Utilizing crosswise self-growing connection as load-bearing element in building design

Abstract

Within the context of utilizing alternative materials to promote sustainable construction in the building industry, Living Architecture is a concept that uses living organisms as the building materials. As a rather unconventional approach, it possesses benefits such as low cost, not requiring considerable workforce or industrial material, carbon-free, and its ability to return to nature when no longer in use. As an essential part of Living Architectures, scientists and engineers have recognized the importance of fusion between trees. For instance, in projects such as the Baubotanik Tower and the Living Tree Pavilion, a design premise is that the structure becomes ready when the fusion between trees could provide sufficient strength to support the structure. However, little research has been conducted in terms of the fusion processes and the mechanical behaviours of tree connections, which is an essential step prior to designing Living Architecture. With crosswise tree connection as the main focus of this thesis, the following research question is formulated: What are the mechanical behaviours of a self-growing crosswise tree connection when utilized as load-bearing elements in a building structure, and how can such connections be modelled during the preliminary design phase of a building structure? To help answering the research question, literature studies are conducted. In chapter 1, from a botanical perspective, it is concluded that, in the event of two stems contacting with one another, the two stems would gradually fuse together. Such an event is triggered by the abrasion on the bark due to the rubbing between the stems; later, due to the secondary growth, fibers from both stems deviate and join together to form common growth rings, such event can also be verified by the micro-CT scans that have been conducted on the crosswise tree connections. Additionally, due to the nature of the connection, a certain eccentricity from the stems' piths exists, and it is important to acknowledge that the length of such eccentricity stays constant from the start of the fusion process. To investigate the influence of tree growth on the mechanical properties of trees and tree connections, which is needed for design purposes, chapter 2 investigates the growth model and the tapering geometry of living trees. With the help of the Urban tree growth model published by the United States Department of Agriculture and a series of simple tapering equations, the growth parameters of a growing tree can be computed. Part-II of the thesis explores deeper in terms of the mechanical behaviours of such crosswise connections. A simple analysis of two trees connected in a crosswise manner is first conducted in chapter 4. It is concluded that, under loading, the connection can be subjected to tensile stress perpendicular to the grain and rolling shear stress. As two of the weakest strength properties of wood, the crosswise connections should be treated with care by future designers. As a part of an entire building structure, it is essential to determine the rotational stiffness and strength of a structural connection; therefore, in chapter 5, an experimental design is proposed for such purpose. Due to the irregular geometry of the crosswise connection, the experiment makes use of digital image correlation so that the rotational stiffness of the connection under loading can be obtained. Since conducting the actual experiment falls out of this thesis scope, a finite element modelling analysis with similar boundary conditions is conducted in chapter 6. With the results obtained from FEM, it is found that, due to the nature of the connection, complex torsional behaviours occur to the connection. Additionally, with the connection under loading, it is found that non-uniform stress distribution and stress concentration occurs along the interface between two stems. Comparing with the strength properties of wood, it is discovered that the first incident that causes failure is tension perpendicular to the grain, which is the weakest strength property of wood. To conduct preliminary structural design and verification, wireframe modelling approach is often utilized. In chapter 7, it is concluded that it is most suitable to model the crosswise connection with a separate beam element that connects the two stems, for its ability to capture the complex torsional behaviours described in chapter 6.
Lastly, after investigating the local behaviours of crosswise connection, Part-III
investigates the feasibility of conducting a preliminary structural design and verification with such connections. With the case study analysis conducted in chapter 8, it is concluded that, by appropriately capturing the load effects on growing trees and tree connections, designers are able to predict when the structure would reach sufficient strength to be in service. This thesis positions itself as a part of a broader spectrum that examines the feasibility of utilizing tree connections as the load-bearing elements in structures, which can be seen as a step towards sustainable construction.