Chapter 7: Timber

Sources of Timber

The source of timber supply is the trees in the forest.

Cross-section of a Tree

Classification of Timber

• Mode of Growth

a. Exogens (Exogenous) – These trees grow outward.

Coniferrous Trees

Deciduous Trees

b. Endogens (Endogenous) – These trees grow inwards.

Classifications of Timber

• Modulus of Elasticity

Group A: E = 12.5 GPa

Group B: E = 9.8 GPa to 12.5 GPa

Group C: E = 5.6 GPa to 9.8 Gpa

• Durability

High Durability

Moderate Durability

Low Durability

Classifications of Timber

• Seasoning characteristics

High refractory (Class A)

Moderately refractory (Class B)

Non refractory (Class C)

• Grading

Select-Grade

Grade I

Grade II

Identifications of Timber

• General Properties

Color – a darker color in wood indicates greater durability.

Odor – it is present only on freshly cut trees.

Hardness – is the ability of wood to withstand indentations caused by harder bodies.

Density – densest woods are generally the strongest.


Grain - Depending on the actual alignment, the grain may be straight, spiral, interlocked, wavy or irregular.

• Spiral Grain

• Interlocked Grain

• Wavy Grain

Texture - In hardwoods, the texture depends upon the size and distribution vessels and rays. In softwoods, it is determined by the size and distribution of tracheid.


Luster- is the ability to reflect light beam pointedly.

Workability- the relative case in which wood is shaped cut and fastened together than the others.

Warping- is the general term used to describe any variation from a true surface.

Moisture content- is a percentage of the mass of water over the mass of wood fibre in a piece of timber.

Specific Gravity- is the ratio found by dividing the weight of a substance by the weight of an equal volume of pure water.

Structure of Softwoods

• Tracheids - These are hollow, needle-shaped units closely packed together so that a cross-section through them resembles a honeycomb.

• Storage Tissues (Rays and Wood Parenchyma Cells) - They are too small to be seen individually and are not of much in identification.

• Resin Canals - these canals are not cells but are cavities in the wood lined by the parenchyma cells.

• Growth Rings - these are normally distinct in softwoods.

Structures of Hardwoods

• Pores or Vessels – these are tubular elements running in a vertical direction and serve to conduct water and mineral nutrients.

• Inclusions – the pores or vessels are normally open to enable conduction but many times, particularly in the heartwood, they become filled with inclusions of various types.

• Parenchyma or Soft Tissues - these are storage tissues and consist of small, thin-walled, rectangular or brick-shaped cells. These are vertically aligned.

• Rays - are groups of horizontally aligned parenchyma cells running in a radial direction and their function is horizontal conduction and storage.

Structures of Hardwoods

• Fibres – These are vertically-aligned, thick-walled elements with pointed, tapering ends which make up the bulk of the weight of the wood.

• Intercellular cells – These are long tubular cavities seen in the horizontal or vertical direction.

Felling and Conversion

• Felling and Transporting the Timber

Felling is done by a clean saw cut as near to the base as possible.

Transporting the log to saw mill is done either by road transport or dragging (by elephants) to the nearest river and floating them down in rafts.

Felling and Conversion

• Sawing

Methods of Sawing

Ordinary or Flat Sawing

Tangential Sawing

Radial Sawing

Quarter or Rift Sawing

Combination Sawing

Moisture of Timber

• Moisture Content - It is one of the most important characteristics of timber that affects its use on the site performance.

• Equilibrium Moisture Content (EMC) - moisture content of wood changes in response to the temperature and humidity of its surroundings.

• Fibre Saturation Point (FSP) – when the cell cavities are empty but the cell walls still retain their bound water.

Moisture of Timber

• Shrinkage and Swelling - Shrinkage occurs as moisture content decreases, while swelling takes place when it increases.

Moisture of Timber

• Timber Drying :

Natural Drying or Air Drying

Artificial Drying or Kiln Drying

Mechanical Properties

• Tensile Strength – Timber is stronger in tension along the rain but it’s quite difficult to determine this because of the difficulties in conducting test.

• Compressive Strength – The strength along the grain is important for columns, props, and post.

• Shear Strength – Shear strength is important in the case of the beam and slabs.

• Bending Strength or Flexural Strength – This refers as the strength of the timber as a beam.

Mechanical Properties• Cleavability – High resistance for cleavage is important for

nailing and screwing while low splitting strength is important for used as firewood.

• Brittleness – is use to describe the property of suffering little deformations before breaking.

• Torsion Strength – is used to determine the torsion strength of the timber and the specimen is loaded up to failure.

• Hardness – is important in case of timber for paving blocks flooring bearings and other similar purposes.

• Stiffness - This property is important to determine the deflection of a timber under a load

Defects in Timber

Defects caused in timber may be basically classified into two categories:

Internal Defects – due to factors affecting the growing tissue or the tree.

