Table of Contents
5.1 Classification of Trees
The word timber is derived from an old English word timbrian which means to build. Trees are classified according to their mode of growth.
Figure 5.1 — Classification of Trees and Comparison of Soft Wood vs Hard Wood
Do you know? A single tree can cool the summer heat for an entire day and night and is found better than 20 air-conditioners running for 20 hours. A hectare of trees will produce about 10 tonnes of oxygen which is enough for 45 persons to live for one year.
5.2 Structure of a Tree
From the visibility aspect, structure of a tree divided into two categories: (i) Macrostructure (ii) Microstructure
Figure 5.2 — Cross-Section of an Exogenous Tree Showing All Structural Parts
5.2.1 Felling of Trees
- Trees are knocked down or cut down or caused to fall on the ground — known as felling of trees
- Tree should be cut from a place a little above its roots and very near to the ground level
- Age of trees for felling: 50–100 years
- Season for felling: In autumn and spring, sap is in vigorous motion → felling should be avoided. For hilly areas, mid-summer is proper season; for plain areas, mid-winter would be the proper season.
5.3 Defects in Timber
Defects occurring in timber grouped into five categories:
- Defects due to conversion
- Defects due to fungi
- Defects due to insects
- Defects due to natural forces
- Defects due to seasoning
Figure 5.3 — Defects in Timber: Classification and Types
5.4 Preservation of Timber
Preservation of timber carried out to achieve: (i) increase life of timber structures, (ii) make timber structures more durable, (iii) protect from attack of destroying agencies such as fungi, insects etc.
5.4.1 Requirements of a Good Preservative
- Allow decorative treatment on timber after being applied over timber surface
- Capable of covering a large area with small quantity
- Cheap and easily available; free from unpleasant smell
- Penetrating power into wood fibres should be high — penetrate at least 6 mm to 25 mm depth
- Durable; should not be affected by light, heat etc.
- Non-inflammable; quite efficient in killing fungi, insects etc.
- Safe and harmless for humans and animals
- Should give pleasant appearance to timber; should not affect strength characteristics
- Should not be easily washed away by water
- Should not corrode the metals with which it comes into contact
5.4.2 Types of Preservatives
| Preservative | Description | Use / Note |
|---|---|---|
| 1. AsCu Treatment | Special preservative developed at Forest Research Institute, Dehradun. Composition: 1 part hydrated arsenic pentoxide (As₂O₅·2H₂O) + 3 parts blue vitriol (CuSO₄·5H₂O) + 4 parts potassium dichromate (K₂Cr₂O₇·2H₂O). Mix 6 parts by weight of AsCu in 100 parts by weight of water. | Protection against white ants. Surface can be painted, polished, varnished or waxed. |
| 2. Chemical Salts | Water-borne preservatives; mostly salts dissolved in water. Common: copper sulphate, mercury chloride, sodium fluoride, zinc chloride. Odourless and non-inflammable. | General preservation |
| 3. Coal Tar | Timber surface coated with hot coal tar with help of brush — process called tarring. Has unpleasant smell and appearance. Makes timber unsuitable for painting. | Cheap and fire resistant |
| 4. Creosote Oil | Obtained by distillation of tar. Creosoling process: timber thoroughly seasoned and dried → placed in airtight chamber → air pumped out → creosote oil pumped under high pressure 7–10 kg/cm² at ~50°C → kept 1–2 hours → taken out. Black or brown liquid, weakly affected by water. | Practically doubles life of timber. Used for piles, railway sleepers. Penetration: 1–2 mm only. |
| 5. Oil Paints | Timber surface coated with 2–3 coats of oil paint. Wood should be seasoned first. | Preserves from moisture; makes durable |
| 6. Solignum Paints | Highly toxic in nature; mixed with colour pigment; applied in hot state with brush | Preserves from white ants |
5.4.3 Methods for Preservation
- (1) Brushing (2) Charring (3) Dipping and steeping (4) Hot and cold open tank treatment (5) Injecting under pressure (6) Spraying
- Injecting under pressure: Most effective method of treating timber; usually adopted in creosoling; requires special treatment plant; used for non-durable timbers in places of attack by fungi and insects
- Charring: Rather old method; surface kept wet for half hour then burnt up to depth of about 15 mm over wood fire; charred portion cooled with water. Layer of coal formed — not affected by moisture or white ants, fungi. Used for lower ends of posts for fencing, telephone poles.
