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Chapter 7: Timber
 

Chapter 7: Timber

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    Chapter 7: Timber Chapter 7: Timber Presentation Transcript

    • 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.
    • Identifications of Timber 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.
    • Identifications of Timber 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
    • Classification of Defects in Timber 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.
    • Classification of Defects in Timber 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.
    • Classification of Defects in Timber 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.
    • Classification of Defects in Timber • 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
    • Classification of Defects in Timber • 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.
    • Classification of Defects in Timber • Twisted or Spiral Grain- fibers or wandering hearts are caused by the prevalent wind turning or twisting the tree at its young age constantly in one direction. • Wind Cracks- are shakes on the outside of a log due to the shrinkage of the exterior surface caused by 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.
    • Defects Due to Seasoning • 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.
    • Defects Due to Seasoning • 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 of the 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 of conversion.
    • 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.
    • Evaluation of Defects of Timber • 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.
    • Evaluation of Defects of 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 of water 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 respiration Moisture Suitable Temperature Food 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 kiln Compartment 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.
    • Preservation of Timber • Characteristics of a Good Preservative It should be strongly toxic to insects and fungi It 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.
    • Preservation of Timber • 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 Timber Teak - 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.
    • Availability and Uses of Timber • Common Timber Kail - 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.
    • Availability and Uses of Timber • Advantages of Timber In 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.
    • Availability and Uses of Timber • Advantages of Timber It 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 • Plywood Thickness 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 90 240 x 90 150 x 120 210 x 120 180 x 90 210 x 90 120 x 120 189 x 120 90 x 90
    • Tolerance of plywood Dimension 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 mm 6 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 Hardboards Type (mm) Nominal thickness (mm) Tolerance Medium hardboard 6 8 10 12 ± 0.5 ± 0.7 ± 0.7 ± 0.9 Standard hardboard 3, 4, 5 6 9 ± 0.4 ± 0.5 ± 0.7
    • Wood-based Products • Width and Length of Hardboards Type Width (m) Tolerance on width, Max. (mm) Length (m) Tolerance on length max. (mm) Medium hardboard 1.2 ± 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 to form a slab. • Lamin board – it is a board having core strips, each not exceeding 7 mm in thickness.
    • Wood-based Products • Adhesives Synthetic 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).