Heart

A Quick Summary of Timber Strength Grading

article-image

 

 

The physical and mechanical properties of wood vary greatly - not just between species, but within - from forest to forest, between trees of the same forest and even within a single tree. For structural timber, certain key properties need to be assessed in order to ensure building safety, and economic use of the material. The means by which this is achieved is known as “strength grading”.

In Europe (including the UK), structural timber is graded under the system set out by European standard EN14081. It sorts rectangular cross-section timber into grades based on requirements for three primary grade determining properties for construction; bending strength, bending stiffness and density at a 12% moisture content. There are two ways of grading - visual and machine. These methods sort timber into grades according to a non-destructive assessment that is predictive of the grade determining properties. The collective characteristic properties of the timber sorted into those grades determines the strength class. Strength class is usually specified with reference to EN338 (e.g. C16, C24, D24 and D30), but properties can be declared directly, or by means of a user-defined strength class.

 

As the strength class is assigned to sorted timbers and not individual pieces, grading therefore depends on grading rules and machine thresholds. But it also relies on the population of timber prior to grading. Hence, it is not allowed to visually grade timber rejected by machine grading. This is because machine grading changed the original population as it removed the best pieces from it. A number of additional “visual override” checks are needed to cover things like drying distortion and fissures. It is whether those are carried out after drying that determines whether timber can be described as “dry-graded”.

 
Visual grading is carried out according to grading rules that are usually (but do not have to be) national standards e.g. BS4978 for softwoods and BS5756 for temperate hardwoods. Assignment to a strength class is specific to a combination of grading standard and timber source. The assignments for UK grown timbers (not all of them) are listed in the European standard EN1912. It covers British spruce (Sitka and Norway spruce), British pine (Scots and Corsican pine), larch (European, Japanese and hybrid), and Douglas-fir. The ones for large cross-section Douglas-fir, oak and sweet chestnut are instead in PD6693, which is merely a published document from BSI. Most machine grading of timber in the UK is carried out with x-ray machines, which measures wood density in detail, as well as the size and position of knots. The machines are much faster than the old bending-type machines, which worked by measuring the timber stiffness in mechanical bending (not by proof loading). However, modern grading machines more commonly measure stiffness by the acoustic vibration, allowing them to be much smaller and simpler. There are now many kinds of grading machine, including ones that work on other principles such as slope of grain, but only a few have settings that are approved for UK-grown timber.
article-image

 

The future of timber grading

There is no reason, other than cost, that prevents new test data updating these grading assignments, or adding assignments for other UK grown timber, but with the new generation of portable and lower cost grading machines, it may well make more sense to use a machine assisted visual grading process, to improve yields and assignments. Slovenia has recently introduced a simple grading machine - “STIG” - based on a tablet computer and microphone, which measures the frequency a piece of timber makes when struck with a hammer. This is essentially all that acoustic machine grading involves, and since this is a kind of measurement of the timber’s stiffness, it is usually a more powerful grading predictor than knots, ring width and slope of grain.

Dr Daniel Ridley-Ellis (BEng, MRes, PhD) is Head of Centre for Wood Science and Technology at Edinburgh Napier University.

Dr Daniel Ridley-Ellis

Share this article