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BEC304 TIMBER DESIGN

INTRODUCTION

LEARNING OUTCOMES

SYLLABUS

LESSON PLAN

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Learning outcomes

At the end of the lecture student will be able to;

– Design timber beam, column, wood trusses and timber deck

– Design the glued laminated timber members

– Understand the properties of timber its variations as structural members

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 far from satisfactory

Large quantity are mostly limited to temporary structures such as formwork and structures of minor importance such as roof truss.

Timber being used in a manner not in accordance to good timber practice i.e not utilizing proper treatment, seasoning and good design/detailing practice

The outcome of timber industries

Tarzan suffers the consequences of deforestation

Too many deforestation without replanting

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Sapwood and heartwood wane

Can engineer recognize the timber grade/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

Slope of grain warp

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 strength properties of timber

stringent manufacturing process and the product can be certified

Engineered Wood ProductsEngineered Wood Products

ForintekCanada Corp.

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Laminated veneer lumber(LVL)

Parallel grain lumber

Finger jointing

Glued and pressed

Glued laminated timber (GLULAM)

Can one day in Malaysia we able to see these structures?

Canada train station

Library in Australia

Japan expo centre Germany large

pool

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Need engineers and architects, builders, contractors, housing agencies, timber industries etc to promote the use of timber as structural members.

INTRODUCTION The inherent variability of a material such as timber,

which is unique in its structure and mode of growth, results in characteristics and properties which are distinct and more complex than those of other common structural materials such as concrete, steel and brickwork. Some of the characteristics which influence design and specific to timber are:

The moisture content The difference in strength when loads are applied

parallel and perpendicular to grain direction The duration of the application of the load The method adopted for the strength grading of the

timber.

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Terms for timber

Timber – are described as the normal sawn structural members. Generally, timber will contain of macroscopic defects (cracks, knots etc) of different shapes, sizes and orientation

Wood – refer to small, clear specimens, which are free of any macroscopic defects. So wood is the basic 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 a foundation enable the growing tree to withstand wind forces. They absorb moisture containing minerals from the soil and transfer it via the trunk to the crown

Tree

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Cont.. Trunk provides rigidity, mechanical

strength and height to maintain the crown. Also transport moisture and minerals up to the crown and sap down from the crown

Crown provides as large as possible a catchments area covered by leaves. This produce chemical reactions that form sugar and cellulose which cause the growth of the tree

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 and

bound together by substance called lignin. Cells oriented in the direction of the axis of the

trunk except for cells called rays run radially across the trunk.

A tree produces new layer of wood under the bark in the early part of every growing seasons and the layer is called annular rings, annual rings or growth rings. The age of a tree may be determined by counting its growth ring

In temperate countries, a tree produces a new layer at early part of 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 thick walls and small cavities called summerwood.

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The timber itself can be differentiated into sapwood and heartwoodSapwood The annular band of cross-

section nearest to the bark The living part of the trunk,

where xylem cells are still living

Sapwood is lighter in color compared to heartwood and is 25 – 170 mm wide depending on species

Sapwood acts a medium of transportation for sap from roots to the leaves

sapwood

heartwood

e.g of sapwood trees:

Jelutong, Rubberwood and Ramin

Cont..Heartwood The central core of the wood which is inside the

sapwood 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 attacks by fungi and insect and accepts preservative more easily than heartwood

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Types of woodHardwoods and softwoods

The terms ‘softwood’ and ‘hardwood’ do not indicate softness or hardness of particular timbers. In fact, some hardwoods are softer and lighter than softwoods. The main differences between hardwoods and softwoods are botanical, and relate to the way the tree grows and the timber is laid down:

leaves – Hardwoods have broad leaves and lose their leaves at the end of growing seasons, while softwoods are conifers and have more needle-like leaves and generally evergreen

colour – Hardwoods often have darker coloured wood, while softwoods are invariably light in colour. (Note that there are a number of species of hardwoods with light coloured woods.)

density – Most hardwoods have thicker cell walls than softwoods. Hardwoods often have higher densities than softwoods. Again this is not a definitive test, but it does reflect most of the Australian and Malaysian species.

