Abstract
A systematic investigation is still lacking for tension out-of-plane in cross laminated timber (CLT), as a planar timber construction product. The objectives of the present study are the determination of the tensile properties of CLT made of Norway spruce, the identification of essential product-specific influencing parameters and a comparative analysis with glulam. For this purpose, seven test series were defined, which allowed the determination of the tensile properties on board segments and thereof produced glulam and CLT specimens by varying the number of layers, layer orientation and number of elements within a layer. The orthogonal laminated structure of CLT led to between 50% and 70% higher tensile properties out-of-plane, which is explained by the different stress distribution compared to glulam; the regulation of 30% higher properties than for glulam is suggested. In addition, the lognormal distribution turned out to be a more representative distribution model for characterizing the tensile strength out-of-plane than the Weibull distribution. This was also confirmed with regard to the investigated serial and parallel system effects, in which a clearly more homogeneous behavior was found in CLT compared to glulam, which in turn can be attributed again to the different stress distributions.
Highlights
In timber engineering, the doctrine of avoiding a planned tensile perpendicular to grain stresses wherever possible is well known, especially in combination with shear stresses (Spengler [1]; Hemmer [2]; SIA 265 [3])
It becomes clear that the mechanical properties perpendicular to the grain of usually prismatic structural timber, so-called off-axis properties vary over the cross-section depending on the local annual ring pattern
As there are a number of design situations where this is not possible, reliable tensile properties perpendicular to the grain are needed for the base material and for the structural timber products produced from it
Summary
The doctrine of avoiding a planned tensile perpendicular to grain stresses wherever possible is well known, especially in combination with shear stresses (Spengler [1]; Hemmer [2]; SIA 265 [3]). In the course of this determination, timber is to be regarded as a cylindrically orthotropic material, that is, featuring different material properties in axial, radial and tangential fiber directions, albeit treated and regulated further as transversely isotropic in engineering terms, that is, differentiation in properties is made only in parallel and perpendicular to the grain Following this principle, it becomes clear that the mechanical properties perpendicular to the grain of usually prismatic structural timber, so-called off-axis properties (properties, which follow the cartesian coordinate system of the timber member, that is, the outer, product coordinate system) vary over the cross-section depending on the local annual ring pattern. As a result of the high ratios between modulus of elasticity and shear modulus in the radial-tangential plane, one outcome of this coordinate transformation is what is known as shear-coupling effects; see for example, Dill-Langer [12]
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