Chapter 4 - Timber

7/21/2019 Chapter 4 - Timber

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ECM 206 - CIVIL ENGINEERING MATERIALS TIMBER/WOOD

RT-EC110-ECM206-DIS12-APR2013 1

TIMBER / WOOD

by Dr. Rudy Tawie

INTRODUCTION

-

One of the oldest structural materials still in use.

- In Malaysia, there are more than 2500 species of timber.

-

Timber characteristics and properties are distinct and more complex than those of other

common structural materials such as concrete, steel and brickwork.

- Timber can be used for many purposes in building construction. Some of the applications

are:

o Foundation piles

o Structural components such as beams, columns, etc.

o Roofingrafters, ties, struts, purlins and bracing.

o

Flooring

o Walling

o Ceiling

o Door and windows

- Advantages of timberincludeo high strength/weight ratio (it is light but strong)o

toughnesso

ease of workingo good thermal insulation

o it is more fire resistant than steelo it has attractive appearance

-

Disadvantages of timberincludeo Anisotropy of timber

o Defects in timber reduce its strengtho Limited in sizes (in section and in length) which are commercially availableo The difficulty of drying large sections

-

Timberis a term used to describe wood that has been processed for use

- Processing of timber involves four basic stages:o fellingcut down by chain saw

o conversionlogs cut into sections before seasoningo seasoning timber dried in a controlled mannero gradingcarried out visually or by mechanical means


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ANATOMY OF WOOD

- Trunk (bole)o conducts essential mineral salts and moisture from the roots to the leaves, stores

food materials.-

Growth rings

o occurs in timber grown in climates having distinct seasonal changes.

- Barko Outer bark

a rough textured, dense material that protects the living tree from

extremes of temperature, drought and mechanical damage.

comprises dry dead tissue.o Inner bark

comprises moist, soft material that conducts food synthesised by theleaves to all growing parts of the tree.

- Cambium layer

o

an extremely thin, delicate layer.

Easily damaged

o cell divisions within this layer responsible for growth in the thickness of the tree.

- Sapwoodo the outer or younger growth containing living cells.

- Heartwoodo

comprises the inner layers in which the sapwood has become inert (inactive).

o produces substances such as tannin.

- Rayso comprises narrow bands of tissue running radially across the growth rings.

-

pitho

comprises the core of the tree.

The growth ring has two parts:

- Early woodo Generally softer, weaker and more porous than the latewood.

- Late woodo Grows more slowly

o Often denser, darker and narrower than the early wood.


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Sapwood Heartwood

- Contains starch (food)- Lighter in colour

- More attractive to fungi and certaininsects because it contains sugars,starch and water

- Produces tannins and other phenoliccompounds that are toxic to insects and

fungi.

Composition of wood

- The walls of timber fibres are composed of cellulose and hemicelluloses and these arebonded together essentially by lignin.

-

Sapwood generally contain more lignin than heartwoods.

- The properties of the wood differ greatly in the three main directions. The three

directions are:o Longitudinal (parallel to the direction of the trunk)o radial(perpendicular to the surface towards the core)

o tangential

-

The structural material that can be obtained from the tree is therefore anisotropic.

- The strength of the material depends largely on the direction of the cells, with the greatest

strength being in the direction of the cells, longitudinally.

CLASSIFICATION OF MALAYSIAN TIMBERS

- Malaysian timbers are classified into four categories:

o Heavy Hardwoods (HHW)

o Medium Hardwoods (MHW)


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o Light Hardwoods (LHW)

o Softwoods (SW)

- The classification of the three categories of Hardwoods is based largely on the averagedensity of the timbers at 15% moisture content.

Classification Table

Classification Density range (15% M.C.)

