# Fire Performance of Self-Tapping Screws in Tall Mass-Timber Buildings
## 4 Design Proposal
### Determination of the Withdrawal Resistance of Self-Tapping Screws  
To develop a new calculation design for a better prediction of the fire-resistance for STS connections, it is essential to determine the heat-affected layer that starts to reduce the mechanical properties of timber. When timber is exposed to fire, all free moisture will start to evaporate as the timber temperature rises above 100 \({}^{\circ}\)C [50]. The interaction with  
Figure 17: Measured temperatures of FEM and temperature profiles calculated according to equations for unprotected fastener head exposed under standard fire curve up to two hours of fire exposure. moisture content is a key parameter for wood properties and influences the embedment and withdrawal strengths of fasteners in timber at elevated temperatures. [PERSON] [51] developed a bilinear relationship for the temperature-dependent reduction of the strengths of timber at 100 \({}^{\circ}\)C. This relationship has since been implemented in EN 1995-1-2 [10] for advanced calculations. For instance, the compressive strength of timber at 100 \({}^{\circ}\)C is reduced to 25% of its ambient properties, and then linearly to 0 % at 300 \({}^{\circ}\)C, which represents the char layer. While the char layer may protect well at high temperatures at the surface (as previously discussed), the high thermal conductivity of steel used to connect timber elements can heat up quickly and degrade the strength and stiffness of the wood within. The wood fibers surrounding the fastener need to properly retain its mechanicals properties, given the need of the STS to transfer the load along the length of penetration [12]. In an attempt to properly maintain the temperature-dependent reduction of the compressive strength of timber, [PERSON] [12] recommended that the wood temperature at the interface between the fastener and the timber cover be kept to a maximum of 100 \({}^{\circ}\)C, so that the underlying timber maintains sufficient residual strength for the connection. Indeed, this temperature isotherm depends on the heat flux through the fastener shank and the fire duration, which ultimately affects the heat-affected layer. With the proposed calculation model with Equations (9) and (10), the temperature distribution along the STS can be predicted under different exposures. The temperature threshold suggested by [PERSON] [12] (T \(\leq\) 100 \({}^{\circ}\)C) can be measured at any depth and removed from the fastener's original length of penetration. The residual length of penetration could also be defined as the initial length of treaded shank penetration into the timber element reduced by the zero-strength threaded length, assumed to have no strength and no stiffness. In an attempt to verify the proposed approach under a long fire exposure, the residual length of penetration can be estimated from the small-scale fire tests and evaluated from the load capacity obtained in this paper. Based on the average charring rate in Table 3 for the STS with exposed heads, Equation (9) is first used to predict the position of the 100 \({}^{\circ}\)C isotherm in the timber and is estimated as 75 mm and 112 mm for one and two hours. Beyond that 100 \({}^{\circ}\)C threshold, the mechanical proprieties of the timber are not retained properly. Therefore, the residual length of penetration of the STS corresponds to the initial length of 160 mm subtracted by 75 mm and 112 mm for one and two hours. To evaluate the fire performance of STS as a function of fire exposure time, the withdrawal strength per unit length can be back-calculated with the load-capacity obtained in Table 4 divided by the estimated residual length of penetration. Figure 18 shows the average value for each screw diameter under different fire exposures as well as some values of STS found in the literature at ambient conditions [24; 43]. Figure 18: Withdrawal strength of Ø8 and Ø10 STS under ambient and fire conditions STS with head exposed to fire [24; 43].