Unit introduction
This house uses radiata pine wall frames and roof trusses. All of the pieces that perform a structural function have been stress graded in the sawmill – either visually or mechanically.
Stress grading allows the builder and home owner to have confidence in the structural integrity of the building.
When timber is used in a structure, it needs to have reliable strength properties. The strength of a structural member is what allows it to withstand the stresses that act on it. These stresses come from things like wind gusts that blow from different directions, people or heavy objects inside, and of course, the dead weight of the structure itself.
Stress grading is all about taking into account the things that affect the strength of a piece of timber, and then giving it a grade that matches its ability to withstand stresses.
When the grading is done by a person who physically looks for the strength-reducing characteristics, it's called visual stress grading. When it is done by a machine, it's called mechanical stress grading.
In this unit, we'll be concentrating on the background knowledge needed to visually stress grade softwoods, although we will also briefly look at the principles of mechanical stress grading.
Background knowledge you should already have
Visual stress grading is very much a practical application of wood technology concepts. It will be very helpful for you to complete the unit Selecting timber before undertaking this grading unit, so that you have a good understanding of how certain growth characteristics in trees affect the properties of wood.
You should also get a copy of the Australian Standard covering the visual stress grading of softwood. This is called: AS 2858-2008: Timber – Softwood – Visually stress graded for structural purposes. Each time the Standard is revised, the year is updated – so you can see that the last update was made in 2008. If your company doesn't have a copy that you can borrow, you should buy your own via the Standards Australia International (SAI) Global website.
Working through this unit
There are two sections in the unit Grading structural softwoods:
- Principles of stress grading.
- Assessing characteristics.
Each section begins with Your job, which introduces you to the topics covered and gives you a link to the Task you will need to undertake to demonstrate your understanding. Your trainer may ask you to submit the completed Tasks as part of your assessment evidence for the unit.
Now go to the first section for this unit.