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.     

Cross laminated timber properties including effects of non-glued edges and additional cracks

Cross laminated timber (CLT) is usually comprised of multiple timber layers having alternating grain directions. Because individual boards are glued on their faces between layers, but usually not glued on their edges within layers, those edges define “precracks” in the composite. When exposed to differential thermal and moisture expansion after installation, CLT, like all cross-laminated composites, is prone to formation of “additional cracks”. Confidant CLT design must be able to account for changes in CLT properties during life of a structure caused by such additional cracks. By extending variational mechanics methods for aerospace composites, this paper provides analytical solutions for all in-plane mechanical, thermal expansion, and moisture expansion properties of a three-layer CLT panel. By using the three-layer solution to evaluate effective layer properties as a function of the number of cracks, the analysis can be extended to in-plane mechanical, out-of-plane bending, and expansion properties for CLT panels with any number or arrangement of layers. Some sample calculations are provided along with comments on limitations of the approximations and needs for future work.     

Influence of wood moisture content on bending and shear stiffness of cross laminated timber panels

The bending and shear stiffness of cross-laminated timber (CLT) made from softwood depend on the moisture content of the panel. In principle, the stiffness properties drop with increasing moisture content within the hygroscopic range. However, swelling provokes closing of open gaps of the middle layers and leads to an apparent increase of stiffness due to internal friction. This increase in stiffness is only relevant for low deflections and can provoke misinterpretation of measuring results when deriving stiffness properties of CLT by means of dynamic methods such as modal analysis. Drying, on the other hand causes cracking within CLT, which manifests itself in a distinct reduction of the bending stiffness perpendicular to the grain direction of the face layers. Calculating bending stiffness of CLT perpendicular to the grain direction of the face layers by means of the compound theory not taking into account the face layers therefore is a common and correct procedure.     

Prediction of strength and stiffness of concentrated compressive load applied to the narrow face of cross-laminated timber

European Journal of Wood and Wood Products - The behavior of the concentrated compressive load applied to the narrow face of cross-laminated timber (CLT) panels is an essential parameter in the...     

Cost optimization of cross-laminated timber panels in one-way bending

European Journal of Wood and Wood Products - For the purposes of structural design, cross-laminated timber (CLT) panels are typically chosen from tables detailing standardized layups. While this is...     

Joining wood by friction welding

At the Chair of Timber Constructions of the Swiss Federal Institute of Technology in Lausanne (EPFL) tests were carried out to join wooden work pieces by friction welding without any additional welding deposit. It could be determined that this kind of technology, which is mainly used for thermoplastics and metal, can also be applied to wood. Tests were carried out to determine the influence of the processing parameters like welding pressure, frequency and amplitude of the circular movement on the welding process and the input of energy at the interface. In addition, the resistance of the joint was examined. The development of the shear strength during solidification of the interface as well as the shear strength achievable after a complete solidification of the interface was the objective of the examinations. Furthermore, the microstructure of the welded joint was studied to reveal the manner in which the thermally decomposed wood forms the connection between the welded pieces.     

Hochwertiges Brettsperrholz aus starkem Stammholz

Cross laminated timber (CLT) is an approved, large scale structural element for wood buildings. Its dimensional stability makes it a suitable building material for multilevel constructions. These structural elements, which are light and thereby highly capable of bearing load, are of advantage for such multilevel constructions. The study shows how light CLT with high stiffness can be produced using specially rift sawn boards with high modulus of elasticity and low density from large dimension timber (LDT) and by means of an inhomogeneous layout of the disks with regard to the modulus of elasticity. The modulus of elasticity of these improved disks is 20 to 30% higher than that of standard CLT while they are??9 to 13% lighter than the standard ones.     

Characteristic strength of tooth-plate connector joints

The development of Eurocode 5, the future European timber design code which is based on the partial safety coefficient method, means a change of timber design code formats in a number of european countries. Because of the change in the code format from allowable values towards partial safety coefficients, characteristic values of strength and stiffness of materials and connections are needed. In order to establish characteristic values of the strength and stiffness of tooth-plate connections, existing test data have been evaluated. The results of the evaluation correspond well to a calculation model assuming a load sharing between the connector and the bolt.     

Welding-through doweling of wood panels

Panel products have been joined to levels satisfying the requirements of the relevant standards for metallic connector assemblies by rotation welding of beech dowels through them. The wood panels substrates such as particleboard, OSB and MDF welded to the dowels almost as well as solid timber. Welded-through dowel assemblies presented better stiffness of the test joints than similar assemblies of dowelled solid timber.     

