A stress grade is defined in AS 1720, SSA TIMBER STRUCTURES CODE, as the classification of timber for structural purposes by means of either visual or machine grading.

The stress grade indicates the basic working stresses and stiffnesses to be used for structural design purposes. For example, the basic working stress in bending for F7 grade timber is approximately 7Mpa.

Timber may be stress graded either visually or mechanically.

Visual stress grading.

When timber is graded visually, the resulting stress grade is influenced by both the inherent strength of the species concerned and the quality (or grade) of the particular parcel of timber.

Grade descriptions are listed in Australian Standard specifications for structural timber. They place limits on the size or extent of strength reducing characteristics, such as knots or sloping grain.

The two main specifications are:

• AS 2082, TIMBER-HARDWOOD-VISUALLY STRESS GRADED FOR STRUCTURAL PURPOSES and
• AS 2858, TIMBER-SOFTWOOD – VISUALLY STRESS-GRADED FOR STRUCTURAL PURPOSES.

There are seven strength groups for unseasoned timber (S1 to S7, with S1 being the strongest) and eight for seasoned timber (SD1 to SD8, with SD1 being the strongest). The relationship between these strength groups and the visual grades is illustrated in Table 1 below. An exception to the general rule is Cypress pine. The appropriate stress grades for Cypress pine are F4, F5 and F7. Refer to AS 2858. The relationship between visual structrual grades and strength groups for various species is shown in Table 2.

Mechanical stress grading

Stress grades may also be determined by mechanical grading such as machine stress grading or proof grading.

The basis of machine stress grading is that stiffness of timber is closely correlated to strength. Individual pieces are fed into a machine in a longitudinal direction and continually deflected in the narrow dimension. The relationship between load and deflection gives the stiffness, and hence the stress grade of the piece. Machine grading of radiata pine has led to the introduction of the MGP (Machine Graded Pine) system of grading.

Proof grading allocates a stress grade to a piece of timber if it sustains a specific proof bending stress. The proof stress applied is generally 2.2 to 2.4 times the actual design stress. The load is applied on edge to simulate the usual loading mode in service.

References: NAFI Timber Manual: Timber Datafile P1 – Timber Species and Properties.

Table 1 Strength Group/Stress Grade/Visual Grade Relationship