Reinhard Brandner

Cross laminated timber (CLT): overview and development

Cross laminated timber (CLT) has become a well-known engineered timber product of global interest. The orthogonal, laminar structure allows its application as a full-size wall and floor element as well as a linear timber member, able to bear loads in- and out-of-plane. This article provides a state-of-the-art report on some selected topics related to CLT, in particular production and technology, characteristic material properties, design and connections. Making use of general information concerning the product’s development and global market, the state of knowledge is briefly outlined, including the newest findings and related references for background information. In view of ongoing global activities, a significant rise in production volume within the next decade is expected. Prerequisites for the establishment of a solid timber construction system using CLT are (1) standards comprising the product, testing and design, (2) harmonized load-bearing models for calculating CLT properties based on the properties of the base material board, enabling relatively fast use of local timber species and qualities, and (3) the development of CLT adequate connection systems for economic assembling and an increasing degree of utilization regarding the load-bearing potential of CLT elements in the joints. The establishment of a worldwide harmonized package of standards is recommended as this would broaden the fields of application for timber engineering and strengthen CLT in competition with solid-mineral based building materials.

Stochastic System Actions and Effects in Engineered Timber Products and Structures

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Homogeneous shear stress field of wood in an Arcan shear test configuration measured by means of electronic speckle pattern interferometry: description of the test setup

Abstract Several studies have dealt with the problem of how to measure the shear modulus of small clear wood specimens, avoiding bias from normal compression, bending, tension or torsional stresses. Unbiased results can be used to estimate realistic shear modulus values relevant for timber construction. However, a stress field that contains only uniformly distributed shear stress cannot be achieved. The Arcan shear test is one of the test setups that allows the measurement of more or less homogeneous shear stress and, therefore, shear deformation in a small volume of the specimen at macroscopic material level. The shear deformation of Arcan shear test samples was measured between the two notches by means of electronic speckle pattern interferometry (ESPI). Shear tests were performed in a tension mode, which provided a homogeneous strain field in the reduced midsection. Twisting forces and inhomogeneous shear deformation of the sample volume were assumed, which resulted in different shear deformation on the front and back sides of the sample. In this way, both surfaces of the samples were measured in parallel. Using the mean value of both deformation fields allowed a significant reduction in the coefficient of variation of shear modulus measurements to be achieved in comparison with values gained from a single ESPI measurement.

Spatial correlation of tensile perpendicular to grain properties in Norway spruce timber

Tensile strength perpendicular to grain constitutes one of the most vulnerable properties of timber. Due to versatile influencing parameters this property exhibits a high amount of uncertainty. Thus, progress in modeling, in particular by considering stochastics, is seen as worthwhile. This increases the reliability estimates of timber constructions but also their economic efficiency. Test data of tensile properties determined on consecutive board segments of Norway spruce are analyzed. The data consists of four subgroups, classified in regard to segment length and radial position within the log. The correlation in longitudinal direction of perpendicular to grain tensile strength and elastic modulus as well as of density is examined. This is done depending on the radial position of structural timber within the log. A second-order hierarchical model together with equicorrelation is used. The results outline the applicability of the model and allow the quantification of equicorrelation coefficients of all three properties. The outcome provides a valuable and necessary input for state-of-the-art mechanics-stochastic modeling of the resistance perpendicular to grain tensile strength and elastic modulus of unjointed and jointed structural timber, but in particular of products available in large dimensions, like glued and cross-laminated timber. Additionally, the spatial correlation of density is discussed which is seen as worthwhile for the estimation of group action of fasteners. The necessity to differentiate between the variability within and between segments of structural timber is clearly demonstrated.

