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timbers-lecture 06.ppt

  1. Timber Oh!
  2. General Timber is one of the oldest building materials, along with stone, earth and various vegetable materials. Timber has remained until today the most versatile and, in terms of indoor comfort and health aspects, most acceptable material.
  3. Cont’d Timber is an extremely complex material, available in a great variety of species and forms, suitable for all kinds of applications.
  4. Classification of Timber There are two main groups of timber producing trees:  Softwoods Hardwoods
  5. Softwoods Softwoods are coniferous trees and the timber is not necessarily 'soft'. They are 'evergreen‘, do not shed their leaves in autumn.  Their general characteristics are:  Straight  round but slender  tapering trunk  The crown is narrow and rises to a point.  It has needle like or scale-like shaped leaves.  The bark is course and thick
  6. Hardwoods Hardwood trees are:  Broadleaf and generally deciduous.  Their timber is not necessarily hard.  The general characteristics are: Stout base which divides into branches to form a wide, round crown.  The leaves are broad  The bark may be smooth or course and varies in thickness and colors.
  7. Timber for construction Timber for building construction is divided into two categories:  Primary timber Secondary timber
  8. Primary timbers Primary timbers are:  Slow-grown  Aesthetically appealing  Have considerable natural resistance to biological attack, moisture movement and distortion.  Expensive and in short supply.
  9. Secondary timbers Secondary timbers are: Mainly fast-grown species Low natural durability With appropriate seasoning and preservative treatment, physical properties and durability of secondary timbers can be greatly improved.
  10. Advantages of timber Timber is suitable for construction in all climatic zones, and is unequaled by any other natural or manufactured building material in terms of versatility, thermal performance and provision of comfortable and healthy living conditions.
  11. Cont’d Most species have very high strength: weight ratios, making them ideal for most constructional purposes, particularly with a view to earthquake and hurricane resistance. Timber is compatible with traditional skills and rarely requires sophisticated equipment.
  12. Cont’d The production and processing of timber requires less energy than most other building materials. Timber provides good thermal insulation and sound absorption, and thicker members perform far better than steel in fire.
  13. Cont’d Demolished timber structures can often be recycled as building material, or burnt as fuel wood, the ash being a useful fertilizer.
  14. Disadvantages of timber  Fire risk of timber members and timber products with smaller dimensions.  High costs and diminishing supplies of naturally resistant timber species.  Failure of joints between timber members due to shrinkage or corrosion of metal connectors.
  15. Susceptibility of cheaper, more abundantly available timber species to fungal decay (by moulds and rot) and insect attack (by beetles, termites, etc.).
  16. Fungi attacked timber
  17. Timber growth
  18. Section of a trunk
  19. Annual or growth rings In temperate climates there are two distinctive growth seasons, spring and summer ~ the spring growth is rapid and is shown as a broad band whereas the hotter, dryer summer growth shows up narrow. In tropical countries the growth rings are more even and difficult to distinguish.
  20. Description of terms • Bark: The outer layer, corklike and provides protection to the tree from knocks and other damage.
  21. Cont’d • Bast: The inner bark, carries enriched sap from the leaves to the cells where growth takes place.
  22. Cont’d  Cambium: Layer of living cells between the bast and the sapwood.
  23. Cont’d  Heartwood: Mature timber, no longer carries sap, the heart of the tree, provides the strength of the tree. Usually a distinctive darker color than the sapwood.
  24. Cont’d  Sapwood: New growth, carries the raw sap up to the leaves. Usually lighter in color than the heartwood, especially in softwoods.
  25. Cont’d  Medullary rays: Food storage cells radiating from the medulla ~ provides a decorative feature found in quarter cut timber.
  26. Cont’d  Pith or medulla: The center of the tree, soft and pithy especially in the branches.
  27. Seasoning of timber Seasoning is the process by which the moisture content of timber is reduced to its equilibrium moisture -MC between 8 and 20 % by weight.
  28. Cont’d Time required for seasoning process depends on:  Timber species and age, Time of harvesting, Climate, Method of seasoning, etc.
