Jan-Willem G. van de Kuilen

Growth reaction patterns of tree height, diameter, and volume of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) under acute drought stress in Southern Germany

Abstract Climate change in Central Europe may come along with acute drought stress, which can severely reduce growth and vitality of forest trees and whole stands. For a tree species such as Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) which is cultivated far beyond its natural range in Europe, knowledge of its behaviour under stress is crucial for the cultivation of Douglas-fir in view of a changing climate. Because of its easy accessibility, growth response to stress was mainly studied based on diameter growth at breast height. In long-term experiments on one dry and one moist site in Southern Germany, stem analyses of 133 mature and even-aged Douglas-firs were performed. The short-term growth reaction pattern under acute drought stress of 2003 had not only consequences on diameter but more pronounced effects can be observed when studying tree height: Respecting the different age trends by previous detrending, height increment only reacted more sensitive on the dry site. We also showed that extrapolating a particular decline in basal area increment to the whole stem can result in misunderstandings. However, results were less biased, when original data were smoothed or short-term assessment of volume growth was based on basal area measurements. By means of a linear mixed model approach, the influence of site, tree, and stand characteristics on Lloret’s indices of resistance and resilience (Lloret et al. in Oikos 120:1909–1920. doi:10.1111/j.1600-0706.2011.19372.x, 2011) were analysed. For Douglas-fir, site played a crucial role and became more important considering the age trend. On the contrary, the positive influence of site quality on drought tolerance decreased with data processing. However, more growing space by thinning can advance tree resistance and resilience regarding height, diameter, and volume growth. Large individual crown volume improved the growth pattern under drought, and large stand density impaired it. Douglas-fir is obviously equipped with a morphological variability, which fosters lateral rather than vertical growth allocation under severe stress. Silviculture can mitigate stress through the choice of the site and through lower stand densities by thinning. Our refined stress response analysis confirmed a favourable growth and resilience of Douglas-fir even under extreme drought events.

Efficiency of visual strength grading of timber with respect to origin, species, cross section, and grading rules: a critical evaluation of the common standards

Abstract Strength grading is essential for the efficient use of structural timber. Although international standards exist for machine strength grading, visual grading is still regulated based on national rules, which are expected to allow safe and economic grading results. Although there are large differences in the graded output because the species, the cross section, and the origin of the timber influence the results, some of these standards are considered to be applicable universally. The present article demonstrates how the chosen standards influence the grading results. Depending on the parameters, the yields or the mechanical properties are low compared with the declared values. The results also show the efficiency and applicability of different national standards for strength grading of timber from various origins. Furthermore, it is recommended to reconsider the existing limits for source areas and cross sections given in the standard EN 1912.

Influence of cross-section and knot assessment on the strength of visually graded Norway spruce

The strength of graded timber is determined by a multitude of parameters. Properties of interest are the shape of the cross-section and the wood quality. With regard to strength, wood quality is primarily expressed in terms of knots and knot clusters which, together with the cross-section of the timber, are used to calculate knot ratios. By applying the visual grading rules as given in the German standard DIN 4074-1, the influence of different timber sizes on grading results has been analysed. Different grading approaches for joists and boards exist and are taken into account in the assessment of 5,665 specimens originating from various parts of Europe. It was shown that both the cross-section and the grading method have a major influence on the characteristic strength values of Norway spruce. Limitations of the current standard with respect to its applicability to certain cross-sections are exposed. Alternative, simple grading approaches for boards are proposed. They ensure equal strength values and yields comparable to the rather complicated board rules used nowadays.ZusammenfassungDie Festigkeit von sortiertem Schnittholz kann durch mehrere Parameter beeinflusst werden. Querschnittsform und Holzqualität sind hierbei von besonderem Interesse. Im Zusammenhang mit der Festigkeit wird die Holzqualität vor allem über Äste und Astansammlung, die unter Berücksichtigung des Holzquerschnitts für die Berechnung von Kennzahlen verwendet werden, definiert. Die visuellen Sortierregeln der deutschen Norm DIN 4074-1 wurden angewandt, um den Einfluss des Holzquerschnitts auf das Sortierergebnis zu überprüfen. Unterschiedliche Sortierregeln für Kanthölzer und Bretter wurden beachtet, um 5,665 Prüfkörper aus Europa zu bewerten. Sowohl die gewählte Sortierregel als auch der Querschnitt haben einen wesentlichen Einfluss auf die charakteristischen Festigkeitswerte der Rotfichte. Es wird gezeigt, dass die aktuelle Norm bei gewissen Querschnitten nur eingeschränkt anwendbar ist. Alternative und gleichzeitig schlichtere Sortierregeln für Bretter werden vorgeschlagen. Diese gewährleisten gleiche Festigkeitswerte und ähnliche Ausbeuten im Vergleich zu den momentan verwendeten, komplizierteren Sortierregeln.

Analysis of determination methods for characteristic timber properties as related to growth area and grade yield

Abstract The origin of the raw material is a key aspect for strength grading of timber. Large grading areas are favored by the sawmilling industry as they require less effort in handling and documentation during the production process. However, large growth areas can also cause problems, as too high mechanical properties can be declared or yields may become uneconomical. The presented study presents a method that should allow for timber from different countries to be combined into a single grading area. Additionally, the influence on the yield for guaranteeing timber properties for differently defined populations is analysed. In this process, a number of available calculation methods for characteristic values for modulus of rupture, modulus of elasticity, and density are considered as the determination method also influences the final yield. Non-destructive and destructive test data from 8487 spruce specimens from Europe tested in bending or tension are the basis for the presented study. Based on the grading results the presented method is able to simply identify countries that may be combined. The definition of pan-European grading areas seems problematic if characteristic timber properties need to be guaranteed separately for each individual country as it may result in a severe drop in yield. However, checking timber properties only for the European population is unsatisfying as calculated timber properties considerably vary depending on the origin. As for the calculation method, the preferred method itself seems to have less impact on bending class assignments then on tension class assignments.

