René Steiger

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 study.

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 quality.

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 option.ZusammenfassungDie Verwendung von thermisch modifiziertem Holz erfreut sich wachsender Beliebtheit. Für eine Erweiterung der möglichen Anwendungen auf tragende Bauteile müssen deren Festigkeits- und Steifigkeitseigenschaften bekannt sein. Industriell thermisch behandeltes Buchenholz (Fagus sylvatica L.) in Bauteilabmessungen wurde Biege-, Zug- und Druckversuchen parallel und senkrecht zur Faser unterzogen. Die ermittelten Parameter wurden den Festigkeitsklassen nach EN 338 zugeordnet. Es zeigte sich, dass die verwendete intensive thermische Modifikation zu einer signifikanten Reduktion der Festigkeitseigenschaften führte, während die Steifigkeiten mehr oder weniger unverändert blieben. Besonders betroffen von der Reduktion der Werte waren die Festigkeiten senkrecht zur Faser des thermisch behandelten Holzes. Als großes Manko des Materials erwies sich während der Versuche das spröde Bruchverhalten sowie die große Streuung der Festigkeitswerte. Auf der Basis der Resultate kann das verwendete stark thermisch modifizierte Buchenholz nicht für eine generelle Verwendung in tragenden Bauteilen empfohlen werden. Für spezielle Anwendungen, beispielsweise Stützen oder Fassadenelemente, könnte der Einsatz dieses Materials jedoch eine Option sein.

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 avoided.ZusammenfassungIm Rahmen der Qualitätskontrolle von Brettschichtholz sind Scherprüfungen an Klebfugen durchzuführen. Die bei der Prüfung anzusetzende Belastungskonfiguration ist in den relevanten Prüfnormen lediglich in Form einer Prinzipskizze festgelegt. Heutzutage wird eine Vielzahl unterschiedlicher Prüfgeräte eingesetzt, welche auf Basis der erwähnten Prinzipskizze entwickelt wurden. Beeinflußt durch die konstruktive Umsetzung der Prinzipskizze und durch die Art der Durchführung der Scherprüfung, entspricht der effektiv in der Scherfuge herrschende Spannungszustand nicht reinem Schub, sondern vielmehr einer Kombination von Scherspannung und Spannungen senkrecht zur Scherfläche. In der Scherfuge gleichzeitig mit den Scherspannungen wirkende Zugspannungen senkrecht zur Fuge bewirken ein merkbares Absinken der im Versuch erzielten Scherfestigkeitswerte, z. T. gar unter die eigentliche Schubfestigkeit des Holzes. Auf die Scherfuge senkrecht einwirkende Druckspannungen demgegenüber können zu einer Überschätzung der Scherfestigkeit der Klebfuge führen. Um diesem Problem zu begegnen, wurde ein Prototyp einer Scherprüfeinrichtung entwickelt, welcher den Prüfkörper einem definierten Scherspannungszustand aussetzt. Basierend auf umfangreichen vergleichenden Prüfungen wird gezeigt, dass durch Druckscherversuche unter einem Kraft- zur Faserwinkel von 14° die oben erwähnten Schwierigkeiten umgangen werden können.

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 procedure.ZusammenfassungDie Biege- und die Schubsteifigkeit von Brettsperrholz (CLT) aus Nadelholz sind von der Holzfeuchte der Platten abhängig. Die Steifigkeiten nehmen grundsätzlich mit zunehmender Holzfeuchte ab. Die Quellung der Brettlagen kann jedoch, einhergehend mit der Schließung offener Fugen, die Steifigkeit durch innere Reibung scheinbar erhöhen. Diese Steifigkeitserhöhung ist allerdings nur bei geringem Lastniveau relevant, was zu Fehlinterpretation bei Steifigkeitsmessungen mittels dynamischer Verfahren, wie z.B. der Modalanalyse führen kann. Das Austrocknen von Brettsperrholzplatten andererseits bewirkt Risse in den Brettlagen, was sich v. a. in einer markanten Reduktion der Steifigkeit senkrecht zur Faserrichtung der Decklagen äußert. Daher sollen die Decklagen bei der rechnerischen Ermittlung der Biegesteifigkeiten nach der Verbundtheorie als nicht mitwirkend betrachtet werden.

