Timber

Seasoning is the process of removing moisture (sap and water) from freshly felled timber to a suitable level (about 10-15% moisture content, or equal to the air-dry condition of the place of use) in a controlled way, so the timber becomes stable and durable.

Why seasoning is necessary

1. Reduces weight of timber, so it is easier to handle, transport and use.
2. Increases strength: dry timber is stronger, harder and stiffer than green timber.
3. Prevents shrinkage, warping and cracking after use: moisture loss is completed before fixing, so the shape stays.
4. Resists decay: fungi and insects need moisture (above about 20%) to live, so dry timber lasts longer.
5. Better for treatment: dry timber absorbs paint, varnish, polish and preservatives more easily and holds them.
6. Better for gluing and fastening: nails, screws and adhesives hold better.
7. Improves workability and finish; reduces electrical conductivity, and makes it lighter in colour.
8. Prevents fungal stains and gives a uniform appearance.

Thus seasoned timber is more economical, strong and durable.

Exogenous trees (outward growers, such as sal, teak, pine) increase in diameter by adding new layers of wood each year under the bark. They provide almost all the structural timber.

Cross-section

         _______________
       /  ___________    \
      /  /  _______   \   \   Outer bark
     |  |  /  ___  \   |   |  Inner bark
     |  | |  ( o )  |  |   |  Cambium
     |  | |  pith   |  |   |  Sapwood
     |  |  \_______/  |   |  Heartwood
      \  \___________/   /   Annual rings
       \_______________/    Medullary rays

Macrostructure (components and functions)

1. Pith (medulla): soft central core; it is the first growth and has no strength.
2. Heartwood: dark, dense inner rings formed of dead cells; it gives strength and resists decay, so it is used as timber.
3. Sapwood: light, outer, living rings which carry sap from roots to leaves; weaker and less durable.
4. Cambium layer: thin layer of living cells between sapwood and the inner bark which forms new wood inward and new bark outward every year.
5. Annual rings: concentric rings of one year of growth each (spring wood is light and soft; autumn wood is dark and dense); counting rings gives the age of the tree.
6. Medullary rays: radial lines from the pith to the cambium which carry sap and store food, and bind the rings.
7. Inner bark (bast): carries food from leaves to the tree.
8. Outer bark: protects the tree from weather and insects.

Microstructure

Under the microscope, wood is made of elongated cells (fibres, tracheids and vessels in hardwoods) with hollow cavities, whose cell walls are mainly cellulose and lignin. Softwoods have tracheids (about 90% of the volume) that give strength and carry water; hardwoods have vessels (pores) for water, and fibres for strength.

Preservation is the treatment of timber with chemicals to protect it from decay by fungi, attack by insects (termites, borers) and marine borers, increasing its life. Timber should be seasoned first.

Methods

1. Brushing or spraying: preservative (coal tar, creosote, copper sulphate solution) is applied with a brush or spray on the surface, 2-3 coats. Cheap, but limited penetration.
2. Dipping: timber is dipped in preservative for few minutes/hours (for example in copper sulphate solution); used for small members.
3. Steeping (soaking): timber is soaked in a cold or hot solution of preservative in a tank for days; penetration is better.
4. Hot and cold open-tank treatment: timber is heated in hot preservative (about 90 C) for several hours, then moved to cold preservative, which is drawn into the wood as it cools.
5. Pressure treatment (full-cell/Bethell or empty-cell process): timber in an airtight cylinder is evacuated, preservative is forced in at 8-12 kg/cm², giving deep penetration; the best method but costly.
6. Charring: surface of timber (posts) is burnt to a depth of 15 mm; cheap but of limited use.
7. Pressure-impregnation / diffusion and sap displacement are also used for green wood.

Common preservatives: coal-tar and creosote (oil type), copper-chrome-arsenic (CCA, water-soluble), and organic solvents (pentachlorophenol).

Defects reduce the strength, durability and appearance of timber. They are grouped by cause.

1. Defects due to natural forces (during growth)

• Knots: bases of branches buried in the trunk; they weaken timber and distort grain.
• Shakes: cracks that separate fibres along the grain: heart shake, star shake, cup shake (ring shake), and radial shake.
• Wind cracks and upsets, rind galls, burls, twisted fibres, and water stain.

2. Defects due to seasoning

• Warping (bow, cup, twist, spring), checks, splits, honeycombing, case hardening, collapse.

