3D Finite Element Model for Shear Stiffness of Wood-Wood Connections for Engineered Timber Panels

With the development of automated design and fabrication tools, researchers have shown a growing interest for traditional timber joining techniques. Innovative wood-wood connections, referred to as integral mechanical attachments, have been developed and successfully applied to full-scale building structures. However, it has been shown that these connections highly influence the mechanical behavior of structures. The mechanical behavior of the connections themselves therefore needs to be investigated. Numerical modelling is largely used for complex structural analysis problems when analytical solutions are either cumbersome or non-existent. Furthermore, it has the potential to replace expensive and time-consuming experimental tests, for which a limited number of parameter combinations can be tested. For the numerical modelling of timber connections in particular, various studies have been carried out because of the difficulty to achieve analytical models. However, numerical modelling of timber remains arduous due to the anisotropy and inhomogeneity of timber, with largely different mechanical properties parallel and perpendicular-tograin. In this paper, a 3D finite element model predicting the shear stiffness of wood-wood connections is presented. It is based on models developed by Sandhaas for timber joints [1] and Roche et al. for the study of the rotational stiffness of multiple tab-and-slot joints [2]. In this paper, instead of modelling each layer of engineered timber panels with its orientation (longitudinal layers at 0° and crosswise layers at 90°), a single material with the thickness of the timber panel was used, allowing to extend the model for a larger range of engineered timber panels. The model was compared to experimental tests performed on a shear testing setup. Promising results were obtained when comparing experimental tests to the numerical model. However, the model showed limitations for some configurations of joints.

A Structural Design Methodology for Freeform Timber Plate Structures Using Wood-Wood Connections

Anh Chi Nguyen

Over the past decades, timber has gained popularity as a sustainable building material because of the rising environmental awareness. Furthermore, the resurgence of timber has also been encouraged by the advent of the digital age during the 21st century. The development of computer-aided design programming and digital fabrication tools has stimulated significant advances in both architecture and engineering. Within this context, researchers have shown a growing interest in wood-wood connections, inspired by traditional woodworking joints. Geometrically complex timber structures assembled with joints integrated in their plates have been developed using algorithmic geometry processing. However, although their design and fabrication have been automated, research focused on automated numerical tools for their structural analysis has been very limited. This thesis is providing a design methodology for the structural analysis of timber plate structures composed of a large number of discrete planar elements and wood-wood connections. A finite element model, in which the semi-rigid behaviour of the connections is implemented using springs, is proposed. It is applied to a specific case study, namely the Annen Plus SA head office project in Manternach, Luxembourg, which consists of a series of double-layered double-curved timber plate shells. A design framework is introduced to automate the generation of the model and integrate structural analysis into the existing design and fabrication workflow. The numerical model was built for both small- and large-scale structures through custom scripts and subsequently assessed through experimental investigations: first, laboratory tests were performed on small assemblies with simplified geometry; secondly, a displacement study was carried out onsite on a 24 m span structure. Results obtained with the semi-rigid spring model were found to be in good agreement with experimental tests. The proposed model was therefore validated for the serviceability limit state. Furthermore, the semi-rigidity of the connections in translation as well as in rotation was shown to highly influence the model and is therefore crucial for the accuracy of the model. Based on experimental tests observations, an alternative structural system was proposed and compared to the initial one through numerical investigations within the proposed design framework. A significant influence on the structureâs performance was found, demonstrating the possibilities for structural optimisation. Finally, a three-dimensional finite element model for wood-wood connections was investigated. It aimed to predict their semi-rigid behaviour, generally characterised through experimental tests, necessary for their implementation in global models. The material model was evaluated based on shear load tests performed on different engineered wood products. Stiffness and load-carrying capacity of the connections were approximated with numerical simulations. However, experimental tests remain necessary to precisely predict the behaviour of the joints. This thesis highlights the importance of adopting an integrated design strategy encompassing engineering and fabrication aspects for geometrically complex timber structures, as well as establishing a link between local behaviour of the connections and global behaviour of the structure. The gained knowledge can facilitate the design and realisation of large-scale freeform timber structures with wood-wood connections.