External Defects – due to external agents or subsequent treatment of timber.

Classification of Defects in Timber

• Knots - are dark, hard pieces occurring as signs of branches broken or cut off.

According to size (diameter)

Pin Knot – not more than 6.5mm

Small Knot - more than 6.5mm but not more than 20mm

Medium Knot- more than 20mm but not more than 40mm

Large Knot- more than 40mm


According to form

Round or Oval Knot- a knot cut more or less at right angles to its long axis so that the exposed is round or oval.

Splay or Spike Knot- a knot cut approximately parallel to its long axis so that the exposed section has an elongated shaped.


According to quality

Live Knot- a knot whose fibers are completely intergrown with those of the surrounding wood.

Sound Knot- a live knot free from decay.

Tight Knot- a live or dead knot held firmly in place

Dead Knot- a knot whose fibers are not intergrown with those of the surrounding wood.


According to quality

Loose Knot- a dead knot which is not held firmly in place.

Knot Hole- a hole left as results of the removal of a knot.

Decayed Knot- a knot softer than the surrounding wood, and containing decay.


• Shakes- or cracks are caused by the rupture of tissues resulting in partial or complete separation of the fibers along the grain.

Star Shakes

Heart Shakes

Cups Shakes

Ring Shakes

Radial Shakes


• Rind Galls- these are typical curved swelling formed upon the trunk of the tree by

successive layer of the sapwood.

• Upsets- are clipping or buckling of fibers caused by crushing of fibers when the trunk is felled violently over a rocky surface.


• Twisted or Spiral Grain- fibers or wandering hearts are caused by the prevalent wind turning or twisting the tree at its young age constantly inone direction.

• Wind Cracks- are shakes on the outside of a log due to the shrinkage of the exterior surface causedby the atmospheric influences.

Defects Due to Seasoning

• Bow- a curvature of the timber in the direction of its length.

• Cup- a curvature in the transverse direction of the timber.

• Twist- a spiral distortion along the length of the timber.

• Spring- a curvature of the timber in its own plane.


• Case Hardening- it is due to the unequal drying of the exterior surfaces under compression and the interior surfaces under tension due to rapid drying.

• Split- separation of the fibers along the grain and extends from one end of the plank to the other.

• Honey Combing- separation of the fibers in the interior due to drying stresses.


• Collapse - the cells of timber are flattened due to excessive shrinkage.

Defects Due to Manufacturing or Use

• Chipped or Torn Grain- a defect caused by the breaking away of timber below the surface ofthe dressing by the action of the planning tool.

• Chip Mark- indentation on the finished surface of the timber caused by the chips or other small pieces of the timber being carried around on the planning knife edges.

Defects Due to Manufacturing or Use

• Wane- the original round surface of a tree which remains on finished plank.

• Boxed Heart- the pith at the center fully enclosed within one piece in the process ofconversion.

Evaluation of Defects of Timber

• Knots- tend to weaken timber in tension but may improve the strength in compression.

• Checks, Splits and Shakes- these reduce the shearing resistance of the wood.

• Compression Wood- increase density and shrinkage along the grain and decreased shock resistance.

• Tension Wood- it has high longitudinal shrinkage tending it to warp and split.

• Sap Rot- this results in decreased shock resistance and increased absorption.

• Slope of Grain- this lowers the tensile strength parallel to the grain and the modulus of rupture which increases in the case of radial slope of grain.


• Loose Grain- weaker than closed grain timber.

• Wane- this reduce the mechanical properties and volume.

• Worm Hole- deep, large worm holes spoil the appearance and reduce the mechanical properties of timber.

• Pith Pockets- affect the appearance and reduce the mechanical properties of timber.

• Boxed Heart- in cut sizes of timber, it does not affect the quality appreciably.

• Discoloration of Heartwood- it does not affect the mechanical properties and this defect can be ignored in preservative treated timber.


• Mould- this essentially spoils the appearance of the wood.

• Sap Stain- which the stain is the most important, does not affect the strength properties of timber.

Decay of Timber

• Decay or Rot - is usually caused by the presence of sap or by the alternate dry and wet condition of the timber.Forms of Decay

Wet rot

Dry rot

Due to fungal attack

Due to insects

Decay of Timber

Wet Rot - This is a decomposition of sap and fibers of the living tree set up by the access ofwater through wounds in the barks.

Dry Rot - This is a decomposition of converted timber induced by the commonest and most destructive dry rot fungi or weeping fungus.

