Robert Widmann

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.

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.