Structural performance of rounded dovetail connections: experimental and numerical investigations

Rounded Dovetail Connections (RDC) are a relatively new wood-to-wood connection concept that, despite the lack of design guidance in standards, has become popular in timber construction due to the widespread of modern milling machinery. Because of the anisotropic nature of wood and the complex stress-strain state in RDC, the question of their dimensioning is very complex. Experimental and numerical investigations were carried out on full scale RDC used to connect two timber members as joist to beam connections subjected to quasi-static shear loading. The influence of two geometric parameters was investigated: the dovetail height (varied between 109 and 189?mm) and the flange angle (varied between 5 and 20°). Both, serviceability and ultimate limit states were studied using analysis of variance. It was found that the joint capacity (i) depends on the dovetail height, with an optimum of approximately 2/3 of the beam height and (ii) can be considered almost independent of the flange angle. The development and implementation of a numerical model for the design process of RDC was examined and good agreement between experimental and numerical load deformation curves validated the model, thus making it suitable for developing a method to predict RDC capacity. The paper proposes a probabilistic method to predict the capacity of RDC taking into account the scale sensitivity of the material strength, which is modelled using Weibull statistics, and considers not only the magnitude of the stress fields, but also the volume over which these stress peaks act. The proposed method has immediate actionable application for the improvement of RDC design.     

Prefabricated timber walls anchored with glued-in rod connections: racking tests and preliminary design

A new beam and post system for multi-storey timber buildings has been developed in Sweden. The building is braced with timber walls constructed from two Kerto-Q LVL boards glued and screwed onto a glulam frame. The walls are prefabricated off-site and can be connected to the foundation using either glued-in steel rods with metric thread or nail plates. Introductory racking tests of full scale walls anchored with glued-in threaded rods were performed. The paper presents the results of the experiments and discusses the use of the transformed section method to predict racking capacity of the anchored wall. To evaluate the strength of the glued-in rods, a newly proposed model was employed. An analytical study was conducted to investigate the role of the sheathing and the contribution of the axial force on the racking capacity of the walls. The wall panels tested in this experimental programme showed high strength and stiffness under racking load. The anchoring joints with glued-in steel rods with metric thread demonstrated a high load-carrying capacity with, however, large scatter and a brittle failure mode characterized by pull-out from the timber member. The transformed section method was successfully used to predict the racking capacity of timber walls anchored with glued-in steel rods.     

Strength and stiffness of ring and shear-plate connections

The development of Eurocode 5, the future European timber design code which is based on the partial safety coefficient method, means a change of timber design code formats in a number of European countries. Because of the change in the code format from allowable values towards partial safety coefficients, characteristic values of strength and stiffness of materials and connections are needed. In order to establish characteristic values of the strength and stiffness of ring and shear-plate connections, existing test data have been evaluated. The results of the evaluation correspond well to a mechanical model assuming a shear block failure of the wood as the governing failure mode.     

Derivation of buckling design curves via FE modelling for in-plane compressed timber log-walls in accordance with the Eurocode 5

In ‘Blockhaus’ systems the structural capacity derives from surface interactions and friction mechanisms between multiple timber logs stacked horizontally one upon each other. Unlike masonry or concrete walls, timber log-walls are characterized by the absence of a full structural interaction between the basic components, hence resulting in ‘assembled’ rather than ‘fully monolithic’ structural systems characterized by high flexibility of timber and usually high slenderness ratios. The current Eurocode 5 for timber structures, however, does not provide formulations for the prediction of the critical load of log-haus walls under in-plane compressive loads. In this work, based on past experimental tests and detailed Finite-Element (FE) models, extended numerical investigations are performed on timber log-walls. A wide number of configurations (more than 900) characterized by different geometrical properties, timber log cross-sections, number and position of door and window openings, presence of in-plane rigid (RF) or fully flexible (FF) inter-storey floors, as well as initial curvatures and/or load eccentricities, are analyzed under monotonic in-plane compressive load. Careful consideration is also given to the influence of additional out-of-plane pressures (e.g., wind pressures) combined with the in-plane compressive load. In accordance with the buckling design approach proposed by the Eurocode 5 for timber columns, non-dimensional buckling curves are then proposed for timber log-walls under in-plane compression. These curves are based on an accurate calibration of the k _c buckling coefficient and the related imperfection factors on the results of the numerical parametric study. The developed simple and conservative approach for the design of log-walls can be proposed for implementation in the new generation of the Eurocode 5.     

Embedding strength in joints of fast-growing Argentinean Eucalyptus grandis with dowel-type fasteners. Analysis according to the criterion adopted by European standards

The present paper reports on the results of an investigation regarding the determination of the embedding strength of sawn timber of Argentinean Eucalyptus grandis with dowel-type fasteners. The importance of accurately knowing this property has increased with the increasing demand for this material for structural purposes. An empirical research project with 294 specimens prepared for testing under the embedding action of rigid fasteners (nail, bolt and dowel) was carried out according to the procedures prescribed in EN 383 (2007). The results showed no significant influence of pre-drilling on the embedding strength for nailed joints, which is not in line with the European experience and the corresponding criterion adopted by the Eurocode?5 (2005). The characteristic values of the embedding strength estimated based on the timber density and by applying the corresponding equation from the Eurocode?5 (2005) for bolted joints were 45% and 176% greater than those obtained in this project for the specimens loaded parallel and perpendicular to the grain, respectively, and similar percentages were found for dowelled joints (45% and 126%). These results showed that the reliability of structural design may be strongly affected if the embedding strength of this timber species is estimated by following the procedures adopted by the Eurocode?5 (2005) for dowel-type connections.     

