Structural Timber Connections with Dowel-Type Fasteners and Nut-Washer Fixings: Mechanical Characterization and Contribution to the Rope Effect

Dowel-type fasteners are one of the most used type of connections in timber joints. Its design follows the equations included in the Eurocode 5. The problem with these equations is that they do not adequately contemplate the resistive capacity increase of these joints, when using configurations which provoke the so-called rope effect. This effect appears when using threaded surface dowels instead of flat surface dowels, expansion kits or nut-washer fixings at the end of the dowel. The standards consider this increase through a constant value, which is a poor approximation, because it is clearly variable, depending on the joint displacement and because is much bigger, especially when using nut-washer fixings. It is also very important because of the rope effect trigger interesting mechanisms that avoids fragile failures without warning of the joints. For these reasons, it is essential to know how these configurations work, how they help the joint to resist the external loads and how much is the increase resistance capacity in relationship with the joint displacement. The methods used to address these issues consisted of a campaign of experimental tests using actual size specimens with flat surface dowels, threaded surface dowels and dowels with washer-nut fixings at their ends. The resistance capacity results obtained in all the cases has been compared with the values that will come using the equations in the standards. After the tests the specimens were cut to analyze the timber crushings, their widths, the positions and level of plasticizations suffer in the steel dowels and in the washer-nut fixings and the angle formed in the dowel plastic hinges. With all this information the failure mode suffered by the joints has been identified and compared with the ones that the standards predict. The results for the size materials and types of joints studied shows that the crush width average values go from 20 mm with flat surface dowels, to 24 mm in threaded to 32 mm in threaded with washer-nut fixings. The rope effect force/displacement goes from 100 N/m in threaded surface dowels to 500 N/m in threaded with washer-nut fixings. Finally, the load capacities are on average 290% higher those indicated in the standard. The main conclusion is that the rope effect force should be considered in the standards in more detail as a function of multiple variables, especially the displacement of the joint.

Finding the Key: Designing Timber Connections for CLT Panels

<p>In a climate where standard methods of construction are being challenged, developments in engineered timbers are allowing mass timber construction to be explored as a sustainable alternative to traditional building methods. Cross- laminated timber (CLT) is at the forefront of this evolution and, with the advancement in computational design and digital fabrication tools, there lies an opportunity to redefine standard construction. This project explores how digital modelling and advance digital fabrication can be combined to generate a connection system for CLT panels. The advantages of CLT and mass timber construction are numerous and range from environmental and aesthetic benefits to site safety and cost reduction benefits. There are, however, issues that remain surrounding the connections between CLT panels. Steurer (2006, p.136) stated that, “Progress in engineered timber construction is directly related to developments in connector technology.” This thesis creates connections inspired by traditional Japanese joinery that have been adapted to be used for the panel construction of CLT structures. Using CLT offcuts as a primary connection material, the system not only reduces waste but also mitigates thermal bridging and lowers the number of connection points whilst increasing the ease of building and fabrication. The connections are first considered at a detail scale. They use the literature review and case studies as a base for design before being tested using digitally fabricated prototypes. These prototypes are evaluated against a framework created in line with the aforementioned criteria. Within this framework, the connections are analysed against existing connection systems as well as previous designs to establish a successful system. The connections are then evaluated within the context of a building scale and considers large-scale fabrication and on- site assembly whilst continuing to focus on the reduction of waste. This research found that the simplicity of the connections is key to a successful system as this allows for faster and cheaper fabrication and installation. However, there is still further research needed surrounding large-scale fabrication and the structural capacity of timber connection systems.</p>

Finding the Key: Designing Timber Connections for CLT Panels

<p>In a climate where standard methods of construction are being challenged, developments in engineered timbers are allowing mass timber construction to be explored as a sustainable alternative to traditional building methods. Cross- laminated timber (CLT) is at the forefront of this evolution and, with the advancement in computational design and digital fabrication tools, there lies an opportunity to redefine standard construction. This project explores how digital modelling and advance digital fabrication can be combined to generate a connection system for CLT panels. The advantages of CLT and mass timber construction are numerous and range from environmental and aesthetic benefits to site safety and cost reduction benefits. There are, however, issues that remain surrounding the connections between CLT panels. Steurer (2006, p.136) stated that, “Progress in engineered timber construction is directly related to developments in connector technology.” This thesis creates connections inspired by traditional Japanese joinery that have been adapted to be used for the panel construction of CLT structures. Using CLT offcuts as a primary connection material, the system not only reduces waste but also mitigates thermal bridging and lowers the number of connection points whilst increasing the ease of building and fabrication. The connections are first considered at a detail scale. They use the literature review and case studies as a base for design before being tested using digitally fabricated prototypes. These prototypes are evaluated against a framework created in line with the aforementioned criteria. Within this framework, the connections are analysed against existing connection systems as well as previous designs to establish a successful system. The connections are then evaluated within the context of a building scale and considers large-scale fabrication and on- site assembly whilst continuing to focus on the reduction of waste. This research found that the simplicity of the connections is key to a successful system as this allows for faster and cheaper fabrication and installation. However, there is still further research needed surrounding large-scale fabrication and the structural capacity of timber connection systems.</p>

