Comparison between wet and dry timber visual strength grading according to the Spanish (UNE 56544) and German (DIN 4074-1) standards

D F Llana,M Esteban,F Arriaga,G Íñiguez-González

doi:10.3989/mc.2019.03319

D F Llana, M Esteban + Show 2 more

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https://doi.org/10.3989/mc.2019.03319

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Abstract

Visual strength grading is widely used to estimate mechanical properties of structural timber. National visual grades are allocated to strength classes according to European standard EN1912. The recent discussion about the proper function of visual strength grading standards and the assignment of strength classes shows the need for further research in this field. Spanish-sourced radiata, Scots, Salzmann and maritime pine timber samples were visually graded in wet and dry condition according to the Spanish UNE56544 (2011) and German DIN4074-1 (2012) standards. Rejection was far higher dry due to distortion (warping) parameters. However, this rejection could be significantly mitigated by adopting a higher twist limit (2 mm / 25 mm width). UNE 56544 is more suitable for visually grading these species because it was specifically designed for them and a Spanish source. However, both standards underestimated the Scots pine rejection pieces which mechanical properties fulfill the MEG and S10 grades.

Highlights

Visual strength grading is widely used to estimate mechanical properties of structural timber
The main concern involved doubts about the implications of revising the standard. This would involve more than including new national grades approved since 2012 by the Task Group 1 (TG1) of the Working Group 2 (WG2) “Solid Timber” of Technical Committee 124 “Timber Structures” of the European Committee for Standardization (CEN/TC124)
According to clause 5.2, Note 2 of EN 1912:2012 (1) “The assignments of grades, species and sources to strength classes given in this document should be reassessed when this document is reviewed, or sooner if there is reason to suspect that the mechanical properties and/or density of the timber have changed, or the basis for the existing assessment no longer represents the current situation, e.g. if there has been a change in the source”

Summary

Introduction

Visual strength grading is widely used to estimate mechanical properties of structural timber. UNE 56544 is more suitable for visually grading these species because it was designed for them and a Spanish source Both standards underestimated the Scots pine rejection pieces which mechanical properties fulfill the MEG and S10 grades. The TG1 of CEN/TC124/WG2 raises concerns that some of these “long experience of use” grades may be unsafe, and it considers limiting the grades without test data according to EN 384:2016+A1:2018 (3) to the C24 strength class. Vega et al (9) proposed combining visual and machine grading with the influence of dimensions Taking this uncertainty about the proper functioning of visual grading standards into account, the authors consider it to be of interest to compare two national European standards, a Spanish one UNE 56544:2011 (4) and a German one DIN 4074-1:2012 (10), grading wet and dry sawn timber, as well as studying their exactitude in discerning by mechanical properties. Based on British standard CP112 Part 2:1971 (20) Argüelles and Arriaga (21) published a visual grading proposal with four visual grades for sawn timber

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