State-of-the-Art Review of Approaches for the Design of Timber Beams with Notches

AbstractNotches are details that significantly reduce the load-carrying capacity of beams made of timber. The amount of reduction was the subject of investigation in numerous studies from an experimental or theoretical point of view. The singular stress distribution around the notch corner can be described by equations based on linear elastic fracture mechanics, fracture energy, or complex solutions. From experimental tests, simplified equations can be proposed. Standards, codes, and handbooks for the design of timber structures are based on such equations. When comparing the dimensions of notched beams tested in experiments with those that are relevant in practical applications, specimens of small dimensions were primarily tested. The application of design approaches verified by tests on specimens of small dimensions and the use of material properties determined from small clear specimens do not account for size effects that are important when working with beams of large dimensions. The influence of dura...

Bending Strength and Stiffness of Glulam Beams Made of Thermally Modified Beech Timber

The paper describes tests carried out on structural glued laminated timber (glulam) beams and finger-jointed boards made out of thermally modified hardwood (beech, fagus sylvatica) in the following named as TMTB. The finger joints were bonded with a two-component PRF adhesive and the lamellas were edge-bonded using a two-component MUF adhesive. The finger jointed lamellas were tested in tension, flatwise- and edgewise bending. While automatically produced finger joints mostly showed unsatisfactory strengths, it was possible with manually produced finger joints to achieve higher strength values. Fifty glulam TMTB beams were produced to evaluate their load carrying behaviour. The beams were tested in 4-point bending and the integrity of the glue lines was verified by means of delamination tests and shear tests. Usually it is expected that combining lamellas of a certain strength class to a glulam beam will enhance certain characteristic mechanical properties of the final product compared to the properties of single boards. The results of the tests could not confirm this behaviour for the TMTB glulam beams even if the bond lines proved to be of a satisfactory quality. Hence, a structural use of TMTB glulam seems to be restricted to a limited range of applications.

Fully Threaded Self-tapping Screws Subjected to Combined Axial and Lateral Loading with Different Load to Grain Angles

In order to benefit from the advantages of fully threaded self-tapping screws as reinforcing elements, it is essential to have detailed knowledge about the strength and stiffness of screws with different shank to grain angles subjected to combined axial and lateral loading. In this paper a design model is proposed for the calculation of the load-carrying capacity and stiffness of screws with different shank to grain angles subjected to loads perpendicular to the grain by accounting for the effective embedment of the screw in the timber. The proposed model is based on commonly used material properties and other established design models and fits well the influence of the angle between shank and grain direction on the joint’s capacity and stiffness. However, detailed knowledge about the specific input parameters in the design approaches is necessary in order to achieve a reliable prediction of the load-carrying capacity and stiffness of the individual screw.

Impact of growth characteristics on the fracture perpendicular to the grain of timber

The natural material wood is commonly graded with regard to the parallel to the grain strength and stiffness properties and taking into account different growth characteristics such as knots and grain deviations. In this paper the impact of knots and grain deviations on the fracture perpendicular to the grain of timber is analysed by means of numerical models. The results are used for the calibration of an analytical model. With this model it is possible to evaluate the impact of growth characteristics on the perpendicular to the grain fracture and compare the results with test data from literature. The evaluation shows that certain growth characteristics increase the strength perpendicular to the grain. This is in contrast with current grading procedures, where such growth characteristics are considered as being strength decreasing. The results are compared with a model for the description of the effects of growth characteristics on the distribution characteristics of the strength perpendicular to the grain. This strongest link model can be used to describe phenomena with a parallel system of failure events.

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.