Heavy Hardwood 800 - 1120 kg/m3

Medium Hardwood 720 - 880 kg/m3

Light Hardwood 400 - 720 kg/m3

Softwood(Bontanical distinction)

CLASS OF SOME STRUCTURAL TIMBERS

Heavy

Hardwoods

Medium

Hardwoods

Light

Hardwoods SoftwoodsBalau

Balau red

Belian Bitis

Chengal

Giam

Kekatong

Keranji

Malangangai

Merbau

Penaga

PenyauResak

Tembusu

Alan batu

Bekak

Derum

Entapuloh

Geriting

Kandis

Kapur

Kasai

Kayu malam

Tulang daing

KelatKeledang

Kempas

Keruing

Keruntum

Kulim

Mata ulat

Mempening

Mengkulang

Meransi

Merawan, Gagil

Merbatu

Merpauh

MertasNyalin

Pauh kijang

Perah

Petaling

Punah

Ranggu

Rengas

Semayur

Senumpul

Ara

Alan bunga

Babai

Bayur

Berangan

Bintangor

Binuang

Dedali

Durian

Geronggang serungan

GerutuJelutong

Jongkong

Kedondong

Kelumpang

Kembang semangkok

Ketapang

Kungkur

Laran

Machang

Mahang

Medang

Melantai

MelunakMempisang

Meranti bakau

Meranti, dark red

Meranti, light red

Meranti white

Meranti yellow

Merawan

Merbulan

Mersawa

Damar minyak, Malayan kauri

Podo

Sempilor


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Simpoh

Tampoi

Tualang

Nyatoh

Pelajau

Penarahan

Perupok

Petai

Pulai

Ramin, Melawis

Rubberwood

Sengkuang

Sentang

Sepetir

Sesendok

Terap

Terentang

White seraya

The distinction between the terms, Hardwood and Softwood, do not always related to

hardness. Thus, a softwood is not necessarily soft and similarly, a hardwood is not necessary

hard.

-

- It is based on the botanical convention.

o Hardwoods (angiosperms) Not all hardwoods are hard broad-leaved trees most are deciduous

losing its leaves in autumn and growing new ones in the spring.

o Softwoods (gymnosperms) not all are soft

some softwood are very hard, strong and durable

needle-like leaves

Coniferevergreens

Tropical trees have leaves all year round

STRESS GRADING

-

All timbers used for structural work should be stress graded.

- There are two methods of stress grading:

o Visual stress grading

A grading method that involves the visual judgement of the quality of

timber.


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o Machine stress grading

Grading is done by mechanical means that evaluates the Modulus of

Elasticity value of timber

- The timber shall be graded in accordance with the Malaysian Grading Rules for sawnhardwood timber by graders registered with Malaysian Timber Industry Board (MTIB).

- Malaysian structural timbers are graded into the following grades:

o Select structural

Intended for special purposes, particularly when the strength/weight ratioof the timber is to be a maximum e.g. towers for transmission lines and

trusses of very long span. 80% strength ratio

o Standard structural Should be specified for normal purposes. 63% strength ratio

o

Common building

Intended for wooden members used in less important parts of building

frames (not designed by calculations). 50% strength ratio

STRENGTH GROUPING

- Species having similar strength and stiffness properties are grouped together forsimplicity in design.

- Common grade stresses are allotted to each group. Seven such strength groups areformed, namely SG1, SG2, SG3, SG4, SG5, SG6 and SG7 in order of decreasing

strength.

- The groups formed are based on the weakest species in the particular group.

