timber lesson 1

8/10/2019 Timber Lesson 1

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BEC304

TIMBER DESIGN


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INTRODUCTION

LEARNING OUTCOMES

SYLLABUS

LESSON PLAN
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Learning outcomes

At the end of the lecture student will be ableto;

Design timber beam, column, woodtrusses and timber deck

Design the glued laminated timbermembers

Understand the properties of timber itsvariations as structural members


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SYLLABUS CONTENT

1. Introduction to timber as structural member

2. Introduction of relevant timber design codes

3. Beam


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Timber building

Architect Wooi Lok Kuang


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Dewan Filharmonik - Petronas


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Current scenario in

timber construction Usage as the principle material in the

construction industry is very limited and farfrom satisfactory

Large quantity are mostly limited totemporary structures such as formwork andstructures of minor importance such as rooftruss.

Timber being used in a manner not inaccordance to good timber practice i.e notutilizing proper treatment, seasoning andgood design/detailing practice


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Sapwood and heartwood wane

Can engineer recognize the timbergrade/species/quality ??

Mixed species

Grain - // or

Irregular growth of timber-The effect is lesser if axially loading but poor in

bending resistanceStrong in parallel to grainWeak in tension perpendicular to grain


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Slope of grainwarp

Checks/cracks

Need to understand these facts in design for

long life of timber structures


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To reduce the uncertainty in the utilisationof timber in construction, need

timber/timber product; Less variability in strength and

dimension

Less effect of strength reducing

characteristics on the strengthproperties of timber

stringent manufacturing processand the product can be certified


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Engineered Wood Products

ForintekCanadaCorp.


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Laminated veneer lumber(LVL)

Parallel grainlumber

Fingerjointing

Glued and

pressed

Glued laminated timber (GLULAM)


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Can one day in Malaysia we able tosee these structures?

Canada train station

Library inAustralia

Japan expo

centre Germany largepool


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Need engineers and architects,builders, contractors, housing

agencies, timber industries etc topromote the use of timber asstructural members.


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Terms for timber

Timberare described as the normalsawn structural members. Generally,

timber will contain of macroscopicdefects (cracks, knots etc) of differentshapes, sizes and orientation

Woodrefer to small, clearspecimens, which are free of anymacroscopic defects. So wood is thebasic materials obtain from trees


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The structure of timberA tree has three subsystems:

roots trunk and crown;

Roots-spreading through the

soil as well as acting as afoundation enable the growingtree to withstand wind forces.They absorb moisturecontaining minerals from thesoil and transfer it via the trunkto the crown


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Tree


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Cont.. Trunk provides rigidity, mechanical

strength and height to maintain thecrown. Also transport moisture and

minerals up to the crown and sap downfrom the crown

Crown provides as large as possible a

catchments area covered by leaves. Thisproduce chemical reactions that formsugar and cellulose which cause thegrowth of the tree


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As engineer we are mainly concernwith the trunk of the tree.

The cross-

section of a

trunk


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Composition of wood

Long thin tabular cells made up of cellulose andbound together by substance called lignin.

Cells oriented in the direction of the axis of the

trunk except for cells called rays run radiallyacross the trunk.

A tree produces new layer of wood under thebark in the early part of every growing seasons

and the layer is called annular rings, annual ringsor growth rings. The age of a tree may bedetermined by counting its growth ring


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In temperate countries, a tree produces a new layer at early partof growth seasons and ceases at the end of growth seasons or

during winter months (eccentric rings) In tropical countries, trees growth throughout the year- more

uniform wood cells Annular ring is divided into two layers: inner layer made up

relatively large cavities called springwood and outer layer of thickwalls and small cavities called summerwood.


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The timber itself can bedifferentiated into sapwood and

heartwoodSapwood The annular band of cross-

section nearest to the bark

The living part of the trunk,where xylem cells are stillliving

Sapwood is lighter in colorcompared to heartwood and is25170 mm wide dependingon species

Sapwood acts a medium oftransportation for sap from

roots to the leaves

sapwood

heartwood

e.g of sapwood trees:

Jelutong, Rubberwood

and Ramin


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Cont..Heartwood

The central core of the wood which is inside thesapwood is heartwood.

The physiological dead part of the xylem

Cells are lignified and presence of extractives. Heartwood functions mainly to give mechanical

support or stiffness to the trunk

Sapwood has lower natural resistance to attacksby fungi and insect and accepts preservativemore easily than heartwood


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Types of wood

Hardwoods and softwoods The terms softwood and hardwood do not indicate softness or

hardness of particular timbers. In fact, some hardwoods are softerand lighter than softwoods. The main differences betweenhardwoods and softwoods are botanical, and relate to the way the

tree grows and the timber is laid down: leavesHardwoods have broad leaves and lose their leaves at

the end of growing seasons, while softwoods are conifers and havemore needle-like leaves and generally evergreen

colourHardwoods often have darker coloured wood, whilesoftwoods are invariably light in colour. (Note that there are a

number of species of hardwoods with light coloured woods.) densityMost hardwoods have thicker cell walls than softwoods.

