Strength grading of Norway spruce structural timber: revisiting property relationships used in EN 338 classification system

Solid timber for structural applications has to be strength graded prior to its use. In order to remain economic the grading process usually focuses on the most important physical and mechanical properties: density, modulus of elasticity (MOE) and bending strength. Based on respective limits given in standards, the timber is assigned to strength classes. Additional mechanical properties such as tensile and compression strength parallel to the grain are derived from the basic property values by empirical relationships. The objective of this study was to review some of these property relationships based on recently compiled large data sets as a contribution for a future revision of the grading standards. Based on mechanical tests of Norway spruce structural timber with different cross-sections, the following characteristic values and property relationships were evaluated: (a) strength and MOE in bending, (b) in-grade characteristic values of bending strength, bending MOE and density, (c) relationship of characteristic values of tension and compression strength parallel to the grain with respect to the corresponding characteristic value of bending strength, (d) ratio of fifth percentiles and mean values of density and MOE, as well as (e) the ratio of MOE in bending, tension and compression. Mechanical tests were accompanied by measurements of density and ultrasonic wave speed. Resulting dynamic MOE was partly used as an indicator of timber qualit

Quality control of glulam: shear testing of bondlines

In quality control of glulam, shear testing of bondlines is required. However, in relevant standards the method of applying shear stress to the bondline is only exemplified by a principle schematic. Based on this schematic a considerable variety of test equipments is in use nowadays. Depending on the respective construction of the test equipment as well as on the execution of testing, the resulting stress in the bondline is not pure shear but rather a combination of shear and normal stresses. When the normal stresses are acting as tensile stresses perpendicular to the bondline, the registered shear strength values range considerably below the pure shear stress level, whereas compression stresses perpendicular to the grain lead to an overestimation of the shear strength of the bondline. To overcome this deficiency, a prototype of a modified shear test device was developed, which ensures a clearly defined state of shear loading of the specimens. Based on extensive comparative testing it is shown that by performing compression tests at an oblique angle to the grain of 14° the above mentioned problems can be avoide

Assessment of resonance wood quality by comparing its physical and histological properties

The quality of wood used for music instrument making (resonance wood) is determined by assessing six physical properties: density, modulus of elasticity, sound velocity, radiation ratio, emission ratio, and loudness index. This can easily be done by means of measurements of the resonance frequency and the corresponding damping factor. The method described here is based on vibrational analyses, adapted from standard non-destructive testing of solid material, so as to provide information both for scientific studies and for violin making. The above six properties were assessed in samples of resonance wood of different quality and in normal (control) wood of Norway spruce and sycamore. The differences observed between the samples correlated with anatomical or histological characteristics of the wood. A sample of best-quality Norway spruce resonance wood showed a high radiation ratio in the axial direction, which correlated with the presence of small wood cells with thin cell walls. In "curly maple”, a high sound velocity in the radial direction correlated with the presence of broad xylem rays. The influence of external factors like wood moisture content or the geometry of the system is discussed within the context of the present stud

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 procedur

Zerstörungsfreie Bestimmung elastischer Eigenschaften quadratischer 3-schichtiger Brettsperrholzplatten mit symmetrischem Aufbau

Zusammenfassung: Die Steifigkeiten von Holzwerkstoffen, wie z.B. von Brettsperrholzplatten, zählen bei statischen Berechnungen zu den wichtigsten Materialeigenschaften. Bei Platten sind es insbesondere die zwei Elastizitätsmoduln (Eii) in Plattenebene und die drei Schubmoduln (Gij). Um diese Parameter zerstörungsfrei und mit wenig Arbeitsaufwand bestimmen zu können, wurde ein Verfahren entwickelt, mit dem auf Grundlage von Schwingungsanalysen alle fünf Steifigkeiten in nur einem Versuch bestimmt werden können. Das Verfahren wurde auf 24 aus zwei verschiedenen Herstellerwerken stammenden, quadratischen Brettsperrholzplatten mit Seitenlänge 2.5m und einer Dicke von 70mm mit zwei unterschiedlichen, symmetrischen Aufbauten angewandt. Die so experimentell ermittelten Steifigkeiten wurden mit Biegeversuchen verifiziert und mit auf Grundlage der Verbundtheorie berechneten Steifigkeiten verglichen. Für alle Platten konnten unabhängig von deren Produktionsmethode und Aufbau vier Steifigkeiten sicher bestimmt und die Richtigkeit der Werte verifiziert werden. Die Bestimmbarkeit der Steifigkeit G23 ist von der Geometrie der Platte abhängig. Bemerkenswert ist die Tatsache, dass in der Praxis oft angewendete Steifigkeitsberechnungen, die unter der Annahme durchgeführt wurden, dass das Rohmaterial der einzelnen Schichten der Festigkeitsklasse C24 entspricht, zu optimistische Werte liefern könne

Mechanical properties of thermally modified beech timber for structural purposes

The use of thermally modified timber for structural purposes is of increasing interest. In order to guarantee sufficient reliability in terms of load bearing capacity and fitness for use the strength and stiffness properties of this modified wood have to be assessed. Industrially produced, thermally modified structural timber members made of beech (Fagus sylvatica L.) were subject of the tests presented in this paper. Bending, tension parallel and perpendicular to grain and compression parallel and perpendicular to grain properties were determined. The derived mechanical properties were benchmarked to the European EN 338 strength class system for structural timber. It turned out that the used strong thermal treatment of the raw material resulted in a significant reduction of most of the strength properties. However, stiffness properties were not affected. In particular the strength properties perpendicular to grain suffered a lot due to the thermal modification whereas compression strength parallel to grain was unchanged. The main drawbacks found along the experiments were a pronounced brittle behaviour of the specimens and big variations in strength. For the determination of strength values it is proposed not to use correlations as provided in European standards but to test and state these properties discretely. On the basis of these results a general use of strongly thermally modified beech as structural timber cannot be recommended. However, for selected purposes, like e.g., for structural façade elements or for columns, the use of this material might be an optio

