# Fire Performance of Self-Tapping Screws in Tall Mass-Timber Buildings
## 3 Results and Discussion
### Effect of the Withdrawal Resistance on Self-Tapping Screws
For instance, the decrement was 23% between ambient conditions and one hour of exposure, and 63% between ambient conditions and two hours of exposures for \(\mathcal{O}\)8 mm STS connections, respectively. This reduction for inclined full-thread screws is consistent with the decrement of the pull-out tests. Furthermore, the same failure mode was obtained as at ambient conditions with the reduced section of gluam after one and two hours of standard fire exposure, which confirm that the residual section of the element maintained properly its structural capacity. As noted previously, there are currently no design provisions for STS in CSA O86-19 [8; 24]. It has been suggested that the withdrawal resistance equation provided in the CCMC evaluation report [20] can be used as a viable model for STS with consideration of the group effect for screwed connections in CSA O86-19 [8; 29]. The factored withdrawal resistance, \(\mathrm{P_{rw,\alpha}}\), loaded at an angle (\(\alpha\)) relative to the grain direction, can be calculated using Equation (4). \[\mathrm{P_{rw,\alpha}}=\phi\frac{0.8\cdot\delta(\mathrm{b}\cdot 0.84\cdot\mathrm{p})^{2} \cdot\mathrm{d}\cdot\mathrm{l_{ef}}\cdot 10^{-6}}{\sin^{2}\alpha+\frac{4}{3}\cdot \cos^{2}\alpha}\cdot\mathrm{K_{D}}\cdot\mathrm{K_{SF}} \tag{4}\]  
Figure 12: Load-displacement plots of \(\mathcal{O}\)8 \(\times\) 300 mm ASSY VG CSK (FT) connections at 45 degrees. \begin{table}
\begin{tabular}{c c c c c c} \hline  & & \multicolumn{4}{c}{**Load-Carrying Capacity**} \\ \cline{2-6}
**Fastener Type** & \multicolumn{2}{c}{**Ambient Conditions**} & \multicolumn{2}{c}{**One Hour of Exposure**} & \multicolumn{2}{c}{**Two Hours of Exposure**} \\ \cline{2-6}  & **Max Load (kN)** & **CoV** & **Max Load (kN)** & **CoV** & **Max Load (kN)** & **CoV** \\ \hline \(\mathcal{O}\)8 \(\times\) 300 mm FT & 56.3 & 2.8\% & 43.6 & 5.7\% & 20.8 & 0.7\% \\ \hline \end{tabular}
\end{table} 
Table 5: Average withdrawal resistance per connection for 4 \(\times\)\(\mathcal{O}\)8 mm screw at 45 degrees in accordingly presuming a charring rate of 0.7 mm/min under fire conditions. where \(\varphi\) is a resistance factor (0.9), 0.8 is an adjustment factor for standard term loading, \(\delta\) is a material adjustment factor (82 for \(\rho\geq 440\) kg/m\({}^{3}\)), b is 1 for DFir-L, SPF, SPF, WRC, and Hem-Fir, \(\rho\) is the mean oven-dry relative density (470 kg/m\({}^{3}\)), 0.84 is an adjustment factor to convert the mean oven-dry oven density to the fifth percentile value, d is the outside screw diameter (8 mm), \(\lambda_{\text{F}}\) is the length of threaded shank penetration (subtracting length of the tip) (mm), \(\alpha\) is the screw angle relative to grain direction (45 degrees), \(\text{K}_{\text{D}}\) is a load-duration factor, and \(\text{K}_{\text{SF}}\) is a service condition factor. Then, the ultimate capacity of inclined screws, N\({}^{\prime}\), will be limited by withdrawal resistance in the side member, in the main member or screw tensile fracture in the shear plane [27]. Based on MTC Solutions Design Guides [29], the withdrawal resistance cannot exceed the tensile capacity of the screw, which is the un-factored tensile capacity of 15.12 kN for \(\mathcal{O}\)8 mm ASSY, as provided by the manufacturer [20]. One can observe that reducing the timber cross-section decreases the length of the threaded shank penetration into the main timber member and by weakening its withdrawal strength.