Long-term Performance of Timber-concrete Composite Systems with Self-tapping Screws Connections
Creators: Esmaeildoust, Shadi
Publisher: name: University of Alberta Library
Year: 2024

<p>Timber-concrete composite (TCC) systems combine timber and concrete to leverage their respective advantages. This composite structure enhances strength and stiffness, making TCC systems suitable for a range of applications. Despite their advantages, the design of TCC systems is often governed by Serviceability Limit States due to the rheological characteristics of the materials involved. Long-term deformations in TCC systems are influenced by sustained loading and environmental changes, as both timber and concrete are susceptible to creep and shrinkage thus effective connection between these materials is crucial for ensuring the systems’ optimal short- and long-term performance. Recent research has shown that self-tapping screws offer superior load-bearing capacity and reduced slip compared to traditional connectors, especially when inserted at an angle to the shear plane. Despite this, there remains a gap in understanding the long-term performance of these connections, and current design standards do not fully address this aspect, highlighting the need for further investigation into long-term deformations of TCC. This study was divided into experimental and analytical phases. Experimental work involved the fabrication of TCC connection specimens with self-tapping screws inserted at various angles to the shear plane and with different nominal diameters. Preliminary tests assessed the shear strength, stiffness, and failure modes of different configurations and the results were compared with analytical models. Next, the specimens were subjected to a sustained load for six months to determine the creep coefficient, with the creep curves extrapolated to predict the 50-year creep coefficient. Destructive tests followed to evaluate the impact of long-term loading on specimen strength and stiffness. Short-term tests showed that inclined screws provided greater strength and stiffness but exhibited reduced ductility compared to perpendicular screws, in which the failure mode was the formation of two plastic hinges. Long-term creep tests, conducted under uncontrolled indoor environmental conditions, revealed that minor fluctuations in temperature and humidity did not influence the connection’s creep coefficient. Results also revealed that 90-degree screws showed more instantaneous slip and a faster initial increase in creep coefficient, due to their dependence on bearing resistance, while inclined screws benefited from both bearing and withdrawal resistances. Also, the generalized Kelvin model effectively captured the creep behaviour. While the six-month sustained loading did not affect the connection strength, it did reduce connection stiffness, highlighting the need for better predictive models that account for long-term deflections. An analytical model was used which was capable of modeling the final midspan deflection of TCC systems considering the sustained load and concrete shrinkage. This model showed that neglecting the concrete shrinkage can lead to underestimation of the final deflection. The parametric study based on this analytical model showed that geometric parameters, particularly span, had greater impact on long-term deflection than material properties. The type of connection also influences the total deflection, with inclined screws reducing deflection due to their higher stiffness and lower creep coefficient. However, concrete shrinkage exerts a greater impact on stiffer connections, necessitating a careful balance between these parameters. </p>