Hardwoods often have higher densities than softwoods. Again thisis not a definitive test, but it does reflect most of the Australianand Malaysian species.


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Characteristics ofsoftwood

Quick growth rate, treescan be felled after 30 yearsresulting in low densitytimber with relatively lowstrength.

Generally poor durabilityqualities unless treated withpreservatives

Due to speed of felling, they

are readily available andcomparatively cheap-i.erubber trees.

Characteristics ofhardwood

Slow growth rate, takestime to matureover 100years results in higherdensity and strength.

Generally good durabilityless dependency onpreservatives

More expensive thansoftwood


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Microstructure of

softwood and hardwoods

tracheids


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Microstructure of softwood Softwood derives its strength from

a matrix of cellulose andhemicellulose molecules boundtogether with lignin.

Consists of single cells calledtracheids, which are like straws inplan

Tracheids function as conductionand support The remainder are parenchyma,

ray, resin and pith cells thatprimarily store and transit food.

Rays run in radial direction and

allow the convection of liquids towhere they are needed

The tracheids' vertical orientation with the trees' trunk explains the

bending strength of wood "parallel with the grain direction" and its

susceptibility to splitting "perpendicular to the grain direction."

Per unit of weight, softwood is stronger than steel.


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Microstructure of

hardwood More complex than softwood. With additional thick walled cells

called fibres providing thestructural support and thin walledcells called vessels providing

medium for food conduction Also consists of distributed

parenchyma cells, and ray cellswide enough in some species tobe seen easily with the naked eye.

The fibre's vertical orientation with the trunk explains the high

bending strength of hardwoods "parallel with the grain direction"

and its susceptibility to splitting "perpendicular to the grain

direction."


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Physical Properties of

timber Moisture contentbehaviour of timber unlike other materialsignificantly influenced by the existence and variation of itsmoisture. The moisture content as determined by oven drying ofa test piece

w = 100 (m1m2)/m2Where:m1 is the mass of the test piece before drying (in g)m2 is the mass of the test piece after drying (in g)

Moisture contained in green timber is held both within the cells(free water) and within the cell walls (bound water)

The condition in which all free water has been removed but thecell walls are still saturated is known as the fibre saturation point(FSP)


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At levels of moisture above FSP, thephysical and mechanical properties remain

constants. Variations of moisture below FSP cause

considerable changes to properties such asweight, strength, elasticity and shrinkage

and durability. Equilibrium MC at room temperature in

timber/wood can be achieved by seasoningit after being cut from tree.


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Seasoning- is the controldrying.

Methods:

Air seasoning

in which the timber is stacked andlayered with air-space in open sidedsheds to promote natural drying

Relatively inexpensive with very little loss

in the quality of timberDisadvantage- space is unavailable for

long period and limited control in thespace between the layers and the stacks.
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Kiln drying

Timber is dried out in a heated,ventilated and humidified oven.

Requires specialist equipment andmore expensive in terms of energyinput

Offer control environment toachieve the required reduction inmoisture content much quicker.


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Timber defects

Defects in timberwhether natural orcaused during

conversion orseasoning, will havean effect on structuralstrength as well as

fixing, stability,durability and finishedappearance of timber


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Types of defects Natural defectsoccurs during growing period Chemical defects-occurs when timber is used in

unsuitable positions or in association with othermaterials. Timbers such as oak and western redcedar contain tannic acid and other chemicalswhich corrode metals.

Conversion defectsdue to unsound practice in theuse of milling techniques or to undue economy inattempting to use every possible piece of timberconverted from trunk

Seasoning defectsrelated to the movement occurs

in timber due to change in moisture


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Seasoning Defects intimber Caused by differential drying out due to

uneven exposure to drying agents such aswind, sun and applied heat can results in anumber of defects

Distortion due todifferential directionalshrinkage


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Seasoning defects


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Natural and conversion

defects


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Densitybest single indicator of the properties of timber and majorfactor determining its strength. Specific gravity or relative density isa measure of timbers solid substance. Basic specific gravity of timberis expressed at certain moisture content and generally ranges from0.29 to 0.81.

Grain- often used in reference to annual rings and to indicate thedirection of fibers. Timber grain angle can be estimated by visualinspection or using Scriber (MS554). grain is the longitudinaldirection of the main elements of timber, these main elements beingfibres or tracheids and vessels in the case of hardwood.

It can be done by seasoning in air for several days at room temp orin a kiln. MC can be measured using weight difference method orusing moisture meter. MC can affect the physical and mechanicalproperties of wood


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Scriber-to determine slope ofgrain

Sl f h


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Slope of thegrain

Depends on theway timber is cut

Slope of grain


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Slope of grainCont..

Variation due to :

Poor cutting

Irregular growth of

timber The effect is lesser if

axially loading but

poor in bendingresistance


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Axes directions (a) Principle direction androtation angle in Cartesian coordination foruniaxial anisotropic timber (b) The direction ofprinciple axes for general anisotropic timber


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Other factors

Position in tree and condition ofgrowth- high density near the butt

and near the pith and low near the topand away from pith. Timber structurefrom trees depends on soil type, tree

spacing, sunlight, temp Defects