Classification of timber(IS:399)
The terms timber and wood are often used synonymously, but they have distinct meaning 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.Folowing is the classification of timber as pre IS: 399,except the classification of timber based on grading which is given in IS:6534.
On the basic of its Postion
Standing timber implies a living tree.
Rough timber forms a part of the felled tree.
Converted timber or Lumber are long of timber sawn into planks,posts,etc.
On the Basic of Grading (IS :6534)
All grading specifications are clearly distinguished between structural or stress grading, and commercial or utility grading based on Indian Standard classification.
Structural 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 graded on the basic 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 structural grading is further divided as:
1.Grading based on known effect of defect and estimating accumulative value.
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 classfiaction 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 defect.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 defect and fixing the permissible number of standard volume of area or the material in each grade.This system is prevalent in 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.