Structural fire performance of wood-steel-wood bolted connections with and without perpendicular-to-wood grain reinforcement

Abstract

Purpose In fire condition, the time to failure of a timber connection is mainly reliant on the wood charring rate, the strength of the residual wood section, and the limiting temperature of the steel connectors involved in the connection. The purpose of this study is to experimentally investigate the effects of loaded bolt end distance, number of bolt rows, and the existence of perpendicular-to-wood grain reinforcement on the structural fire behavior of semi-rigid glued-laminated timber (glulam) beam-to-column connections that used steel bolts and concealed steel plate connectors. Design/methodology/approach In total, 16 beam-to-column connections, which were fabricated in wood-steel-wood bolted connection configurations, in eight large-scale sub-frame test assemblies were exposed to elevated temperatures that followed CAN/ULC-S101 standard time-temperature curve, while being subjected to monotonic loading. The beam-to-column connections of four of the eight test assemblies were reinforced perpendicular to the wood grain using self-tapping screws (STS). Fire tests were terminated upon achieving the failure criterion, which predominantly was dependent on the connection’s maximum allowed rotation. Findings Experimental results revealed that increasing the number of bolt rows from two to three, each of two bolts, increased the connection’s time to failure by a greater time increment than that achieved by increasing the bolt end distance from four- to five-times the bolt diameter. Also, the use of STS reinforcement increased the connection’s time to failure by greater time increments than those achieved by increasing the number of bolt rows or the bolt end distance. Originality/value The invaluable experimental data obtained from this study can be effectively used to provide insight and better understanding on how mass-timber glulam bolted connections can behave in fire condition. This can also help in further improving the existing design guidelines for mass-timber structures. Currently, beam-to-column wood connections are designed mainly as axially loaded connections with no guidelines available for determining the fire resistance of timber connections exerting any degree of moment-resisting capability.

Fire performance and regulatory considerations with modern methods of construction

“…In fire conditions, the time to failure of a timber connection is mainly reliant on the wood charring rate, the strength of the residual wood section, and the limiting temperature of the steel connectors involved in the connection (Petrycki & Salem 2019). In some fire-resistance tests on bolted wood-steel-wood (WSW) connections and bolted steel-wood-steel (SWS) connections, the fire-resistance ratings of all the tested WSW and SWS connections without protection were less than the target rating for code compliance, with the fire performance of the tested SWS connections being about half that of the WSW connection (Peng et al 2012).…”

“…RISC Authority 2022). This is particularly the case with respect to connection performance(Peng et al 2012;Petrycki & Salem 2019), cavities where pyrolysis leading to smoldering combustion may occur if sufficient thermal energy penetrates into the space(Hadden et al 2017), and surfaces where…”

Fire Safe Use of Wood in Buildings

“…Research on screws exposed to fire has shown that charring around screw heads can penetrate 20-30 mm down the shaft and the screws have been shown to not detrimentally influence the fire resistance, provided they are well spaced apart (Hofmann et al, 2016;Petrycki and Salem, 2019;Létourneau-Gagnon et al, 2021). Of importance is that pull-out resistance can reduce where the whole length of the screw is heated.…”

Fire performance of hybrid mass timber beam-end connections with perpendicular-to-wood grain reinforcement

“…Thus, the behaviour of a timber connection subjected to fire is complex and not easily predictable. Over the past decade, a number of experimental studies were conducted to investigate the fire performance of timber connections (Owusu, 2019;Palma et al, 2013;Peng et al, 2012;Petrycki and Salem, 2019). However, to fully understand and assess the fire performance of such connections, further experimental studies need to be conducted.…”