Decay of Timber

Due to fungal Attack – A fungus starts its life as a tiny seed or spore, one of millions produced at the fruiting stage.o Fungi are system plant organisms which live on and

attack timber causes it to rot or decay.o Fungi can only exist in following condition:

Air for their respirationMoistureSuitable TemperatureFood Supplies

Decay of Timber

Due to Insects – damage to timber by insects is quite considerable particularly in warm country.

o Types of Insects:

Termites

Beetles

Boarers

Decay of Timber

• Termites

Decay of Timber

• Beetles

Flour like powder

Decay of Timber

• Boarers

BOARS

Seasoning of Timber

• OBJECTIVES OF SEASONING

-To reduce movement of timber, to reduce the tendency to split.

-To make it immune from attack by insects.-To increase strength, durability, workability and

resilience.-To make the timber receptive to finish like

preservatives, paints, and varnish.-To reduce weight and minimize cost of transportation.-To make the timber burn readily, if used as a fuel.

Seasoning of Timber

• Principles of seasoning

Three factors controlling this process:

Relative Humidity

Rate of Air Circulation

Temperature

Seasoning Characteristics of Timber

• Class A (Highly refractory woods) – These timbers are slow and difficult to dry if the final product is to be free from defects, particularly cracks and splits.

• Class B (Moderately refractory woods) – These timbers show a moderate tendency to crack and split during seasoning.

• Class C (Non-refractory woods) – These timbers can be rapidly seasoned to be free from defects even by applying high temperatures in industrial kilns.

Methods of Seasoning

• Natural Seasoning - is a slow process and the reduction in moisture content to the desired level may not be attainable by air seasoning alone.

• Artificial or Kiln Seasoning - is a quicker method of seasoning timber t the desired moisture content under highly controlled conditions. Progressive kilnCompartment kiln

• Electrical Seasoning - in this method high frequency alternate electric current is passed through timber.

• Chemical Seasoning - If the outer layers of timbers are treated with such solutions, the vapor pressure of the outer layers is reduced and a vapor pressure gradient is set up.

Preservation of Timber

• Preservatives Types:Type 1 (Oil Type) – coal tar creosote with or without

admixture of petroleum, coal tar, fuel oil or other suitable oils having high boiling range.

Type 2 (Organic solvent type) – consist of toxic chemical compounds dissolved in a non- aqueous solvent like volatile oils, spirits, naphtha etc.

Type 3 ( Water-soluble [leachable] ) – these preservatives are inorganic salts soluble in water.

Type 4 ( Water-soluble [Fixed] ) – These consists of the various salt described above with the addition of fixative salt, usually sodium or potassium dichromate.


• Characteristics of a Good PreservativeIt should be strongly toxic to insects and fungiIt should penetrate readily into the timber.It should not be soluble in water nor should it change in

composition under ordinary climatic condition.It should cover a large surface area with a comparatively

small quantity of preservative.It should have no destructive influence on the strength of

the timber.It should be harmless to human beings.


• Characteristics of a Good Preservative

It should not render the timber treated with it more inflammable.

It should not be non-corrosive to metals.

It should be pleasant in color and give an attractive appearance to the wood work.

It should be highly resistant to water and dampness.

It should be economical and available readily.

Preparation of Timber for Treatment

• Methods of Treatment• Surface Application • Full

Cell or Bethel Process

• Soaking • Empty Cell Process

• Hot and Cold Process • Choice of Treatment

• Boucherie Process

• Diffusion Process

• Pressure or Pneumatic Process

Fire-proofing of Timber

• Timber about a particular thickness, cannot support self-sustained combustion and continue to burn only when external heat is applied.

• Timber cannot be made fire proof but can only be made fire resistant.

• It is supposed that the fire-proofing chemicals act in one of the following ways:The melting point of the chemicals is low so that it melts

and forms a barrier to the supply of oxygen to the inside.The chemical decomposes under heat, yielding non-

inflammable gases that dilute the inflammable gases. This retards the ignition of inflammable point.

The chemical vaporizes at sufficiently low temperature, absorbing sufficient heat that the temperature of wood does not rise to decomposition point.

Availability and Uses of Timber

• Common TimberTeak - The sapwood is pale yellowish or greyish-white in

color and is distinguished from heartwood. The heartwood is golden brown and turns a dull deeper brown on exposure.

Deodar - The sapwood is white to creamy white and is distinct from the heartwood which is light yellowish brown turning to pinkish brown on exposure.

Chir - The sapwood is white to creamy white and is distinct from the heartwood which is light yellowish brown turning to pinkish brown on exposure.


• Common TimberKail - The yellowish in white to greyish sapwood is white

to pale-yellowish white in color and is distinct from the heartwood which is light pinkish-red to light red.

Sisso - Sapwood of sisso is pale yellowish or greyish white and is distinct from the heartwood which is golden brown or dark brown in color.

Sal - The sapwood is pale-yellowish or brownish white and is distinct from the heartwood which is brown or reddish brown.

Mango - This is brown in color.


