# Fire Performance of Self-Tapping Screws in Tall Mass-Timber Buildings
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & & \multicolumn{8}{c}{**Withdrawal Resistance**} \\ \cline{3-10}  & & \multicolumn{8}{c}{**Head Exposed**} & \multicolumn{8}{c}{**Tip Exposed**} \\ \cline{3-10}
**Fastener Type** & **Ambient Conditions** & \multicolumn{2}{c}{**One Hour of**} & \multicolumn{2}{c}{**Two Hours of**} & \multicolumn{2}{c}{**One Hour of**} & \multicolumn{2}{c}{**Two Hours of**} \\  & & \multicolumn{2}{c}{**Exposure**} & \multicolumn{2}{c}{**Exposure**} & \multicolumn{2}{c}{**Exposure**} & \multicolumn{2}{c}{**Exposure**} \\ \cline{2-10}  & **Max Load** & **CoV** & **Max Load** & **CoV** & **Max Load** & **CoV** & **Max Load** & **CoV** & **Max Load** & **CoV** \\ \hline \(\mathcal{O}\)8 \(\times\) 160 mm FT & 21.2 & 4.6\% & 15.3 & 0.3\% & 7.93 & 4.7\% & 15.8 & 0.7\% & 9.81 & 0.9\% \\ \(\mathcal{O}\)10 \(\times\) 160 mm FT & 23.0 & 1.6\% & 14.9 & 5.5\% & 9.03 & 3.1\% & 19.7 & 2.8\% & 14.3 & 0.7\% \\ \(\mathcal{O}\)8 \(\times\) 160 mm FT & 12.4 & 4.0\% & 10.3 & 6.4\% & 7.39 & 3.5\% & 10.0 & 0.5\% & 3.61 & - \\ \(\mathcal{O}\)10 \(\times\) 160 mm PT & 20.7 & 1.7\% & 13.7 & 7.8\% & 7.54 & 9.2\% & 15.6 & 4.1\% & 8.59 & 8.2\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Average withdrawal resistance for \(\mathcal{O}\)8 and \(\mathcal{O}\)10 STS under ambient and fire conditions. sure. Figures 10 and 11 show the load-displacement curves for the STS pull-out tests at elevated temperatures. As opposed to the results obtained from the pull-out tests in ambient conditions, withdrawal tests in fire conditions exhibited a greater plastic behaviour in the screws with longer exposures. Long exposure times decreased the mechanical properties of the steel for a more asymptotic response up to the maximum withdrawal resistance. This behavior causes further strain after the yield point is reached, which is characterized as a plastic deformation [45,46]. In this study, a more nonlinear behavior response up to the failure point was observed when exposed to longer times because the elevated temperature became deeper along the STS shank. However, the temperature-dependant reduction of the mechanical properties of the steel is not significant in comparison to the reduction of the withdrawal strength of the timber, as explained by [PERSON] et al. [47]. Therefore, the results suggest that the failure of STS connections is influenced by the strength reduction of the timber as it heats up (i.e., residual cross-section of the timber element and localized charring) due to the heating area of the steel element. Figure 11: Load-displacement curves for \(\mathcal{O}\)10 mm screws: (**a**) Exposed head; (**b**) Exposed tip. Solid line for ambient conditions, dashed line for one hour of exposure and dotted line for two hours of exposure. Figure 10: Load-displacement curves for \(\mathcal{O}\)8 mm screws: (**a**) Exposed head; (**b**) Exposed tip. Solid line for ambient conditions, dashed line for one hour of exposure and dotted line for two hours of exposure. ### Effect of the Withdrawal Resistance on Self-Tapping Screws  
The gluam sections were reduced accordingly to replicate one and two hours of standard fire exposure, and the resulting lengths of penetration of the STS at those times, as mentioned previously in Section 2.3.2. Failure mode due to withdrawal of the screw was observed for all vertical load tests at ambient conditions, similarly to the pull-out tests. The average withdrawal resistance for the screw connections tested in the vertical load tests are presented in Table 5.  
Figure 12 showed the load-displacement curves per connection for 4 \(\times\)\(\mathcal{O}\)8 mm screws at an angle of 45 degrees by reducing the section of gluam after one and two hours of standard fire exposure. Reducing the fastener's length of penetration into the timber, the withdrawal resistance of STS decreases, thus reducing the connection capacity.