Gerhard Schickhofer

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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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.

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.

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.