Strength Groups of Timber

SG1 SG2 SG3 SG4 SG5 SG6 SG7

A. Naturally Durable TimberBalauBitis

Chengal

BelianMata ulat

kekatong

BekakDelek

Keranji

GiamMalabera

MerbauResak

TeakTembusu

B. Timber requiring treatmentDedaruKempas

MerbatuMertas

AgohoBalau, red

KelatKembang semangkokKulimPauh kijangPenyau

BeranganDedali

DerumKapurKasaiKeruntumMempening

Alan bungaBabai

Balek angin bopengBintangorBrazil nutGerutuKayu kundur

BayurDamar minyak

DurianJelutongJenitriJongkongKasah

AraBatai

GeronggangLaranPelajauPulaiSesendok


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PerahPetaling

RangguDurian batuTualang

MeransiMeranti bakau

MerawanMerpauh

NyalinPerupok

Punah

RengasSimpoh

KedondongKeledang

KeruingKetapangKungkurMelunak

Mempisang

MengkulangMeranti, dark redMeranti white

NyatohPenarahanPetaiRamin

RubberwoodSengkuangSepetir

MachangMedang

Melantai/KawangMeranti, light redMeranti, yellowMersawa

Terap

Terentang

NOTES:1. For naturally durable timbers, sapwood should be excluded. If sapwood is included, preservative treatment is necessary

(Source MS360, 1986)2. For timber requiring treatment, they should be amenable to preservative treatment.

DESIGN STRESSES

The following stresses form the basis of design for various timber components.

- Basic stresso stress that can safely be permanently sustained by solid timber containing no

visible strength reducing characteristics.

- Grade stress

o

Stress which can safely be permanently sustained by material of a specificsection size and of a particular strength group or species and grade.

- Wet stress/Green stresso

Stress applicable to material exposed in conditions which would result in timberhaving a moisture content exceeding 19%.

- Dry stresso Stress applicable to material exposed in conditions which would result in solid

timber having moisture content not exceeding 19% in service.

- Permissible stress

o

Stress that can safely be sustained by a structural material under a particularcondition.

-

Strength ratio

o

The ratio of the grade stress to basic stress.


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DEFECTS IN TIMBER

- Defects can affect the quality of the timber, either by reducing its strength or marring itappearance.

- Natural defects

o

Knots The most obvious natural defect. A knot is the part of a branch which became enclosed in a growing tree.

- Conversion defects

o Wane Bark or lack of wood on the edge or corner of a piece of wood resulting

from the piece being sawn from near the outer circumference of asawlog.

o Sloping grain

Results from spiral growth or from conversion which is not parallel to

the axis of the tree.


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- Seasoning defects

o Surface check

o

Twist

o Cupping

o Honeycombing

o Cut surfaces

o

Case hardeningo

Spring

o

Bow

- Deterioration in use

o

Weathering

Water

Flowing water

o

Leaching of soluble colouro

Erosion of surface

Wetting and drying

o Expansion and contraction causing mechanical failure informs of cracks.

Sunlight

Fading of colour

Embrittlement of surface

o Fire

Timber is combustible and evolves smoke and toxic gases. Timber members survive longer in fires than equivalent unprotected steel

members.

o

Fungi Decay

The decomposition of wood substance by fungi. Requirements for growth of fungi:

Suitable food i.e. timber

o In all species growth is more rigorous in sapwood thanin heartwood.

Suitable moisture content of timber


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o At least 20 per cent moisture content is required forcolonization although fungi cannot live in saturated

conditions.

Suitable temperature

o Optimum temperature is about 30C. Growth is about

twice as rapid at 21C as at 10C.

Oxygen

o Marine borers

Extremely destructive particularly in warm salt waters.

Gribble (limnoria lignorum), a crustacean

o The gribble creates small tunnels in the near surfacelayers of exposed timber, weakening the timber which

may be abraded by water action, exposing fresh surfacesfor attack.

Shipworm (teredo navalis), a mollusc

o The shipworm bores into timber through a very small

entry hole which is almost impossible to detect.

o Wood-destroying insects

Termites

a serious problem in many warmer temperate countries.

Beetles

In the UK beetles are the chief pest.

Unlike wood-destroying fungi, beetles rarely cause structural

failure.

in the UK the principle beetles in buildings are

o

The common furniture beetle

o

Death watch beetleo

Power post beetleo The house longhorn beetle

PROPERTIES OF TIMBERS

- Properties vary widely between species, between tress of any one species, in differentparts of a tree and in different directions.

o Density

Expressed as mass per unit volume

The densities of timbers vary widely.

Numerous properties e.g. strength, shrinkage, stiffness and hardness,increase with increasing density.

o Thermal insulation

Timber is a good insulator.

o Thermal movement

Thermal expansion is relatively small.