Measurement of moisture-related strain in bonded ash depending on adhesive type and glueline thickness

Abstract Structural wood-adhesive bonds (WAB) have to be durable while subjected to considerable stresses caused by mechanical loads and moisture content changes. To better understand the moisture-related durability of WABs, knowledge is important of how moisture changes generate strain in the bond. In this paper, strain on end-grain surfaces of bonded ash specimens was analyzed by means of digital image correlation. Strains were generated by wood shrinkage, and the evaluation was focused on shear strain (SStr). The bond lines were studied depending on the adhesive type – phenol resorcinol formaldehyde (PRF), melamine urea formaldehyde (MUF), polyurethane (PUR), and emulsion polymer isocyanates (EPI). Moreover, three different glueline (GL) thicknesses of MUF were taken into consideration. Comparing the adhesive types, SStr distributions (SStrD) were strongly influenced by adhesive elasticity. MUF and PRF bonds were quite rigid and were associated with pronounced strain amplitudes in and close to the GL together with strain dissipation reaching deep in the wood. PUR and EPI adhesives were more elastic and therefore allowed for smoother strain transition showing less distinct strain peaks. GL thickness had significant impact on SStrD. A high strain level and direct strain transition between adherends was found for the 0.01 mm GL, whereas a pronounced strain decrease was observed in the 0.1 and 0.2 mm GLs. This indicates different stress levels in the wood-adhesive interface dependent on GL thickness.

Critical section effect during derivation of settings for grading machines based on dynamic modulus of elasticity

ABSTRACT Testing of the critical section between the load points in four-point bending testing is required according to [DIN EN 384:2010–08, Structural Timber – Determination of Characteristic Values of Mechanical Properties and Density (Brussels European Committee for Standardization)]. However, strength grading machines based on measurements of the dynamic modulus of elasticity (MOEdyn) benefit as the strength of the weakest section is not always measured when deriving settings. According to the current procedure for derivation of settings, violation of the critical section requirement is accepted. The extent of such violation and its impact on grading settings of strength classes are quantified in two ways using about 3500 recently tested specimens of Norway spruce: empirically and modelled. The results show that for strength grading machines based on MOEdyn, the settings would be higher if the weakest section was always placed between the inner load points. The current procedure, however, leads to lower settings and hence better yields in practice. The settings for machines based on MOEdyn are up to more than 20% higher if the lowest bending strength of the entire board is applied. To treat different grading principles equally, a correction factor for settings of MOEdyn-based grading machines should be introduced – especially for lower strength classes. A simplified factor between 1.00 and 1.20 for standardization is recommended.

Slope of grain measurement: a tool to improve machine strength grading by detecting top ruptures

Severe local fiber deviation in timber boards can lead to reduced strength values. As grading machines do traditionally not detect these deviations, such boards are typically assigned to higher strength values than they actually possess. Based on statistics, these cases are taken into account in machine grading. Slope of grain can be measured on the surface by laser diffraction. The aim of this study is to estimate the reducing effect of local slope of grain on tensile strength, which is caused by top ruptures and the impact on the grading results of machine strength grading. This is investigated in full-scale tensile tests. In total, 192 test specimens of Norway spruce (Picea abies) with more or less pronounced local slope of grain were selected, measured and strength-tested to show the influence of local slope of grain on tensile strength. The utilized timber was sawn from relatively small diameter logs. The grain angle was determined on all four longitudinal sides of the boards with the help of a dense matrix. A model is presented for estimation of the grain deviation over the cross section of the sawn timber. These values are converted into a strength-reducing factor taking into account the ratio of tensile strength parallel and perpendicular to the grain. The introduced strength prediction algorithm is tested for the investigated sample. Improvement of the reliability and economy of timber grading using grain angle as an additional parameter will be possible. Furthermore, it is shown that different strength-reducing features such as top ruptures in sawn timber can be located on the specimens, and the strength-reducing effect can be estimated. Local slope of grain around knots is not part of this study.

Aspects of the Difference between the Local and Global Modulus of Elasticity of Structural (hardwood) Timber

The modulus of elasticity of structural timber (MOE) may be determined by 2 methods according to the European standard EN 408 [1]. The ratio between the so-called local and global MOE that is found from tests according to these two methods, cannot be explained by the ratio between the MOE and shear modulus G that is assumed in the strength class tables of EN 338 [2]. The relationship between MOElocal and MOEglobal from EN 384 [3] is not consistent with the shear values given in [2]. In this study the shear modulus for samples of the tropical hardwood species massaranduba and of softwood species spruce was determined. The shear modulus G was found to be not related to the MOE and was shown to be constant at around 550 N/mm2 for massaranduba and 190 N/mm2 for spruce. With these values, the ratios between MOE-local and MOE-global that were found in the test series could be explained. The found values for the shear moduli differ from previous research. The study concludes that it is unclear which parameters determine the magnitude of the shear modulus of a single piece of timber and that this needs to be investigated.