  29. Reasons for seasoning Seasoning makes the timber suitable for the environment and intended use. The MC of timber needs to be reduced for the following reasons:
  30. Cont’d • Makes the timber more resistant to biological decay, increases its strength, stiffness and dimensional stability, and reduces its weight (and consequently transportation costs).
  31. Cont’d • Seasoned timber show fewer tendencies to warp, split or shake. • Seasoned timber although lighter will be stronger and more reliable.
  32. Cont’d • The sap in timber is a food for fungi and wood parasites. Remove the sap and the wood will be less attractive to these dangers! • For construction grade timber the timber must be below 20% MC to reduce the chances of fungi infestations.
  33. Cont’d • Dry and well seasoned timber is stronger. • Dry and well seasoned timber is easier to work with and consequently safer especially machine working. • Timber with higher moisture content is difficult to finish i.e. paint, varnish, etc.
  34. Methods of seasoning timber There are two main ways of seasoning timber:  Air seasoning  Kiln seasoning Both methods require the timber be stacked and separated to allow the full circulation flow of air, etc. around the stack.
  35. Air seasoning Air seasoning is done by stacking timber such that air can pass around every piece. Protection from rain and avoidance of contact with the ground are essential.
  36. Kiln seasoning There are two main methods used in kiln seasoning:  Compartmental  Progressive Both methods rely on the controlled environment to dry out the timber. and require the following factors:
  37. Cont’d Kiln seasoning require the following factors:  Forced air circulation by using large fans, blowers, etc. Heat of some form provided by piped steam. Humidity control provided by steam jets. The amount and duration of air, heat and humidity again depends on species, size, quantity, etc.
  38. Compartmental kiln A compartment kiln is a single enclosed container or building, etc. The timber is stacked and the whole stack is seasoned using a program of settings until the whole stack is reduced to the MC required.
  39. Progressive kiln A progressive kiln has the stack on trolleys that ‘progressively’ travel through chambers that change the conditions as it travels through the varying atmospheres. The advantage of this system, although much larger, has a continuous flow of seasoned timber coming off line.
  40. Cont’d Kiln seasoning achieves accelerated seasoning in closed chambers by heating and controlling air circulation and humidity, thus reducing the time by 50 to 75 %, but incurring higher costs. An economical alternative is to use solar heated kilns.
  41. Preservative treatment Seasoning alone is not always sufficient to protect secondary timbers from fungal decay and insect attack. Protection from these biological hazards is effectively achieved by preservative treatments with certain chemicals. There are many chemicals, used singly or in combination, which preserve timber against insect and/or fungal attack.
  42. Properties of timber Timber, as a natural material is variable! The disadvantages of variability are overcome by selection or grading processes and by the application of safety factors in structural calculations. Strength Durability Permeability Fire resistance
  43. Strength The structural strength of timber is a measure of its ability to resist external forces, such as compression, tension and shear. The density is reliable indicator of many structural and mechanical properties. There is a particularly strong relationship between density and compressive strength, bending strength and hardness.
  44. Cont’d Density ranges from an average of 160 kg/m3 to 1040 kg/m3. There is a marked difference in strength properties depending upon whether they are measured parallel to or perpendicular to the grain of the timber. The tensile strength of most timbers parallel to the grain is three to four times the compressive strength. The tensile strength parallel to the grain can be thirty times as high as perpendicular to it, while for compressive strength the ratio is of the order of six to one.
  45. Cont’d For most timbers: (Tensile strength parallel to the grain) = 3 to 4 times (the compressive strength) (The tensile strength parallel to the grain) =30 times as high as perpendicular to it) (The compressive strength parallel to the grain) =6 times as high as perpendicular to it)
  46. Factors affecting strength The strength of a piece of timber is affected by characteristics such as  Knots  Direction and slope of grain (diagonal or sloping grain reduces strength, particularly bending and stiffness),  Moisture content (generally timber is more flexible when wet but increases in strength as it dries): The strength of timber is broadly related to its density.
  47. Durability The durability of timber is a measure of its resistance to attack by insects and fungi. The most effective means of preventing fungal attack is to ensure that the moisture of timber remains below 22% when there is not enough moisture for the fungus to survive. Insect attack is often associated with fungal decay.
