Lecture 4 - Timber (2015)

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LECTURE 4

TIMBER

Presented by: Mr. Milton McIntyreOct. 2015

University of Technology, Jamaica

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Classification of trees

Trees are classified as Exogen and Endogens.Exogens are those that grow diametrically, by addingnew cells in a layer between the existing wood and the

bark. Almost all commercial wood are considered to beExogens.Endogens are those that add new living fibre to the oldby allowing new fibre to intermingle with the old, thusproducing growth both diametrically andlongitudinally. Endogens are usually small plants eg:corn, cane, bamboo and palm.

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Structure of Exogens

In the trunk there are three mainsections, the heartwood , which isphysiologically inactive,the sapwood , where all conduction

and storage occurs, and the bark ,which protects the interior of thetree trunk.Wood is an anisotropic material.

Its anisotropic nature affectsphysical and mechanical propertiessuch as shrinkage, stiffness andstrength.

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Heartwood is the inner part of the trunk made ofdead tissue. Its primary function is to providemechanical support to the tree. Heartwood that is the

older wood and is darker, drier and harder than theouter part.Pith is the soft center of the log surrounded by the annual

rings. The number of rings approximately represents the

age of the tree.Sapwood is the woody layer just beneath thecambium layer and is used to transport moisturethrough the tree.

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The region of sapwood cells, which are often light incolour, lies outside the heartwood cells. Sapwood is lessdense than heartwood and is less resistant to decay and

insect attack than heartwood, which is more durable(not stronger) in use.

Bark is a flaky layer of corky material that protects thetree. The inner bark, nearest the cambium, is aliveand provides protection around the tree.

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The outer bark is not alive and protects the tree fromattack by various insects and diseases.

Cambium is a thin (microscopic) layer of wood cells thatexists inside the bark. The growth of wood takes placecontinuously under the bark in the cambium layer resultingin a ring known as “ Annual (growth) ring”. The width of

ring depends on the rate of growth of the tree.

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Structure of Exogens7

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Wood is a natural, renewable, organic substance with awide variety of uses. Of particular interest are trees thatgrow to sufficient height with diameters large enough tobe useful in the production of lumber and wood products

Wood typically consists of tiny, hollow, fibrous cells thatare composed of cellulose and lignin. The typicalcomposition of wood is 60% cellulose, 28% lignin, and12% other materials. Lignin acts primarily as a cement tohold the wood cells together, thus supporting the tree.Cellulose is made up primarily of carbohydratesproduced by photosynthesis .

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Hardwood Trees – trees with broad leavesthat shed in winter.

Example: Oak, Maple,Aspen, beech, birch,basswood, poplar, elm,cherry, etc.

Types of Exogenous trees

Wood is broadly classifiedinto 2 categories that is:

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Types of Exogenous trees

Softwood Trees – Any species that haveneedlelike leaves (conifers)

and that are generallyevergreen.

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Sawn Lumber13

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Cutting techniques

Round logs are cut into regular, rectangularpieces, which offer greater flexibility of use. The

different methods of cutting are:

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Plain sawn (Flat)15

The most common method ofsawing ( found in most solid timberflooring)

The most straightforward way tocut rectangular-profiled timberboards by cutting a log lengthwisewith a series of parallel cuts (which

is tangentially to a tree's growthrings, creating the familiar "flame-shaped" or "cathedral" grain)It provides excellent yield, widest

boards and least waste.

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Quarter sawn (edge)16

The method that cut logs intoquarters then sawing itperpendicular (90˚) to the growthrings.Produces radial (vertical) grains.This method yields fewer andnarrower boards per log than plain

sawing.Most popular for decorativeapplications such as cabinet faces.

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Rift sawn17

Similar to quarter sawn (cutperpendicular to the rings)Produces narrow boards with

accentuated vertical or "straight"grain patterns (more uniform).It provides very poor yield and leaveslots of waste. Therefore, not a very

popular method used.Favoured for fine furniture and otherapplications where matching grain isimportant.

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Seasoning

After cutting, lumber is seasoned. Seasoning is definedas the process of controlled drying of lumber toincrease its structural properties. Lumber is dried to:

Reduce shrinkage while in useReduce checking and warpingIncrease its mechanical properties

Increase its resistance to decayPrepare it for further treatment with preservativesReduce its weight for transportation.

