DEFECTS OF TIMBER

CONTENTS 1.) DEFINITION2.) CLASSIFICATION OF DEFECTS a.) DEFECTS DUE TO NATURAL FORCESb.) DEFECTS DUE TO FUNGIc.) DEFECTS DUE INSECTSd.) DEFECTS DUE TO CONVERSIONe.) DEFECTS DUE TO SEASONING

1). DEFINITION- A DEFECT IS AN IRREGULARITY OR ABNORMALITY OCCURING IN OR ON WOOD WHICH IS RESPONSIBLE FOR ITS STRENGTH REDUCTION, LOWERING OF DURABLITY, LOWERING OF UTILITY, POOR APPERANCE AND DECAY.

2.) CLASSIFICATION OF DEFECTS a.) DEFECTS DUE TO NATURAL FORCES

RIND GALLS KNOTS

SHAKES

TWISTED FIBRES UPSETS

BURLS

CHEMICAL STAIN

CHEMICAL STAIN- THE WOOD IS SOMETIMES DISCOLOURED BY THE CHEMICAL ACTION CAUSED WITH IT BY SOME EXTERNAL AGENCY.THIS IS KNOWN AS CHEMICAL STAIN.

RIND GALLS- THE RIND MEANS BARK AND GALL INDICATES ABNORMAL GROWTH.HENCE PECULIAR CURVED SWELLING FOUND ON THE BODY OF TREE ARE KNOWN AS RIND GALL.THEY DEVELOP AT POINTS FROM WHERE BRANCHES ARE IMPROPERLY CUT OFF OR REMOVED.THEY ARE RARELY FOUND IN A TREE AND THE TIMBER IN THIS PART IS VERY WEAK AND NOT DURABLE.

KNOTS- THESE ARE THE BASES OF BRANCHES OR LIMBS WHICH ARE BROKEN OR CUT OFF FROM THE TREE.THE PORTION FROM WHICH THE BRANCH IS REMOVED RECIEVES NOURISHMENT FROM THE STEM FOR A PRETTY LONG TIME AND IT ULTIMATELY RESULTS IN FORMATION OF DARK HARD RINGS WHICH ARE KNOWN AS KNOTS.AS CONTINUITY OF WOOD FIBRES ARE BROKEN BY KNOTS, THEY FORM A SOURCE OF WEAKNESS.

SHAKES- THESE ARE LONGITUDINAL SEPERATIONS IN WOOD BETWEEN THE ANNUAL RINGS.THESE ARE CRACKS WHICH PARTLY OR COMPLETELY SEPARATE FIBRES OF WOOD.THE SEPERATIONS MAKE THE WOOD UNDESIRABLE WHEN APPERANCE IS IMPORTANT .

TWISTED FIBRES- THESE ARE KNOWN AS WANDERING HEARTS AND CAUSED BY TWISTING OF YOUNG TREES BY FAST BLOWING WIND.THE TIMBERS WITH TWISTED FIBRES IS UNSUITABLE FOR SAWING.

UPSETS- THESE INDICATE WOOD FIBRES WHICH ARE INJURED BY CRUSHING OR COMPRESSION.THE UPSETS ARE MAINLY DUE TO IMPROPER FELLING OF TREE AND EXPOSURE OF TREE IN ITS YOUNG AGE TO FAST BLOWING WIND

BURLS- THEY ARE PARTICULARLY FORMED WHEN A TREE RECIEVES SHOCK OR INJURY IN ITS YOUNG AGE.DUE TO ITS INJURY, THE GROWTH OF TREE IS COMPLETELY UPSET AND IRREGULAR PROJECTIONS APPEAR ON THE BODY OF TIMBER

b.) DEFECTS DUE TO FUNGI

BLUE STAIN

BROWN ROT

DRY ROT

HEART ROT

WET ROT

WHITE ROT

BLUE STAIN- THE SAP OF WOOD IS STAINED TO BLUISH COLOUR BY THE ACTION OF CERTAIN TYPE OF FUNGI.

BROWN ROT - THE TERM ROT IS USED TO INDICATE DECAY OR DISEASE OF TIMBER,THE FUNGI OF CERTAIN TYPE REMOVES CELLULOSE COMPOUNDS FROM WOOD AND HENCE WOOD ASSUMES THE BROWN COLOUR.

WHITE ROT- IT IS JUST OPPOSITE OF BROWN ROT.IN THIS CERTAIN TYPE OF FUNGI ATTACK LIGNIN OF WOOD AND WOOD ASSUMES THE APPEARANCE OF A WHITE MASS CONSISTING OF CELLULOSE COMPOUNDS.

HEART ROT- THIS IS FORMED WHEN BRANCH HAS COME OUT OF THE TREE.IN SUCH CASE,THE HEART WOOD IS EXPOSED TO ATTACK OF ATMOSPHERIC AGENTS.ULTIMATELY THE TREE BECOMES WEAK AND IT GIVES HOLLOW SOUND WHEN STRUK WITH HAMMER.