Relationship between timber quality and the influence of moisture content above fibre saturation on mechanical properties in beams of fast-growing Argentinean Eucalyptus grandis

The present paper reports on the results of an investigation regarding the relationship between timber quality and the influence of moisture content above fibre saturation on bending strength and stiffness in structural-sized beams of fast-growing Argentinean Eucalyptus grandis. An empirical research project with a sample containing 96 pairs of structural-sized beams (seasoned and unseasoned) was carried out. Results showed that the influence of moisture content on mechanical properties was related to timber quality and it increased with timber quality but this relationship did not clearly emerge from the results obtained by visually strength grading the sample with the most important strength and stiffness reducing growth characteristic (the presence of pith), whereas it was better revealed by machine grading the sample with the most important single parameter (modulus of elasticity). Results also showed that decisions related to structural design with timber of high quality exposed to service class 3 according to European standards or installed near fibre saturation are situated on the unsafe side if the strength and stiffness values are obtained by multiplying those corresponding to seasoned timber by a constant factor representing the average behaviour of this material. Even though this average behaviour compared relatively well with the results obtained by applying the criteria adopted by standards of overseas and Latin-American countries, which do not consider timber quality for this purpose, the reliability required by structural design accounts for the convenience of taking into account the relationship between timber quality and the influence of moisture on mechanical properties where timber of high quality is concerned.     

Preliminary evaluation of a density-based lumber grading method for hem-fir CLT manufacturing

European Journal of Wood and Wood Products - In the manufacturing process of cross-laminated timber (CLT), lumber selection and grading are essential steps for the quality control of CLT. In this...     

Evaluation of density and strength of Norway spruce wood by near infrared reflectance spectroscopy

The non-destructive evaluation of wood properties by Near Infrared Reflectance Spectroscopy (NIR) has been assessed. The surfaces of specimens of clear wood (Picea abies) were NIR-scanned, and the results compared to such properties as moisture content, density, compression strength and chemical and biological degradation. In addition, the NIR-scans of clear wood specimens were compared to the bending strength of the structural timber from which it had been cut. The NIR dependency of surface roughness was investigated and found to be of minor importance. NIR calibrations for moisture content (MC<30%), density, compression strength and chemical degradation proved that the NIR technique is an excellent non-destructive method (r ²≧0.9; independent test set). Even biological degradation was determined with a most promising accuracy (r ²=0.75; independent test set). For the prediction of the bending strength of timber NIR proved less efficient. However, NIR still contributed to timber strength prediction at the same level as annual ring width, the parameter which is presently visually assessed by timber graders. It is concluded that the NIR method is very versatile in the non-destructive evaluation of wood. The results merit further investigations in order to develop proper models and instrumentation for commercial use.     

The evaluation of panel bond quality and durability of hem-fir cross-laminated timber (CLT)

To better use the second-growth wood resources in value-added applications, this work addressed the manufacturing aspects of cross-laminated timber (CLT) products from western hemlock (Tsuga heterophylla (Raf.) Sarg) and amabilis fir (Abies amabilis (Dougl.) Forbes) (or hem-fir) harvested from coastal British Columbia, Canada. Small CLT billets (nominal 610 mm?×?610 mm) were made to examine CLT bond quality and durability through block shear and delamination tests. Two types of adhesives, single-component polyurethane (PUR) and emulsion polymer isocyanate (EPI) and two critical applied pressure parameters (0.28 and 0.83 MPa) were adopted to manufacture hem-fir CLT. It was found that the adhesive type and applied pressure significantly affected wood failure percentage (WFP) and delamination of hem-fir CLT. When PUR adhesive was used, CLT made at 0.83 MPa pressure yielded significantly higher WFP and lower delamination than that made at 0.28 MPa pressure. The results demonstrated that despite the fact that hem-fir lumber is not particularly specified in the current North American CLT standard, it could be used for manufacturing CLT with the required panel bond quality.     

Leichtbauplatten aus Holzwerkstoffen—Teil I: Eckverbindungen

This paper describes the main properties of corner joints of lightweight panels made of wood based materials. With this investigations the suitability of the lightweight panels for application in furniture industries should be proofed. The observed properties of the tested specimens are the ultimate load and the deformation at the moment of break. For testing specimens with corner joints through lamello, a standard jointing system in the furniture industry, were produced. Altogether 70 specimens were produced, while following specimen parameters varied: panel thickness, used glue system, ambient climate, form of corner joint and direction in which the force effects the corner joint. The results show a relation between the failure load and the ambient climate, the used glue system and the form of load. A high correlation between ultimate load and the deformation at the moment of failure at tensile load could be observed for the specimens made of 38 mm thick lightweight panels. This correlation is a very important finding to evaluate the deformation properties of the lightweight panel as construction material for furniture.