26 November 2020
Grade in Britain: enabling a wider range of home-grown species
Photo: Strength grading is based on data from mechanical testing. Allan Shedlock, Edinburgh Napier University.
The UK’s forests provide about one third of the UK sawn wood market and about one third of this homegrown timber is sawn for construction. The majority of this is Sitka spruce (Picea sitchensis), but this is not the only home-grown species available. With calls to diversify the forests, Sitka it is set to become a less ubiquitous, although still very important, tree and timber species in years to come. Recent years have seen a broadening of strength grading methods.
For historical and practical reasons, strength grading is done according to one of two approaches: visual strength grading and machine strength grading. The underlying principle is, however, the same for both, and in Europe this is covered by BS EN 14081-1 and supporting standards.
- Visual strength grading works by assessing features such as the size and position of knots, the ring width, and the slope of grain.
- Machine strength grading methods have expanded from the original mechanical stiffness measurements (bending graders) to incorporate a range of sensing technologies including x-ray scanning, acoustic velocity, slope of grain and digital image recognition.
The strength classes achieved by grading depend not just on the quality of the timber, but also on the grading approach. Knowing what is possible for home-grown timber is useful, since habitual specification of the most common BS EN 338 strength classes (C24 and C16) may limit what can be done with home-grown timber – for reasons that do not have much to do with the ability of the timber to actually do the job.
Visual strength grading
Visual grading is carried out according to grading rules that are usually (but do not have to be) national standards (such as BS 4978 for softwoods and BS 5756 for temperate hardwoods). Assignment to a strength class is specific to a combination of grading standard and timber source.
The assignments for UK-grown timber were established many years ago, but not all of them are listed in the European Standard BS EN 1912. It covers British spruce (a mixture of Sitka and Norway spruce), British pine (Scots and Austrian/Corsican pine), larch (European, Japanese and hybrid) and Douglas fir. The assignments for oak, sweet chestnut and large crosssection Douglas fir are instead in the British Standards Institute (BSI) published document PD 6693. Assignments repeated on other documents, including this one, should be checked against the latest standards since things can, and do, change. Because the current visual grading rules have certain predetermined thresholds of the visual criteria, the range of available strength classes is very limited. Visual criteria such as knots and slope of grain are typically not as powerful indicators of strength as people tend to assume, so the grading assignment is conservative on the safe side. However, since the visual grading approach is simple to apply it is still very useful, especially for smaller mills and less common species.
Machine strength grading
Unlike visual strength grading, machine strength grading is done by machines that have the ability to vary the grading thresholds to potentially grade to any strength class. This allows the commonly specified strength classes C24 and C16 to be targeted for any species, but the grading yield is not necessarily most efficient for those particular grades and the grade’s design properties may be quite a lot lower than the actual properties of the timber. As with visual grading, machine grading settings are specific to species (or species combination) and growth area. Listed below are summaries of current grading possibilities, but these do not necessarily correspond to what is available on the market. Indeed the large sawmills in the UK are mostly set up to grade a single grade with reject, and do so according to the maximum yield and majority market grade.
Grading possibilities for UK home-grown species
British spruce, a mixture of Sitka spruce (Picea sitchensis) and Norway spruce (Picea abies) grown in the UK and Ireland, has the widest range of grading options.
- Visual grading to BS 4978 or IS 127 assigns to C14 and C18.
- Machine grading is usually done to the single grade C16, which needs hardly any machine reject since even before grading the timber overall has the required properties. With the right grading machine, yields of about 25% C24 with 75% C16 and minimal machine reject are achievable, and while likely not economical on an industrial scale for primary processing, this may be viable for one-off projects. Machine settings do go up to C27 and TR26 although the yield for those is only small.
- Settings are available for the grading machines by:
- Tecmach (Cook Bolinders)
- Measuring and Process Control (Computermatic/ Micromatic)
- Microtec (Goldeneye 702, Goldeneye 706, Viscan, Viscan plus, Viscan compact, Viscan portable with and without balance)
- Brookhuis (MTG920, MTG960, MTGbatch922/926/962/966), Luxscan Weinig (EScan FM/F/FWM/FW) and Dynalyse (Precigrader).