3. Defects due to conversion

• Chip marks, torn grain, diagonal grain, wane (rounded edge with bark).

4. Defects due to fungi and insects

• Dry rot, wet rot, blue stain, and attack by termites, powder-post beetles and marine borers.

Note: Wood with such defects is graded lower or rejected.

Good timber for engineering use should have these characteristics:

1. Colour and appearance: uniform, dark colour in heartwood; shining surface when cut; sweet smell, with clear ringing sound when struck.
2. Structure: compact, fine and uniform grain, regular annual rings with narrow width, dense medullary rays, and a hard texture.
3. Sound heartwood: timber from mature trees (not young or old) with a high proportion of heartwood.
4. Free from defects: no knots, shakes, cracks, decay or insects.
5. Strength: high strength in compression, bending and shear; resilient and tough.
6. Seasoned: low moisture content (10-15%).
7. Durable: resistant to fungi, insects, and moisture changes; weathers well.
8. Elasticity and hardness: good elasticity, high hardness to resist wear.
9. Workability: easy to saw, plane, nail and polish; takes paint well.
10. Dimensionally stable: small shrinkage and swelling; no warping.
11. Fire resistance, weight: heavy species are generally stronger; adequate resistance to fire.

Timber is sold in the market in the following forms:

1. Logs: felled, trimmed trunks of trees, with branches removed.
2. Baulk: log roughly squared by axe or saw, with sides larger than about 200 mm.
3. Plank: sawn timber 50-150 mm thick and more than 100 mm wide.
4. Board: sawn timber with thickness less than 50 mm and width over 100 mm.
5. Battens: sawn pieces up to 50 mm thick and 50-200 mm wide.
6. Scantling: sawn pieces of small square cross-section, with sides of about 50-150 mm.
7. Poles/Posts: round, slender trunks used for scaffolding and electric lines.
8. Veneers: thin sheets of 0.4 to 6 mm cut by peeling/slicing.
9. Plywood: layers of veneer glued with grains at right angles.
10. Fibre board, particle board (chipboard), block board, laminated board, hardboard, and MDF.
11. Sawdust, wood wool, bark and cork as by-products.

Seasoning is the controlled removal of moisture from timber to bring its moisture content to 10-15%. The methods are classified as natural and artificial.

A. Natural seasoning

1. Air seasoning: logs are cut into planks and stacked in a shed, well ventilated and protected from sun, rain and wind.

     (shade roof)
   ===================
   --- boards ---        <- sticker (spacer)
   --- boards ---        <- sticker
   --- boards ---
   ___ ___ ___ ___ ___   <- raised platform
   |  |  |  |  |  |       (300 mm above ground)

• Planks are piled on raised platforms with spacers (stickers) of 25 mm so air circulates.
• It takes 6 months to 2-3 years. It is slow but cheap and gives a uniform result.

2. Water seasoning: logs are submerged in running water for 2-4 weeks, with the thick end facing upstream. The sap is washed away, then the logs are dried in air. It is quick and reduces shrinkage, but may weaken the timber.

B. Artificial seasoning

1. Kiln seasoning: timber is stacked in a closed chamber (kiln) in which hot air with controlled humidity and temperature circulates. Temperature is raised in stages (about 35 C to 80 C) with humidity reduced slowly. Done in 4-10 days (depends on thickness). Controlled, fast, but costly.

   Hot air in -> [ stacked timber ] -> moist air out
        ^                                  |
        +----- Fan, heater, steam sprays --+

2. Chemical (salt) seasoning: timber is soaked in a solution of urea or common salt, which draws moisture from the inner part and slows down surface drying, which reduces cracking. 3. Electrical seasoning: a high frequency current passes through the timber and heats it; very fast but costly (used in special cases). 4. Boiling: timber is boiled in water, then dried in air. 5. Smoke seasoning for small items is used in rural areas.

Comparison

Hardwood comes from broad-leaved (deciduous) trees such as sal, teak, oak, shisham and mahogany. Softwood comes from needle-leaved (coniferous) trees such as pine, deodar, cedar and fir.

Uneven or fast drying of timber causes the following defects.

