WHAT ARE THE CLASSIFICATION OF TIMBER ON THE BASIS OF GRADING (IS: 6534-1971)

TIMBER CLASSIFICATION ON THE BASIS OF GRADING

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 used in various engineering works. The classification of timber based on grading which is given in IS: 6534.

STRUCTURAL GRADING

this type of grading 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 gradedb 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: 

1. Grading based on known effects of defects and estimating accumulative value.

2. Machine grading.

COMMERCIAL GRADING

Commercial grading 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 

This classification is based on dimensions and general appearance. The dimensions of lengths, widths and thicknesses of converted materials are measured. This system is prevalent is Kerala and Mysore.

Grade B

This classification is based on the best ultimate use of the material. Such a system is mostly in Andhra Pradesh and some parts of Tamil Nadu. Here, each grade is further divided into A, B and C classes to indicate occurrence of defects. Only two lengths are recognized, long (L) which is 5m and above, and short(S) that is under 5m. Each log is stamped such as BAL (Beam, A-class, long), PBS (Plank, B-class, short), etc. Some times another letter is also added indicating the species, e.g. 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

This classification is based on evaluation of units of defects and fixing the permissible number of standard volume of area or the material in each grade. This system is prevalent in Bombay region and is increasingly adopted in Indian Standards and is recognized internationally.