# Fire Performance of Self-Tapping Screws in Tall Mass-Timber Buildings
## 1 Introduction
The results showed that the STS increase the connection's time to failure at elevated temperatures compared to bolt-type fasteners. The latter were burning the glue-laminated timber pilot holes and locally weakening the connection. Although STS are a promising alternative for the fire-resistance of connections in mass timber construction, limited information can be found on the fire performance of screwed connections during fire exposure up to two hours. As of today, timber design standards including North American and European standards do not yet provide design provisions for STS. As such, designers would typically use product evaluation reports from the Canadian Construction Materials Center [20] or the International Code Council Evaluation Services [21] for use in Canada or in the United States of America, combined with the connection design provisions found in its respective applicable standard. The same applied in Europe where a European Technical Approval [22] is used to design an STS connection using the relevant provisions in EN1995-1-1 [23]. In Canada, wood screws with a diameter greater than 5.48 mm use the lag screw design provisions in CSA O86-19 [8], but these equations under-estimate the withdrawal strength of STS [24]. In fire conditions, Annex B of CSA O86-19 [8] provides a design methodology to calculate the structural fire-resistance of large cross-section timber elements. This method presumes that metallic fasteners located within the reduced cross-section are deemed thermally protected from fire and are thus not affected mechanically. It does not address failure modes, interactions between materials and reduced capacities under fire. On the other hand, EN 1995-1-2 [10] provides a method, called the \"Reduced Load Method\", to design timber connections exposed to a standard fire exposure, which follows the principles of the reduced properties method for the determination of cross-sectional properties for the load-bearing capacity of structural elements. For standard fire exposure, the characteristic load-carrying capacity of a connection with fasteners solicited in shear in fire condition (\(F_{V,Rk,fi}\)) should be estimated as,  
\[F_{V,Rk,fi}=\eta\cdot F_{V,Rk} \tag{1}\]\[\text{with }\eta=e^{-k\cdot t_{d,fi}} \tag{2}\]  
where \(\eta\) is a reduction factor, \(F_{V,RK}\) is the characteristic lateral load-carrying capacity of the connection with fasteners in shear at normal temperature, \(k\) is a parameter that accounts for the type of fastener (as per EN1995-1-2) and \(t_{d,fi}\) is the fire-resistance design of the unprotected connection, in minutes (as per EN 1995 1-2) [10]. This fire-resistance design of the connection loaded by the mechanical design effect considered in a fire situation should be taken as given by Equation (3). \[t_{d,fi}=-\frac{1}{k}\ln\frac{\eta_{fi}\cdot\gamma_{M,fi}}{\gamma_{M}\cdot k_{ fi}} \tag{3}\]  
where \(\eta_{fi}\) is the reduction factor for the design load in a fire situation; \(\gamma_{M}\) is the partial factor for the connection; \(\gamma_{M,fi}\) is the partial safety factor for timber in fire; \(k_{fi}\) is a strength and stiffness conversion factor (as per EN 1995 1-2) [10]. In EN 1995-1-2 [10], the reduced load method yields variable results, and the analytical results overestimate the fire resistances of connection configurations tested [13]. The method is also limited to fire resistance up to one hour, which would automatically exclude its applicability to buildings required to provide greater fire resistance such as in the case of tall mass timber buildings in many countries. As such, the main objective of this research is to evaluate the fire resistance of mass timber STS connections for fire exposure up to two hours in an attempt to develop design principles facilitating the determination of their structural fire resistance.