Timber as construction material in civil engineering.pptx

Timber as construction material in civil engineering

1. Timber as building material

2. TIMBER Wood can be defined as the material that forms the trunks and the branches of the trees. Timber is wood cut from the trunk which can be used for constructing houses, boats, furniture and so on. 2

3. INTRODUCTION Timber is not only one of the oldest building materials , along with stone, and earth material, but has remained until today the most versatile and, in terms of indoor comfort and health aspects, most acceptable material. However, timber is an extremely complex material , available in a great variety of species and forms, suitable for all kinds of applications . Although only a small proportion of the timber harvested is used for building. Since timber cannot be completely replaced by other materials, it shall long remain one of the most important building materials. 3

4. USES OF TIMBER Building construction House post-construction Beams and rafters Construction of bridges and boats Furniture and instruments Railway sleepers and packing cases Toys and engraving work Railway coach wagons Formwork of cement concrete 4

5. CLASSIFICATION OF TREES Endogenous: The trees which grow inwards in longitudinal fibrous mass are called endogenous trees. Ex: bamboo, cane , palm etc Exogenous: The trees which grow in outwards across horizontal section of stem are called exogenous trees. These trees are fit for engineering construction. Conifers or Evergreen: •They give soft wood. •They have pointed leaves. Ex: Deodar , Pine, Chir, Kail , etc. Deciduous or Broad leaf Trees: •These have hard wood. •These have broad leaves. Ex: Teak, Sal, Shisham , etc. 5

6. COMPARISON OF SOFTWOOD AND HARD WOOD Item Soft woods Hard woods Annual rings Distinct Indistinct Color Light Dark Density Low High Fire resistance Poor More Weight Light Heavy Strength Strong for direct pull and weak for equal resisting thrust or shear Equally strong for resisting tension, compression and shear Structure Resinous and split easily Non-resinous and close grained 6

7. 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 . Texture - In hardwoods , the texture depends upon the size and distribution of vessels and rays . In softwoods , it is determined by the size and distribution of tracheid . 7

8. STRUCTURE OF TREE From the visibility aspect, the structure of a tree can be divided into two categories Macro structure Micro structure 8

9. MACRO STRUCTURE OF WOOD Pith: The innermost central portion or core of the tree is called pith or medulla. Heart wood: The inner annual rings surrounding the pith is known as heart wood. It imparts rigidity to tree. It is inactive in nature. Sap wood: The cuter annual rings between heart wood and cambium layer is known as sap wood. Sapwood is less resilient to decay and insect attack. Cambium layer: Thin layer of sap between sap wood and inner bark is known as cambium layer Inner bark: The inner skin or layer covering the cambium layer is known as inner bark Outer Bark: The outer skin or cover of the tree is known as outer bark Medullary rays: The thin radial fibres extending from pith to cambium layer are known as medullary rays 9

10. MICRO STRUCTURE OF WOOD The structure of wood apparent only at great magnifications is called micro structure. Conductive cells Mechanical cells Storage cells 10

11. CHARACTERISTICS OF GOOD TIMBERS Appearance : A freshly cut surface of timber should exhibit hard and of shining appearance. Color : A color should preferably be dark. Defects : A good timber should be free from series defects such as knots, flaws, shakes etc. Durability : A good timber should be durable and capable of resisting the action of fungi, insects, chemicals, physical agencies, and mechanical agencies. Elasticity : The timber returns to its original shape when load causing its deformation is removed. Fire resistance : A dense wood offers good resistance to fire. Hardness : A good timber should be hard. 11

12. Shape : A good timber should be capable of retaining its shape during conversion or seasoning. Smell : A good timber should have sweet smell. Unpleasant smell indicates decayed timber. Sound : A good timber should give a clear ringing sound when struck. Strength : A good timber should be sufficiently strong for working as structural member such as joist, beam, rafter etc. Structure : The structure should be uniform. Toughness : A good timber should be tough (i.e.) capable of offering resistance to shocks due to vibration. 12

13. Water permeability : A good timber should have low water permeability, which is measured by the quantity of water filtered through unit surface area of specimen of wood. Weight : The timber with heavy weight is considered to be sound and strong. Working conditions : Timber should be easily workable. It should not clog the teeth of saw. Mechanical wear : A good timber should not deteriorate easily due to mechanical wear or abrasion. 13

