In the construction industry, the terms ‘timber’ and ‘wood’ are frequently interchanged, yet they carry specific meanings. Wood constitutes the sturdy, fibrous material found within a tree beneath its bark. On the other hand, timber refers to wood that maintains its innate physical structure and chemical composition, rendering it suitable for diverse engineering applications. The classification of timber, as per IS: 399, excludes the grading-based classification detailed in IS: 6534.
Classification of Timber on the Basis of Position
On the basis of its position, timber can be categorized as follows:
Standing Timber: This term refers to a living tree still rooted in the ground.
Rough Timber: Constituting a segment of the felled tree, rough timber retains its natural state and has not undergone any processing.
Converted Timber or Lumber: These are logs of timber that have been sawn into planks, posts, and other such forms for various applications.”
Classification of Timber On the Basis of Grading (IS: 6534)
Grading in accordance with IS: 6534 distinguishes between two main categories: structural or stress grading and commercial or utility grading as per the Indian Standard classification.
Structural Grading, also known as stress grading, involves grading materials based on visible defects that are known to impact the strength properties of the material. Stress grading, on the other hand, evaluates materials considering the maximum principle stresses they can endure. This category further breaks down into:
- Grading based on known effects of defects and estimating accumulative value.
- Machine grading.
Commercial Grading, also known as yard grading or utility grading, evaluates materials based on their utility and pricing considerations. This grading category is subdivided into the following classes:
Grade A: Classifies materials based on dimensions and overall appearance, measuring lengths, widths, and thicknesses of converted materials. This system is predominant in Kerala and Mysore.
Grade B: Classifies materials based on their best ultimate use. Primarily used in Andhra Pradesh and some parts of Tamil Nadu, each grade further divides into A, B, and C classes to indicate the occurrence of defects. This grade recognizes only two lengths: long (L), which is 5m and above, and short (S), under 5m. Logs are stamped as BAL (Beam, A-class, long), PBS (Plank, B-class, short), etc. Sometimes, an additional letter indicating the species, e.g., T for teak, is also added.
Grade C: Evaluates materials based on qualitative assessment of defects and an approximate estimation of usable material. Prevalent in Madhya Pradesh.
Grade D: Assesses materials based on units of defects and determines the allowable amount of standard volume or area for each grade. Widely adopted in the Bombay region, this system is increasingly incorporated into Indian Standards and holds international recognition.
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Classification of Timber On the Basis of Modulus of Elasticity
The recommended species of timber for construction purposes are classified according to their modulus of elasticity as follows:
- Group A: Modulus of elasticity in bending above 12.5 kN/mm².
- Group B: Modulus of elasticity in bending above 9.8 kN/mm² and below 12.5 kN/mm².
- Group C: Modulus of elasticity in bending above 5.6 kN/mm² and below 9.8 kN/mm².
Classification of Timber On the Basis of Availability
Timber availability is classified into three grades—X, Y, and Z—based on the annual volume sourced (Availability ):
- X: Most common, comprising 1415 m³ or more per year.
- Y: Common, ranging between 355 m³ to 1415 m³ per year.
- Z: Less common, representing below 355 m³ per year.
These grades are determined by figures provided by forest departments, indicating the annual volumes of timber available, allowing for a clear classification based on availability.
Classification of Timber On the Basis of Durability
Timber durability is assessed through burial tests, using specimens sized at 600 × 50 × 50 mm, buried in the ground to half their lengths. Observations at intervals determine their condition, enabling the calculation of their average lifespan. Timbers are classified according to these observations:
- High Durability: Timber with an average life of 120 months and over.
- Moderate Durability: Timber lasting less than 120 months but at least 60 months.
- Low Durability: Timber with an average life of less than 60 months.
This classification system is based on observational data, offering clear categorization of timber based on its durability.
Classification of Timber On the Basis of Characteristics
Timber classification based on its behavior during air-seasoning practice is categorized into three groups:
- Highly Refractory (Class A): Timber in this category is slow and challenging to season but tends to be free from defects.
- Moderately Refractory (Class B): Timber classified as moderately refractory can be seasoned without surface defects if precautions are taken to prevent rapid drying.
- Non-Refractory (Class C): Timber in this category can be quickly seasoned without defects.
This classification system assists in understanding how different timbers respond to normal air-seasoning practices, aiding in appropriate handling and treatment.
Classification of Timber On the Basis of Treatability
The classification based on treatability depends on the heartwood’s resistance to preservatives under a working pressure of 1.05 N/mm², categorized as:
(a) Easily Treatable: Heartwood that readily accepts preservatives.
(b) Treatable but Complete Preservation Challenging: Heartwood that can be treated, but achieving full preservation is difficult.
(c) Partially Treatable: Heartwood that only partially accepts preservatives.
(d) Refractory to Treatment: Heartwood resistant to preservative treatment.
(e) Very Refractory to Treatment: Heartwood where preservative penetration is practically nil from the sides and ends.
This classification aids in understanding how different types of heartwood respond to preservative treatments, guiding appropriate treatment methods based on their treatability.