Abundant topological outliers in social media data and their effect on spatial analysis

Twitter and related social media feeds have become valuable data sources to many fields of research. Numerous researchers have thereby used social media posts for spatial analysis, since many of them contain explicit geographic locations. However, despite its widespread use within applied research, a thorough understanding of the underlying spatial characteristics of these data is still lacking. In this paper, we investigate how topological outliers influence the outcomes of spatial analyses of social media data. These outliers appear when different users contribute heterogeneous information about different phenomena simultaneously from similar locations. As a consequence, various messages representing different spatial phenomena are captured closely to each other, and are at risk to be falsely related in a spatial analysis. Our results reveal indications for corresponding spurious effects when analyzing Twitter data. Further, we show how the outliers distort the range of outcomes of spatial analysis methods. This has significant influence on the power of spatial inferential techniques, and, more generally, on the validity and interpretability of spatial analysis results. We further investigate how the issues caused by topological outliers are composed in detail. We unveil that multiple disturbing effects are acting simultaneously and that these are related to the geographic scales of the involved overlapping patterns. Our results show that at some scale configurations, the disturbances added through overlap are more severe than at others. Further, their behavior turns into a volatile and almost chaotic fluctuation when the scales of the involved patterns become too different. Overall, our results highlight the critical importance of thoroughly considering the specific characteristics of social media data when analyzing them spatially

Comparison of bending stiffness of cross-laminated solid timber derived by modal analysis of full panels and by bending tests ofstrip-shaped specimens

The design of cross-laminated solid timber (CLT) as load-bearing plates is mainly governed by serviceability criterions like maximal deflection and susceptibility to vibration. Hence, predicting the respective behavior of such plates requires accurate information about their elastic properties. According to product standards, the bending stiffness of CLT has to be assessed from 4-point bending tests of strip-shaped specimens, cut from the CLT panels. By comparing elastic properties of CLT derived by means of modal analysis of full panels with the results of bending tests on 100mm and 300mm wide strip-shaped specimens it is shown, that by testing single 100mm wide strip-shaped specimens bending stiffness of full panels cannot be assessed correctly, whereas single 300mm wide strips or averages of 5 to 6 100mm wide strip-shaped specimens lead to acceptable results. Hence, strip-shaped specimens should only be used in the course of factory quality control or when assessing the bending stiffness of parts of CLT panels used as beam-like load-bearing elements but not to derive bending stiffness of gross CLT panels. Verification by carrying out static bending tests of gross CLT panels under different loading situations showed that alternatively to tests on strip-shaped specimens or estimations with the compound theory, the overall stiffness properties of CLT can be derived directly by a modal analysis of full-size panel

Versuche und Berechnungen an allseitig gelagerten 3-schichtigen Brettsperrholzplatten

Zusammenfassung: Im Rahmen eines umfangreichen Forschungsprojektes wurden Brettsperrholzplatten (BSP), die im Bauwesen auch als statisch tragende Bauteile eingesetzt werden, untersucht. Quadratische dreischichtige BSP mit Seitenlänge 2,5m und Dicke 70mm von zwei Herstellern wurden statisch senkrecht zur Plattenebene bis zum Bruch geprüft. Untersucht wurden drei verschiedene Laststellungen und zwei Querschnittsaufbauten. Ultraschallmessungen an den Einzelschichten vor der Verklebung dienten zur Schätzung der Elastizitätsmoduln der Einzelschichten. Die Elastizitätsmoduln und Festigkeiten der verklebten Platten parallel und senkrecht zur Faserrichtung der Deckschicht wurden zusätzlich in Balkenversuchen ermittelt. Ergebnisse und Erkenntnisse aus den Ultraschallmessungen, Balken- und Plattenversuchen werden präsentiert. Verschiedenste Balken- und Platten-Berechnungsmodelle für die BSP werden vorgestellt. Die Versuchsresultate werden mit den Berechnungen verglichen. Ein Berechnungsvorschlag für die Praxis wird präsentier

Integrity check of structural softwood glue lines: correspondence between delamination and block shear tests

In factory production control (FPC) of glued-laminated timber the integrity of glue lines according to product standards has to be checked by carrying out either delamination or block shear tests. The chosen test method should provide reliable and reproducible results, and from a practical point of view should be as cheap and easy to perform as possible. An extensive experimental study aiming at systematically comparing the outcomes of delamination and shear tests on structural softwood glue lines using different wood species and adhesives was carried out. In the delamination tests, the length of open glue lines was assessed after subjecting the test specimens to vacuum- and pressure-soaking and to subsequent drying. In the block shear tests, the shear strength and wood failure percentage were determined. Only if the benchmark values in the product standards were met or exceeded, the respective specimen passed the test and the glue lines were regarded as sound. As a main overall result it turned out that the outcomes of both test methods generally agree well, but will not necessarily lead to the same pass/fail conclusion in any case. Therefore, for FPC the choice of their application should be defined more specifically