14. SEASONING OF TIMBER As fresh timber which is obtained from trees contains about 30 to 40 % sap or moisture . This sap is very harmful for the life of a timber . Therefore, it is necessary to remove that sap by applying some special methods. All those methods which are used for removing the sap from timber are collectively termed as seasoning of timber . Advantages of seasoned timber: It has reduced weight, It is strong and durable, It has resistance to decay or rot, It takes high polish, It is easier to work, Its life is more. 14

15. TYPES OF TIMBER SEASONING 15

16. NATURAL SEASONING The seasoning of timber is carried out by natural air . Sawed timber is stacked in layers on a platform of brick or concrete pillars. Each layer is separated by spacers . The stack is covered by a roof of a suitable material to protect it from, fast blowing wind, rain and heat. The moisture content that can be achieved by this method is about 18-22%. This is a simpler and more economical but slower method of seasoning timber but produces stronger timber than that obtained by artificial seasoning. 16

17. WATER SEASONING The timber is cut into pieces of suitable sizes and immersed wholly in water, preferably in the running water of a stream. The timber is taken out after a period of 2 – 4 weeks. During this period, the sap contained in the timber is washed away by running water. The timber is then allowed to dry under a shed. This is done to dry out the water that has replaced the sap in the timber and season it completely. 17

18. ARTIFICIAL SEASONING With artificial seasoning, the moisture content of the wood can be reduced to any desired level and the timber produced is less liable to insect and fungi attacks. 18 Kiln Seasoning Quick method of drying timber to the required moisture content. In this method timber is subjected to hot air in air tight chamber. The hot air circulates in between the timber logs and reduces the moisture content . When the required temperature is obtained, moisture content and relative humidity gets reduced and timber gets seasoned. Even though it is costly process it will give good results strength wise . The time required for this seasoning is 3 to 12 days. This is quick process.

19. Chemical Seasoning In case of chemical seasoning , timber is stored in suitable salt solution for some time. The salt solution used has the tendency to absorb water from the timber . So, the moisture content is removed and then timber is allowed to drying. It affects the strength of the timber . Electric Seasoning In the method of electrical seasoning timber is subjected to high frequency alternating currents. The resistance of timber against electricity is measured at every interval of time. When the required resistance is reached seasoning process is stopped because resistance of timber increases by reducing moisture content in it. It is also called as rapid seasoning and it is uneconomical . 19

20. TYPES OF TIMBER AND ITS USES 20

21. INDUSTRIAL TIMBER Scientifically prepared timber in a factory. They possesses desired shape, appearance, strength, etc. Veneers Plywoods Compressed wood boards Compreg timbers Block board and lamin board Glulam Flushdoor shutters Hardboard 21

22. VENEERS These are thin sheets or slices of wood if superior quality whose thickness varies from 0.40 mm to 6 mm or more. They are obtained by rotating a log of wood against a sharp knife of rotary cutter or saw. The veneers after being removed are dried in kilns to remove moisture. Veneers may be glued with suitable adhesives on the surface of inferior wood. Used to produce plywood, batten boards and lamina boards. May be fixed on corners or bent portions. Indian timbers suitable are mahogany, oak, rosewood, sissoo, teak, etc. 22

23. PLYWOOD Plywood is a panel formed of three or more layers of veneer glued together, usually with the grain of adjacent veneers running at right angles to each other, held in position by the application of suitable adhesives. The perpendicular placing increases longitudinal and transverse strengths of plywood. The plywood are available in different commercial forms such as batten board, lamina board, metal faced plywood, multiply, three-ply, veneered plywood, etc. According to CPWD specifications, plywood for general purposes are of following three grades: Boiling Water resistant Warm water resistant Cold water resistant 23

24. Followings are the uses and applications of plywood: To make light partition or external walls To make formwork, or a mould for wet concrete To make furniture, especially cupboards, kitchen cabinets, and office tables As part of flooring systems For packaging To make light doors and shutters 24 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

25. COMPRESSED WOOD Compressed wood is an engineered, man-made wood product made of wood fibers, sawdust, wood particles, wood shavings, and a veneer facade . It has a wide range of common applications, most notably in furniture, cabinetry, and trim work. Its many distinct advantages over natural wood make compressed wood ideal for many applications. Compressed wood is usually made from wood chips, shavings, sawdust, or fibers, which are mixed with synthetic resin and compressed under heat. Types Particle board Medium-Density Fiberboard (MDF) High-Density Fiberboard (HDF) 25

26. Methods of sawing logs for Timber Qualities of boards 26

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