Cross laminated timber (CLT) is leading the evolution of wood construction throughout the world. As atwo-dimensional plate-like construction product, the in-plane elastic constants of CLT panels are the fundamental parameters for serviceability design. The elastic constants including moduli of elasticity (MOE) in major and minor strength direction ( and y) and in-plane shear modulus ( xy) of full-size CLT panels with different dimensions and layups from three CLT producers were measured by a non-destructive test (NDT) method developed by the first author. In total, 51 CLT panels were tested with most of the testing conducted at CLT mills. The measured values were used to examine the existing effective stiffness prediction models of CLT. Results show that k-method can be used for predicting and y values of industrial size CLT with a large length/ width to thickness ratio. xy cannot be well predicted by k-method and is greatly affected by edge bonding and gaps. Gamma method and shear analogy method can include the effect of transverse shear to different extents into account in predicting apparent or y. Shear analogy method appears to predict closer apparent to the measured values than gamma method for CLT with small length to thickness ratio. However, the effect of transverse shear on apparent y is not as much as predicted by shear analogy method for CLT panels with width from 1 to 3 meters. NDT by modal testing was proven to be an efficient mechanical property evaluation method for full-size CLT panels.
Twenty real dimensions beams from the glued laminated timber were tested in our previously works. Twenty advanced FE models were created precisely according to tested beams. Input files for FE models are lengths of segments and local moduli of elasticity. The segment is part of lamella between two finger joints. Each local modulus of elasticity was obtained via non-destructive penetration test. The output for comparison between real beam and FE model is displacement in half span. The quality of input data file from experiments is very important for the good agreement between real tested beams and FE models. In advanced FE models is described distribution of local moduli of elasticity via distribution function. The solution is based on the LHS. Accuracy of each distribution function is dependent on the number of measured local moduli of elasticity. In presented work was used probabilistic approach for determination of corresponding number of penetration tests as function of segments lengths. Results of this analysis will be used in the latter series of bending tests of new real dimensions beams and corresponding advanced FE models.
At the institute of structural engineering at the ETH Zurich multiple of investigations are conducted to analyse the material properties of Norway spruce timber boards. The investigations are part of the research project “Influence of varying material properties on the load bearing capacity of glued laminated timber (glulam)”. The majority of the investigations are non-destructively.
The investigations are taking place on 400 timber boards. On all specimens the moisture content, the density, the Eigenfrequency and the longitudinal ultrasonic runtime was investigated. Further all knots with a diameter larger then 10mm are measured. Thereby the position and the size of all the knots are documented. Subsequently on 200 selected boards non-destructive tensile test are performed to analyse the local young modulus. Herewith it was particularly focused on the investigation of the stiffness of areas having knots or knot clusters and areas without knots. The strains are measured with an optical coordinatemeasurement device. In the last part of the experimental investigation the deformation and failure behaviour of significant knot clusters is analysed. The strains are measured with digital image correlation.
Focus of the entire experimental analysis was the investigation of the young modulus and the quantifications of its variability within timber members and between timber members. Within this study a database was produced to evaluate existing test methods for the estimation of the young modulus. Further, the results can be used as a basis for further investigations on the variability of structural timber.
The paper concentrates on the determination of local elastic moduli of timber in the fiber direction. To that end a single commercially produced glued timber beam was subjected to 3600 penetration measurements. The beam was first covered by a regular grid of monitoring points at which the depth of indentation was measured. The pin was shot into the wood with a given energy (Pilodin 6J). We expect the measured elastic moduli to serve as an input for advanced finite element simulations on the bases of stochastic analysis. In such a case the local measured moduli represent in a given segment of each lamella an ensemble of data characterized by a selected probability distribution. These distributions are then employed in the LHS based stochastic simulation to provide probability distribution of the maximum deflection for a given load level. Apart from that it appears meaningful to compare independently the probability distributions of the elastic moduli for segments of the lamella (these may considerably differ owing to the specifics of the production of structures made from glued lamella timber) with statistical data from the whole beam. Based on the measured data the correlation matrix relating statistical dependence of individual segments can be estimated thus improving the quality of the stochastic model.