TIMBER STRUCTURAL GRADING – A SIMPLE EXPLANATION
- July 20, 2017
- Posted by: Programmed Timber Supplies
- Category: Uncategorized
When specifying timber for use in a structural application certain design properties must be known so the correct dimension and stress grade is used.
A structural timber stress grade is made up of a number of different design characteristics which includes bending strength, bending stiffness, compression strength, tensile strength, shear strength etc. Each timber stress grade has a set minimum value for each of these criteria. For a piece of timber to have a stress grade all of the minimum criteria for that grade need to be met.
One or more of these criteria will be relevant and which ones are dependant on the application. For example a truss chord will primarily be designed or specified in relation to bending stiffness (to ensure a deflection limit is not exceeded), a wall stud is primarily designed with consideration of compression parallel to grain.
The Australian Standard AS 1720.1 contains the individual characteristic values for design relevant for both ‘MGP’ and ‘F’ stress grades. All producers need to ensure whatever stress grade they stamp their timber will meet at least the minimum requirement for all characteristics of that grade. There are several production methods producers can use to show they have an adequate grading process in place. These methods for solid softwood are covered in standards AS/NZS 1748, AS 2858 or AS 3519.
On top of all of that the grading result needs verification testing with most producers using the standard AS/NZS 4490 for that purpose and AS/NZS 4063 for method.
Basically if you are going to produce or sell structural timber in Australia you need to be doing it right.
The conversion in the 1990’s to ‘MGP’ grades by the major Australian softwood producers had a number of advantages for them. Although there was no issue either technically or structurally with the ‘F’ grade system some of the individual design characteristics were seen as ‘light on’ when compared with others. It was felt better utilisation of product could be achieved with a realignment of the characteristics. It was a view of ‘let’s use the full structural potential of the fibre rather than being too conservative’.
In this process it was important that more was known about the characteristics and their relativity to each other. Considerable testing of timber from various locations was done to establish correlation between characteristics. Correlations and averages were determined according to forest location and tree growth. Each area had its own values.
Programmed Timber Supplies sources feedstock from a number of locations which makes it difficult to implement a ’MGP’ type grade system but alternatively rely on the actual properties of each component. Additionally Programmed Timber Supplies manufacture ‘components’ so they know the end application and what are the relevant attributes that make each component truly ‘fit-for-purpose’. This is a manufacturing advantage over a sawmill that never knows what will be the specific application for each segment of each board.
The ‘F’ grade system used by Programmed Timber Supplies is supported by their ability to design in the actual requirements of each and every one of the 25 million components they currently process each year. The use of high speed scanning technology permits this product certainty and also allows inclusion of such attributes as end zone grading (for nail plate or end grain nailing application), nominated point load specification, non defect board density for joint strength certainty etc.
Especially for wall frame and roof truss fabricators the buying in of grade certain common components from Programmed Timber Supplies allows for greater plant output by allowing them to only concentrate on cutting the job critical components.