Mechanical and physical properties of timber can vary greatly from species to species. But they can also vary greatly within one species itself. Especially fast-growing plantation timber, such as Sengon, which has a high degree of juvenile timber, has a great range of properties (Please also see the article on Juvenile Timber).
For structural timber, it is essential that certain key characteristics need to be evaluated before it can be used for construction.
Structural timber in Europe is graded under the system that is set out by the European standard EN14081-1:2016 (Timber structures — Strength graded structural timber with rectangular cross section). The standard is sorting rectangular cross-sectional timber into different strength classes, based on the three primary determined parameters such as bending strength, bending stiffness and the density at 12% equilibrium moisture content. The grading is either done visually or per machine.
The grading process categorizes timber into different strength grades. The collective characteristic properties, determine the strength grade that can be allocated. The strength classes, according to the Eurocode, are divided into D-Classes and C-Classes:
- D-Classes refer to Deciduous Tree Species
- C-Classes refer to Coniferous Tree Species
The Classes are usually specified with reference to EN 338. A common structural grade in Europe is C24.
|Minimum Value for C24 Sawn Timber|
|Characteristic Bending Strength||24 N/mm2|
|Mean Modulus of Elasticity in parallel bending||11,000 N/mm2|
|Mean Density at 12% EMC||420 kg/m3|
Farmers as well as sawmill operators that are looking to venture into structural timber processing need to understand the importance of strength grading and also the limitations and possibilities of their wood species for structural applications. Besides the strength grading, there are also other parameters to be considered such as knots, wood grain deviations and extensive research & development is necessary. Table 1 above, however, provides an initial guideline whether a timber species may be suitable to achieve C24 or not.