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Structural Properties & Grading
When considering structural properties it is important to realise there are many commercially available species. Strength varies for each depending on whether it is tested in the longitudinal, tangential or radial axes. The timber industry has simplified these variables by developing a system of timber stress grading. Stress grades give an indication of strength 'in bending'. The process of grading begins by identifying a timbers 'strength group' which clusters species of similar strength together (e.g. based on modules of rupture and elasticity). 'Structural grades' break the strength groups up into smaller groups to deal with reduced strength through natural imperfections in the tree. From this a stress grade is assigned (signified by an 'F' for sawn timber or 'GL' for glue laminated timber). Twelve grades exist and those relating to hardwoods and Cypress are shown in Table 9. These grades are utilised by structural designers to assess the capacity of elements to perform in given applications (with the assistance of AS 1720.1).

Approaches to Stress Grading
Grading can be carried out at any stage in the chain of production e.g. on logs, unseasoned or seasoned sawn timber. It can be carried out in any one of the following three ways, according to need and circumstance.

Visual grading
Visual grading involves a trained 'grader' who first identifies the timber strength group, then looks for characteristics that influence structural grade, then assigns a stress grade. The two main guiding documents for this are: AS 2082 (for hardwood) and AS 2858 (for Cypress).

Machine grading
Machine grading utilises a computer controlled machine that measures the stiffness of timber.
This can be directly correlated to strength where upon a grade is assigned. Individual pieces are fed into the machine in a longitudinal direction, which continually deflects the timber by a given load. Stress grades are assigned based on the measured stiffness. The main guiding document for this method is AS 1748.

Proof grading
Proof grading allocates a stress grade based on a piece of timber being able to sustain a
specific proof bending stress. The proof stress is generally 2.2 to 2.4 times the actual design stress. The main guiding document for this method is AS 3519.

Grading in Pactice
Machine grading is useful where economies of scale and the characteristics of the timber permit. It allows grading to take place as an inline part of the milling process and is less reliant on human error. Even so, machine grading is sometimes hard to undertake on high strength hardwoods – the machines have difficulty bending the timber. In addition, machines cannot check for decay and susceptibility to the Lyctus borer. As such it is not necessarily the most efficient approach and so visual grading is still common. Table 6 provides an account of common methods and commonly available stress grades for Cypress and hardwoods (seasoned and unseasoned).