Timber Engineering and Wood Technology

h i g h l i g h t s Verification of a novel test configuration for in-plane shear properties of CLT. Conclusive examination procedure for net-, gross-shear and torsion. Identification of main influencing parameters from a comprehensive parameter study. Proposal for characteristic shear properties for CLT with/without edge bonding. Conclusive design concept for in-plane shear of CLT.

This paper focuses on the point-to-point X-RAD connection system for the construction of cross laminated timber buildings in earthquake prone areas. Tests on X-RAD connectors subjected to "shear-tension" and "shear-compression" loading configurations are presented as supplement to the experimental data provided in previously published research. Tests aimed at characterizing the behavior of timber walls assembled with the system and loaded by lateral force are also reported. A capacity domain that allows for quick safety checks for any given load combination on the connector was derived from the experimental outcomes and is presented herein. An extremely simplified, yet effective numerical approach (the connector behavior is reproduced by 3-link elements) is proposed and compared with the experimental evidence. The 3-link model was able to reproduce with sufficient accuracy both, the elastic and post-elastic response of the walls. In the discussion section, particular attention is given to the system capability of dissipating energy and exhibiting ductile behavior.

The stock of deciduous tree species is raising. Some of them feature outstanding high mechanical properties and are thus predestined for load bearing applications in timber engineering. Although the pressure to use these species is growing, the timber industry and engineering sector still stays observant. For some products made of deciduous timber costs are reported, which prevent their competitiveness against products made of coniferous timber and other building products. One way to improve the competitiveness and use of products made of deciduous timber is the development of innovative and specific single fastener and joint solutions with the aim to maintain at its best the high load bearing capacity of these products also at the joints. In this paper the outcomes from a comprehensive testing campaign conducted in frame of the long-term research project hardwood_SCREWS and for two types of self-tapping screws exposed to axial loads are reported. One screw is of commercial type and used as benchmark. The other screw is an innovative alternative and especially developed for applications in deciduous timber. Three groups with specific parameter settings and aims were tested to analyse the following influences on insertion and withdrawal properties: the wood anatomy, the thread-grain angle and the predrilled insertion. The degree in relationships between investigated physical/mechanical properties is quantified by power coefficients. Compared to coniferous timber, these power coefficients outline a disproportional capacity of screws inserted in deciduous timber. Following this, the paper concludes with a generic model approach for calculating the withdrawal strength of axially-loaded self-tapping screws inserted in structural timber or glued laminated timber products made of either coniferous or deciduous timber.

The raising stock of hardwood and the increasing popularity to build with timber are strong drivers to use hardwood as structural material on a regular basis. Despite its outstanding mechanical properties, economic boundary conditions in producing structural hardwood products consequence, however, rather a niche application. Efficient fastener and joint solutions which are optimised for hardwood are one possible solution to increase the share of hardwood in structural applications. Within this contribution a new, innovative and for hardwood optimised screw is benchmarked with a commercial screw. The influence of wood anatomy, thread-grain angle and predrilling on the withdrawal strength of single screws are analysed. For groups of screws, the minimum edgedistance and spacing perpendicular to the grain are determined and various possibilities for endgrain joints investigated. A generic approach for predicting the withdrawal strength of axially loaded self-tapping screws inserted in structural timber or glued laminated timber products from either soft-or hardwood is presented. Models to account for the influence of density, thread-grain angle and predrilling are proposed and regulations for the geometric requirements and execution of groups of screws in end-grain joints discussed.

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 ...

The project WoodC.A.R. investigates the capabilities of wood and engineered wood-products (EWPs) for their application as a load-bearing material in automotive applications. For crash-relevant components, materials have to provide a high impact bending energy over a wide range of climatic conditions. This study investigates the effect of temperature on the bending behavior of solid birch wood beams (800 × 90 × 43 mm3) under quasi-static and dynamic loading. Specimens were exposed to a three-point bending test with lateral confinement, replicating the hypothetical installation environment in a car, at five temperature levels: −30 °C, 0 °C, +30 °C, +60 °C, and +90 °C. A cylindrical impactor (D = 254 mm, m = 91 kg) was propelled against the center of the beam with an initial velocity of 8.89 m/s (dynamic) and at a constant velocity of 10 mm/min (quasi-static), respectively. Specimens were conditioned in a freezer and a climate chamber, respectively. Temperature was monitored prior and ...

The committor function is a central object of study in understanding transitions between metastable states in complex systems. However, computing the committor function for realistic systems at low temperatures is a challenging task, due to the curse of dimensionality and the scarcity of transition data. In this paper, we introduce a computational approach that overcomes these issues and achieves good performance on complex benchmark problems with rough energy landscapes. The new approach combines deep learning, importance sampling and feature engineering techniques. This establishes an alternative practical method for studying rare transition events among metastable states of complex, high dimensional systems.

