Timber Engineering materials notes

7 TIMBER 7.1 Timber: source, types, classification, characteristics, advantages and uses Timber is a material that has many characteristics that make it a good material for construction of buildings. The material has a very high strength, especially when compared with its low weight. The material is, however, very anisotropic with different properties in different directions due to its make-up of oriented fibers. The strength parallel to the fiber direction is very good while the strength when loaded perpendicular to the fiber direction is very low. This low strength perpendicular to the fiber direction needs to be addressed when designing timber structures. Properties of timber/wood Physical Properties : 1. Density - light — very light: density less than 550 kg/m3 , eg. bamboo - moderately heavy: density 550 kg/m3 — 750 kg/m3 , eg. deodar - heavy-very heavy: density greater than 750 kg/m3 , eg. Sal 2. Bulk Density (Defined asthe mass of many particles of the material divided by the total volume they occupy) - Depends on air voids present in wood. 3. Moisture Movement - Wood is liable to shrink or swell with the movement of moisture. 4. Shrinkage : - Is a Reduction in linear and volumetric dimension due to drying of moisture. 5. Swelling (Increase in size) - It is increase in linear and volumetric dimension on absorbing moisture. 6. Sound conductivity - Has high Sound Conductivity - Sound velocity in wood is 2-17 times greater than in air. 7. Thermal Conductivity - Has very low thermal conductivity Mechanical and thermal properties of timber/wood:  As wood is natural material there is different mechanical properties for different wood  Mechanical properties are governed by soil type, climate and other factors  Properties affected by specific gravity, moisture content, knots and other defects and slope of grain.  Wood can’t resist fire and burnt easily  It will be better to take precautions to save timber structure from thermal action Characteristics of good timber  It should be free from sap and be from heart of a sound tree.  It should have straight and close fibres.  It should give a clear ringing sound when struck. Dull heavy sound is a sign of internal decay. 93  It should be of uniform dark colour. Light colour usually indicates timber with low strength.  It should have regular annual rings.  Timbers with narrow annual rings are generally the strongest.  Freshly cut surface should give sweet smell.  It should have bright and smooth surface when planed. Dull appearance is a sign of defective timber.  Teeth of saw should not get clogged while sawing.  Out of same variety of timber, darker and heavier pieces are stronger.  It should be free from dead knots, from too many knots, shakes or other defects.  It should have firm adhesion of fibers and compact medullary rays.  A good timber should be durable. It should be capable of resisting the actions of fungi, insects, chemicals etc.  A good timber should be capable of retaining its shape during conversion or seasoning. It should not bow or warp or split.  A good timber should be capable of offering resistance to shocks due to vibrations. Advantages of timber  availability: source and transportation  strength: light weight and higher strength  easy to handle and planed easily  joining: simple  Repair and modification: easy  superior, thermal insulation, sound and electrical resistive  uses: furniture and decorative purpose  strong and flexible  corrosion resistance: excellent  higher shock and impact strength .  Quick and easy to build.  Cheaper than other types of building materials.  Easy to heat and cool depending on the weather.  More eco-friendly than other materials  Can last many years if proper maintenance is carried out Uses of timber It is mainly used for following categories of works  It is used for construction purposesincluding building construction, houseposts, beams, rafters, bridges, piles, poles etc.  It is used for furniture and cabinet making.  It is used for light packing cases.  It is also used for heavy packing cases such as machinery and similar stores.  It is used for manufacturing agricultural implement sand tool handles.  It is used for manufacturing veneers, plywoods, boards etc. 94 Disadvantages of timber:  Defects in timber can reduce the life of wood.  Affected by relative humidity, fungi, insects  Modification of general arrangement drawings if based on masonry construction  Transportation and carriage access  Exposure to weather before enclosed  vulnerability to decay of timber when exposed to excessive moisture.  