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Seasoning

Two methods are practiced;Air drying

Kiln dryingChemical

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Timber is one of the most popular material in lightconstruction because of :

a) simplicity in fabricationb) Lightnessc) Reusabilityd) insulation from heat, sound & electricity

e) aesthetically pleasing appearancef) resistance to oxidation, acid attack, salt

attack and salt waterg) environmental compatibility

Timber21

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Advantages

Other advantages of using wood as an engineeringmaterial include:the low energy content needed for production,

the low cost of production,wood is an environmentally friendly material,wood is a renewable material. When trees grown in

sustainable forests are cut down, more trees areplanted, keeping the trees from extinction andmaintaining the levels of oxygen production by livingtrees.

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Advantages

wood has a very high specific strength due to its lowdensity and reasonable strength,wood‟s low density also makes it easier to transport,

there are very low costs associated with the disposal ofwood,wood is not electrically conductive,most woods are non-toxic,wood is low in thermal conductivity,nails and screws do not measurably weaken wood, if putin with care, showing that wood is very resistant to stress

concentrations.

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Disadvantages

However wood also has disadvantages as an engineeringmaterial which generally stop its use as a high-tech material.These include:

there is large variability in properties between species and,depending on growing conditions and the position of the woodwithin a trunk, within a species.wood is dimensionally unstable, as water changes itsdimensions.wood‟s strength decreases when wet. time-dependent deformation such as creep and visco-elasticityoccur in wood. Creep occurs due to movement of the non-

crystalline (amorphous) sections of the cellulose microfibrils.

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Disadvantages

It is highly combustible.It is susceptible to termites, woodworm and infestations.It cannot be used at high temperatures.

It is susceptible to rot and disease.It is highly anisotropic, although this can be limited by theuse of plywood. Plywood involves assembling layers ofwood with orthogonal grain orientation, decreasing the

anisotropy.

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Mechanical Properties

The mechanical properties of timber that are ofconcern are:

Tensile strength

Compressive strengthShear strengthBending strengthModulus of Elasticity

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Tensile strengthWood exhibits its highest strength values in tensionparallel to the grain. Tensile capacity perpendicular to

the grain is considerably lower than that of the parallelgrain. Variation in the grain, knots, and a differentmoisture content certainly would produce differentvalues. The difference in strength is caused primarily by

the lack of primary lateral connections between theindividual longitudinal wood fibres.

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Compressive strengthThe capacity of wood in compression parallel to thegrain is conditional to the ability of the wood cells to

resist buckling. As loading increases, the microscopicbuckling that occurs in the cell walls is magnified to thepoint where failure will occur across the wood section.Compression perpendicular to the grain causes

flattening of the wood cells, and a large amount ofdeformation can occur without any clear maximumload.

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Shear strength

Three basic types of shear can occur in wood: shearparallel to the grain, shear perpendicular to the

grain, and rolling shear.Shear parallel to the rain occurs in a plane parallelto the wood grain, with the resulting slidingoccurring in the same direction.Shear perpendicular to the grain occurs whenfailure occurs in a plane normal to the grain.

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Rolling shear also occurs in a plane parallel to hegrain but the direction of sliding is at right angles tothe grain in a plane parallel to the grain direction.

Wood has a high resistance to shear perpendicular tothe grain, and usually some other form of failure occursbefore this type of shear failure occurs. Shear parallel

to the grain must be considered when designing sectionsin flexure, and rolling shear must be dealt with whendesigning sections using plywood.

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Bending StressWhen loads are applied, structural members bend, producingtension in the fibres along the faces farthest from the appliedload and compression in the fibres along the face nearest tothe applied load. These induced stresses in the fibres aredesignated as bending stresses. Bending stresses in sectionsare influenced by the fact that wood has differentcharacteristics in tension and compression.

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The modulus of elasticity (E) measures the amount apiece of lumber will deform in proportion to anapplied load under elastic range stresses. It is a

measure of stiffness and not a strength property.

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Physical & mechanical properties of wood differ fromspecies to species & also within species. Among the factorinfluencing it‟s properties are:

DensityMoisture ContentTemperatureGrain structurePosition in treeCondition of growthDefectsCreep

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Defects in Timber

Lumber defects are nothing more than imperfections in wood.Regardless of type of wood used, some or all of them areusually a little less than perfect.

Defects can be either "man made" or naturally occurring. Manmade defects can be attributed to wood movement or a changein moisture content.Naturally occurring defects are more "organic”. Natural defects

are always environmental, attributed by either soil composition,changes in weather conditions, foreign invaders or growthpatterns.

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Defects in Timber

The common defects in timber are:Knots, Blue Stain, Pitch, Spalt, Wormhole, Shake,

Checks, Split, Wane, Bow, Crook, Cup, Twist,

Machine burn

Read up on each wood defect.

