This report represents the results of the activities performed in working group 1, Basis of Design. The most important task of working group 1 was the defragmentation and harmonization of techniques and methods that are necessary to prove the reliable, safe and economic application of timber materials or products in the construction industry.
This report is structured into five parts. At first general principles regarding the design formats are addressed (Part I). Afterwords timber specific aspects regarding code calibration (Part II) and serviceability (Part III) are summarized. In Part IV other demanding issues for the implementation into Eurocode 5 are addressed. Here also summaries of joint activities with other working groups on cross laminated timber and timber connections are presented. The report concludes with a guideline for data analysis (Part V).
At the Institute of Structural Engineering at the ETH Zurich numerous of investigations are conducted to analyse the load bearing capacity of glued laminated timber beams. The investigations are part of the research project ’Influence of varying material properties on the load bearing capacity of glued laminated timber (glulam)’. The investigations are taking place on 24 glulam beams with well-known material properties. The glulam beams are fabricated out of 400 timber boards. From those boards the material properties are investigated non-destructively within a former research project. During the glulam fabrication it is particularly focused to keep the information of the timber boards; i.e. after the glulam fabrication the position of each particular timber board within the glulam beam and thus the position of each particular knot is still known. The glulam beams are investigated during a 4-point bending test. On the glulam members the load bearing capacity, the bending stiffness and the density is measured. Furthermore local strains within the glulam beams are investigated using an optical coordinate-measurement device. Following the test the failure is investigated in detail. Hereby the type of failure (knot cluster, finger joint, clear wood) and the amount of failure (number of damaged lamellas) is documented. Afterwards the failed glulam beams are loaded again to analyse the remaining bending strength and the corresponding remaining bending stiffness. The major aim of the experimental analysis is the investigation of the load bearing capacity of glulam beams with well-known local material properties. The gained results can be used for an investigation of the influence of local weak zones, such as knot clusters or finger joints, on the load bearing capacity of glulam. In addition a data basis is produced to develop a new model (or to evaluate existing models) for the estimation of the load bearing capacity of glulam.
Society of Wood Science and Technology International Convention
Research Status
Complete
Summary
The application of deconstructable connectors in timber-concrete composite (TCC) floors enables the possibility of disassembly and reuse of timber materials at the end of building’s life. This paper introduces the initial concept of a deconstructable TCC connector comprised of a self-tapping screw embedded in a plug made of rigid polyvinyl chloride and a level adjuster made of silicone rubber. This connection system is versatile and can be applied for prefabrication and in-situ concrete casting of TCC floors in both wet-dry and dry-dry systems. The paper presents the results of preliminary tests on the shear performance of four different configurations of the connector system in T-section glulam-concrete composites. The shear performance is compared to that of a permanent connector made with the same type of self-tapping screw. The failure modes observed are also analyzed to provide technical information for further optimization of the connector in the future.
Cross-laminated timber panels offer an effective option for timber structures; they allow biaxial load transfer and have good dimensional stability. However, practical transportation and handling limits size of the panel and a stiff connection between the panels is required to effectively utilise biaxial properties. In this paper, a dovetail splice joint for timber panels is presented using cross-banded LVL with cast concrete grout interlayer. The interlayer allows a tight fit, which is important for stiffness, but also avoiding installation problems due to manufacturing tolerances and moisture-induced dimensional changes. The mechanical behaviour of the dovetail joint was investigated experimentally for various geometries. Furthermore, a numerical model was developed that shows a wide agreement with the experiments, especially in the cases with governing failure in the LVL. Using the numerical model, a parameter study was performed where the influence of the connection length (number of dovetails) and the joint geometry on the strength and stiffness properties was investigated. Besides the optimal geometrical configurations of the dovetail joint, also a significant increase of the strength and stiffness properties with increasing connection length was identified.
Design of modern timber floors is often governed by the vibration serviceability requirements. One way to improve vibration serviceability is through the design of two-way floor systems. In this paper, the behaviour of two-way LVL–concrete composite plates and a plate strip is investigated experimentally, with an emphasis on the performance of proposed dovetail joint for connecting the adjacent LVL panels. The investigations consist of the experimental modal analysis and static load deformation tests, performed under multiple support conditions. The results show a significant two-way action, indicated by about 45% higher fundamental natural frequency when four edges are supported instead of two. The point load deflection in the centre of the plate was reduced of about 9%. Furthermore, a numerical model for two-way TCC plates was developed and results show a wide agreement with the experimental behaviour, except for discrepancies related to deflections on the plate edge. The results from the experimental and numerical investigations indicate that the dovetail joint can produce a stiff connection, such that the LVL layer could be regarded as continuous in the connected direction.
