The strength of Australian timber is described in terms of "stress grades". Stress Grading is a way of classifying timber, either visually or by mechanical means to indicate the basic properties to be used for structural design purposes. The stress grade is designated in a form such as "F14", which indicates that the timber's working stress in bending is approximately 14 MPA (Megapascals).
The timber stress grade is influenced by:
The inherent strength of the species concerned:
eg. Iron Bark (Strength Group 1) is stronger than Blackbutt (Strength Group 2), and Slash Pine (Strength Group 5) is stronger than Radiata Pine (Strength Group 6), and as all of the species of hardwood which Outdoor Structures supply for structural purposes are a minimum (& mostly) Strength Group 2, for simplicity, we use this group as our standard.The quality of the sample of timber being examined:
i.e. The stress grade will vary within one timber species depending on the size and extent of faults such as knots, gum pockets, or sloping grain. For a sample of timber from Strength Group 2 to be graded as F14, it must not have:For a sample from the same species to be graded as F17, it must not have:A knot more than 1/4 of the width, and/or
Loose gum veins 1/6 of the length, and/or
Want & wane 1/5 of the cross section
A knot more than 1/3 of the width, and/or
Loose gum veins 1/4 of the length, and/or
Want & wane 1/4 of the cross section
In most cases, the size and grade of timber required for a particular application is determined from tables contained in Technical Publications such as the TRADAC Timber Framing Manuals. Stress grading only guarantees the strength of the timber at the time of grading and does not guarantee other characteristics such as appearance, durability under adverse conditions, and shrinkage.
