TIMBER AND GRADING
 
If timber is to be used in construction, it must be graded for strength and stiffness. Grading into different strength classes allows the best possible use to be made of the various timber dimensions. This ensures that timber that is unsuitable due to low strength, twist, warp or springs is not used structurally.
 
Three methods of grading structural timber are employed:
  • Machine stress grading
  • Visual grading
  • Proof grading

MACHINE STRESS GRADING

Research has shown that timber strength is directly related to its stiffness - the stiffer it is, the stronger it is likely to be. This principle has been applied to the development of machinery and systems which accurately and consistently classify South African pine timber into structural grades.

The graph deals with the relationship between strength and stiffness. This characteristic strength value is taken at the 5th percentile of the strength of the material, that is the value which is exceeded by 95% of the timber tested. A safety factor of 2,2 is then applied to give the working stress used for design purposes.

Small static machines in which each piece of timber is individually tested, or high-speed dynamic testing machines which grade timber at speeds of up to 60m per minute, may be used for grading. Both these work on the same simple principle (shown below).

Stress-grading machines measure the deflection of the timber at a certain load and, given the dimensions of the piece, the stiffness can be calculated. In the simple static machines this is done by setting the scale on the dial gauge. The dynamic machines use computer systems to calculate the stiffness and indicate the grade, e.g. M4, by automatically spraying the timber with coloured dyes which are coded to the various grades (see below)

Machine stress-grading of structural timber is covered by SABS specifications1783 and the SABS code of practice D149.