# Fire Performance of Self-Tapping Screws in Tall Mass-Timber Buildings
## 3 Results and Discussion  
### Effect of Temperature Distribution along the Fasteners  
Exposing the tip or the head of the STS did not have much influence on the temperature distribution along the fastener shank. When the tips were exposed, thermocouples at 120 mm of the STS showed the same temperatures of 68 \({}^{\circ}\)C as those when the heads were exposed after two hours, as illustrated in Figure 7. The low temperatures are most likely explained by the thermal properties of the gulam's slow heat penetration along the screw, regardless of the exposed side. Adding a timber cover at the exposed tip had a positive impact on the thermal distribution along the screw shank. For instance, the temperature difference (\(\Delta\)T) at mid-length showed a lower temperature when the exposed side was initially protected by 20 mm of gulam, even after two hours (see Figure 7). This effect was also noted from the thermocouples at the exposed tips until about 90 min when the temperatures of 750 \({}^{\circ}\)C were similar as the exposed head. Covering the screw with a timber cover slowed the thermal penetration under the exposed surface and maintained lower temperatures along the screw. Figure 7 also shows that the threaded length of the screws did not have a significant influence on the temperature distribution. For similar temperatures of 750 \({}^{\circ}\)C at the head of the fasteners, the temperatures of 218 \({}^{\circ}\)C at the mid-length and 72 \({}^{\circ}\)C at the tip showed the same measurement for the partial-thread screws and the full-thread screws. Figure 6: Major parameters used for FEM modeling: (**a**) Temperature–time curve applied at the surface of the specimen; (**b**) Meshing. In Figure 8, higher temperatures were reached through the fastener shank with increasing fire duration, regardless of fastener types. With continued heating, the heat flux through the fastener shank depends on the fire duration conducted by a heating rate ranging between 10 and 50 \({}^{\circ}\)C/min for the unprotected steel member [37]. Longer exposure causes higher temperatures through the screw and rising the thermal capacity of fastener into timber, as obtained. The shank diameter of the fastener was also influencing the temperature distribution along the screws as well. The larger heated area of the fastener increases the amount of heat rate per unit area through the fastener shank, resulting in an increasing heat flux into steel element. For instance, the \(\mathcal{O}\)10 mm screws showed higher temperatures along the shank than those with \(\mathcal{O}\)8 mm screws. Furthermore, and as expected, the bolt-type fasteners reached much higher temperatures during the tests due to the larger thermal capacity of the bolt diameter. For similar temperatures at the fastener heads, the temperature distribution along the bolt shank was significantly greater than that of the smaller-diameter STS (see Figure 8). The average mid-length bolt temperature of 165 \({}^{\circ}\)C was reached after 30 min of radiant heat exposure while the same shank temperature was reached after 90 min for the \(\mathcal{O}\)10 mm STS. After two hours of exposure, the temperatures at the tip of the shank remained lower than 73 \({}^{\circ}\)C for the STS, whereas it was greater than 165 \({}^{\circ}\)C for the bolts. Using smaller and more slender screws thus limited the temperature distribution under 100 \({}^{\circ}\)C after two hours compared to bolts, wherein temperatures reached much higher levels due to the larger thermal capacity of the steel shank. Figure 8: Distribution of temperatures on unprotected fastener heads under a constant heat flux. Figure 7: Time–temperature curves of \(\mathcal{O}\)10 \(\times\) 160 mm screws with the exposed heads and the exposed tips. [PERSON] et al. [18] and [PERSON] [38] have also evaluated the temperature distribution for STS and bolts under one hour and 30 min of standard fire exposure, respectively.