5.5 Fire Resistance of Timber
- Timber classified as refractory and non-refractory w.r.t. fire resistance
- Refractory timber: non-resinous; does not catch fire easily. Examples: sal, teak, etc.
- Non-refractory timber: resinous; catches fire easily. Examples: chir, deodar, fir, etc.
- To make timber more fire-resistant, two methods adopted:
- 1. Application of Special Chemicals: Two coats of solution of borax or sodium arsenate at 2% strength are quite effective. When temperature rises — chemicals melt or give off gases which hinder or forbid combustion. When wood treated with antipyrine, it does not inflame at high temperature, merely moulds.
- 2. Sir Abel’s Process: Timber surface cleaned and coated with dilute solution of sodium silicate; cream-like paste of slaked fat lime is applied; finally concentrated solution of silicate of soda applied. Quite satisfactory in making timber fire-resistant.
5.6 Seasoning of Timber
By seasoning, the excess water of timber is extracted in such a way that the moisture content in timber corresponds to the required moisture content for the environments in which it is to be used.
5.6.1 Objects of Seasoning
- (i) To allow timber to burn readily if used as fuel
- (ii) To decrease weight of timber and thereby lower cost of transportation and handling
- (iii) To make timber safe from attack of fungi and insects
- (iv) To reduce tendency of timber to crack, shrink and warp
- (v) To make timber fit for receiving treatment of paints, preservatives, varnishes etc.
- (vi) To impart hardness, stiffness, strength and better electrical resistance to timber
Figure 5.4 — Methods of Seasoning and Comparison: Natural vs Kiln Seasoning
5.6.3 Conversion of Timber
The process by which timber is cut and sawn into suitable sections is known as the conversion.
| Method | Description | Advantage / Note |
|---|---|---|
| Ordinary Sawing (Bastard/Flat/Slab sawing) | Saw cuts tangential to annual rings; right through cross-section. Log moved forward and backward on platform of sawing mill. | Very easy and quick; most economical; minimum wastage of useful timber. However, planks obtained liable to warp and twist as result of unequal shrinkage. |
| Quarter Sawing | Saw cuts tangential to annual rings; they meet each other at right angles | May produce fine figure wood when adopted for timber having no distinct medullary rays |
| Tangential Sawing (Plain/Flat grained) | Saw cuts tangential to annual rings; they meet each other at right angles. Adopted when annual rings are very distinct and medullary rays are not clearly defined. | Planks obtained warp too much because section is weak as medullary rays which impart strength are cut |
| Radial Sawing (Rift sawing) | Saw cuts made radially in parallel direction to medullary rays | Used for conversion of hard timber; shrinks and warps to less degree; distorted to minimum; strongest method |
Strength of sawing (descending): Radial > Quarter > Ordinary > Tangential | Allow for shrinkage: About 3 mm to 6 mm
5.7 Market Forms of Timber
| Term | Definition (from source) |
|---|---|
| Batten | Timber piece whose breadth and thickness do not exceed 50 mm |
| Baulk | Roughly squared timber piece; removing bark and sap wood; one cross-sectional dimension exceeds 50 mm; other exceeds 200 mm |
| Board | Plank i.e. timber piece with parallel sides; thickness less than 50 mm; width exceeds 150 mm |
| Deal | Piece of soft wood with parallel sides; thickness 50–100 mm; width does not exceed 230 mm |
| End | Short piece of batten, deal, scantling etc. |
| Log | Trunk of tree obtained after removal of branches |
| Plank | Timber piece with parallel sides; thickness less than 50 mm; width exceeds 50 mm |
| Pole | Sound long log of wood; diameter does not exceed 200 mm; also known as a spar |
| Quartering | Square piece of timber; length of side being 50 mm to 150 mm |
| Scantling | Timber piece whose breadth and thickness exceed 50 mm but less than 200 mm in length |
5.7.1 Industrial Timber
Veneers
- Thin sheets or slices of wood of superior quality; thickness 0.40 mm to 6 mm or more
- Obtained by rotating a log of wood against a sharp knife of rotary cutter
- Veneers after being removed are dried in kilns to remove moisture
- Used to produce plywoods, battenboards and laminboards
- Indian timbers suitable for veneers: mahogany, oak, rosewood, sissoo, teak etc.