If left untreated, timber become water logged and crack or chip.  Timber is more susceptible to fire than other building materials unless treated and maintained. Types of timber/wood Wood is a product obtained from the tree. In simple classification of tree, it can be classified as: a. Standing timber - if a tree is living then the timber is termed as Standing Timber. b. Rough timber - if a tree is fallen then the timber is termed as Rough timber. c. Converted timber - If a tree is modified into desired size and shape then the timber is termed as Converted timber. On the basis of mode of growth, the types of trees are: 1. Endogenous Tree 2. Exogenous Tree Endogenoustree  Trees grow in inward direction with the addition of fibrous (like of jute) layer at tip portion, i.e the growth is longitudinal.  Trees are hollow at the core and have flexible stem. e.g-Bambo, coconut, Sugarcane etc.  For construction purpose only bamboo can be used.  Are very flexible due to which they are not generally used for construction material. Exogenoustree  Trees increase in bulk by growing outwards and distinct consecutive rings are formed in the horizontal section of such trees.  These rings are called annual rings, because one such ring added every year, such trees grow bigger in diameter as well.  Life of trees can be find by counting the annual rings. e.g – Sisham, Sal, Pine etc.  Timber from these suitable for construction purpose These are further classified as 1. Deciduous tree 2. Coniferous tree 95 Deciduoustree  Are characterized by flat and broad leaves which fall in autumn season and new one appear in spring season  Hard wood is obtained by such tree which can be used for superior structural and engineering work. e.g – Sal, Sisham, Teak etc. Coniferous tree  Trees are characterized by needle shaped thin leaves which are evergreen  Trees bear conical shaped fruits and yield resins ( viscous substance )  Soft variety of woods are obtained  Used is general constructional purpose ex Pine, deodar etc Hard Wood A hard wood possesses the following characteristics: (a) The wood is comparatively heavier and is darker in colour. (b) The annual rings are not distinct. (c) It contains a large percentage of acid. (d) It is hard and difficult to work upon. (e) It resists shearing stresses. (f) It is close-grained and strong. (g) It is non resinous. Shisham, Sal, Teak, Oak, Mahagony and Babul are examples of hard wood. Soft Wood A soft wood possesses the following characteristics. (i) It is light in weight and colour. (ii) Annual rings are very distinct. (iii) It is comparatively weaker and splits easily. (iv) It has straight fibres. (v) It is resinous, i.e. contains resins and turpentine. It has a peculiar fragrance. (vi) It is strong for resisting tensile forces. (vii) It is weak in directions across the grains. (viii) Its texture is soft and regular. Spruce, Deodar, Chir, Kail and Walnut etc., are examples of soft wood. 7.2 Growth of a tree The roots of the tree suck a solution of salts from the soil in spring season. These salts are food for the tree and roots transmit the same through the trunk of tree to its branches and leaves. This solution of salts looses some of the moisture because of evaporation and absorbs carbon dioxide from the air. This action in the presence of sun makes the solution a bit viscous. This transformed viscous solution is known as sap. This viscous sap descends below the bark and leaves a thick layer in autumn. Layer of sap left below the bark getstransformed to wood and is known as cambium layer. A fresh layer is thus added on the outside of the tree every year forming a new annual ring. The new ring represents a year’s growth of tree. Medullary rays carry the sap from below the bark to the interior thereby nourishing the tree. Structure of a Exogenous tree A tree basically consists of three parts, viz, trunk, crown and roots. From the visibility aspects, the structure of a tree can be divided into two categories:  Macrostructure  Microstructure Macrostructure The structure of wood visible to the naked eye or at a small magnification is called macrostructure. Figure 9-1 shows the macrostructure of exogenous tree. These parts of the cross-section are described below: Pith or Medulla It is the first formed portion of the stem of tree and it consists entirely of cellular tissues. The pith contains a large amount of fluid and nourishes the plant when the plant is young. It dies up and decays when the plant becomes old and sap is then transmitted by the woody fibres deposited round the pith. Annual Rings The rings of woody fibre arranged in concentric