Development of non-metallic and adhesive-free timber-timber moment connections using compressed wood connectors
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Increasing the sustainability credentials of building materials is one of the main challenges in the built environment. Over the last few decades, there has been a renewed interest in the use of timber in construction in response to the challenges of global warming and climate change. The ongoing transition towards a bio-based and circular economy has contributed to significant developments in the mass timber products market and associated novel connection typologies. These advancements have resulted in the use of traditional carpentry type connections being replaced by fast, efficient, and cost-effective modern connections using mechanical fasteners and synthetic adhesives. However, metallic fasteners have high embodied energy and synthetic adhesives have negative implications for end-of-life disposal of such products. Therefore, it is favourable to replace these materials with sustainable wood-based connectors. Today, robotic fabrication is a key enabler of innovation in mass timber construction with advanced production technologies using computer numerically controlled (CNC) tools. This has facilitated the development of cost-competitive dowel-laminated timber (DLT) products using low-cost precision machining and assembly thereby eliminating the need for synthetic adhesives. By replacing metallic connection systems with wood-based alternatives, all-wood high-performance building systems, with enhanced environmental and health benefits, are then possible. Enhancement of the properties of wood using modification techniques, such as thermo-mechanical compression, is an emerging field. Thermo-mechanically compressed wood (CW) has increased density, decreased porosity and significantly improved material strength, stiffness and dimensional stability when compared to uncompressed wood. Despite several studies focusing on the manufacturing and basic material properties of CW, only a few studies have examined the suitability of CW connectors in an all-wood timber connection for a mass timber structure. The objective of this research is to develop a novel approach to produce non-metallic and adhesive-free timber moment connections incorporating thermo-mechanically CW connectors. To achieve this objective, an extensive experimental programme was undertaken, which included material characterisation tests on CW connectors, tests on the long-term swelling and shrinkage-behaviour of CW and the development and experimental testing of timber-CW dowel moment connections, namely beam-beam and beam-column connections. The material characterisation tests demonstrated that CW offers superior mechanical properties when compared to uncompressed softwoods and hardwoods when comparing embedment strength, yield moment capacity, cross-grain shear capacity and bending strength and could be used as an ideal wood-based connector material in the form CW dowels and CW plates to connect glulam, DLT and CWDLT elements in mass timber structures. A key finding from the long-term swelling and shrinkage-behaviour of CW has shown that “springback”, which is the expansion of CW in response to external climate conditions, is a beneficial characteristic of CW in connections as it facilitates form and friction fit with the timber substrate and yields a higher pull-out strength of the dowel. The experimental characterisation of timber-CW dowel connections has enabled the evaluation of the load-carrying capacity, moment capacity, rotational stiffness, ductility ratio and failure modes of a range of semi-rigid type timber-CW connection design, which have been developed to connect both glued-laminated members and adhesive-free CWDLT members. Furthermore, the developed timber-CW connection systems were successfully implemented within portal frames manufactured using both glued-laminated and CWDLT members. The test results have shown that the lateral load-carrying capacity and moment capacity of the frames with timber-CW connections and CWDLT members are comparable to frames with timber-CW connections and glued structural members. This indicates that an all-wood connection concept would be a suitable choice in heavy timber structures with CWDLT members as well as with solid timber and glulam members. Also, the suitability of current design rules in Eurocode 5, which is the European standard governing the design of timber structures, supplemented by design rules from other research studies was assessed for designing and predicting the moment capacity and rotational stiffness of the novel timber-CW connections. It is anticipated that the current study will provide an improved understanding of the structural behaviour of CW connectors and timber-CW connections that may widen the knowledge base and help structural engineers to design timber-CW connections. It is also anticipated that the material characterisation tests performed will provide the necessary input data for numerical models to further examine the connection designs developed in this study and contribute to the optimised design of such connections with increased reliability. The findings are important to develop safe, sustainable, recyclable, and energy-efficient timber connection systems and mass timber products using CW connectors.
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