TIMBER AND GRADING (continued)
Research has quantified the strength-reducing effect of defects such as knots and slope of grain. Similarly, the relationship between the density and the strength of the timber has been established, and this information has been used to write visual grading rules.
By visually inspecting each board, only those pieces with defects of a permissible size are marked grade V4 or V6, depending on the density of the timber. For details see SABS 1783.PROOF GRADING
Proof grading subjects a piece of timber to a given load. Pieces which carry the proof load without fracture are then regarded as proof graded.
This system is often used to grade battens and branding and has recently been extended by the SABS as a method of grading larger section sizes into their various structural grades.
Proof grades for structural use are stamped P5, P6, etc.
THE ADVANTAGES OF MACHINE STRESS-GRADING
Machine stress-grading is highly effective, providing a reliable and consistent method of grading timber into grades with known working stresses. Timber that has been graded mechanically is most commonly used in engineered connector plate roof trusses.
Timber stiffness is assessed along the length of the boards being tested. The machine maintains constant load levels and measures the stiffness variable consistently and accurately. The machine is regularly calibrated and can be relied upon to hold its calibration through many production shifts.
Because statistical methods need only resolve the variability between stiffness and the strength properties, machine stress-grading is the most efficient means of grading, requiring a single pass to separate timber into a number of stress-grades. Visual methods of stress-grading are inherently less efficient. They rely on the assessment of a number of indicators of strength - such as knots, density and slope of grain - all of which can only be defined in large step specifications. It is also much more difficult to use them to discriminate efficiently between a number of grades.
As machine stress-grading is based on timber strength properties obtained from the continuous evaluation of its stiffness, errors in dimensioning will effect the stiffness calculation exponentially, and hence the predicted strength. It is therefore essential to size timber to fairly precise tolerances prior to machine stress-grading. The maximum tolerance on sizing is 0,5mm on both thickness and width.
Sizing of the timber to these tolerances can be done by fine sawing or planing, the latter method being the most common. The intention is not to remove all skips or to create a furniture quality finish, but simply to size the timber to the necessary narrow tolerance limits not normally achievable by rough sawing. Thus timber to be used on exposed trusses may require further surfacing.