# Fire Performance of Self-Tapping Screws in Tall Mass-Timber Buildings
## 3 Results and Discussion
### Effect of Fasteners on the Timber Charring Rate  
The average values of the charring depth and rate for all the tests were measured and are presented in Table 3. The length of localized charring along the fastener's shank is also presented. As one would expect, the glulam charring rate was not constant throughout the fire exposure. Due to the evolution of the insulation effect given by the charcoal formed at the exposed surface of the specimens, the charring rate decreased into timber, as also reported in the literature [39; 40; 41]. The thickness of the charred layer increased with longer exposures times limiting the heat penetration, and ultimately, the temperature profile into the timber. Moreover, as the specimen chars, its surface moves away and the incident heat flux imposed at that moving surface is reducing, thereby reducing the resulting charring rate. For instance, the charring rate in the first hour observed for the exposed STS head is consistent with those in various glulam specimens exposed to a constant heat flux without fastener reported by [PERSON] et al. [40]. However, the charring rate after an exposure of two hours is lower than that after an exposure of one hour. Due to its insulative effect, the charred wood prevented the charring depth of the specimens and decreased the heat penetration inside the timber under longer time exposures. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Time Exposure**} & \multirow{2}{*}{**Fasteners**} & \multirow{2}{*}{**Exposed Side**} & \multicolumn{2}{c}{**Charring Depth**} & \multicolumn{2}{c}{**Charring Rate**} \\ \cline{3-6}  & & & **Charred Layer (mm)** & **Local Charred (mm/min)** & **Charred Layer (mm/min)** & **Local Charred (mm/min)** \\ \hline \multirow{3}{*}{One hour} & \(\mathcal{O}\)8 mm screws & Head Tip & 34 & 18 & 0.57 & 0.30 \\  & & Tip & 28 & - & 0.46 & - \\ \cline{2-6}  & \(\mathcal{O}\)10 mm screws & Head Tip & 36 & 22 & 0.60 & 0.36 \\  & & Tip & 28 & - & 0.47 & - \\ \hline \multirow{3}{*}{Two hours} & \(\mathcal{O}\)8 mm screws & Head Tip & 62 & 21 & 0.52 & 0.18 \\  & & Tip & 61 & 17 & 0.51 & 0.14 \\ \cline{1-1} \cline{2-6}  & \(\mathcal{O}\)10 mm screws & Head Tip & 61 & 28 & 0.51 & 0.23 \\ \cline{1-1}  & & Tip & 57 & 13 & 0.42 & 0.11 \\ \cline{1-1} \cline{2-6}  & \(\mathcal{O}\)15.9 mm bolts & Head & 74 & 140 & 0.62 & 1.17 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average values of charring depth and charring rate for all the tests exposed to a constant heat flux. Furthermore, on the insulation effect of the char layer, the charring rate also influenced the relative exposure of wood and steel elements. By exposing an initial 20 mm of gluam cover between the exposed surface and the tip of the fastener, a char layer is formed, and its low thermal conductivity protects the underlying timber and steel against heat [14]. For instance, the charring rate of the STS with exposed head was higher than the charring rate of the STS with exposed tip (see Table 3). With continued heating, the thermal degradation of the initial gluam cover prevents the exposure of the steel element while the unprotected fastener heads increase warming inside. The fastener contributes by increasing the charring depth due to the larger thermal capacity. A deeper char layer was formed around the fasteners shank while the three unexposed sides remained unchanged on the surface (see Figure 9). Due to the high conductivity of the steel, the heat transmitted through the fastener shank leads to higher temperatures along the cross-section area and locally increases wood temperatures. Moreover, this gradient of heat transfer through the fastener shank is controlled by the slenderness of the fastener (i.e., the ratio of their diameter to their length). The combination of the heated surface and the ability of the fastener to penetrate the timber further away from the exposed surface gives the capacity of the thermal properties of timber to prevent its propagation along the fastener [14].