circle around the pith are known as annual rings because one such ring is added every year. Heart Wood Inner most rings surrounding the pith constitute the heart wood. This wood is darker in colour, stronger, more compact and durable. Sap Wood The outer annual rings between heart wood and cambium layer of the tree constitute the sap wood which transmits the sap from roots to branches. Compared with heart wood, sap wood is lighter in colour, weaker and more liable to decay. Sap wood is also known as alburnum. Cambium Layer Outermost ring between the bark and sap wood which is not yet converted into wood is known as the cambium layer. Inner Bark 96 The inner skin or layer covering the cambium layer is known as inner bark. It gives protection to cambium layer from any injury. Outer Bark The outer skin or cover of the tree is known as outer bark. It is the outermost protective layer and it sometimes contains cracks and fissures. It consists of cells of woody fibre and is also known as cortex. Medullary Rays These are thin horizontal veins radiating from the pith towards the bark. They carry sap from outside to the inner parts of tree and nourish it. They keep the annual rings tightly gripped together. Microstructure The structure of wood apparent only at great magnifications is called microstructure. When studied under a microscope, it becomes evident that wood consists of living and dead cells of various sizes and shapes. 3. Defects in timber Defects are either due to growing phases of tree or defect occurring after cutting trees. Defects reduce the usefulness of wood as aesthetic appearance as well as strength 1. Heart Shake  This defect usually occurs in over matured trees due to shrinkage of heart wood  These cracks are mostly confined to heart wood portion and diminishing towards outward. 2. Star Shake  It is confined in the outward portionand diminishing towards center.  It occurs due to frost action and strongheat of sun.  It is the most serious defect because ittends to separate single log into smallpieces. 3. Radial Shake  These are radial cracks extending fromcentre towards bark .  These are similar to star shake but areirregular and numerous. 97  It occurs due to drying of sap or moisture. 4. Cup or Ring Shake  Is the separation of tissue along annual rings  If the shake is whole round the annual ring then it is called ring shake and if it is half way round the ring then it is called cup shake.  It is due to unequal seasoning and change in humidity. 5. Rind Gall  Is the curved irregular swelling on surface.  Due to development of new sap layer on wound left after branches have been irregularly cut.  New layer fails to unite with old one and decay starts from that point 98 6. Knots  This defect occurs due to formation of annual rings at right angles to surface and also due to twisting of fibers.  The roots of the branches yet embedded(Enclosed firmly) by the tissue and knots are formed  Knots reduce the aesthetic value and effects tensile strength  Knots may be round or oval shaped 7. Twisted fibers  This defect occurs due to wind velocity constantly turning trees inone direction only. 8. End Splits  Occurs only at the ends of log due to exposed conditions duringseasoning.  May be prevented by painting the ends of log or covering withmetallic cap. 99 100 9. Druxiness and Foxiness  Druxiness is a term that is defined as a disease that isfound in timber or wood. It is caused or brought about bya wound located in the layer known as the cambium which is attacked by fungus.  Foxiness is caused due to poor ventilation during storageor due to over maturity of the tree These are stains or patches on the surface of timberwhich destroy the appearance and may cause decay of timber. Stain may be in form of white patches or mottled reddish colour. 7.4 Seasoning of timber The art of seasoning is to extract the moisture under controlled conditions as nearly as possible at a uniform rate from all parts of timber and to leave the remaining moisture that cannot be extracted, uniformly distributed throughout the mass. Irregular drying will cause irregular shrinkage resulting in the setting up of internal stresses between the fibres. When these stresses become strong enough to overcome the cohesion of the fibres then the timber warps and shakes are formed. Objectives of Seasoning (i) Seasoning makes timber resistant to decay. (ii) Seasoning makes timber lighter. (iii) It becomes easier to paint and polish seasoned timber. (iv) It is easier to treat seasoned timber with preservatives. (v) Seasoned timber becomes stronger and more stable. (vi) Seasoning