ENGINEERING MATERIALCHAPTER NO.5

TIMBER

INSTRUCTORENGR.MIR WAJAHAT ALI

KARDAN INSTITUTE OF HIGHER EDUCATIONDEPARTMENT OF CIVIL ENGINEERING

INTRODUCTION

• Wood suitable for building or other engineering purposes is called timber.

• When it forms part of living tree it is called standing timber.

• When the tree has been felt is called rough timber.

• When it has been sawn to various market forms such as beams, planks, battens it is called converted timber

TIMBER

CLASSIFICATION OF TREES

• Depending on the mode of growth trees are classified into two categories as:a. Endogenous b. Exogenous

a. Endogenous trees: • The trees that grow inwards in a longitudinal fibrous

mass such as banana, bamboo, palm and cane. • Even though the stem of trees of this class is light

and though yet it is too flexible that it is not suitable for engineering works with exception to bamboo.

CLASSIFICATION OF TREES

b. Exogenous trees: • The trees that grow outward by the addition of

one concentric ring every year. These rings are known as annual ring.

• Since one ring is added to the tree every year so the number of annual rings in the stem of tree indicates its age in years.

• The timber obtained from this class of trees is extensively used in engineering works.

CLASSIFICATION OF TREES

BAMBOO TREE BANANA TREE

DEODAR TREE

CLASSIFICATION OF TREES

• Timber available from exogenous trees is further classified into two categories:

a. Conifers or evergreens b. Deciduousa. Conifers or evergreens: These trees yield soft

wood. These are trees with pointed leaves. Deodar, pine, chir, and kaii belong to this class.

b. Deciduous: these trees are having broad leaves, yielding Hard wood. Teak, sal, shisham belong to this class.

GROWTH OF TIMBER TREE• In spring season roots of the tree suck a

solution of salts from the soil. 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.

GROWTH OF TIMBER TREE• In autumn viscous sap descends below

the bark and leaves a thick layer. Layer of sap left below the bark gets transformed to wood and is known as cambium layer.

• It goes on gaining the strength with the passage of time. A fresh layer thus added on the outside of the tree every year forming a new annual ring. The new rings represents a years growth of tree.

STRUCTURE OF TIMBERStructure

STRUCTURE OF TREE

• Pith or medulla: It is the first formed portion of the stem of tree. It consists of entirely cellular tissues.

The pith, which when the plant is young contains a large amount of fluid and nourishes the plant. It dies up and decays when the plant becomes old. Sap is then transmitted by the woody fibers that deposit about the pith.

STRUCTURE OF TREE

• Annual ring: 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: innermost ring surrounding the pith constitute the heart wood. The wood is darker in color, stronger more compact and durable.

STRUCTURE OF TREE

• Sap wood: outer annual rings of the tree constitute the sap wood which transmits the sap from roots to branches. Compared with heart wood sap wood is lighter in color, weaker and more liable to decay.

• Bark: it is outermost protective covering of cells and woody fibers on a tree. In course of time older layers split and scale off.

STRUCTURE OF TREE

• Cambium layer: outermost one ring between the bark and sap which is not yet converted into wood is known as cambium layer. If the cambium layer is exposed by removing the bark, the cells cease to be active and results in death of tree.

• 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.

Medullary Rays

CHARACTERISTICS OF GOOD TIMBER

• It should be from the heart of a sound tree and be free from sap.

• It should have straight and close fiber• It should be of uniform color• It should give a clear ringing sound when struck.

Dull heavy sound is a sign of internal decay.• It should have regular annual ring.• Timber with narrow annual rings are generally

the strongest.

CHARACTERISTICS OF GOOD TIMBER

• Freshly cut surface should give sweet smell.• Teeth of saw should not get clogged while sawing.• Out of same variety of timber darker and heavier

pieces are stronger.• It should have bright and smooth surface when

planed. Dull appearance is a sign of defective timber.

• It should have firm adhesion of fibers and compact medullary rays .

DEFECTS IN TIMBER

• These defects are mostly of two types:a. Those develop during the growth of tree andb. Those developed after the tree has been felled.a. Defects developed during the growth of treeStar shakes:

These are radial splits wider on the surface of the tree and becoming narrower as they moves towards the centre. They are caused by severe frost or by severe heat of the sun

Star shakes: These are radial splits wider on the surface of the tree and becoming narrower as they moves towards the centre. They are caused by severe frost or by severe heat of the sun

Heart shakes: These are wide splits running right through the heart wood of the tree. These splits from the pith running towards the sap wood. These are caused by the shrinkage of interior parts or by decay because of accumulated mixture.

Cup shakes or ring shakes

These are curved splits separating one annual ring from the adjacent one either wholly or partly. These are caused by strong winds and by the excessive frost action on the moisture present in the tree.

Twisted fiberFibers are twisted by strong winds turning the tree constantly in one direction. Trees in exposed position or on hill tops are the most affected.

