Constructiemateriaal

Civil Engineering and Geoscience

Wood properties from roundwood to timber engineering

Measuring and assessing wood properties during the production chain is getting more and more important for an optimal use of the resource. Over the years, research has been performed with the focus on establishing important wood properties, with the final goal of an optimized use in timber engineering. It is recognized that not all research results are easily translated into applications or code provisions. Timber grading, the conversion of grading results into strength class assignments, mechanical properties perpendicular to the grain of soft- and medium dense hardwoods are presented. The influence of density and fastener steel grade on the load carrying capacity of joints is discussed. Time-to-failure behaviour of joints is presented and it is shown that slightly more penalizing duration of load factors are required for joints behaving more brittle. Numerical modelling of joints is shown applying a modified Hill-criterion and a continuum damage mechanics model. Steel & Composite Structure

The residual load carrying capacity of timber joints

Timber joints that have been preloaded for 2 to 8 years have been short term tested in accordance with EN 26891. The applied load levels varied between 30% and 50% of the average short term strength. The study comprised nailed, toothed-plate and split-ring joints. All joints were made of spruce and loaded in tension. The test results indicated no strength loss during this period. The strength of the preloaded joints was at least equal to the average short term strength of joints with no pre loading prior to testing. Actually, the results indicate a slight increase in strength. The development of the strength of the joints in time is modelled with an exponential damage equation. The parameters of the damage equations have been determined on the basis of time to failure tests on the same types of joints.Civil Engineering and Geoscience

Duration of Load Effects in Timber Joints

Civil Engineering and Geoscience

Creep of timber joints

A creep analysis has been performed on nailed, toothed-plates and split-ring joints in a varying uncontrolled climate. The load levels varied between 30% and 50% of the average ultimate short term strength of these joints, tested in accordance with ISO 6891. The climate in which the tests were performed varied between about 40% and 90% relative humidity, which coincides with Service classes 1 and 2 of Eurocode 5 for timber structures. A large scatter in creep factors was found with the highest average creep values for the 30% load levels. In order to analyse the influence of moisture variations, a creep model was developed containing the effects of mechanical creep, the influence of yearly shrinking and swelling as well as mechano-sorptive creep. Furthermore, a method has been presented in order to be able to derive creep factors to be applied in design calculations, based on the creep factors determined in laboratory creep tests, taking into account the design stiffness values for these joint types.Design and ConstructionCivil Engineering and Geoscience

Time-of-flight modeling of transversal ultrasonic scan of wood

Time-of-flight is a time for an ultrasonic pulse to cross a sample. It contains valuable information about the mechanical properties of a material. For the ultrasonic pulse propagating in wood perpendicular to the grain the relation between the time-of-flight and the elastic constants is rather complex due to the strong anisotropicity of wood. With the help of some assumptions this relation can be established from the elastic theory. The analytical calculation results in a function which represents a change of time-offlight when the direction of propagation shifts from the radial to the tangential direction while scanning a board crosswise. The function takes into account the location of the pith and the geometry of the growth rings. The measurement performed on a sample of European spruce confirms the theoretical prediction.Structural EngineeringCivil Engineering and Geoscience

Pre-grading of Douglas-fir logs for glulam lamella production

Structural EngineeringCivil Engineering and Geoscience

Strength grading of timber in Europe with regard to different grading methods

Structural timber in Europe is either graded visually or by machine. Both grading methods are applied to limit the variation in engineering properties of sawn timber. The obtained grading results are largely dependent on the method chosen. In addition, parameters such as species, source, and cross-section of the timber, as well as the applied grading rules also play a role. To what extent these parameters - depending on the chosen grading method - actually affect timber properties and yields is of interest for both producers and users of sawn timber. For analysing the different grading results, laboratory data of 10704 spruce specimens were evaluated. The used cross-section has a major influence on the grading result. Furthermore, the used grading rule and the method applied to determine characteristic values are essential for the grading result. The origin of the timber influences the grading results of both grading methods. While the yields for machine grading are always higher than for visual grading, both grading methods are prone to fall short of the declared properties. It is recommended to adjust the normative framework as well as to regulate both grading procedures similarly.Structural EngineeringCivil Engineering and Geoscience

Development of a high-capacity engineered wood product-LVL made of European beech (fagus sylvatica l.)

Structural EngineeringCivil Engineering and Geoscience

Material model for wood

Wood is highly anisotropic and shows ductile behaviour in compression and brittle behaviour in tension and shear where both failure modes can occur simultaneously. A 3D material model for wood based on the concepts of continuum damage mechanics was developed. A material subroutine containing the developed model was implemented into a standard FE framework. Eight stress-based failure criteria were derived in order to formulate piecewise defined failure surfaces. The damage development of wood was controlled by nine damage variables. Embedment tests using three different wood species (spruce, beech, azobé) were carried out whose results were compared to modelling outcomes. The failure modes could be identified and the general shape of the load-displacement curves agreed with the experimental outcomes up to a numerical limit.Structural EngineeringCivil Engineering and Geoscience