Cutting Edge Fabrication: Investigating Timber Sheet Materials with Robotic Fabrication

<p>Timber sheet materials have been used in the same manner for decades despite having a vital role in the construction industry. This often results in indistinguishable surfaces with no identity. The research developed in this thesis is the creation of a workflow to create a self-supporting structure from sheet materials using robotic fabrication and computational tools. Timber sheet materials is the key focus for this research, as timber is a material that can be altered in a variety of ways. Japanese timber connections were a strong influence for this research, due to its prolonged life span and sustainable advantages. In the past, timber fabrication techniques have been limited due to design limitations. Current technology, specifically parametric software combined with the robotic arm was explored to find how it can create timber connections to connect sheet materials at different angles. This method was utilised to repurpose the concept of sheet materials towards a complex structure, which adopted the idea of mass customisation over mass production. Prototypes of timber connections were created to develop an outcome that will structurally support itself. The outcome of each prototype was evaluated and compared with one another to establish which connection would be most suited to bring forward to the self-supporting structure. Computational simulations were used to explore individual structures which created panels that were automatically flattened in the software. This allowed the digital file to be transferred to the robotic arm to be milled. Using the robotic arm was an advantage, as it can rotate around six-axis giving multiple degrees of design freedom which broadened the range of construction techniques that can be used with sheet materials. There is a high chance of human error with manual labour, therefore precision is a positive attribute of the robotic arm. The precision helped minimise waste compared to manual labour. This thesis presented an opportunity for the design/construction industry to adopt a new workflow to bring leading-edge technology to focus on sustainable materials and to steer away from the repetitions evident in buildings today.</p>

Cutting Edge Fabrication: Investigating Timber Sheet Materials with Robotic Fabrication

<p>Timber sheet materials have been used in the same manner for decades despite having a vital role in the construction industry. This often results in indistinguishable surfaces with no identity. The research developed in this thesis is the creation of a workflow to create a self-supporting structure from sheet materials using robotic fabrication and computational tools. Timber sheet materials is the key focus for this research, as timber is a material that can be altered in a variety of ways. Japanese timber connections were a strong influence for this research, due to its prolonged life span and sustainable advantages. In the past, timber fabrication techniques have been limited due to design limitations. Current technology, specifically parametric software combined with the robotic arm was explored to find how it can create timber connections to connect sheet materials at different angles. This method was utilised to repurpose the concept of sheet materials towards a complex structure, which adopted the idea of mass customisation over mass production. Prototypes of timber connections were created to develop an outcome that will structurally support itself. The outcome of each prototype was evaluated and compared with one another to establish which connection would be most suited to bring forward to the self-supporting structure. Computational simulations were used to explore individual structures which created panels that were automatically flattened in the software. This allowed the digital file to be transferred to the robotic arm to be milled. Using the robotic arm was an advantage, as it can rotate around six-axis giving multiple degrees of design freedom which broadened the range of construction techniques that can be used with sheet materials. There is a high chance of human error with manual labour, therefore precision is a positive attribute of the robotic arm. The precision helped minimise waste compared to manual labour. This thesis presented an opportunity for the design/construction industry to adopt a new workflow to bring leading-edge technology to focus on sustainable materials and to steer away from the repetitions evident in buildings today.</p>

Experimental investigation on the effect of accelerated ageing conditions on the pull-out capacity of compressed wood and hardwood dowel type fasteners

The widespread use of adhesives in timber construction has negative implications for the end-of-life disposal or re-use of the structural timber components. To promote the circular bioeconomy, it is preferable to substitute adhesives with more sustainable alternatives such as wood-based connectors. Today, robotic fabrication technologies facilitate the development of dowel-laminated timber (DLT) products whereby hardwood dowels are used to connect timber laminates as a substitute to adhesives. In recent years, thermo-mechanical densification of wood has resulted in significant improvements in the mechanical performance of the wood. This modified product often termed compressed wood (CW) has a shape-recovery effect which may be beneficial for the development of DLT products and timber-timber connections with improved friction fit with time. To test the hypothesis, accelerated ageing tests were carried out on CW-timber and hardwood-timber dowel type connections subjected to variable climate conditions. Finally, the capacity of the connections or friction fit was assessed using pull-out tests. Results show that the shape-recovery effect leads to the continuous expansion of the CW dowels and facilitates a friction fit with the timber substrate yielding higher pull-out loads when compared to hardwood dowels.