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Materials UK timber

Photo: Edinburgh Napier University

Sitka spruce This comparison takes Sitka spruce (Picea sitchensis) (BS EN 13556 marking code PCST) as the benchmark, a species for which rapid growth and ability to thrive on sites where other trees struggle are distinct advantages. Contrary to popular belief, fast tree growth per se does not cause inferior wood properties, and the assumption that fast growth causes low density leading to low strength is, at best, unhelpfully incorrect. Napier has been researching Sitka since 2003, and now has a great deal of information about its properties, and how they are influenced by forest management and climate.3 The characteristic density of ungraded UKgrown Sitka spruce ranges from about 310 to 350 kg/m3, characteristic strength from about 13 to 18 N/mm2, and mean stiffness from about 7 to 9 kN/mm2. It is therefore the stiffness that tends to limit the structural grading, although when grading to C16 alone the machine reject is minimal. For this C16-graded timber, strength is actually higher than www.trada.co.uk

required for C18 and density is the least critical property (sufficient for C20), but still low enough to make Sitka’s high strength-to-weight ratio another one of its practical advantages. However, the extra strength and density cannot be considered in design without using the bespoke timber grade for UK spruce, C16+, established by Napier. With the right grading machine, yields of about 25% C24 with 75% C16 and minimal machine reject are achievable4, and while likely not economic on an industrial scale for primary processing, this may be viable for one-off projects. Grades of up to C27 and TR26 can be achieved in small amounts, which although impractical, illustrates the range of timber properties within a species from even a relatively small growth area such as the UK. Visual grading to BS 4978 assigns Sitka spruce to C14 and C18. The coefficient of determination (R-squared) between strength and density in UK-grown Sitka is only about 0.2, due partly to this species growing relatively dense but less strong wood in the early years of growth. >> Timber 2019 Industry Yearbook

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Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Timber 2019 Industry Yearbook  

The annual publication of the Timber Research and Development Association (TRADA) includes topical and technical features on all aspects of...

Profile for trada
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