1. Warping: change of shape due to uneven shrinkage, and may be bow (along the length), cup (across the width), twist (spiral) or spring (edgewise).
2. Checks: cracks across the annual rings on the surface, caused by rapid drying of the outer part.
3. Splits: a check which passes through the thickness from one face to the other, mainly at the ends due to quick drying.
4. Honeycombing: internal cracks and cavities, caused by interior drying after the outer layer has hardened (stress from case hardening).
5. Case hardening: the surface dries and hardens (set) while the core is wet, so the surface is under tension, causing cracks later.
6. Collapse: flattening of cells due to fast drying of wet wood at high temperature.

Why timber is widely used

Timber is used in all engineering works because it is:

• readily available and renewable, and easy to cut, shape, join and finish with simple tools;
• light in weight but has high strength-to-weight ratio, good in tension and bending;
• a poor conductor of heat, sound and electricity, and so comfortable and insulating;
• good at absorbing shock and vibration, and resistant to some chemicals;
• cheaper than steel in many places (Nepal has plentiful forests), and attractive in appearance;
• reusable and easy to repair, and suitable for doors, windows, roofs, floors, formwork, scaffolding, bridges and furniture.

Seasoning method (brief)

Seasoning removes moisture to 10-15% so that timber does not shrink, warp or decay.

1. Natural (air) seasoning: the converted timber is stacked on raised platforms under a shed in a well-ventilated place, with spacers between the layers, for 6 months to 2 years.
2. Water seasoning: logs are immersed in running water for 2-4 weeks to wash out the sap, then dried in air.
3. Kiln seasoning: timber is stacked in a chamber with hot air of controlled temperature and humidity for 4-10 days; this is quick and controlled.
4. Chemical and electrical seasoning are also used in special cases.

Seasoning is the process of drying freshly felled timber in a controlled manner to reduce its moisture content to about 10-15%, near the moisture of the surrounding air in use.

Why it is needed

• Reduces weight and increases strength and hardness.
• Prevents shrinkage, warping and cracking after fixing.
• Prevents decay by fungi and insects (need moisture above 20%).
• Makes timber easy to treat with paint, polish, and preservatives, and to join with nails and glue.

Methods

1. Natural seasoning

• Air seasoning: planks are stacked on raised platforms under cover, with spacers for air circulation. It takes months to years; cheap and uniform.
• Water seasoning: logs are immersed in running water for 2-4 weeks, the sap is washed out, and then dried in air.

2. Artificial seasoning

• Kiln seasoning: timber is stacked in a closed chamber and dried by hot air of controlled humidity and temperature in 4-10 days.
• Chemical seasoning: soaking in a salt or urea solution to reduce surface cracks.
• Electrical seasoning: high-frequency current heats the timber; very fast but costly.
• Boiling: timber is boiled in water or steamed, then air dried.

Defects that occur while the tree is growing (from natural forces and growth faults) are:

1. Shakes: cracks that separate the fibres along the grain, of several types:
   • Heart shake: radial cracks starting from the pith in over-mature trees.
   • Star shake: several radial cracks from the bark inward, from frost or sun.
   • Cup shake (ring shake): cracks along the annual rings, curved separation of rings.
   • Radial shake: fine radial cracks from the circumference.
2. Knots: the bases of branches embedded in the trunk (live or dead, pin, spike); they disturb the grain and reduce strength.
3. Rind galls: curved swellings where a branch was cut off and the bark covered it.
4. Burls (excrescences): abnormal growths on the trunk surface.
5. Upsets (rupture): fibres crushed by wind or by bad felling.
6. Wind cracks and twisted fibres (spirality): due to winds, and spiral growth.
7. Water stain, Foxiness, Callus: stains from wet conditions or injury.

Defects caused by natural forces (wind, frost, sun, lightning or abnormal growth) in the standing tree are:

1. Knots: bases of branches enclosed within the wood; they cut across the grain and reduce strength.
2. Shakes: heart, star, cup and radial shakes, which are cracks along the fibres due to frost, sun heat, wind or over-maturity.
3. Wind cracks and upsets: crushed or broken fibres from strong winds or rough felling.
4. Rind galls: swellings where a branch was lost and the bark folded over.
5. Burls and twisted fibres: abnormal growths or spiral grain due to constant wind in one direction.
6. Callus, water stain and lightning damage.

Plywood is a manufactured board made by gluing together thin layers of wood (veneers, plies) in an odd number (3, 5, 7...) with the grain of each layer at right angles to the next one, under heat and pressure. It is strong, stable and resists splitting and warping; used for doors, partitions, furniture and shuttering.

