This section contains information on characteristic stresses of sawn timber.
Characteristic stresses are those stresses that can safely be sustained by a particular size and grade of timber at a specified moisture content under briefly applied loads.
As defined in NZS 3603, characteristic stress or strength is an estimate of the lower 5th-percentile value determined with 75% confidence, from tests on a representative sample of full size test specimens.
For stiffness properties (ie modulus of elasticity), the characteristic value is the mean value.
The most reliable characteristic stresses for structural timber products are those derived from testing sawn timber in the sizes and grades used in structures (in-grade testing).
Because of the costs involved this has been done only on a few sizes of radiata pine sawn timber in bending and tension.
The more traditional method is to modify strength properties obtained from small clear (defect-free) specimens of timber.
With recent moves away from visual grading to a system of verified strength classes, the need for derivation of characteristic stresses for structural timber has diminished.
However, if a producer wants to market non-standard grades or species for engineering uses, it is necessary to derive characteristic stresses for design, from the results of in-grade testing as described in the following section.