• Advantages of TimberIn terms of specific strength.Timber can be used for both load bearing structures and

non-load bearing instruments.It can be easily converted to any shape and size.With the advent of superior adhesives and other forms of

timber connectors.The wastage in timber construction is less because all

wastes can be put one use or the other.


• Advantages of TimberIt has high resale and salvage value.

Timber has better insulating properties than any other materials.

Durability of timber can be very high, if properly seasoned and preserved.

Timber constructions are light in weight and can be made economical.

For aesthetic and decorative appearance.

Wood-Based Product

• Veneers - thin sheet of wood, 0.4 mm to 0.6 mm in thickness obtained by different knife cutting process.

Rotary veneers Cutting sliced veneers

• Plywood - is made by gluing together veneers.

Wood-Based Product

• Plywood

Plywood grades and sizes

Boiling water proof (BWP) grade

Boiling water resistance (BWR) grade

Warm water resistance (WWR)

Cold water resistance (CWR) grade

Wood-Based Product

• PlywoodThickness

Board Thickness (mm)

Board Thickness (mm)

3-ply 3, 4, 5, 6 9-ply 12, 15, 16, 19

5-ply 5, 6, 8, 9 11-ply 19, 22, 25

7-ply 9, 12, 15, 16 Above 11-ply As ordered

Wood-Based Product

• Plywood

• Dimensions (sizes of plywood)

(cm x cm) (cm x cm)240 x 120 180 x 90240 x 90 150 x 120210 x 120 180 x 90210 x 90 120 x 120189 x 120 90 x 90

Tolerance of plywoodDimension Nominal size Tolerance

Length Up to and including 120 cm

Above 120

+ 3 mm - 0 mm

+ 6 mm- 0 mm

Width Up to and including 90 cm

Above 90 cm

+ 3 mm - 0 mm

+ 6 mm- 0 mm

Thickness Up to and including 5 mm6 to 9 mm

Above 9 mm

± 10%± 7%± 5%

Wood-based Products

• Advantages of plywood

It has a good strength both across as well as along the grain.

Plywood has better splitting resistance due to grains in adjacent layers running at the right angle and nailing and screwing even closed to the edges in safe.

Can easily be bending to a curvature limited by the tensile and the compressive strength of the other plies.

The face piles made at different heartwoods show great aesthetic variety in grain pattern and color.

Wood-based Products

• Hardboard - manufactured from raw materials like wood waste, veneer manufacturing, furniture making.

Sizes of hardboards

Medium hardboard

Normal hardboard

Tempered hardboard

Wood-based Products

• Thickness of HardboardsType (mm) Nominal thickness

(mm)Tolerance

Medium hardboard 68

1012

± 0.5± 0.7± 0.7± 0.9

Standard hardboard 3, 4, 569

± 0.4± 0.5± 0.7

Wood-based Products

• Width and Length of HardboardsType Width (m) Tolerance

on width,Max. (mm)

Length (m) Tolerance on length

max. (mm)Medium

hardboard1.2 ± 3 1.2, 1.8, 2.4 ± 3

Standard hardboard

1.2 ± 3 3.0, 3.6, 4.8 ± 3

Tempered hardboard

1.2 ± 3 5.5 ± 3

Wood-based Products

• Particle boards - is a board manufactured from particles of wood or other lingo cellulose materials.

• Block board – these are boards having a core made up strips of wood each not exceeding 25 mm.

Grade 1

Grade 2

Wood-based Products

• Dimensions

Thickness (mm): 12, 16, 20, 25, 30, 32, 35, 38, 40, 45, 50

Length (mm): 300, 270, 240, 210, 180, 150, 120

Width (mm): 150, 120, 90

Wood-based Products

• Other boards• Batten board – it is a board having a core made

up of strips of wood usually 8 cm wide each laid separately or glued or otherwise joined toform a slab.

• Lamin board – it is a board having core strips, each not exceeding 7 mm in thickness.

Wood-based Products

• AdhesivesSynthetic Resins – it has a composition consisting

substantially of phenolic or amino plastic resins including a hardening agent, fortifier, filler or extender. Four types of synthetic resin

Boiling Water Proof (BWP)Boiling Water Resistant (BWR)Warm Water Resistant (WWR)Cold Water Resistant (CWR)

Wood-based Products

Classification According to their Uses

Gap filling adhesive – suitable use in situation where the surfaces to be joined may or may not be in continuous contact.

Close contact adhesive – where the surfaces to join can be brought into closed contact by means of adequate pressure.

Wood-based Products

• Anima Glue – it is generally obtained from hides and bones of animals.

• Vegetable Glue – it is made from natural grains and starch-bearing roots, potato, corn, wheat, etc. with acid like Sodium Hydroxide (NaOH).

Chapter 7: Timber

Business

Transcript of Chapter 7: Timber