Structural fire performance of CLT-concrete composite floor systems with innovative notch shear connection configurations – an experimental and analytical study

Abstract

Mass timber products, such as cross-laminated timber (CLT), are gaining momentum due to their high level of prefabrication and environmental sustainability. To further enhance their flexural performance through composite action, CLT–concrete composite floor systems can be an optimal solution to achieve flexural efficiency under service loads, while maintaining sufficient ductility at the ultimate limit state, particularly under fire exposure. Introducing an interlayer between the supporting CLT panel and the top concrete layer can enhance thermal and acoustic performance and increase strength and stiffness by extending the moment arm, particularly under fire conditions. This new study evaluates the fire endurance of six full-scale CLT-concrete composite floor slabs with four notch shear connection configurations. The first configuration used whole-wide strip notches with a 25-mm-thick insulation layer, increasing fire resistance from 60 minutes (the bare CLT panel's fire resistance) to 90 minutes. The other three configurations featured innovative individual-notch shear connections with a 50-mm-thick insulation layer, achieving up to twice the fire-resistance duration (120 minutes) and applied load capacity (10.0 kPa) compared with the bare CLT reference specimens, thereby demonstrating the superior performance of the composite system. Full-scale composite slabs (5300 mm long, 5000 mm clear span, and 900 mm wide) were constructed using 143-mm-thick, 5-ply CLT panels, topped with a 65-mm-thick concrete layer (minimum compressive strength 30 MPa), and tested at the Lakehead University Fire Testing and Research Laboratory (LUFTRL). The fire endurance and residual bending experiments, together with the analytical models verified in this research, were designed to address persistent design gaps in CLT–concrete composite floors exposed to fire. The addressed gaps include the absence of a well-defined structural model for shear connection mechanisms and ongoing uncertainty about the necessity of a ductile failure model, such as for individual-notch shear connections, for ultimate limit-state conditions under both ambient and fire exposure. Furthermore, incorporating an interlayer system into the shear connection structural model has demonstrated improvements in the overall efficiency of composite floor systems under both ambient and fire conditions. The analytical model developed in this study uses the stress-strain relationships of concrete and CLT materials to calculate composite yielding moments in high- and low-slip zones along the slab span. These moments were applied within a virtual work framework, incorporating a non-parabolic deflection curve to define the overall ductility of the composite action. The analytical model effectively identifies both ductile and non-ductile failure modes, such as the non-ductile tensile-flexural failure of timber and compression failure of the concrete slab. Additionally, it predicts the ductility of the composite floors based on yielding in the high-slip zone, resulting in a non-parabolic deflection curve. The predictions of the analytical model align with experimental results on the fire resistance of CLT–concrete composite floors, providing valuable insights into asymmetric failure modes arising from varying charring rates at the slab ends, which are essential for predicting lower-boundary conditions in fire simulations. The unique outcomes of this pioneering experimental and analytical study support innovations in Canadian wood construction by developing timber–concrete composite (TCC) slab-type floors with novel individual-notch shear-connection configurations that enhance robustness, sustainability, material performance, and fire safety in mass-timber buildings.