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.

The application of vertical forests in the building industry is a popular development with promising advantages regarding sustainability. However, placing trees along a façade gives additional loads which often results in extra material use. This is undesired concerning building design, costs, and the environment. To mitigate these adverse effects, trees could be used as load-bearing structural elements. To increase the stability of such elements, it might be advantageous to connect the trees to each other. Such connections can naturally be created with inosculations: self-growing connections in which the bark and inner tissue of two trees merge. Applying living trees as structural elements requires a deeper understanding of both the botanical and structural behaviour of trees. Likewise, more must be known about the botanical and structural behaviour of self-growing connections. This research expands the knowledge on the topic of using living trees as structural elements, both as single tree elements and self-growing interconnected tree elements. This aim is reached by proposing a structural model of (interconnected) trees, which is verified by comparing the outcome of the model with the outcome of winching tests. Furthermore, a design is made in which living trees are used as structural elements in a vertical forest case study. This lays the groundwork on how to approach a living tree design, both from a structural and botanical point of view. The structural model is verified by carrying out winching tests on the Living Tree Pavilion, including one single tree, one pair of cross-connected trees and one pair of parallel-connected trees. Additionally, winching tests are performed on a tree in an airpot, in which boundaries constrain the root system. During the winching tests, a force is applied to the tree system, and the elongation is measured on several locations of the tree, providing insight into the strain distribution. Additionally, the displacement of the trees is measured at the height of the force application. Based on geometry measurements, the trees are modelled as solids in a finite element software. From the models of the interconnected trees, a compound solid is created which behaves like a single solid. The winching load is applied to the models to allow for a comparison between the results of the winching tests and the models. The geometry measurements show indications that leaning trees create an oval cross-section, which is influenced by the presence of inosculations. The winching tests show that an unconstrained root system is stiffer than a root system constrained by an airpot. Furthermore, the tests show that interconnected trees do not have favourable stiffness qualities compared to single trees. A comparison between the model and test results indicates that the finite element model is a plausible representation of reality for the single tree, the parallel-connected trees, and the out-of-plane results of the cross-connected trees. The finite element model fitted poorer with the in-plane winching test results of the cross-connected trees. More research is needed to determine whether diverging tree characteristics, in the direction that is rarely subjected to loads, could explain the discrepancy in measured and modelled behaviour. Two designs are created in which living trees of the Wonderwoods vertical forest carry the loads of a plant container. There are three reasons why cross-connected trees are not favourable over a design with single trees. First, the single trees can bear the plant containers at a younger age. Second, the risk of trees not creating suitable inosculations is high. Third, as interconnected trees share one container, the competition for space can become fierce. This research concludes that a system of interconnected trees as structural elements is not favourable over a system of single trees. This is mainly because no clear advantages in terms of strength and stability could be found in both the winching tests and the design for Wonderwoods.