By Dr Brand Wessels, University of Stellenbosch
Photo: Brand Wessels
Yes, it is true. South Africa’s pine plantations are the source of the best structural timber in the world. From 01 August 2021, sawmills producing structural timber must implement a continuous quality control grading system using the SANS 1783 specifications to underscore this fact.
This process means our northern hemisphere colleagues are wrong in their belief that their slow-growing pines and spruces are far superior to wood from southern hemisphere plantations.
A continuous quality control system in structural grading puts South African sawmillers at the forefront of producing safe and reliable structural timber.
A committee chaired by George Dowse and consisting of representatives from industry, the auditing bodies SATAS and SABS, Abe Stears and Francois Louw edited the draft standards and guided them through the SABS registration process.
The South African National Standard (SANS) 1783 has been available for the past two years and could be applied voluntarily but will become compulsory from August 2021.
What is good structural timber?
The answer to this question lies in the safety of the products. The smaller the chance of failure of timber in a structure, the better the wood.
It is essential to note 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 wood. Ultimately, the safety of structural timber depends on it having a predictable strength rather than a high strength. Deformation also plays a role since structural elements often limit how much the wood can deform under load.
Stress grading
In most countries, there are legal requirements for structural timber. Stress grading according to standards is widely accepted to be the only way to assure the mechanical performance of wood.
Currently, there are two ways to do the grading. Human operators can perform grading based on visual characteristics such as knots and density. When it comes to determining the physical strength of the wood, sawmills use machines equipped with various sensors or mechanical devices.
Through extensive testing, the non-destructive measured properties of timber demonstrate its strength and stiffness characteristics. Therefore, most grading systems depend on a consistent relationship between the measured properties and the mechanical performance of the wood.
Wood properties change over time
The most significant risk for users of structurally graded timber is that the relationship between wood properties measured during grading and the mechanical properties of wood changes over time.
It may happen due to changes in plantation management regimes, rotation ages, new genetic material or even climate change. In most cases, however, large scale destructive testing takes place very rarely. The properties of South African structural grades were, for instance, last determined in large scale testing in the 1980s.
SANS 1783-5-1 & SANS 1783-5-2
To minimise the risk of substandard structural timber entering the market, the South African sawmilling industry, through Sawmilling South Africa (SSA), decided to develop two quality control standards for structural timber graders: SANS 1783-5-1 and SANS 1783-5-2.
Photo: George Dowse
Research at the Department of Forest and Wood Science at Stellenbosch University convinced Sawmilling South Africa (SSA) to appoint Peter Muller to coordinate, develop and write the draft standards.
George Dowse chaired the committee industry representatives, the auditing bodies SATAS and SABS, Abe Stears and Francois Louw that edited the draft standards and guided it through the SABS process.
The two standards have been available for the past two years and could be applied voluntarily but will become compulsory from August 2021.
The standards require continuous sampling and destructive testing of graded timber to ensure that the measured grading properties keep good predictors of strength and stiffness. The manufacturer must test one piece of lumber for every 1000 pieces produced.
Statistical quality control charts track the performance of the grading system, and as soon as the performance becomes “out-of-control”, the grading system needs to be adapted.
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!
Note: Edited by Joy Crane
