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Mechanics and Dimensioning of Cross Laminated Timber Plates: Plate Bearing Capacity and Introduction of Load Transmission Problems

In modern engineering carried out by engineers, cross laminated timber plates (CLT plates) in terms of plane-like structural elements fulfil not only purely load-absorbing functions, i.e. load absorption parallel to the plate plane (disk strain) and orthogonally to the plate plane (plate strain), but also meet physical building and architectural (space-limiting) requirements. Compared to conventional homogenous–isotropic plate building components made from steel, glass, or plastics, CLT plates exhibit typical and comparably complex inner structures. These inner structure are produced through an alternating sequence of individual layers that are mostly arranged orthogonally to each other and which are glued together completely face to face in direction of thickness. The board layers themselves consist of parallel adjacent individual boards whose connection remains unglued along the narrow sides of the boards, but can also be in a glued version.

This situation requires a problem-specific structural as well as a detailed mechanical description of the elastic and, as a further consequence, of the elastic-plastic bearing behaviour while taking into consideration the mentioned structural heterogeneities. Modern structural mechanics do not offer any suitable methods here, and therefore, based on the three-dimensional continuum mechanics, totally new theoretical structural models must be developed in a first step and must be practically numerically implemented in a second step. In the past of timber engineering, the scientific exploration of plane-like supporting elements mainly focused on an experimental examination plus accompanying theoretical back-up of the basic case of a one-axe plate bearing capacity. However, the newly developed or existing plate models only allow for limited valid statements on the bearing strength (bearing strength and deformation) of CLT plates. The reason for this can be found in the fact that, with the existing classical plate models, the individual board layers are described in terms of homogenous individual layers with smeared orthotropic mechanical specific parameters. Heterogeneities, such as board seam, ruptures, gluing joints between the boards, etc. can in this way not be grasped qualitatively nor, therefore, qualitatively, in their typical structural mechanical effects.

In the course of a current research project, the linear elastic disk bearing capacity behaviour (deformability) of orthogonal lattice constructions with and without openings is being examined for the principle case of pure shear strain. These findings concerning the shear bearing behaviour of constantly shear-strained building components via the CLT element thickness render valuable information on the drill bearing behaviour, i.e. on the changeable linear shear strain exercised over the thickness of the building component of a multiple-glued CLT element. The regular geometrical structure of the CLT element leads to complex shear bearing mechanisms that are based on the disk shear strain and plate drill strain. A detailed knowledge of such bearing mechanisms is necessary for an understanding of the bearing effects and thus for enabling an efficient economic exploitation as well as a clarification of special effects. Practical building special effects here are openings and notches, apertures, concentrated load introductions along the rims, combined cross sections, participating widths for ripped plates, and so on. Due to the assumed ideal homogenous structure of the individual layers and the assumed smeared elastic couplings of the individual layers, in the existing classical plate models, torque interactions of adjacent individual layers cannot be determined. The mentioned torque interactions between the wide sides of the board neighbouring in direction of thickness represent an indispensable inner equilibrium of forces that is vital for the bearing capacity.

This newly occurring bearing strength can therefore be grasped by means of a suitably extended, non-classical combined disk-plate theory, and a general finite structure element for practical application can be formulated. Crucial starting points for the implementation of this extended disk-plate theory are currently being created in the running project. The work to be tackled can use various simplified derived CLT models, and subsequently these can be compared to newly-to-be-developed models for verification. These models can be placed in a “hierarchical chain“ ranging from the simplest case of a homogenous overall plate model with homogenised plate stiffnesses to a progressive layered model, to the complete 3D continuum model. The first model type is striven for in direct practical engineering application carried out by engineers, while the second model serves the examination of special effects in the course of scientific research, thus indirectly serving practical engineering applications by engineers. The second model type serves the verification and calibration of the striven-for first-mentioned structural mechanical models. Additionally to elastic stiffness examinations for disk and plate structures, a measuring concept for the dimensioning of multi-layer plane-like timber structures with random boundary and strain, with the focus being on the shear and drill strain, shall be elaborated. The striking main idea here lies in introducing a so-called "strain intensity factor", which is applied to the characteristic tension conditions occurring in the individual glued areas, in a suitable form. Comprehensive preparation work, particularly, of an experimental nature concerning these themes have already been carried out in some earlier projects. By means of the carried-out torsion and shifting experiments on insulated board crossings and the knowledge obtained from the tension images of numeric simulations as well as groups of knots, the full dimensioning concepts can be worked out for calculating the CLT elements on the basis of the cut sizes at the glued surfaces. A random sampling verification of the theoretical examinations in the context of large-scale experiments is envisaged provided it is still an objective once the theoretical numeric working steps have been completed.