Glued laminated timber (GLT) is a structural product composed of several layers of timber boards glued together. GLT components have many advantages, such as the larger range of available component dimensions to choose from, the environmental sustainability or the efficient ratio between weight and load-bearing capacity. Because of that, GLT beams have been established as one of the most important products in timber engineering in the last decades.
As a natural grown material, timber properties exhibit higher variability, compared with other building materials. The variability is pronounced not only between different structural elements but also within single elements, the latter being highly related to the occurrence of knot clusters. Due to the highly inhomogeneous structure of timber, the prediction of the material properties of GLT beams is affected by large uncertainties. In the presented thesis, the influence of varying material properties on the load-bearing capacity of GLT beams was investigated. Thus the thesis contributes to develop the quality of GLT beams, in terms of reliability and efficiency.
Detailed, non-destructive investigations of altogether 400 timber boards were performed. Thereby, different strength and stiffness related indicators, such as the position and characteristic of knots, or the eigenfrequency, were assessed. Furthermore, non-destructive tensile test were performed to estimate the stiffness properties of knot clusters. Out of the investigated timber boards, GLT beams having a precisely-known beam setup were fabricated. As a result, the exact position of each particular timber board (and each particular knot cluster) within the GLT beams was known. Afterwards, bending tests were performed to estimate the load-bearing capacity of these GLT beams. Thereby, the influence of knot clusters and finger joint connections on the deformation and failure behaviour was investigated.
In addition to the experimental investigations, a probabilistic approach for modelling GLT beams (referred to as GLT model) was developed. Thereby, at first, timber boards are simulated according to their natural growth characteristics. Afterwards, out of the simulated timber boards, virtual GLT beams are fabricated. Finally, the load-bearing behaviour of these GLT beams is estimated by using a numerical model. To assure the quality of the numerical model, it was validated with the test results. Using the GLT model, the influence of different parameters, such as the position and characteristics of knots, or the quality of finger joint connections, on the load-bearing capacity of GLT beams was investigated.
One further goal of this thesis was the investigation of machine-grading indicators, that are measured during the grading process. Therefore, all the investigations presented in this thesis are conducted for indicators measured in laboratory and machine-grading indicators. The same applies for the GLT model, which was also developed for both types of indicators.
At the institute of structural engineering at the ETH Zurich multiple of investigations are conducted to analyse the material properties of Norway spruce timber boards. The investigations are part of the research project “Influence of varying material properties on the load bearing capacity of glued laminated timber (glulam)”. The majority of the investigations are non-destructively.
The investigations are taking place on 400 timber boards. On all specimens the moisture content, the density, the Eigenfrequency and the longitudinal ultrasonic runtime was investigated. Further all knots with a diameter larger then 10mm are measured. Thereby the position and the size of all the knots are documented. Subsequently on 200 selected boards non-destructive tensile test are performed to analyse the local young modulus. Herewith it was particularly focused on the investigation of the stiffness of areas having knots or knot clusters and areas without knots. The strains are measured with an optical coordinatemeasurement device. In the last part of the experimental investigation the deformation and failure behaviour of significant knot clusters is analysed. The strains are measured with digital image correlation.
Focus of the entire experimental analysis was the investigation of the young modulus and the quantifications of its variability within timber members and between timber members. Within this study a database was produced to evaluate existing test methods for the estimation of the young modulus. Further, the results can be used as a basis for further investigations on the variability of structural timber.
The aim of this study is to investigate the economic, environmental and technological challenges, as well as the environmental potential, of prolonging the life cycle of construction materials with focus on structural timber in Finland. To achieve this, a literature review was conducted along with interviews with actors pertinent to timber construction. Moreover, a case study of life cycle environmental impact assessment was conducted to quantify the potential of reusing timber to abate global warming and other environmental burden. The literature review highlighted the possibility of reusing structural timber, but pointed to the need for efficient and standardized assessment criteria. The interviews indicated interest towards the concept of circular economy applied to construction and demolition wood material, although this appears to be driven more by policy and regulation rather than for business reasons. Therefore, a reconfigured conceptual framework to achieve circularity for wood is proposed, where material brokers would be used to connect different actors along the value chain. The paper concludes with a case study showing that reusing structural timber components can result in a significant reduction of the environmental burden.