  48. Permeability Permeability is an important factor in the treatment of timber with chemicals such as preservatives and flame retardants. Permeability varies enormously between species although the sapwood of all specious is more permeable than the heartwood.
  49. Fire resistance Fire resistance is an important consideration in using timber. Although wood is used as a fuel, large sections of timber are difficult to ignite and the charcoal produced on the surface provides protection for the wood underneath.
  50. Defects of timber Since timber is a natural product, developed through many years of growth in the open air, exposed to continual and varying climate conditions, it is prone to many defects. Defects cannot be corrected and therefore each individual piece must be inspected before use and judged on its own merits.
  51. Cont’d Defects can be caused during growth, during drying, through insects, through fungi or during subsequent handling or machining, and each should be known, so that imperfect pieces can be detected and rejected.
  52. 1. Shrinkage When timber is seasoned and its moisture content (MC) is reduced below the Fiber Saturated Point (FSP) continued drying will cause dramatic change such as increase in strength but also distortion and shrinkage.
  53. 2. Cupping Because of the varying shrinkage rates tangential boards tend to cup because of the geometry of the annual rings shown on the end grain. It can be seen that some rings are much longer than the others close to the heart. Therefore they will be more shrinkage at these parts than the others cupping is the result. In square section timber cut from the same place, diamonding is the result.
  54. 3.Knot Knots are the result of the trees attempt to make branches in the early growth of the tree. They are the residue of a small twig, shoot, etc. that died or was broken off by man or an animal in the wood or forest. The tree subsequently continued its growth over this wood. The knot may be live, sound, or tight or if it has become separated and is contained in residue of bark, dead.
  55. 4. Splits A separation of the wood fibers along the grain forming a fissure that extends through the board from one side to the other. It is usual in end grain and is remedied by cutting away the defected area. All boards should have an allowance so that some end grain may be cut away because of possible shakes or splits.
  56. 5. Checks and end checking A separation of the fibers along the grain forming a fissure which shows up on one face or at the end grain but does not continue through to the other side.
  57. 6. Wind or Twisting Spiral or corkscrew distortion in a longitudinal direction of the board. Due to the board being cut close to the centre of the tree which has spiral grain. The board is of not much use but small cuttings may be obtained from it with careful selection.
  58. 7. Bow Bowing is concave/convex distortion along the length of the board. It is a seasoning and or storage defect caused by the failure to support the board with stickers at sufficient intervals. The boards own weight and probably those above it bears down and the resultant bow is inevitable. This defect can and should be avoided by careful use of stickers supporting the board at the correct width.
  59. 8. Spring Spring is concave/convex distortion along the length of the board again but this time the distortion is in the flat plane of the board. Boards with this defect may have been cut from near the heart of the board and is the result of growth stresses being released on conversion. Useable timber may be recovered from these boards by cutting a straight edge and re sawing. The grain direction however may not be satisfactory for aesthetics and care should be taken for placing the possible short grain figure where stability is required.
  60. 9. Shakes Shakes are separation of the fibers along the grain developed in the standing tree, in felling or in seasoning. They are caused by the development of high internal stresses probably caused by the maturity of the tree. The shake is the result of stress relief and in the first place results in a single longitudinal crack from the heart and through the diameter of the tree.
  61. 10. Defects through insects Termites or white ants attack timber structures and are a serious problem. The species that causes the damage live in the ground. Precautions involve treating timber with a preservative or avoiding direct timber contact with the ground.
  62. CONVERSION OF TIMBER As soon as possible after felling the tree should be converted into usable timber. There are two main methods of converting timber: Through and through (or Plain) and Quarter also referred to as rift sawn.
  63. Through and through Through and through  produces mostly tangentially sawn timber and some quarter sawn stuff.  Tangential timber is prone to cupping but it is stronger when placed correctly. Because of this it is used extensively in the construction industry and especially for beams.
  64. Quarter sawn Quarter sawn  Is far more expensive because of the need to double (or more) handle the log. There is also more wastage.  It is however more decorative and less prone to cup or distort. Note also there are two ways of sawing the quarter.
