Andreas Ringhofer

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.

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.

Investigations Concerning the Force Distribution along Axially Loaded Self-tapping Screws

Self-tapping screws, as simple fasteners with a high load carrying potential if stressed axially, are frequently applied in timber engineering as tensile joints in wide span GLT truss systems or as reinforcements against stresses perpendicular to grain. In fact, force distribution along axially loaded screws has a very important influence on the joint behaviour. Some models based on Volkersen’s theory combined with fundamentals of linear elastic fracture mechanics already exist for glued-in rods or lag screws.

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.

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.