WET ROT- SOME KIND OF FUNGI CAUSEDCHEMICAL DECOMPOSITION OF WOOD OF TIMBER AND IN DOING SO CONVERT TIMBER INTO GREYISH BROWN POWDER.IT IS KNOWN AS WET ROT.

DRY ROT- SOME TYPES OF FUNGI FEED ON WOODS AND DURING FEEDING THEY ATTACK ON WOOD AND CONVERT IT INTO DRY POWDER FORM.THIS IS KNOWN AS DRY ROT. c.) DEFECTS DUE INSECTS

BEETLES

MARINE BOARERS TERMITES

DEFECTS CAUSED BY BEETLES - THEY FORM PIN HOLES OF SIZE ABOUT 2MM DIA IN WOOD- TUNNEL FORMATION IS DONE IN SAP WOOD BY LARVAE OF BEETLE- CONVERSION OF TIMBER INTO FLOUR LIKE POWDER- THEY DO NOT DISTURB OUTER SHELL OR COVER

DEFECTS DUE TO MARINE BOARERS - THEY ARE FOUND IN SALTY WATER- THEY FORM TUNNELS OR BORES TO TAKE SHELTERS- DIAMETER AND LENGTH OF HOLES ARE AS HIGH AS 25MM AND 60 MM RESPECTIVELY- AFFECTED WOOD LOOSES ITS COLOUR AND STRENGTH- NO TIMBER IS COMPLETELY IMMUNE FROM ATTACK OF MARINE BOARERS

DEFECTS CAUSED BY TERMITES - LIVES IN COLONY AND VERY FAST IN EATING AWAY THE WOOD FROM CORE OF CROSS-SECTION.- MAKES TUNNELS IN DIFFERENT DIRECTIONS AND USUALLY NOT DISTURB THE OUTER SHELL OR COVER.- THE TIMBER PIECE ATTACKED BY TERMITES MAY LOOK SOUND UNTILL IT COMPLETELY FAILS- FEW GOOD TIMBERS LIKE TEAK, SAL, ETC CAN RESIST THE ACTION OF TERMITES

d.) DEFECTS DUE TO CONVERSION

CHIP MARK

DIAGONAL GRAIN

TORN GRAIN

WANE

CHIP MARK- THIS DEFECT IS INDICATED BY MARK OR SIGNS PLACED ON FINISHED SURFACE OF TIMBER.THEY MAY BE FORMED BY PLANING MACHINE.

WANE- THIS DEFECT IS DENOTED BY PRESENCE OF ORIGINAL ROUNDED SURFACE ON MANUFACTURED PART OF TIMBER.

DIAGONAL GRAIN- THE DEFECT IS FORMED DUE TO IMPROPER SAWING OF TIMBER.IT IS INDICATED BY DIAGONAL MARKS ON STRAIGHT GRAINED SURFACE OF TIMBER.

TORN GRAIN- DEFECT CAUSED WHEN A SMALL DEPRESSION IS FORMED ON A FINISHED SURFACE OF TIMBER BY FALLING A TIMBER OR SO.

e.) DEFECTS DUE TO SEASONING

TWIST

CUP

BOW

CHECK AND SPLIT

HONEY COMBING

BOW- THIS DEFECT IS INDICATED BY CURVATURE FORMED IN DIRECTION OF LENGTH OF TIMBER.

CUP- THIS DEFECT IS INDICATED BY CURVATURE FORMED IN TRANSVERSE DIRECTION OF TIMBER.

CHECK AND SPLIT- CHECK IS A CRACK WHICH SEPERATES FIBRES OF WOOD.IT DOES NOT EXTEND FROM ONE END TO THE OTHER WHILE SPLIT IS WHEN CHECK EXTENDS FROM ONE END TO OTHER.

TWIST- WHEN A PIECE OF TIMBER HAS SPIRALLY DISTORTED ALONG ITS LENGTH,IT IS KNOWN AS A TWIST.

HONEY- COMBING- DUE TO STRESS DEVOLOPED DURING DRYING, VARIOUS RADIAL AND CIRCULAR CRACKS DEVELOP IN THE INTERIOR PORTION OF TIMBER, WHICH RESEMBLES WITH HONEY-COMB TEXTURE.

GROWTH CHARACTERISTICS OF TIMBER

Timber is a natural material, and each piece tells part of the story of the tree that produced it. Branches that are engulfed by the growing tree cause knots. Insect attack causes the tree to lay down extra wood to protect the tree giving a lump or burl, or may produce a resin or sap pocket. Fires cause damage that is often reflected in resin pockets, or may result in the tree trying to heal itself by laying down a new cambium that traps bark. These, and other experiences contribute to the character of each piece of wood, and makes each length of timber unique or individual. This individuality gives anything built from the wood a character of its own.Architects can choose to express this individuality by selecting feature timber that displays these growth characteristics. On the other hand, some appearance grades limit the extent of natural growth characteristics and result in clear timber. Clear timber may be chosen where it is necessary to match the color and appearance of adjacent pieces to give the impression of an unbroken expanse of uniform timber.

