Implications in the design of a method for visual grading and mechanical testing of hardwood structural timber for designation within the European strength classes

Guillermo Riesco Muñoz,Francisco Pedras Saavedra,Andrés Remacha Gete

doi:10.5424/fs/2011202-9771

Abstract

The international strength class system (EN 338:2003) is based on a testing standard (EN 408:2004) that is not easy to use with hardwoods, although some of this type of timber, such as European oak (Quercus robur L.), is highly appreciated for construction purposes. This study proposes specifications for sampling design, visual grading and mechanical testing of oak wood. With this aim, 27 adult oaks were felled and sawn into pieces. A sample of quarter-sawn planks was selected, then air-dried and planed to 50x100x2,000 mm. The beams were visually graded in accordance with ten standards, and were mechanically tested, at an average wood moisture content of 21%. The Spanish visual grading standard UNE 56544:2007 was the most effective for categorizing the pieces. According to this standard, 39% of the sampled beams were suitable for structural use. The mean value of modulus of elasticity (11,702 N mm-2) and characteristic density (714 Kg m-3) enable preliminary designation of the wood in visual grade ME-2, to European strength class D40. However, the bending strength was too low for inclusion of the timber in the European strength classes. Criteria are proposed for elaborating visual grading rules for oak wood and for structural beam testing in order to allocate oak visual strength grades into strength classes.

Highlights

Oak (Quercus robur L.) wood has traditionally been used for structural purposes because of its mechanical strength and acceptable durability (Van Acker et al, 1999; Riesco Muñoz, 2001)
The aim of the present study is to present criteria for visual grading and mechanical testing of oak timber of Spanish origin destined for construction, to enable assignation of the visual qualities to the European system of strength classes outlined in the European standard EN 338:2009
Visual grading standards for structural wood members should be elaborated in collaboration with the relevant institutions and timber industries, with specific criteria for oak timber

Summary

Introduction

Oak (Quercus robur L.) wood has traditionally been used for structural purposes because of its mechanical strength and acceptable durability (Van Acker et al, 1999; Riesco Muñoz, 2001). It has commonly been used in Spain to construct flooring and roofing on fabric or stone structures (Nassar, 1996). In the 20th century, use of oak in structures decreased due to the increased presence of concrete, steel and graded coniferous wood. An additional restriction to the use of oak is the large variability in the species with regard to density and other characteristics related to quality (Zhang et al, 1994; Degron and Nepveu, 1996). The demand for good quality oak is increasing (Iglesias, 1998). Sawn oak timber of Spanish provenance is expected to be sold on the European market in light of the predicted deficit in hardwood timber in the intermediate term, as a result of the destruction of broadleaved forests outside of Europe (Bermúdez, 1997)

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