Zidi Yan

Modular, adaptable, prefabricated timber construction offers a promising avenue to embody circular economy (CE) principles within the Australian building and construction (B&C) sector by creating buildings that can be structurally and spatially flexible, thus able to adapt to occupants changing needs over an entire lifetime. Adaptable design ensures buildings can grow as families expand and downsize when children move out. Additionally, demountable building systems can be relocated either as a whole or in parts. They can also be repeatedly reused, thereby reducing construction and demolition waste while retaining value through CE closed-loop reuse. However, this concept remains theoretical and experimental in the existing B&C sector and has not yet been fully realised. The logical progression of this research project is to realise modular off-site construction by using a renewable resource such as timber, which stores carbon while in use, and extending the service life of those timber elements through adaptable modular design. The combination of Design for Adaptability, Disassembly, and Re-configuration (DfADR) principle as a design framework, panelised light timber framed offsite construction as a fabrication mode, and timber as a structural material presents a great opportunity for transitioning the current 'one-off', make-waste-dispose building practice towards a sustainable and regenerative CE framework, where buildings are conceived as 'material banks' that can be reshaped, reassembled, and relocated. Despite the distinct advantages of the P-LTF building system, there still are some challenges and research gaps. The selection of appropriate reversible connectors is a crucial precondition for achieving the primary goals of DfADR, and adaptable design. Therefore, this PhD research project aimed to develop and evaluate reversible connections for adaptable modular timber buildings that can be manufactured off-site and re-used multiple times within a CE framework. This research particularly focuses on solving the technical issues for the implementation of the concept of DfADR from a structural engineering perspective, such as development and experimental testing of reversible timber connections, and systematic design of an adaptable panelised light timber framed (P-LTF) building to implement these reversible connections.

However, given that most of the reversible timber connectors are designed in an European and North American context, made for mass timber construction, and typically fastened with proprietary screws, it is of great importance to examine the feasibility of applying these connectors in the specific P-LTF building system and assess the proprietary screws' structural and durability performance in Australian timber materials. To this end, it is necessary to understand the parameters influencing the withdrawal strength of screws, especially since the current Australian timber standard AS1720.1 is inadequate for addressing the structural capacities of modern fasteners and connectors. Thus, 662 axial screw withdrawal tests were conducted and evaluated in this PhD research, which led to a regression analysis that extended and modified the existing European empirical model to more closely and conservatively match the Australian context. These works constitute the first research objective of this thesis: to assess the structural and durability performance of modern screws in the Australian context. The screw withdrawal tests validated the impact of influencing parameters on screw withdrawal strength and the regression model adjusted the parameters to more accurately reflect the behaviour of modern screws in Australian timber. Additionally, the durability tests indicated that chemical corrosion can greatly affect screw withdrawal strength.

Furthermore, two commercial timber connectors initially designed for mass timber construction were selected and experimentally tested within the context of the Australian P-LTF system. These works constitute the second research objective of this thesis which evaluate existing reversible connections within the prefabricated P-LTF building system. The test results showed that the selected connectors performed very well even in applications different from their original design intent. However, tight-fit connections encountered issues with disassembly due to ambient moisture-induced swelling and shrinkage. Following this, recommendations were made after determining these connectors as unsuitable for the P-LTF system.

Subsequently, an innovative reversible panel-to-panel connector, was designed in collaboration with a European company, SHERPA Connection Systems GmbH, and experimentally tested. The novel University of Queensland (UQ)-SHERPA Panel Connector (USPC) aims to address the challenges previously identified and is designed to facilitate the DfADR design principle. To further address assembly and positioning difficulties, two types of screwed carpentry castellation joints for wall panel-to-floor cassette and roof cassette-to-ridge beam connections were developed and tested. Experimental tests were conducted to determine the best screw installation layouts and the inclination angle of the castellation shear blocks in order to balance ease of installation with structural performance. These works constitute the third research objective of this thesis: to develop and test novel reversible timber connections for P-LTF building system. Overall, the connectors showed excellent performance in the configuration of P-LTF building system, addressing the limitations found in previous commercial timber connector tests.

Lastly, a comprehensive P-LTF system was developed, integrating innovative new connectors, existing commercial connectors, and standardised building components. This new P-LTF system evolved from a conventional system to become adaptable with relatively minimal effort, thus enhancing its commercial and industrial viability. The development addresses challenges related to spatial adaptability and construction efficiency in line with the DfADR principle. Furthermore, it supports long-term adaptability and resilience through ease of assembly, disassembly, reassembly, and reconfiguration.