The growth characteristics affect the appearance of the timber and also its behavior in some circumstances:

Cell structure and grainExtractivesNatural featuresCell structure and grainAll the cells that constitute wood material are formed in the narrow cambium layer, which separates the wood from the bark. The walls of these cells are made up of a network of complex chains of cellulose molecules, which are calledmicrofibrils. When the formation of the cell wall is complete, lignin gradually extends through the cell walls, acting as a bonding agent that glues the cells together. The newly formed cells on the inner side of the cambium, become one of the following types of wood tissue:cells fibers,and in hardwoods,vessels. Each of these tissue types performs one or more of the following specific functions: vessels occur inhardwoodtimber only and conduct water and dissolved mineral salts from the roots to the leaves. These transmitted fluids provide the basic raw materials for photosynthesis. Cells store food materials and water, and in softwood species also perform the same conduit function as the vessels do in hardwoods fiber is a collection of cells and provides mechanical strength of wood.

Most of the cells are vertical in the tree trunk and together form fibers which are elongated particles with the long axis parallel to the longitudinal axis of the trunk (and the lengths of timber when sawn). These fibers form the grain of the timber.Other cells are laid down radially. (They go from the centre of the tree outwards to the bark much like spokes in a wheel.) The rays hold the other fibers together, transfer shear in the growing tree, and ultimately affect shrinkage and other properties of sawn wood.Apart from extractives, there are three main chemical components of wood: cellulose, hemicelluloses and lignin: Cellulose represents about 4045% of wood and is a long chain molecule (one dimensional) generally oriented parallel to the long axis of the cell, Hemicelluloses about 15-30% of hardwoods and about 20% of softwoods. It is a derivation of cellulose that is a two dimensional (planar) organic molecule, Lignin comprises 2230% and is an amorphous substance (like a gel).

Extractives

As the many capillaries that are present between the microfibrils become filled with extractives, the sapwood changes into heartwood. Whilst extractives are not generally a part of the essential wood structure, they are of major importance to wood properties, contributing to color, taste, odor, density, durability, flammability and moisture absorbency (hydroscopicity).The main types of extractives are polyphenols, oils, fats, gums, resins, waxes, small amounts of starches and some inorganic chemicals. Extractives can be used to make a variety of products including wood glues and stains.Natural features

KnotsWhere a branch starts to grow out of the trunk of a tree, the newly laid-down wood cells in the trunk have to bend around the branch. This leads to a knot in timber that is cut through the young branch.

Knots are a natural feature of timber and in many cases, enhance the appearance of the wood. They invariably reduce the strength of the timber due to two effects: The wood in the centre of the knot can have similar properties to juvenile wood with the low strength and durability associated with core wood. Effectively a knot introduces some loss of section. Knots near the edges of the piece can cause deviations of grain near the corners and this can have a significant effect on strength.The effect of knots on structural performance is a function of where the knots are in the cross-section, and on the size of knot:Even though many arris knots (knots that pass right through a corner) are small, they can reduce structural performance as they are in a very critical location. However, knots near the centre of the piece (well away from the corners) can have little effect on the performance of the timber while enhancing its appearance.As the grain in the wood around the knot is deflected around the knot, it leads to a localized slope of grain in the timber adjacent to the knot. The slope of grain does not necessarily detract from appearance, butslope of grainat the edge of a piece of timber can significantly reduce its tensile or bending strength. In appearance products, the slope of grain can affect the way in which stains and clear surface treatments are absorbed by the timber. This produces attractive effects in the way the timber reflects light.Softwoodspecies tend to have dark colored knots, so they are often quite obvious, but in many hardwoods, the knots are virtually the same color as the parent wood, and less noticeable. In some hardwood appearance products, knots may only be noticed once a stain and clear finish is applied. (The different grain direction in the knot accepts the stain and finish differently.)

Slope of GrainLocalized slope of grain can be caused byknots.

Slope of grain can also be caused by a slight bend in the tree, which means that when a straight board is cut out of it, there is a bend in the grain. This tends to be a longer feature and may go unnoticed in an appearance product.

Checks and splitsChecksare small cracks in the wood (often as a result of drying or seasoning). They are not deep, and do not continue through the depth of a board. They can make wood unsuitable for appearance use, though they can be filled with appropriate colored or clear fillers. They reduce the contact between adjacent wood grains, but generally have no affect on structural properties.Splits are deep cracks that penetrate the entire depth or thickness of the wood. They go from one side of the piece to the other. They can occur in wood due to inappropriate handling or by drying stresses. They are more likely to occur near the ends. They generally make the timber unsuitable for use in appearance applications, and reduce shear strength of structural timber. In some applications, where a rustic appearance is required (eg, unfinished timber poles, or rough sawn timber), some limited splitting will be appropriate.InclusionsSap veins, bark and other inclusions appear as dark, sometimes soft stripes parallel to the grain. These are all growth characteristics that are sandwiched between the growth rings as the wood is laid down.In limited quantities, these can enhance the appearance of the timber, and where they are accompanied by a surface check or depression, it can be filled with clear filler.

Where the length of the inclusion is within limits, it has little effect on the strength of the timber. A long inclusion can lead to a discontinuity across the grain that reduces the shear strength of the timber.