stopsshrinkage of timber on drying. (vii) Seasoned timber has better electrical resistance. Methods of Seasoning 1. Natural seasoning or Air Seasoning 2. Artificial seasoning or Kiln Seasoning 3. Water Seasoning 4. Boiling or Steam Seasoning 5. Chemical or Salt Seasoning 6. Electrical Seasoning Natural Seasoning or Air Seasoning The log is converted by sawing it into battens and planks etc. as soon as possible after felling of tree. These are then stacked on a well drained place in the shade. Care should be taken to ensure free circulation of fresh air all around each piece while stacking. The stacking should be done on masonry or concrete supports a few centimeters above the ground. Care should be taken not to expose the freshly converted timber stacked for seasoning to severe winds or to sun. This process of seasoning timber is the best as it gives very strong and durable timber, but it is extremely slow. It takes more than six months for timber to season in moderate climates. Kiln Seasoning or Artificial Seasoning This method of seasoning speeds up the seasoning process. This method of seasoning is a must for large scale production of seasoned timber. Kiln seasoning is done in a chamber equipped with arrangements for heating and humidifying the air to required conditions of relative humidity and temperature and for its circulation across the timber stacked in the chamber for seasoning. Usually, it is steam that is used for heating and humidifying the air in the kiln. The seasoning of the timber is started at a comparatively lower temperature and high humidity. As the timber dries, these conditions are gradually altered until at the end of the seasoning. The temperature of the air inside the chamber is fairly high and the humidity is low. The kiln charge is allowed to cool inside the kiln to within 15 to 20o C of the outside temperature before removal. Seasoning of timber by this method takes about four to five days under normal conditions. Water Seasoning Log of wood is kept completely immerged in running stream of water. Sap, sugar etc are leached out of wood and replaced by water. Then log is kept out of water after 2 to 4 weeks and placed in air. Quick process and also remove organic matters materials. Elasticity may reduce and make timber brittle. Boiling of timber in water is another rather quick process. It takes long time to be dried but less than air seasoning. Boiling or Steam seasoning Submersion in boiling water or the application ofsteam speed the drying of wood. This method is said to cause less shrinkage but it is expensive to use, and reduces the strength and elasticity of the timber. Chemical or salt seasoning Salt seasoning is the submersion of wood in a solution of urea, sodium nitrate or sodium chloride, all of which act as dehydrating agents. Then the wood is air dried. Electrical seasoning Electrical seasoning involves running an electrical current through the timber causing heat to build up drying the wood. This method is expensive but is fast and uniform quality. 7.5 Preservation of timber Timber has to be protected from the attack of insects, e.g. white ants etc., and from internal decay due to dry and wet rots. Perfect seasoning is the most effective means of preservation. Timber should be so used that either it is wholly dry and well ventilated or is wholly under water. It will not decay when kept under water but it will become soft and weak. 101 102 Proper damp proofing of the building and providing free circulation of air around the built in portions of timber are essential for the preservation of the timber used. However, when these conditions cannot be obtained then preservatives have to be applied for preservation. Timber should be well seasoned before the application of preservatives as otherwise the preservatives would block the pores of timber thereby causing its decay due to the entrapped moisture. Direct contact with lime mortar should be avoided while using preservative with masonry. Methods of Preservation of Timber Following are some of the common methods of preservation adopted (i) Charring (ii) Tarring (iii) Painting (iv) Creosoting (v) Wolman salt (vi) Ascu treatment (vii) Fire proofing of timber Charring Lower ends of the posts that are to be embedded in ground are generally charred with a view to prevent dry rot and attack of worms. It is done by quenching the ends of posts in water after they are charred on wood fire to a depth of 1.5 cm. Tarring It consists in coating with tar or tar mixed with pitch. Embedded portions of timber fence posts, ends of door and window frames, battens and beams