Proof-grading commences with the preliminary sorting of the ungraded timber being produced by the mill into sub-groups, each of which will become a stress grade classification. An accepted and well-documented grading method is used for the preliminary sort. When machine proof-grading procedures are used, after the preliminary sorting, every piece of timber in each sorted group is tested by the application of a predetermined bending load, known as the proof load.
Each target stress grade has its own proof stress, which is used with the cross-sectional dimensions to give the proof load. As the proof load is applied, each piece of timber is loaded on edge and continuously stressed in bending to the proof stress.
The proof stress is specified in AS 3519 and is significantly higher than the stress at serviceability loads for the particular grade being assigned. Pieces that survive the proof load without failure, excessive deformation or other signs of damage are deemed to qualify for the stress grade that corresponds to the applied proof stress.
Machine stress-grading and machine proof-grading differ in the following ways:
- Timber loaded about minor axis (on flat)
- Small loads applied - timber is loaded at much less than design strength
- Intention is to find poor pieces by measuring stiffness
- High speed operation - timber in the machine for typically 1 second
- Timber loaded about major axis (on edge)
- High load applied - timber is loaded at loads near the design strength
- Intention is to find poor pieces by breaking them
- Low speed process - timber may take 15 to 20 seconds to pass through the machine
Two grading 'checks' are applied to any batch of timber undergoing machine proof-grading. The first is that the proof stress will normally break the weaker pieces of timber which do not meet the grade requirements. The second is concerned with the fact that the proof load is usually estimated to break 1% to 5% of the pieces passing through the grading machine. However, in the event of too many pieces breaking, then the timber producer is prompted to improve the efficiency of the preliminary grading. A very small number of pieces of timber which have passed the test, may have been overstressed during application of the proof stress, thereby affecting their future performance.
While machine proof-grading is not as widely used as visual and machine stress-grading methods, it is used by a number of producers in limited applications for grading of some Australian hardwoods and cypress pine and radiata pine underpurlins. It may also find application in the grading of treated timber.
The concept of proof loading to confirm design strength properties can be applied to structural assemblages or complete structures AS 1720.1 D4. This check is suitable for high volume products that require a high level of reliability. Proof-loading is currently used in the production of some timber utility poles.
Australian Building Codes Board website - the Australian Building Codes Board (ABCB) is a joint initiative of all levels of Australian Government and includes representatives from the building industry.