The Cook Bolinders and Computermatic/Micromatic are bending machines (stiffness) and the Goldeneyes are x-ray machines (knots and density). All the other machines are of the acoustic longitudinal resonance type, with or without mass or density measurement (stiffness). The Goldeneye 706 combines x-ray with acoustic measurement.
Larch in the UK is a mixture of European larch (Larix decidua), Japanese larch (Larix kaempferi) and hybrid larch (or Dunkeld larch) (Larix x marschlinsii, syn. eurolepis).
- Visual grading of UK-grown larch to BS 4978 assigns to C16 and C24.
- Larch can achieve C20 or C22 as a single machine grade with minimal machine reject. With the right grading machine, yields of about 30% C27 with 70% C16 and minimal machine reject are achievable. Grades of up to C35 can be achieved in small amounts.
- Settings are approved for the same machines as listed above for spruce.
- Work is currently underway at National University of Ireland (NUI), Galway, to extend larch settings to Ireland and the expectation is that the timber is very similar to that in the UK.
Most recently, work by NUI Galway and Edinburgh Napier University has established new grading machine settings for Douglas fir (Pseudotsuga menziesii) grown in UK and Ireland.
- Visual grading of UK-grown Douglas fir to BS 4978 assigns to C14 and C18, or C24 for large cross-section (PD 6693).
- The machine yield of C18 as a single grade is about 95%. With the right grading machine, yields of about 65% C24 with about 25% C16 and about 10% machine reject are possible. Grades of up to C40 can be achieved in small amounts.
- Settings are approved for all the Microtec and Brookhuis machines listed above, and are currently in development by Dynalyse for the Precigrader.
British pine is a mixture of Scots pine (Pinus sylvestris) and Austrian/Corsican pine (the listing is P. nigra) grown in the UK.
- British pine can be visually graded by BS 4978 to C14 and C22.
- Currently the only machine grading settings are for the Cook Bolinders and Computermatic/Micromatic bending machines and they cover only C16 and C24.
UK-grown oak visually graded by BS 5756 is assigned to D24, D30 and D40 for large cross-section.
Sweet chestnut is assigned to D24.
There are currently no machine grading settings for UK-grown hardwoods.
Approaches to timber strength grading
One problem with the current system of strength grading is that it is well suited to large sawmills grading a handful of the main species, but not well suited to smaller sawmills, especially when they grade a larger range of species, perhaps for specific building projects rather than the open market. It also, currently, cannot be used to grade recycled timber – or indeed even new timber that has already been graded (without reduction in cross-section as per BS EN 14081-1). This is true of both visual grading and machine grading, and is because grading works on populations of timber, rather than individual pieces. This is also why the grading is linked to species and source.
Portable acoustic grading machines are small, simple to use, and relatively inexpensive. This makes them good candidates for smaller producers, especially when grading timber for specific buildings, rather than putting graded timber on the open market. Work at Edinburgh Napier University has been anticipating this in its development of special strength classes that make better use of the real properties of homegrown timber. These tailored strength classes now exist for spruce, larch and Douglas fir.
On the other hand, many uses of timber do not have high requirements for the timber properties and for this there is a good case for a much simpler approach to grading. There is no need to reject timber that is perfectly adequate for the job, simply to achieve a familiar strength class that is higher than is actually needed. Visual grading does not need to be so strict as we are used to, and a simple set of visual strength grading rules that can be conservatively applied to a wide range of species may be the way forward for minor species that are not commercial enough to warrant the usual, expensive, testing work to establish grading assignments and settings.
Wood may be renewable, but it is not unlimited. We should therefore be careful that the way we think about grading does not:
- unnecessary limit what timber can be used for
- cause rejection of timber that would still be perfectly adequate
- artificially limit the range of species we use.
While commodity strength classes are good for easy trade, it also makes sense to make better use of the real properties of the timber where possible – especially where the convenience of general trade is not needed. On the other hand there is also a case for a simple, conservative approach to allow timber to be used that would not otherwise be cost effective to grade. Strength grading is not about the grade itself – strength classes are simply shortcut ways of describing design properties.
About the author
Head of the Centre for Wood Science and Technology, Edinburgh Napier University
Convenor of CEN TC124 WG2 TG1
This is an extract from the Timber 2020 Industry Yearbook. Download the full article, including supporting images, references and further reading, here
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