GRADE IN BRITAIN: MAKING MORE OF UK WOOD
With industry demand for timber huge, Dan Ridley-Ellis, Head of the Centre for Wood Science and Technology, Edinburgh Napier University, summarises how different homegrown species can be graded.
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 but this is not the only homegrown 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 homegrown 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 homegrown 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 cross-section 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.
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.
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.
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