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Degradation/Decay of Timber36

Type Agent Environmental Factors

Biological fungi (dry rot, wet rot, moulds andothers) bacteria; actinomycetes;lichens, mosses and algae wood-boring insect larvae(woodworm, death watch beetle andothers) carpet beetle, moths, book liceand silverfish termites

moisture and humidity air movement temperature light dust food source

Chemical acids, alkalis and solvents pollution remedial treatment

Physical mechanical abrasion, general

handling and others, decompositionby physical agents such as prolongedheating, fire and moisture

normal use, visitor wear

accidental damage sunlight, heating, fire, damp

Radiation ultraviolet light exposure to sunlight

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Physical degradation

Wood is a highly stable polymer, that does not degrade on itsown.Exposure to sunlight causes a change in colouration -heartwood lightens (mahogany, oak) – some darken (teak)

Exterior exposure is the most severe & in a few months„weathering‟ will take place Light, rain & wind all contribute to the weathering process – silver grey appearance

Produces loss of surface integrity due to the breakdown oflignin under the action of ultraviolet light

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Further exposure will cause the shortening of the chain lengthin the cellulose – erosion of the cell wall will resultTimber becomes brittle & resistance to load is reduced as thedamaged lignin cannot fully transfer the stressThe surface is built to protect & filter the UV light therebyslowing the effects of weathering. (slow process – 1mm/20years)The application of surface protection is recommended – weathered surface must be cleaned prior to treatment

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Timber heated to 120°C for a period of about onemonth will experience a loss in strength of about 10%- small increases in temperature above this value willaccelerate the processBrowning of the timber takes place indicating thethermal damage & a caramel like odour can bedetected – degradation of the hemicelluloses – continued exposure will affect the celluloseTimber is stressed under load for long periods – creep

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Duration of load, creep & the associated loss ofstrength with time – 50 years of loading – strengthapproximately 50%Designers apply time modification factors

Compression failure – can occur naturally due to theformation of „kinks‟ in the cell walls under highcompressive stress or as brittle heart due to growthstresses in the centre of the trunkService conditions can induce over stressing of the cellwalls due to longitudinal compressionResults in reduced tensile strength & a major loss oftoughness

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Chemical degradation

Generally timber is highly resistant to various chemicalsTimber is more resistant to mild acids than cast iron or mildsteelTimber has lower resistance to alkalis – dissolves lignin &hemicellulosesIron salts are acidic in the presence of moisture & leads tohydrolytic degradation of the timber – softening &discolouration in the area of iron fasteningsCorrosion of certain metal fittings causes chemical decay oftimber – „nail sickness‟ (chemical rot) – electrochemical effectcontrolled by the availability of oxygen

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Biological degradation

Wood being of organicorigin is a suitable food formany organism

When the tree is alive it hasits own resistance (producescertain chemicals)

After felling- undergodeterioration.

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Bacteria degradation

Very slow degradationOnly few cellulytic bacteria can degrade woodContact with soil or water saturated conditions – Favourable

Causes - Softening of surface layersExcessive shrinkage on drying

Attack- mostly in Heart woodWood is called- Wet wood

Lower density, toughness, compression strengthBut have higher permeability to preservatives

Bacteria- play a dominant role in rapid decomposition of fungusattacked wood - Assignment

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Effects of Decay and discolouration

Distinct change in colour

Characteristic odour is given off by wood

Zonation lines may be formed by some fungi

Reduces strength, hardness, elasticity of wood

Decrease in specific gravity occurs

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Treatment

Timber needs to be chemically treated for two reasons:To prevent destruction from fungi and insectsTo inhibit combustion

Treatment for fungi and combustion is carried out with one ofthe following types of chemicals:Pentachlorophenol (Penta)Creosote

Inorganic arsenicals (waterborne)If protected from fire, insect & fungal attack, timber structurescan survive for extremely long periods (in excess of 2000years)

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Engineered Wood Products

Lumber is the traditional form of application ofwood but recently several other products haveemerged.

Assignment

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Wood Construction

Some typical terms used in wood construction:Rafters/Joists – for roof/floor; a series of beamsparallel to each other that support floor or roofloads.Studs – vertical slender member used as support forloads bearing walls or partitions. Usually spaced at

16” or 24”. Stringer – a cross beam supporting load from flooror roof joists.

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Timber

For submission on October 21, 2015 by 3 pm.1. Write short notes on the different methods of seasoning lumber.2. Select any seven wood defect and write a short paragraph on

each.3. Write a short paragraph on the fac tors that influences the

properties of wood.4. Write short notes on the various typ es of biological

degradation/decay of wood.5. Identify and briefly state three products of wood that are used

extensively in the construction sector.6. Read up on the different types of wood treatments, including the

pros and cons associated with each..No late submission will be marked.

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Lecture 4 - Timber (2015)