# Fire Performance of Self-Tapping Screws in Tall Mass-Timber Buildings
## 1 Introduction  
Due to the urbanization and the densification of cities [1], high-rise buildings are being planned and constructed in many countries. Increasing the number of floors in buildings imposes additional performance requirements to the gravity and lateral load-resisting systems. Hybrid steel-wood connections in mass timber construction provide great strength and excellent ductility [2]. The lightness of mass timber elements with the ductility of steel fasteners allows desirable connections by reducing the weight of the structural frame and by providing the ability of the structure to deform without collapse, as required in the National Building Code of Canada (NBCC) [3]. However, using combustible materials as part of the primary structural frame bring concerns for a fire-safe design. Building elements are required to provide sufficient fire-resistance to prevent collapse and safe means of evacuation to the occupants. Typically, full-size standard fire-resistance tests are required in most countries to demonstrate code compliance [4]. The most widely used time-temperature curves in standard fire tests are ASTM E119 [5], CAN/ULC S101 [6], and ISO 834 [7]. Based on these test specifications [5, 6, 7], the fire resistance of building elements can be achieved by supporting the design load for the entire duration required of the fire test. However, these standard tests apply to single elements such as floors/ceilings, walls, beams, and columns. They do not address connections. A common practice is that connections between elements are deemed to be designed so that they provide at least the same fire-resistance as the elements they support as specified in Annex B of the Canadianstandard for wood engineering design (CSA O86-19) [8]. With one hour or higher of fire-resistance required in tall mass timber buildings in many countries [3; 9; 10; 11], it is essential to determine the thermomechanical behavior of timber connections for long fire exposure. Several researches were performed in the past decades in an attempt to increase the knowledge on the fire behavior of timber connections [12; 13; 14; 15]. Although a large amount of information is publicly available in the literature, the results are often limited to short fire exposure (under one hour) and to traditional fasteners such as bolts and dowels. Under fire conditions, the larger heated area of these fasteners (i.e., bolts and dowels) has a negative impact on the fire resistance of the connections due to the heat being conducted into the timber, resulting in a localized reduction of the timber strength and stiffness [12; 13]. Connections with slot-in steel plates can achieve a high fire-resistance because the steel plates are thermally protected by the timber side members [13]. The rate of heat conduction within the connection region is dependent on the relative exposure of wood and steel elements. By exposing the thickness of the side members, a char layer is formed, and its low thermal conductivity protects the interior of the timber and steel plates against heat [14]. The lower temperature profile behind the char layer also prevents the reduction of the strength of timber as it heats up [12]. With new innovative fasteners such as self-tapping screws (STS), the design principles of timber connections have greatly changed [16]. With their excellent withdrawal resistance, STS are often used for reinforcing the weak points of timber connections and for transferring axial loads by the higher strength and stiffness of inclined STS [17]. Moreover, under fire exposure, [PERSON] et al. [18] tested the use of STS in a beam-column connection and the results show that the use of smaller, longer, and more slender STS is favorable to limit the charred depth and the distribution of the temperature along the fasteners. Additionally, [PERSON] and [PERSON] [19] tested bolted connections with STS used as perpendicular-to-wood grain reinforcement.