of wood fiber strength and specific gravity due to elevated temperatures [14; 15]. Increasing the wood temperature at the vicinity of the fastener shank locally weakens the wood fibers surrounding the threaded shank and decreases the ability of timber to resist withdrawal forces. With the heating rate dependent on time exposure, higher and deeper temperature profiles are obtained through the fastener shank; therefore, the overall withdrawal resistance decreases as less threads are fully secured in unaltered wood fibres that are still at ambient conditions. In other words, the initial depth of STS penetration is reduced by decreasing the withdrawal strength of the timber under expo