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Effects of grading procedures on the scatter of characteristic values of European grown sawn timber

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Abstract

The natural scatter in mechanical properties of sawn timber must be reduced by grading the material either visually or mechanically. Depending on the grading procedure, the scatter of these properties varies. This study deals with their variation as influenced by the grading procedure. The effect of the grading principle is analyzed based on 4,893 sawn timber specimens from several European natural forests with widths up to 167 mm and depths up to 284 mm and using the method given in EN 14081-2:2010, CEN, Brussels (2010). Grading models for visual grading and machine grading are derived considering different source countries, strength classes and strength class combinations. Material safety factors for the graded material are then estimated in accordance with ISO 2394 (1998) to evaluate the grading outcomes. Analyzing and comparing the lower 5th-percentile to the requirements of EN 384: 2009, CEN, Brussels (2009), it is found that the actual strength for class C24 can be up to 20 % lower than required by the standard. This is true, regardless of whether the timber is graded visually or by an advanced grading machine using dynamic modulus of elasticity and knots. Low strength values can be expected especially in cases where a batch of timber is graded into a single strength class and reject only. High coefficients of variation of the graded material lead to the conclusion that high material safety factors are needed. On the contrary, if the material is graded by a machine and into more than two strength classes in one pass, it can be shown that the required material safety factors can be lower.

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Acknowledgments

This research is partly based on a feasibility study made by the European wood industries under the Roadmap 2010 Building With Wood programme, and this project is supported by the industry via CEI-Bois. CEI-Bois is the main industrial financier of this project. The Gradewood-project is realised by eight research institutes: Technische Universität München, VTT Technical Research Centre of Finland, BRE, FCBA, SP Wood Technology, HFA Holzforschung Austria, University of Ljubljana, and Swiss Federal Institute of Technology ETH. It belongs to the Wood Wisdom. Net-programme and is funded by national technology development bodies: Federal Ministry of Education and Research (Germany), Tekes Finnish Funding Agency for Technology and Innovation (Finland), Vinnova (Sweden), Ministry of Agriculture (France), and Slovenian Research Agency (Slovenia). The contributions from funding organisations and other support are gratefully acknowledged.

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Authors and Affiliations

  1. Holzforschung München, Technische Universität München, Munich, Germany

    Peter Stapel & Jan-Willem G. van de Kuilen

  2. Delft University of Technology, Delft, The Netherlands

    Jan-Willem G. van de Kuilen

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  1. Peter Stapel
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  2. Jan-Willem G. van de Kuilen
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Correspondence to Peter Stapel.

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Stapel, P., van de Kuilen, JW.G. Effects of grading procedures on the scatter of characteristic values of European grown sawn timber. Mater Struct 46, 1587–1598 (2013). https://doi.org/10.1617/s11527-012-9999-7

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  • DOI: https://doi.org/10.1617/s11527-012-9999-7

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