Resistance and Failure Modes of Axially Loaded Groups of Screws

Screwed connections provide high resistance in strength and stiffness. Arranged to a group the screws interact and influence each other in dependency of their in-between spacings. A test setup was found to investigate (i) the influence of the spacings in-between the screws, and (ii) the anchoring depths on the failure modes and resistances of groups of screws. We conducted tests on axially loaded and under a stress-fiber angle of 90° placed groups of screws in solid timber (ST) and glued laminated timber (GLT) of Norway spruce. Steel fracture, withdrawal failure and also block shear failure mode, till now for self-tapping screws not considered by design codes, were observed. Additionally and based on a simple mechanic load shearing consideration model for the block shear failure mode was developed for the investigated axially loaded groups of screws. Verification with test results confirms congruent but conservative results.

Probabilistic models for the withdrawal behavior of single self-tapping screws in the narrow face of cross laminated timber (CLT)

Cross laminated timber (CLT) and self-tapping screws have strongly dominated the latest developments in timber engineering. Although knowledge of connection techniques in traditional light-frame structures can be applied to solid timber constructions with CLT, there are some product specifics requiring additional attention; for example in positioning of fasteners, differentiation in the side face and narrow face of the panels and the influence of potential gaps. The load–displacement behaviour of single, axially-loaded self-tapping screws positioned in the narrow face of CLT and failing in withdrawal was investigated. For the first time a multivariate probabilistic model was formulated together with models relating the parameters with the thread-fibre angle and the density. Different types and widths of gaps, initial slip and / or delayed stiffening as well as softening after exceeding of the maximum load can be considered. Beyond the scope of this contribution, the probabilistic model is seen as a worthwhile basis for investigations into the withdrawal behaviour of primary axially loaded, compact groups of screws positioned in timber products and subjected to withdrawal failure.

Length Effects on Tensile Strength in Timber Members With and Without Joints

Strength properties in timber disperse remarkable. A regressive course of strength with increasing volume is observed. This phenomenon is known as size effect, dominated by the stochastic part, the dispersion in strengths locally. Although boundary conditions of this theory are violated, traditionally, size effects have been modelled by means of Weibull’s brittle failure theory. We address stochastic length effects on the tensile strength parallel to grain of timber members with and without finger joints by means of a probabilistic approach. For jointed members regulations of minimum requirements on finger joint tensile strength are discussed. We assume lognormal distributed strengths and use a second-order hierarchical model together with equicorrelation to account for within and between members’ strength variations. As outcome, the effect of length on the mean and 5 %-quantile of tensile strength is quantified. Simplified models and parameters for the design of timber structures are provided. Minimum requirements on finger joint tensile strength, relevant for modelling of timber products as well as factory production control, are defined.

Samples of iid Lognormals: Approximations for Characteristics of Minima

We concentrate on characteristics of minima XN from samples of iid lognormals X1i ∼ 2pLND. We demonstrate that the distribution of XN for 2 ≤ N ≤ 1,000 may be fitted more accurately by 2pLND than by the limiting Gumbel distribution. An extended power model is established to represent the quotients CN/C1, where CN is the mean, standard deviation, or p-quantile of XN and C1 is the corresponding characteristic of X1i. Our empirical comparisons show that this model provides not only more accurate estimates than alternating approximations but it is also much simpler than its competitors.

Serial Correlation of Withdrawal Properties from Axially-Loaded Self-Tapping Screws

Previous investigations outline the applicability of a two-level hierarchical stochastic material model combined with equicorrelation for the description of timber strength and elasticity, by explicit differentiation in variation within and between timber elements. Consequently, as far as withdrawal of primary axially-loaded self-tapping screws is concerned, the load bearing capacity of screw groups in laminated timber products depends on their positioning relative to the product layup. We analyse the first time the applicability of a two-level hierarchical model on withdrawal strength, stiffness properties and density. By testing a saturated data set, the hypothesis of equicorrelated withdrawal properties could not be rejected. Test setup, examination and accompanied epistemic uncertainties in analysing the stiffness properties are seen as general reason for their relatively high variation and consequently low correlation, whereas the high equicorrelation of withdrawal strength is explained by the homogeneous test material. However, in reality screw groups are influenced by unavoidable flaws which provoke higher variation and lower correlation. In view of previous investigations on timber strengths, an equicorrelation for withdrawal strength in the range of 0.40 to 0.50 (0.60) appears more reasonable.