Characteristics of softwood

Quick growth rate, trees can be felled after 30 years resulting in low density timber with relatively low strength.

Generally poor durability qualities unless treated with preservatives

Due to speed of felling, they are readily available and comparatively cheap-i.erubber trees.

Characteristics of hardwood

Slow growth rate, takes time to mature – over 100 years results in higher density and strength.

Generally good durability less dependency on preservatives

More expensive than softwood

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Microstructure of softwood and hardwoods

tracheids

Microstructure of softwood Softwood derives its strength from

a matrix of cellulose and hemicellulose molecules bound together with lignin.

Consists of single cells called tracheids, which are like straws in plan

Tracheids function as conduction and support

The remainder are parenchyma, ray, resin and pith cells that primarily store and transit food.

Rays run in radial direction and allow the convection of liquids to where they are needed

•The tracheids' vertical orientation with the trees' trunk explains thebending strength of wood "parallel with the grain direction" and itssusceptibility 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 the structural support and thin walled cells called vessels providing medium for food conduction

Also consists of distributed parenchyma cells, and ray cells wide enough in some species to be 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."

Physical Properties of timber

Moisture content – behaviour of timber unlike other material significantly influenced by the existence and variation of its moisture. The moisture content as determined by oven drying of a test piece

w = 100 (m1 – m2)/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 the cell walls are still saturated is known as the fibre saturation point (FSP)

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At levels of moisture above FSP, the physical and mechanical properties remain constants.

Variations of moisture below FSP cause considerable changes to properties such as weight, strength, elasticity and shrinkage and durability.

Equilibrium MC at room temperature in timber/wood can be achieved by seasoning it after being cut from tree.

Seasoning- is the control drying.Methods: Air seasoning in which the timber is stacked and

layered with air-space in open sided sheds to promote natural dryingRelatively inexpensive with very little loss

in the quality of timberDisadvantage- space is unavailable for

long period and limited control in the space 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 and

more expensive in terms of energy input

Offer control environment to achieve the required reduction in moisture content much quicker.

Timber defects Defects in timber

whether natural or caused during conversion or seasoning, will have an effect on structural strength as well as fixing, stability, durability and finished appearance of timber

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

unsuitable positions or in association with other materials. Timbers such as oak and western red cedar contain tannic acid and other chemicals which corrode metals.

Conversion defects – due to unsound practice in the use of milling techniques or to undue economy in attempting to use every possible piece of timber converted from trunk

Seasoning defects –related to the movement occurs in timber due to change in moisture

Seasoning Defects in timber Caused by differential drying out due to

uneven exposure to drying agents such as wind, sun and applied heat can results in a number of defects

Distortion due to differential directional shrinkage

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Seasoning defects

Natural and conversion defects

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Density – best single indicator of the properties of timber and major factor determining its strength. Specific gravity or relative density is a measure of timber’s solid substance. Basic specific gravity of timber is expressed at certain moisture content and generally ranges from 0.29 to 0.81.

Grain- often used in reference to annual rings and to indicate the direction of fibers. Timber grain angle can be estimated by visual inspection or using Scriber (MS554). grain is the longitudinal direction of the main elements of timber, these main elements being fibres or tracheids and vessels in the case of hardwood.

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

Scriber-to determine slope of grain

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Slope of the grain

Depends on the way timber is cut

Slope of grain Cont..Variation due to : Poor cutting Irregular growth of

timber The effect is lesser if

axially loading but poor in bending resistance

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Axes directions (a) Principle direction and rotation angle in Cartesian coordination for uniaxial anisotropic timber (b) The direction of principle axes for general anisotropic timber

L

T

R

(b)(a)

2

y 1

x

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Other factors

Position in tree and condition of growth - high density near the butt and near the pith and low near the top and away from pith. Timber structure from trees depends on soil type, tree spacing, sunlight, temp

Defects