By Brand Wessels
This question might elicit a condescending smile from our northern hemisphere colleagues who might believe the wood of their slow-growing pines and spruces are far superior to structural timber from that of southern hemisphere plantations.
A bending test machine at a sawmill used for continuous quality control testing. Image by SSA
But, in general, they will be wrong. There is a very good argument for calling structural wood that has been graded according to the latest SANS 1783 specifications as “the best”. Below I will set out the reasons why South African sawmillers can, from this year, lay a credible claim to this title.
Firstly, how can the performance of structural wood be quantified? In other words, when is structural timber good and when is it poor? This is an easy question and is largely based on safety. The smaller the chance of failure of timber in a structure, the better the timber. It is important to notice that stronger timber does not necessarily mean better timber. Timber can be very strong but will still fail if the engineer uses a design value higher than the strength of the timber. Safety of structural timber therefore depends on having a predictable strength rather than a high strength. Deformation also plays a role since structural elements often have a limitation in how much it is allowed to deform under load.
To ensure that timber has predictable mechanical performance, it is stress graded – in most countries this is a legal requirement for structural timber. Grading can be performed by human operators based on visual characteristics such as knots and density or by machines using various types of sensors or mechanical devices. Through large scale testing the non-destructively measured properties of timber is related to its strength and stiffness characteristics. Most grading systems therefore depend on a consistent relationship between the measured properties and the mechanical performance of the timber. The biggest risk for users of structural graded timber is that the relationship between wood properties measured during grading and the mechanical properties of wood change over time. This might happen due to changes in plantation management regimes, rotation ages, new genetic material or even climate change. In most cases large scale destructive testing takes place very rarely. The grade properties of South African structural grades were, for instance, determined in large scale testing in the 1980s.
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- A bending test machine at a sawmill used for continuous quality control testing. Image by SSA
In order to minimise the risk of substandard structural timber entering the market, the South African sawmilling industry, through SawmillingSA, decided a few years ago to develop two quality control standards for structural timber graders (SANS 1783-5-1 and SANS 1783-5-2). These standards require continuous sampling and destructive testing of graded timber in order to ensure that the measured grading properties keep being good predictors of strength and stiffness.
For every 1 000 pieces of lumber produced, one piece needs to be tested. Statistical quality control charts are used to track the performance of the grading system and as soon as the performance becomes ‘out-of-control’, the grading system needs to be adapted. These standards are a product of cooperation between many people and organisations. The standards were initially proposed as a result of research at the Department of Forest and Wood Science of Stellenbosch University. Mr Peter Muller was then asked by SawmillingSA to develop and write the draft standards. A committee consisting of various industry representatives, auditing bodies SATAS and SABS (Messrs. Abe Stears and Francois Louw) and chaired by Mr George Dowse, edited the draft standards, and guided it through the SABS process. The standards have been available for the past two years, and could be applied voluntarily, but will become compulsory from August 2021.
For every 1 000 pieces of lumber produced, one piece needs to be tested. Image by Chris Wilton | Unsplash
The use of a continuous quality control system in structural grading put South African producers right at the forefront of producing safe and reliable structural timber. To the best of our knowledge there is only one other country where continuous sampling and testing of all structural timber is required, and that is New Zealand. Unlike the rugby world cup, we must share this title with our great foes!