Rind galls:These are peculiar swelling caused generally by the growth of layers of sapwood over wounds remaining after a branch of tree has been imperfectly cut off.

Knots A knot is either a root of a branch that is embedded in the stem with the formation of annual rings at right angle to those of stem or the tissues set in concentric circles

b. DEFECTS DEVELOPED AFTER THE TREE HAS FELLED.

Dry rot:It is attack on timber by fungus. The fungus reduces the timbers to a dry powder. To prevent dry rot only well seasoned timber should be used. There should also be free access to fresh air to all parts of timber.

Wet rot:It is decay of timber due to alternate wetting and drying. To prevent wet timber should be prevented against alternate wetting and drying. It should be so used that it should be wholly submerged in water or it is always dry.

SAWING OF TIMBER

• After felling if the logs are not cut for sometime, then because of drying of moisture from the outer rings, when the moisture still retains in the centre of the log, outer rings shrink without proportionate shrinkage of central portion.

• As in uncut log shrinkage takes place in circumferential direction so it results in star shakes( cracks on the surface of log narrowing as they move inwards.

METHODS OF SAWING TIMBER

• Ordinary sawn or flat sawn:Parallel cuts made throughout the length of

the logCutting parallel slices of planksEasiest and economical methodShrinkage of sapwood more than the

heartwoodCausing warp and twisting of planks

Methods of sawing timber(ordinary sawn)

Methods of sawing timber

• Quarter sawingTendency to cup i.e. to curve in a transverse

directionWhen applied to wood, not having distinct

medullar rays this method produces very fine wood

Methods of sawing timber(Quarter sawn)

Methods of sawing timber

• Rift or Radial SawingTimber cut parallel to medullar rays and

perpendicular to annual rings least shrinkage but most wasted limited rift is adopted

Methods of sawing timber(rift or radial sawn)

Methods of sawing timber

• Tangential sawingBoards or planks sawn tangentially to annual

ringsNot suitable for flooringPlanks cut by this method warp too much

Methods of sawing timber (tangential sawn)

SEASONING OF TIMBER

• Newly felled tree contains a considerable amount of sap. If this sap is not removed, the timber is likely to wrap, crack and shrink. It may even decay.

• 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 timber.

OBJECTIVES OF SEASONING OF TIMBER

1) Wet timber is an easy prey to decay by fungi, borers, termites etc. Seasoning thus makes timber resistant to decay.

2) Seasoning makes timber lighter.3) It becomes easier to paint and polish

seasoned timber.4) It is easier to treat seasoned timber with

preservatives.

OBJECTIVES OF SEASONING OF TIMBER

5) Seasoned timber becomes stronger and more stable.

6) Seasoning stops shrinkage of timber on drying.

7) Seasoned timber has better electrical resistance.

SEASONING METHODS

• There are two main methods of seasoning:1) Air seasoning or natural seasoning2) Kiln seasoning or artificial seasoning1)Air seasoning method:As soon as after felling, the log is converted by

sawing into battens and planks.These are then stacked on a well drained place

in the shade.

Air seasoning method

• There should be free circulation of fresh air all around each piece of timber.

• The stacking should be done on masonry or concrete supports a few inches above the ground.

• Care should be taken not to expose the freshly converted timber stacked for seasoning to severe wind or to sun.

Air seasoning method

• This process of seasoning timber is the best as it gives very strong and durable timber.

• But it is extremely slow process. It takes more then six months for timber to season in moderate climate.

Air seasoning method

Air seasoning method

Kiln seasoning• Kiln or artificial method of seasoning speeds

up the seasoning process. For large scale production of seasoned timber kiln seasoning is must.

• Kiln seasoning is done in a chamber equipped with arrangements for heating and humidifying the air for required condition of relative humidity and temperature for its circulation across the timber stacked in the chamber for seasoning.

Kiln seasoning

• Usually it is steam that is used for heating and humidifying the air in the kiln. The seasoning of timber is started at relatively lower temperature and high humidity.

• At the end this situation is reverse, the air inside the chamber is relatively high and the humidity is low.

• Seasoning of timber by this method takes four to five days under normal condition.

Kiln seasoning

PREVENTING DRYING OF LOGS(TIMBER)

• After the tree has felled its bark is removed and it is roughly squared and sawn as quickly as possible to avoid cracking of timber and to speed up the seasoning.

• There is possibility of faster evaporation of moisture from ends of timber piece and if not checked then it is likely to result in cracking and splitting of ends.

• To avoid this , we used to paint the ends of logs or planks with sealing liquid like tar or asphalt etc,

PREVENTING DRYING OF LOGS(TIMBER)

• If however the timber cannot be converted into planks after felling then rapid drying of timber should be minimized by storing them completely submerged under water.

• If the water is not flowing then the water should be changed fortnight(after 14 days) so as to remove the fermenting material.