Timber Classification
The terms timber and wood are often used synonymously, but they have distinct meanings in the building industry. Wood is the hard, fibrous material that makes up the tree under the bark, whereas timber may be defined as a wood which retains its natural physical structure and chemical composition and is suitable for various engineering works. Following is the classification of timber as per IS: 6534.
Timber Classification on The Basis of Grading
All grading specifications are clearly distinguished between structural or stress grading, and commercial or utility grading based on Indian Standard classification.
Structural Grading
It is also known as stress grading. However, there is a small distinction between the two. Structural grading refers to the principle by which the material is graded on the basis of visible defects which have known effects on the strength properties of the material.
Stress grading refers to the principle by which the material is graded by consideration of maximum principle stresses to which it can be subjected.
Structural grading is further divided as:
- Grading based on known effects of defects and estimating accumulative value.
- Machine grading.
Commercial Grading
It is also known as yard grading or utility grading refers to the principle by which the material is graded by consideration of usefulness of the material and price factors. Commercial grading is further divided in the following classes:
Grade A: It is based purely, and sometimes arbitrarily, on dimensions and general appearance. The dimensions of lengths and girths for logs, or lengths, widths and thicknesses of converted material are measured according to specified methods. This system is prevalent in Kerala and Mysore. Under these classifications, teak is placed in four grades with two sub-classes in each grade. In the case of other hardwoods, there are similarly four grades in Mysore (Coorg) but the dimensions are fixed separately for each of the species. In Kerala, there seem to be only two grades of hardwoods
Grade B: It is based on the best ultimate use of logs or converted material. Such a system is mostly prevalent in Andhra Pradesh and some parts of Tamil Nadu, and seems to be one of the quickest systems of grading and marking. The logs are classified into grades on the best use possible as for beams, planks, scantlings, etc, and each grade is further divided into ‘A’, ‘B’ and ‘C’ classes to indicate occurrence of defects. Only two lengths are recognized; ‘long (that is, 5 m and above) and ‘short’ (that is, under 5 m). Each log is thus quickly stamped with the first letter of the grade classification, the sub-class, and ‘L’ or ‘S’ for ‘long’ and ‘short’, for example, BAL and PBS indicate, respectively, ‘ beam, A-class, long ’ and ‘planks, B-class, short’. Sometimes another letter is also added to indicate the species, for example, ‘T’ for teak.
Grade C: This classification is based on qualitative evaluation of defects and rough estimate of out-turn of utilizable material. It is prevalent in Madhya Pradesh.
Grade D: It is based purely on evaluation of ‘units of defects’ and fixing the -number of units permissible for a standard volume in each grade. Such practices are common in the Bombay region; sometimes an estimated out-turn is also indicated in each grade. In general three grades are distinguished for various categories of logs and sawn timber. Sizes and other dimensions are also fixed in a few cases, separately for different species and different depots in the same state. This system is being increasingly adopted in the specifications of Indian Standards Institution, and in international grading specifications. This system has a distinct advantage of evaluating cumulative effect of defects in a particular grade.
Reference
IS – 6534 – 1971