EPFL2020

Displacement Study of a Large-Scale Freeform Timber Plate Structure Using a Total Station and a Terrestrial Laser Scanner

Anh Chi Nguyen, Yves Weinand

Recent advances in timber construction have led to the realization of complex timber plate structures assembled with wood-wood connections. Although advanced numerical modelling tools have been developed to perform their structural analysis, limited experimental tests have been carried out on large-scale structures. However, experimental investigations remain necessary to better understand their mechanical behaviour and assess the numerical models developed. In this paper, static loading tests performed on timber plate shells of about 25 m span are reported. Displacements were measured at 16 target positions on the structure using a total station and on its entire bottom surface using a terrestrial laser scanner. Both methods were compared to each other and to a finite element model in which the semi-rigidity of the connections was represented by springs. Total station measurements provided more consistent results than point clouds, which nonetheless allowed the visualization of displacement fields. Results predicted by the model were found to be in good agreement with the measurements compared to a rigid model. The semi-rigid behaviour of the connections was therefore proven to be crucial to precisely predict the behaviour of the structure. Furthermore, large variations were observed between as-built and designed geometries due to the accumulation of fabrication and construction tolerances.

2020

Upscaling of Friction Welding of Wood for structural Applications

Benjamin Hahn

The present work deals with the large-scale application of the friction welding bonding technology on timber. The investigations that are described in the following were carried out on a prototype of a welding machine that was previously developed for the purpose of this research. By means of this machine timber elements can be welded in a scale that is comparable to structural applications. Detailed experimental experience with the welding of large-sized specimens was not available at the beginning of this work. Preliminary results of initial tests on welded spruce boards indicate a correlation between the size of the sample and the occurrence of larger areas within the interface where no or only an incomplete welding result is attained. Therefore, these scaling effects were investigated in more detail in a first step. The results show that this phenomenon is associated with an internal gas pressure, which is generated at the interface during the evaporation of moisture content and other gaseous by-products. The latter result from the thermal decomposition of wooden cell wall molecules during the friction phase. It can be shown that the scaling effects can be restricted through a reduction of the moisture content or by the arrangement of gas evacuation channels at the interface, which leads to a decrease of the internal gas pressure. Subsequently the potential of the technology with regard to structural applications was estimated and evaluated by means of mechanical studies. First, investigations on the load bearing behaviour of parallel welded connections in form of double lap joints, and in particular the influence of the overlap length on the load capacity, are described. The results show that due to the extremely rigid behaviour of the bond so-called size effects occur, a phenomenon that describes a decrease in nominal strength with increasing size of a load-bearing structure. In addition to the load tests, a numerical strength prediction method is presented, which is based on a probabilistic approach and which allows for a precise prediction of the experimental results and the size effects. Furthermore, it is shown that timber boards with perpendicular fibre orientation can be welded in order to form stiff and resistant connections. Therefore, specimens of different sizes were prepared and tested for their moment transmitting capacity. In this context, a new method was introduced for the determination of a failure criterion of cross-welded joints is presented. Probabilistically determined strength values, which are based on this criterion, show good agreement with the experimental results. The non-linear relationship between the size of the specimens and the nominal strength is confirmed by the results of the numerical calculation. The results from the experiments on cross-wise welded bonds show that the strength of those connections is suitable for an application in timber construction. For this reason small scale prototypes of cross-laminated timber panels were fabricated and examined for their load bearing capacity. For this purpose the most critical load case of in-plane shear is regarded mainly since the load is transmitted only by the moment resisting behaviour of the welded bond. Based on the previously developed probabilistic strength prediction method, the load bearing capacity of the panels can be calculated numerically [...]