In timber engineering, self-tapping screws, optimized primarily for axial loading, represent the state-of-the-art in fastener and reinforcement technology. Their economic advantages and comparatively easy handling make them one of the first choices for application in both domains. This paper focuses on self-tapping screws and threaded rods applied as reinforcement, illustrating the state-of-the-art in application and design approaches in Europe, in conjunction with numerous references for background information. With regard to medium to large span timber structures which are predominately erected by using linear timber members, from e.g. glued laminated timber, the focus of this paper is on their reinforcement against stresses perpendicular to grain as well as shear. However, latest findings with respect to cross laminated timber are included as well.

Timber exposed to compression perpendicular to grain stresses is of concern in all timber constructions. The challenge is the high diversity of possible load configurations, which mandates for a generic approach with basic properties and case individual adaptations of resistance and stiffness. As the consequences of exceeding the allowed stresses are relatively small, there has been a lively debate on whether compression perpendicular to grain should be treated as an ultimate or better as a serviceability limit state. This debate takes also its place in the currently ongoing revision of Eurocode 5 (EC 5), the European design code for timber structures. In the course of this revision regulations for structural timber products not currently anchored in EC 5, in particular for cross laminated timber (CLT), are now being sought. The product CLT is so far lacking in specifications for the testing, basic properties and design. In awareness of the necessity for generally applicable and reliable design approaches, I aim to present the state-of-the-art on compression perpendicular to grain/plane by focusing on the design, i.e. stress dispersion models, and cross laminated timber. In addition, I present the main data and findings from a comprehensive test campaign and adapt van der Put's stress dispersion model for stress dispersion in directions parallel and perpendicular to grain. This new approach allows an explanation of the influences caused by (i) contact area, (ii) load configurations, (iii) support conditions, (iv) layup and thickness of the CLT element, and (v) clear edge distances and clear spacing. The same approach is applicable for both strength and modulus of elasticity. The necessity to consider the orthogonal layup emerged in the case of CLT. I conclude with recommendations for testing and evaluation, propose basic properties and regulations for the main influencing parameters as also a generic design approach generally applicable for all linear and planar, unidirectional and orthogonal laminated structural timber products.

Self-tapping screws are fasteners that are versatilely applicable in timber engineering. For the design of such screw connections, preferential axial-loading, all possible failure mechanisms have to be considered. Recently, in compact groups of axially-loaded screws the block shear failure mechanism, which has not been investigated so far, turned out to fail rather brittle at load levels lower than currently allowed. This failure mechanism is defined as failure of (rolling) shear and/or tension perpendicular to grain planes encompassing the group of screws. This failure mechanism was observed in groups given a number of different parameter settings, i.e. thread-fibre angles of 90°and 45°, glulam, structural timber and cross laminated timber and various group designs. This paper focuses on groups of axially-loaded screws in glulam and solid timber of Norway spruce (Picea abies) and inserted at a thread-fibre angle of 90°. Varying group sizes, loading and supporting distances and group designs, i.e. various penetration lengths l ef and spacing in and perpendicular to grain, a 1 and a 2 , respectively, are analysed by two different "push-pull"-test setups. To predict the block shear capacity and failure characteristics of such groups of screws and to separate this failure mechanism from other failure mechanisms, a mechanical-based block shear model was established. This parallel acting spring model considers load sharing and redistribution between concerned failure planes and depends on a number of material, geometrical and stress distribution parameters. To ensure a reasonable parameter setting, background and potential influencing parameters on each model parameter are discussed. In validation, the model shows overall good predictions of capacities, failure mechanisms and failure sequence for all test series involved. It turned out that the current regulations, comprising the definition of minimum spacing together with minimum edge and end distances, are not sufficient for controlling this three-dimensional block shear failure. In addition, the consideration of the number of screws in the group as well as the penetration length is required. Recent experimental investigations on groups of axially-loaded screws fulfilling all requirements regulated in European standards, e.g. EN 1995-1-1 [3] (Eurocode 5; EC 5) and/or diverse European Technical Approvals/Assessments (ETAs), in particular the requirements on