Merits of timber over steel:

1. Light weight and easy to handle, transport and erect.
2. Easy to work with simple tools (cut, nail, join), and easy to repair or modify.
3. Cheaper in the initial cost in many regions, and locally available (Nepal).
4. Good thermal and electrical insulation: poor conductor, so comfortable in buildings.
5. Does not corrode and is resistant to many chemicals and acid fumes.
6. Good shock and sound absorption and attractive natural appearance.
7. Renewable, and with burnt wood retaining its strength in a fire for some time (a large section chars and keeps its core).

Suitability of timber in structures

Timber is suitable for:

• Roof structures: rafters, purlins, trusses, battens for small and medium spans.
• Floors and partitions: joists, floorboards, panelling.
• Doors, windows and frames.
• Formwork, scaffolding and shoring for concrete work.
• Bridges and piles (light bridges, and timber piles under water, with preservation).
• Furniture and fittings.

It suits the structures in which light weight, quick erection, insulation and ease of working matter, and where members are protected from moisture and fire. It should be properly seasoned and treated, and designed on allowable stresses of the species and grade (IS 883).

Engineering properties

1. Strength in tension, compression, bending, shear and impact (along and across grain).
2. Modulus of elasticity and stiffness.
3. Hardness and toughness.
4. Density (specific gravity) and moisture content.
5. Durability against fungi, insects and weather.
6. Shrinkage and swelling.
7. Workability and finishing property.
8. Thermal, acoustic and electrical insulation.

Uses of timber

• Doors, windows, frames, shutters and floors.
• Roof members: rafters, purlins, trusses.
• Formwork, scaffolding, props and shoring.
• Piles, sleepers for railways, poles, and bridges.
• Furniture, fittings, partitions and panelling.
• Packing cases, tool handles and pulp for paper.

Why seasoning is essential

Seasoning removes the excess moisture so timber becomes lighter, stronger and stable. Unseasoned timber shrinks, warps and cracks after fixing, and rots by fungi and insects. Seasoned timber also takes paint, polish and preservative better.

Modified timber (engineered wood) in buildings

Modified timber is made by processing natural wood for better uniformity, strength and durability:

• Plywood: veneers glued with grains crossed; for doors, partitions, shuttering.
• Veneers and laminated board: thin sheets for facing and decoration.
• Block board: wood strips as core with veneers outside.
• Particle board (chipboard) and fibre boards (hardboard, MDF): made from chips or fibres pressed with resin; for furniture and ceilings.
• Glued laminated timber (glulam) and Cross laminated timber (CLT): layers of boards glued for large beams and floors.
• Compressed wood and impregnated wood: made denser and more durable.

Types of wood

1. Hardwood: from deciduous broad-leaved trees (sal, teak, shisham, oak); dark, heavy, strong, durable; for beams, doors, furniture.
2. Softwood: from coniferous trees (pine, deodar, fir, cedar); light, easily worked, less durable; for roof frames, shuttering, packing.
3. Also by processing: natural timber (sawn), and manufactured or modified wood (plywood, particleboard, fibreboard).
4. By structure: exogenous (outward growing, used as timber) and endogenous (inward growing like bamboo and palm).

Strength along and across the grain

Wood is an anisotropic (direction-dependent) material because its fibres run along the trunk.

• Along the grain the fibres act like bundles of tubes and carry load axially, so strength is high.
• Across the grain the fibres are held only by weak lignin bonds and tend to crush or split, so strength is low.
• Hence timber members are placed so that main stresses act along the grain.

Physical properties

1. Colour, smell and appearance (grain, figure, lustre).
2. Density and specific gravity.
3. Moisture content, hygroscopic nature (absorbs and loses moisture).
4. Shrinkage and swelling (dimensional change).
5. Thermal properties: conductivity, expansion and fire resistance.
6. Electrical and acoustic properties (insulating, sound absorption).
7. Durability: resistance to decay and insects.
8. Workability and finishing.

Mechanical properties

1. Compressive strength (parallel and perpendicular to the grain).
2. Tensile strength.
3. Bending strength (modulus of rupture).
4. Shear strength.
5. Modulus of elasticity (stiffness).
6. Hardness.
7. Toughness and impact strength (resilience).
8. Resistance to splitting, cleavage and fatigue; creep under long-term loads.