Development and Evaluation of Mechanical Joints for Composite Floor Elements with Cross Laminated Timber
Abstract: One of the important issues in the development of a multi-storey timber construction system is the definition of the floor solution. This thesis focuses on the development of composite floor solutions where Cross Laminated Timber (CLT) panels are used as a base element. Preliminary investigations on shear connections between prefabricated concrete beams and CLT panels were performed. The focus is on investigations on glulam-CLT composite beam elements, and the mechanical shear connectors used to achieve composite action. The new shear connections system evaluated in this thesis for glulam-CLT floor elements consists of double-sided punched metal plate fasteners. In order to secure the shear connection made with double-sided nail plates and to improve the shear behaviour of the joint, a combination with inclined self-tapping screws was evaluated through a shear test programme. It was found that the double-sided punched metal plate fasteners and inclined screws can effectively be combined. Bending tests (6.4m span) were carried out using double-sided nail plates as shear connectors, with and without combination with inclined screws. The double-sided nail plate connectors did behave satisfactorily in terms of strength and stiffness for the beam design considered, even when not combined with screws. It is however suggested that further research should be carried out before using double-sided nail plates as the sole shear connection in timber composite floor elements. It was found that, for the design and load case considered, the new shear connection system performs satisfactorily and could fulfil necessary design criteria. It is considered that the shear connection system may be suitable for floor spans up to 7 meters. Further research is necessary in order to assess the behaviour of double-sided nail plates under long term loadings and, combined shear and separation forces. An analytical linear elastic model for the stiffness of inclined screw joints was derived. This model takes into account the dowel and withdrawal action of the screw as well as the friction between the timber members. The linear elastic model is general and allows prediction of the load-deformation behaviour of inclined screw joints in shear tension in the serviceability limit state for different angle inclinations. One of the advantages of this model is that different material properties and geometries can be used in each of the jointed members. The model is found to be appropriate for inclination angles between 30 and 60 degrees between the screw axis and the normal to the shear plane. Further material testing is necessary in order to obtain certain material parameters for this model.
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