Within the last 20 years, cross laminated timber (CLT) has become one of the most important building products in contemporary timber engineering. By the end of this decade, its annual worldwide production volume is expected to exceed 1,000,000 m 3. The industry, engineers, architects and constructors have a strong interest in implementing CLT in European product, design and execution standards. The design of CLT is part of the currently ongoing revision of Eurocode 5. In this context and in addition to the verification of CLT panels in respect of ultimate and serviceability limit states, guidelines on the design of connections in CLT with dowel-type fasteners are of the utmost importance. In this article, we review approaches for calculating characteristic values as provided in literature for single dowel-type fasteners and connections in CLT. We also compare these approaches, with focus on withdrawal and embedment strength of dowels, nails and self-tapping screws, with the current regulations on dowel-type fasteners for solid timber and glulam as formulated in Eurocode 5. These comparisons are made in order to identify the need and potential for revision of current EC 5 equations and state-of-the-art regulations. We summarise regulations on connection design, i.e. minimum spacing, edge and end distances, other additional geometrical conditions, regulations on the effective number of fasteners in a group and minimum penetration depths. Finally, we draw conclusions in respect of the single fastener properties withdrawal and embedment strength and suggest some execution guidelines, which ensure the integrity of CLT structures. Overall, we aim to present a compilation of the current state-of-the-art knowledge on doweltype fasteners in CLT as a basis for implementing design guidelines for CLT in the new connection chapter of Eurocode 5.

The design of cross laminated timber (CLT) structures is not included in the current version of European structural timber design standards of EN 1995 (Eurocode 5, EC 5). Due to the increasing importance of CLT, in different applications such as complete wall or floor elements as well as girders, it is one of the main goals of the currently ongoing revision of EC 5 to provide design principles for CLT structures. In the present paper some general aspects, relevant for the implementation of CLT in European standards in order to be consistent with the general philosophy of the Eurocodes are summarized and discussed. The differences between standard test specimens and structural components as well as the uncertainties related to the production procedure of CLT and the non-standardized test procedure are discussed. An investigation of 12 different test series from five different producers clearly indicates a large variation between different production series. Based on the investigation from the test series a reliability analysis is performed. The results indicate that the same partial safety factor as recommended for glued laminated timber (GLT) is appropriate in order to achieve an acceptable reliability. However, the analysis also indicates the potential for a smaller partial safety factor in the future, in case that the production of CLT is standardized and appropriate standardized test methods for the individual material properties exist.

A systematic investigation is still lacking for tension out-of-plane in cross laminated timber (CLT), as a planar timber construction product. The objectives of the present study are the determination of the tensile properties of CLT made of Norway spruce, the identification of essential product-specific influencing parameters and a comparative analysis with glulam. For this purpose, seven test series were defined, which allowed the determination of the tensile properties on board segments and thereof produced glulam and CLT specimens by varying the number of layers, layer orientation and number of elements within a layer. The orthogonal laminated structure of CLT led to between 50% and 70% higher tensile properties out-of-plane, which is explained by the different stress distribution compared to glulam; the regulation of 30% higher properties than for glulam is suggested. In addition, the lognormal distribution turned out to be a more representative distribution model for character...

Material characteristics are general derived by tests under certain standardized conditions concerning climate, testing procedure, accuracy and size! In regard to mentioned requirements derived material characteristics are only valid for the reference combination of preformulated and standardised constraints. Size effects in general and acc. Bažant (2004) can be divided into mechanically based sub-effects-due to energy release in partial failures and material behaviour (ductile-quasi-brittle-brittle)and statistically based sub-effects-due to randomness of occurring properties and system 2 behaviour (serial-serial-parallel-parallel). Current design of timber structures consider 'size effects' general under assumption of a perfect brittle material behaviour, often independent of loading (tension-bending-compression-shear) and independent of influences due to stochastic (e.g. EN 384, EN 1194, EN 1995-1-1, DIN 1052). Within a finished research project at the competence centre holz.bau forschungs gmbh 'statistical size effects' of linear structural elements have been theoretically examined, influences of system size concerning serial linked elements and components, COV and representative statistical distribution models studied. Additional, comprehensive program by testing finger jointed construction timber (KVH ®) of various lengths in tension parallel to grain has been accomplished. Theoretical examinations clearly reflect the influence of examined distribution model, system size and COV on expectable 'size effect' and lead to relative simple equations to describe the 'length effect' on the mean and 5 %-quantile level. Practical experiments confirmed theoretically gained simplifications and reflect the predominance of stochastic in practical expectable 'length effects'. Proposals for further regulations are given.

The main strength and elastic properties of structural timber products, such as glued laminated timber (glulam; GLT) and cross-laminated timber (CLT), are usually described via load-bearing models, which use the tensile properties parallel to the grain of the base material boards and finger joints as input parameters. These load-bearing models assume that the strength-graded boards will retain their full dimensions in the final product. In some applications or use cases, however, the structural timber products are split lengthwise, e.g., split/resawn glulam, or comprise a random share of in width randomly lengthwise split lamellas. As a result of splitting, the material properties assigned to these boards during the grading process in their full cross-sections are no longer valid. Examples of such structural timber products are the novel flex_GLT-beams which are cut out from large dimensional multi-laminated timber panels. In the following paper, the bending properties and system ef...