built in wall are usually tarred. Tarring is not done in case of those portions of structural members that are open to view, because of unsightly black colour. Painting A paint when applied to timber acts not only as a good preservative but also it enhances the appearance of the surface so treated. Only well seasoned timber should be painted as otherwise the moisture entrapped in the timber, because of the closing of timber bores by paint, would cause decay. Paints however, protect seasoned timber against moisture thereby prolonging its life, e.g. soligum paints have excellent preservative properties and protect timber against the attack of white ants. Creosoting Creosote oil is a dark brown thick oily liquid. Thoroughly seasoned timber dried for 24 hrs before its treatment is placed in an airtight chamber. After the air has been exhausted from this chamber, the creosote oil is then pumped in at a pressure of 9 kg/ cm2 at a temperature of 50o Cso long as the timber is not fully saturated with oil. The oil preserves the timber from rot and from the attacks of white ant. 103 Uses •It is used in case of railway sleepers, piles and transmission poles. Limitations •Undesirable colour and smell, inability to take paint well and the tendency to stainplaster limit its use. Wolman Salt This salt consists of creosote and sodium fluoride and is soluble in water. It is odour less and leaves no stain on wood. After treatment, timber could be painted or varnished. These salts destroy many kinds of fungi that cause timber to rot. This renders the timber extremely fire resistant too. Treatment of timber with zinc chloride, sodium fluoride, magnesium, silico fluoride or copper sulphate renders the timber immune from the attacks of fungi. The timber so treated is capable of being painted on drying. Ascu Treatment Ascu is available in the form of powder and is made up of three chemicals mixed in the ratios given below : (i) 1 part by weight of hydrated arsenic pentaoxide (As2 O5.2H2 O) (ii) 3 parts by weight of blue vitriol (CuSO4 .5H2 O) (iii) 4 parts by weight of potassium dichromate (K2 Cr2 O7 .2H2O) Six parts of this powder are mixed with 100 parts by weight of water. Ascu solution can be applied or sprayed in two coats. To achieve better results, timber may be soaked in the solution and impregnated with it under pressure. The timber should be allowed to dry for three to six weeks. This treatment renders timber immune to the attacks of white ant. Ascu treated timber may be painted, varnished, polished or waxed. The solution is odourless. Fire Proofing of Timber Timber cannot be made completely fireproof, however, by treating as below it can be made fire resistant to a sufficient extent. Soaking timber in ammonium sulphate, ammonium chloride, ammonia phosphate, sodium arsenate, zinc chloride etc. orspraying on timber, solution ofsodium silicate, potassium silicate or ammonia phosphate etc. imparts fire resisting properties. Abel’s methods of fire proofing timber is painting the surface first with a dilute solution of sodium silicate (Na SiO ) then with a 2 3 cream like paste of slaked fat time and in the end with a concentrated solution of silicate of soda. 6. Properties and uses of bamboo  Bamboo, like true wood, is a natural composite material with a high strength-to-weight ratio useful for structures.  In China and India, bamboo was used to hold up simple suspension bridges, either by making cables of split bamboo or twisting of sufficiently pliable(workable) bamboo together.  Bamboo has also long been used as scaffolding; the practice has been banned in China for buildings over six storeys, but is still in continuous use for skyscrapers in Hong Kong. 104  In Japanese architecture, bamboo is used primarily as a supplemental and/or decorative element in buildings such asfencing, fountains, grates and gutters, largely due to theready abundance of quality timber. (Grates - A barrier that has parallel or crossed bars blocks a passage but admitting air.) (Gutters - A channel along the eaves or on the roof; collects and carries away rain water)  Can be cut and laminated into sheets and planks (like board, plyboard).  Bamboo intended for use in construction should be treated to resist insects and rot. The most common solution for this purpose is a mixture of borax and boricacid.  Bamboo has been used as reinforcement for concrete in those areas where it is plentiful, though dispute exists over its effectiveness in the various studies done on the subject.  