Plywoods
- Three or more veneers in odd numbers placed one above other with direction of grains of successive layers at right angles to each other
- Held in position by application of suitable adhesives
- The placing of veneers normal to each other increases longitudinal and transverse strengths of plywood
- Pressure applied on plywood: 7–14 kg/cm²
- Advantages: expansion and shrinkage comparatively very low; elastic; do not split in axial direction; possess uniform tensile strength in all directions
- NOT suitable in situations subjected to direct shocks or impacts
5.8 Qualities of Good Timber
| Quality | Description |
|---|---|
| 1. Appearance | Freshly cut surface of timber should exhibit hard and shining appearance |
| 2. Colour | Colour of timber should preferably be dark. Light colour usually indicates timber with low strength |
| 3. Defects | Good timber should be free from serious defects such as dead knots, flaws, shakes etc. |
| 4. Durability | Should be durable; capable of resisting action of fungi, insects, physical agencies and mechanical agencies |
| 5. Elasticity | Property by which timber returns to original shape when load causing deformation is removed. Elasticity of timber = 1.0 × 10¹ – 1.5 × 10⁴ N/mm² |
| 6. Fibres | Timber should have straight fibres |
| 7. Fire resistance | Timber is bad conductor of heat. Dense wood offers good resistance to fire. |
| 8. Hardness | Should be hard — should offer resistance when being penetrated by another body. Chemicals + density impart hardness. |
| 9. Mechanical wear | Should not deteriorate easily due to mechanical wear or abrasion. Essential where timber subject to traffic. |
| 10. Shape | Should be capable of retaining shape during conversion or seasoning — should not bow or warp or split |
| 11. Smell | Should have sweet smell. Unpleasant smell indicates decayed timber. |
| 12. Sound | Good timber gives a clear ringing sound when struck. Dull heavy sound → decayed timber. |
| 13. Strength | Should be strong for working as structural member (joist, beam, rafter). Capable of taking loads slowly or suddenly. |
| 14. Structure | Should be uniform; fibres firmly added; medullary rays hard and compact |
| 15. Toughness | Should be tough — capable of offering resistance to shocks due to vibration. Essential for tool handles, motor car parts. |
| 16. Water permeability | Should have low water permeability measured by quantity of water filtered through unit surface area of specimen of wood |
| 17. Weight | Timber with heavy weight is considered to be sound and strong |
5.9 Important Indian Timber Trees
| Timber Tree | Use / Characteristics |
|---|---|
| Babul | Bodies and wheels of bullock carts, agricultural instruments, tool handles, well kerbs, making cabinets |
| Banyan | Used for aerial roots; tent poles, well curbs etc. |
| Benteak | Building construction, boat construction, furniture |
| Deodar | Cheap furniture, railway carriages, railway sleepers, packing boxes, structural work |
| Guava | Making toys, handles of instruments, engraving work |
| Hopea | Ordinary house construction, railway sleepers, piles, boat building |
| Kathal | Not attacked by white ants. Used for piles, platforms of wooden bridges, door and window panels |
| Mulberry | Strong, tough and elastic; takes up clean finish; can be well seasoned. Weight after seasoning ~650 kg/m³. Used for baskets and sport goods (hockey sticks, tennis rackets, cricket bats) |
| Oak | Used for preparing sport goods |
| Rosewood or Blackwood | Furniture of superior quality, cabinet work, ornamental carvings |
| Sal | Railway sleepers, ship building, bridges, structural work. Sal poles used as foundation piles. |
| Sissoo | High class furniture, plywoods, bridge piles, sport goods, railway sleepers. Very good material for decorative works and carvings. |
| Toon | Furniture, packing boxes, cabinet making, door panels |
Moisture content in well seasoned timber: 4% to 6%
Chapter 5: Timber — Civil Engineering · Construction Materials
All technical data as per source material and IS specifications