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Shear strength of glued laminated timber made from European beech timber

Schubfestigkeit von Brettschichtholz aus europäischem Buchenholz

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Abstract

This paper describes an investigation into the shear strength of glued laminated timber (GLT) made from European beech. Special consideration was paid to the possible strength influence of a frequently occurring discolouration of the timber, termed red heartwood, which is inherent to the species. The beech lamellae were visually graded according to German hardwood grading standard DIN 4074-5. Grade LS13, conforming to European hardwood strength class D40, was split into two sub-grades LS13− and LS13+. Additionally, modulus of elasticity (MOE) was determined by ultrasound pulse, longitudinal vibration and static tension tests. Sub-grade LS13+ showed a mean density and MOE of 690 kg/m3 and 14,800 N/mm2, respectively.

The GLT shear strength was evaluated by means of four-point bending tests on structural sized I-shaped beams with a depth of 0.6 m and a span to depth ratio of 5:1. The slightly inhomogeneous build-up of the cross-section conformed to glulam strength class GL42c. Two beam samples were investigated, each with seven specimens, where one grouping had no red heartwood and the other with a high red heartwood in the web laminations. Additionally block shear tests on bond line strength were performed with standardized small specimens according to EN 392.

Neither the beam shear capacity tests nor the bond line block shear tests revealed an influence of the red heartwood discolouration on strength. The fifth-percentile value of shear strength of all beams was 3.5 N/mm2. The results of the block shear tests indicate that the present requirements on minimum block shear strength are set too low in the European standard EN 386 with regard to beech GLT.

Zusammenfassung

Der Aufsatz berichtet über Untersuchungen zur Schubfestigkeit von Brettschichtholz (BSH) aus Europäischer Buche. Besondere Beachtung wurde dem eventuellen Festigkeitseinfluss einer häufig auftretenden holzartspezifischen Verfärbung des Holzes, Rotkern genannt, zugemessen. Die Buchenholz-Brettlamellen wurden visuell nach der deutschen Laubholzsortiernorm DIN 4074-5 sortiert. Die Sortierklasse LS13, die der Europäischen Laubholzfestigkeitsklasse D40 zugeordnet ist, wurde in die Klassen LS13− und LS13+ untergliedert. Zusätzlich wurde der Elastizitätsmodul mittels Ultraschalllaufzeit, Längsschwingung und statischer Zugversuche bestimmt. Die Sortierunterklasse LS13+ wies eine mittlere Rohdichte von 690 kg/m3 und einen Elastizitätsmodul von 14.800 N/mm2 auf.

Die BSH-Schubfestigkeitsuntersuchungen wurden mittels Vier-Punkt-Biegeversuchen an großformatigen I-Trägern mit einer Querschnittshöhe von 0,6 m und einem Stützweiten-Querschnittshöhenverhältnis von 5:1 durchgeführt. Der geringfügig inhomogene Querschnittsaufbau entsprach der BSH-Festigkeitsklasse GL42c. Die Untersuchungen umfassten zwei Trägerkollektive, mit je sieben Prüfkörpern, jeweils ohne bzw. mit hohem Rotkernanteil in den Steglamellen. Weiterhin wurden Blockscherversuche zur Klebfugenfestigkeit an kleinen Normprüfkörpern nach EN 392 durchgeführt.

Weder die Schubtragfähigkeitsversuche an den Biegeträgern noch die Klebefugen-Blockscherversuche zeigten einen Festigkeitseinfluss der Rotkernverfärbung. Die 5%-Quantile der Schubfestigkeiten aller Träger ergab sich zu 3,5 N/mm2. Die Ergebnisse der Blockscherversuche weisen darauf hin, dass die derzeitigen Anforderungen an die Klebfugenscherfestigkeit in EN 386 in Bezug auf Buchen-BSH zu niedrig sind.

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References

Referred standards and technical approvals

  • ASTM D143-94 (2007) Standard test methods for small clear specimens of timber. ASTM International, West Conshohocken, PA. doi:10.1520/D0143-94R07

  • ASTM D2555 (2006) Standard practice for establishing clear wood strength values. ASTM International, West Conshohocken, PA. doi:10.1520/D2555-06

  • ASTM D2915 (2003) Standard practice for evaluating allowable properties for grades of structural lumber, West Conshohocken, PA. doi:10.1520/D2915-03

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Acknowledgements

This study was performed at the Institute of Forest Utilization and Work Science of the University of Freiburg and at MPA University of Stuttgart, Department of Timber Constructions, in the frame of the EU-CRAFT-project “Innovation for Beech” and the BMBF project “Large dimensioned timber, sub-work package hardwood products”. Financial support provided by the European Commission as well as by the Federal Ministry of Education and Research (BMBF) is gratefully acknowledged. Many thanks are indebted to the glulam manufacturer Burgbacher Holztechnologie GmbH for the production of the glulam beams and its valuable contributions to the study.

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  1. MPA Stuttgart, Otto-Graf-Institute, Department Timber Structures, University of Stuttgart, Pfaffenwaldring 4, 70569, Stuttgart (Vaihingen), Germany

    Simon Aicher

  2. Institute of Forest Utilization and Work Science, Albert-Ludwigs-University of Freiburg, Werthmannstr. 6, 79085, Freiburg, Germany

    Denny Ohnesorge

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Correspondence to Simon Aicher.

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Aicher, S., Ohnesorge, D. Shear strength of glued laminated timber made from European beech timber. Eur. J. Wood Prod. 69, 143–154 (2011). https://doi.org/10.1007/s00107-009-0399-9

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