Bamboo does have the necessary strength to fulfil this function, but untreated bamboo will swell with water absorbed from the concrete, causing it to crack. Several procedures must be followed to overcome this shortcoming.  Tensile strength: Bamboo has higher tensile strength than steel because its fibers run axially.  Fire Resistance: Capability of bamboo to resist fire is very high and it can withstand temperature up to 4000 C. This is due to the presence of high value of silicate acid and water.  Elasticity: Bamboo is widely preferred in earthquake prone regions due to its elastic features.  Weight of bamboo: Bamboos due to their low weight are easily displaced or installed making it very easier for transportation and construction.  Unlike other building materials like cement and asbestos, bamboo poses no danger to health.  They are cost effective and easy to use.  They are especially in great demand in earthquake prone areas. 7.7 Wood based products Timber which is prepared scientifically in a factory is termed as industrial timber and such Timber possesses desired shape, appearance strength. Veneers: These are thin sheets or slices of 0.40 to 6mm wood of superior quality. Indian timbers, which are suitable for veneers, are mahagony, oak, rosewood, sissoo, teak etc. The process of preparing a sheet of veners is known as veneering. Veneers are used to produce plywoods batten boards and lamin boards. Veneers used for making plywood are known as plies. Plywoods: Plywoods are boards, which are prepared from thin layers of wood or veneers. Three or more veneers in odd number are pressed using adhesives such that the direction of the grains are at Right angles to each other. The odd numbers of the veneers are used in order that the shrinkage is symmetrical about the middle ply. The plywoods are used for various purposes such as ceilings, doors, furniture, partitions, panelling walls, packing cases, railway coaches, formwork for concrete etc. Thickness may vary from 6 to 25mm. 105 Lamin Board A lamin board is a board having a core of strips, each not exceeding 7 mm in thickness glued together face to face to form aslab which in term is glued between two or more veneers, with the direction of the grain of the core strips running at right angles to that of the adjacent outer veneers. The lamin boards are light, strong and do not split or crack easily. They are used for walls, ceilings, partitions and packing cases. Block Board A block board is constructed in the same way as lamin board. In this case core consists of smaller timber block upto 25 mm in width. These blocks are cemented edge to edge and on each face plies upto 3 mm thickness are glued. These are extensively used for construction of railway carriages, busbodies, marine and river crafts and for furniture making, partitions, panelling, prefabricated houses etc. Batten Boards The batten board is a board having a core made up of strips of wood usually 80 mm wide, each laid separately or glued or otherwise joined to form a slat which is glued between two or more outer veneers with the direction of the core running at right angles to that of the adjacent outer veneers. These boards are used for door panels, table tops etc. Fibre boards: These are rigid boards and they are also known as pressed wood or reconstructed wood. The thickness varies from 3mm to 12mm. These are available in lengths from 3 to 4.5m and width varying from 12 to 18m. The weight of fibre boards depends on the pressure applied during manufacture. These are used for:

 (i) For internal finish of rooms such as wall panelling; suspended ceilings. 

(ii) To construct form work for cement concrete.

 (iii) To construct partitions. 

(iv) To prepare flush doors, tops of tables etc. 

(v) To provide an insulating material of heat and sound. 

(vi) To work as paving or flooring material. Impreg timber /impregnated timber Timber which has been made flame-resistant, fungi resistant, or insect-proof by forcing into it under vacuum or pressure a flame retardant or a fungal or insect poison. Compreg timber Compreg timber are processed wood whose cells are impregnated with a resin and compressed, to reduce shrinking and swelling and to increase density and strength. Compreg is a very special wood composite material with exquisite mechanical properties. Compreg indicates that it is both impregnated and compressed. Used in applications where long durability, hardness, and dimensional stability is required e.g. gears, rolls and wear strips of industrial conveyors, woodworking machine tables. Regardless of its hardness, it is comparatively easy to machine, which makes it an excellent choice for manufacturers of exclusive woodworking products.