  65. Tangential boards Tangential boards are the stronger boards and when placed correctly, used for beams and joists. These types of boards suffer from 'cupping' if not carefully seasoned, converted and used properly.
  66. Radial boards Radial boards are cut on 'the quarter' and produce a typical pattern of the medullary rays especially in quartered oak. Such timber is expensive due to the multiple cuts required to convert this board. Quality floor boards are also prepared from this rift sawn timber because it wears well and shrinks less.
  67. TIMBER COMPOSITIES Timber is a natural composite which can be used in its original or sawn sections. Timber can also be converted into particles, strands or laminates which can be combined with other materials such as glues to form timber composite products.
  68. Reasons for transforming timber The principal reasons for transforming timber into composite products include:  to exceed the dimensional limitations of sawn timber  to improve performance; structural properties, stability or flexibility  to transform the natural material into a homogenous product  to utilize low-grade materials, minimize waste and maximize the use of a valuable resource
  69. Classification of Timber Composites Timber composites can be divided into three categories: Layered composites Particle Composites Fiber composites
  70. 1.Layered composites Layered composites are used to produce both sections and sheets. Since composites are often made from relatively small sections, efficient use of the source material can be maximized.
  71. Types of layered composite Layered composites can be classified into three groups: Parallel laminates  Glued laminated timber( glulam),  Laminated veneer lumber (LVL) Cross laminates - Plywood Sandwich panels
  72. Glued laminated timber Glued laminated timber (glulam) is formed by gluing together a series of precision cut small sections of timber to form large cross section structural members of long length. The member can be straight or curved and can be made with a variable section according to the structural requirements.
  73. Glue laminated lumber
  74. Laminated Veneer Lumber Laminated Veneer Lumber (LVL) is manufactured from thin sheets or laminate which are peeled from the log. The veneers are glued together to provide the required thickness and then cut into structural sized sections. A proportion of the laminates within a section may be laid with the grain at right angles to balance the movement characteristics of the section.
  75. 2. Cross laminates Plywood The basic characteristics of plywood are veneers bonded together, most frequently with synthetic glues.  In most plywood the grain of the wood in each veneer is laid at right angles to the adjoining one.
  76. Cont’d  Plywood usually contains an uneven number of veneers so that the properties are 'balanced' about the central veneer or core.  The core in some plywood may be a double veneer. One of the outer veneers may be a decorative.
  77. Cont’d The quality and durability of plywood depends on both the timber species and the adhesive used to bond it. The quality of the face veneer may be of particular significance if the plywood is to be seen.
  78. Plywood
  79. Advantages of Plywood  Plywood is dimensionally stable and can be used for large uninterrupted surfaces.  It is resistant to splitting and can be nailed or screwed close to the edges of the panel.  Plywood panels can, within limits, be bent without cracking to form smoothly curved surfaces.  Common uses for plywood are sheathing, paneling, floors and structural diaphragms, concrete formwork, furniture and fittings.
  80. Sandwich panels Sandwich panels are built up of layers of different materials. Normally the outside layers are of high strength and stiffness with a thicker core of lower strength material.
  81. Sandwich panels
  82. Other types of layered composites Other types of layered composites are produced, for example cores of timber strips produce boards by the name of battenboard, blockboard or laminboard, depending upon the width of the strips used.
  83. 3.Particle Composites Chipboard  Chipboard is produced from dried and graded chips mixed with resin which are formed into boards by curing in a heated press. Board thickness range from 6 - 25 mm, although panels up to 70 mm thick can be produced.  Chipboard has a wide variety of uses in building, such as flooring and cladding. It is widely used pre-painted or faced with decorative wood veneers
  84. Chipboard manufacturing
  85. Cont’d Fiberboard Fibers are produced from chips of wood (mainly from forest thinning) which are reduced to a pulp by mechanical or pressure heating methods. In wet process boards the pulp is mixed with water and other additives, formed on a flat surface and pressed at high temperature. Types: Soft, medium or hard board
  86. Hardboard manufacturing
  87. COMMERSIAL SIZES OF TIMBER Moldings Battens and Stripes Plank
  88. Fire resistance and permeability