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In this study, a numerical model that aimed to predict the strength and stiffness, of a single laterally loaded mechanical fastener in steel-to-cross laminated timber (CLT) connections using a Beam-on-Foundation (BoF) model, is presented. The BoF model was used to predict the ductile failure modes of steel-to-CLT connections and considered the layered structure of CLT. When compared to the European Yield Model (EYM), the BoF model was found to be advantageous as it not only predicted the strength but also the stiffness of the connections. A comparison of the slip modulus and strength from BoF model simulations of experimental tests carried out on steel-to-CLT connections showed good agreement with the experimental results. Using the BoF model, the influence of; (i) density, (ii) deck layer orientation, (iii) embedment behaviour, (iv) dowel diameter, (v) steel plate thickness, and (vi) embedment length, on steel-to-CLT connections was investigated in a parameter study. The results presented herein highlight the benefit of the BoF model that considers CLT layer specific embedment behaviour in determining the shear capacity and stiffness of steel-to-CLT connections.

The rotational stiffness and moment capacity of wall-to-floor connections in platform type CLT structures, as influenced by the connection type, wall and floor thicknesses, and the load level in the CLT wall, were experimentally investigated. For determination of the nonlinear moment-rotation behavior of the connection, a novel test setup with simultaneous loading of the connection by two hydraulic actuators, in combination with digital image correlation (DIC), was used. Findings from this study allow for enhanced engineering design of CLT structures by considering the connection stiffness and its dependence on the investigated influence factors.

The aim of the research presented herein is to investigate the mechanical behaviour of cross-laminated timber (CLT)-to-concrete dowel-type connections. For reliable timber-concrete-composite structures, mechanical connections between the two construction materials are of great importance. This paper investigates the nonlinear load-displacement behaviour, giving access to the stiffness and strength, as well as ductile connection failure modes, of a CLT-to-concrete composite connection using a Beam-on-Foundation (BoF) model. The latter is a numerical model that utilizes non-linear springs for the interaction between the fastener and the surrounding CLT and concrete materials. The influence of: (i) fastener diameter, (ii) initial slip, (iii) concrete embedment properties, and (iv) axial fastener resistance due to friction, on the connection shear capacity and slip modulus, was investigated in a parameter study. The nonlinear load-displacement response, connection stiffness and strength predicted by the BoF model were moreover compared to laboratory tests and the European Yield Model (EYM), which supported the validity of the BoF model. In addition, it was shown that the BoF model could enhance the prediction of the slip modulus compared to the current design regulations in Eurocode 5.

Embedment strength and stiffness of steel dowels in spruce and birch solid wood were investigated in an experimental study, taking into account the moisture and connection assembly history. Thus, in addition to the effect of different mechanical properties of wood at different moisture contents, the effect of changes in the moisture content between the steps of: drilling the dowel hole, assembly of the dowel, and testing of the specimen on the embedment strength and stiffness was studied. Full-hole embedment tests with 12 mm steel dowels showed a decrease in embedment strength with increasing wood moisture content, while the elastic embedment stiffness was not influenced. Drying the wood specimens after the dowel was inserted yielded up to 50% higher elastic embedment stiffness compared with connections drilled and assembled when the equilibrium moisture content was reached. Application of an artificial crack showed only a moderate effect on embedment strength and stiffness, while the ductile embedment behavior was maintained.

As long-term investigations are often too time- and cost-consuming, short-term experimentsare introduced for investigating the behaviour of connections in timber-concrete-compositeelements. The paper presents a proposal for short-term double shear tests with adhesive andmechanical connections for investigating their time-dependent behaviour. The proposedevaluation method determines the elastic and visco-elastic parts of the slip modulus of theconnection. The validity of the identified parameters needs to be verified by long-term tests.

Design for efficient assembly is essential to further enhance the competitiveness of cross-laminated timber building systems for multi-story timber structures. This requires a holistic view from the design of the load bearing structures by structural engineers, over the production, pre-fabrication, and transport to the assembly of the structural elements on-site, which often is done by different companies with input from different stakeholders in the construction process. Especially the design of connections between CLT elements, and CLT and other construction materials and products, as well as the size of CLT elements and possibilities for pre-fabrication are crucial for an efficient assembly process. The paper summarizes findings from expert interviews with a focus on Sweden along the before-mentioned value chain, with the aim to identify current practice and potentials for further improvements. Design for efficient assembly starts at the early-stage design and involves all stakeholders in the design construction process. The reduction of uncertainties in the design and assembly process of multi-storey CLT structures as well as knowledge and experience transfer could lead to more efficient design. The identified requirements for efficient assembly should be combined with a life cycle analysis to quantify the potential for a reduction of the carbon footprint of CLT-based building systems, which is the aim of the ongoing research project ‘Improving the competitive advantage of CLT-based building systems through engineering design and reduced carbon footprint’., KK Synergi: Större konkurrenskraft för KL-träbaserade byggsystem genom effektiv konstruktion och reducerat kolavtryck

In recent years, the use of timber in construction substantially increased due to the material's renewable nature, lower climate impact and increased economic competitiveness. Another driving factor is great improvements in modelling techniques for the design of timber structures. Suitable prediction of the connection behaviour, as a fundamental part of the structural behaviour of timber structures, is crucial for a more economic and reliable design. However, the more realistic and complete connection models, the more complex and difficult to handle they are, which might hinder their practical application. A good trade-off between complexity and computational efficiency can be achieved with the so-called Beam-On-Foundation (BOF) method, which is applied herein in a 2-step hierarchical model to analyse and predict the rotational stiffness, ductile capacity and load distribution among fasteners of four different configurations of moment transmitting beam-to-column timber-to-timber connections. The connection model is validated with experiments on the global response of the connection as well as with a 3-D solid FEM model. The herein proposed connection model well predicted the overall connection response and provided insight into the local fastener behaviour. As compared to the 3-D solid model, which additionally gives access to more realistic local stresses in the timber, the 2-step model is however much more efficient with a great reduction of computation time. This makes the approach suitable for parametric studies and the analysis and engineering design of timber structures.

Material properties of wood under compression perpendicular to the grain and rolling shear are important for the engineering design of timber structures. This regards the short-term stiffness and strength, their dependence on the moisture content of wood, as well as the time-dependent behavior. Norway spruce clear wood properties in the transverse plane of wood were studied inan experimental campaign exploiting an earlier developed biaxial test setup. The moisture dependence of the stiffness and strength and the short-term time-dependent creep deformations under compression in the radial direction and under rolling shear were characterized. Loading and unloading stiffness, as well as the strength, were determined in quasi-static tests at five different moisture contents from 4% to 29%. The elastic and viscous stiffnesses were identified in creep tests at three compressive stress levels of 0.50, 0.75, and 1.00 N/mm2, and at two rolling shear stress levels of 0.33 and 0.50 N/mm2. The test data complements the existing experimental database, especially with novel data regarding the moisture dependence of the rolling shear strength, which showed less moisture dependence than the compressive strength perpendicular to the grain. The results of the creep tests revealed different material properties for the different loading and material directions of wood.