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Expansion joints are not normally required.

o

Behaviour in fire Fire resistance of timber generally compares favourably with other

structural material and is often better than most.

o

Chemical resistance Compared with metals, wood has good resistance to alkalis and weak

acids.

o Strength

Timber has a high strength:weight ratio both in tension and compression

and is elastic. Generally, strength increases with density, particularly within a species.

Strength reduces as moisture content rises.

Temperature rise reduces strength.

Strength is reduced by the particular defects contained in each piece oftimber.

MECHANICAL PROPERTIES

- Wood has nine independent elastic constantso

Moduli of elasticity (EL,ER,andET)o

Moduli of rigidity (GL , GRand GT)

o Poissons ratios (RT , TLand RL)

- The development of the mechanical properties of important species requires thousands of

tests in order to obtain true average values. Several hundred tests have been performed atthe Forest Research Institute Malaysia (FRIM), Kepong.

o

Testing Methods Static bending test

Impact bending test

Compression parallel to the grain

Compression perpendicular to the grain

Shear parallel to the grain

Hardness test

Tension parallel to the grain


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Static bending test Compression parallel to the grain test

Compression perpendicular to the grain test Shear parallel to the grain test


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VARIATIONS IN MOISTURE CONTENT OF TIMBER

- Drying timber from the green to the normal seasoned condition reduces its density by50% or more with consequent shrinkage and increases in strength properties, thermalinsulation, resistance to decay and suitability for impregnation, painting and gluing.

-

MC % = 100 x (weightdry weight) / dry weight

-

Movementso Variations in size of timber in response to variation of its moisture content as a

result of changes in atmospheric humidity.

- Seasoningo It is the controlled reduction of moisture content to a level appropriate to the end

use.

Air seasoning

Process of drying timber by exposure to air.

It is a cheap method but a relatively slow process

Kiln seasoning

Process of drying timber by placing the timber in a kiln (large

oven) and exposing the timber to heat for a prescribed period oftime.

-

Shrinkage

o Shrinkage can result in a number of defects such as twisting, cupping, bowingand cracking.

DURABILITY OF TIMBER

- Some timbers are naturally durable and may be long lasting. Others are very susceptible

to deterioration but they can be treated in some way to improve their resistance to fungaldecay and insect attack, these being the two most serious problems.

WOOD PRESERVATIVES

-

Timber can be treated with chemicals to protect it from fungi, insect or mollusc attack.

Wood preservatives can be classified as follows:

o

Coal tar creosote

Inexpensive preservative

For external use

Unsuitable for internal use due to its unpleasant odour Difficult to paint over satisfactorily

o Organic solvents e.g. organotin, organocopper and organozinc fungicides

Suitable both internal and external uses Penetration is superior to that of creosote


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o Waterborne preservatives

e.g. copper/chromium/arsenic compositions Can be over-painted Odourless Deep penetration

METHODS OF PRESERVATION

- The method used is normally based upon the end-use of the timber, the risk category andthe amenability of the species of timber to treatment.

- The following outline descriptions of methods are arranged in ascending order ofefficiency and effectiveness.

o Brush or spray

The preservative is flooded over the surfaces to encourage absorption.

o

Immersion The period of immersion may vary from only a few seconds to a few

minutes or up to one hour.

o The hot-and-cold open tank method The timber is submerged in a tank of suitable preservative which is then

heated for several hours (bet. 80 and 90C). After that allow absorptionby the timber to take place as the liquid cools.

o Pressure impregnation The timber is sealed in a pressure vessel, air is removed under vacuum;

preservative is forced in under strict control and a second vacuum stage

removes excess liquid.

o The diffusion process The timber is close piled for several weeks during which the preservative

diffuses into the wet timber.

STRUCTURAL APPLICATIONS OF TIMBER

- Marine worko e.g. wharves, piers, sheet piling, cofferdams

o

Nowadays concrete and steel is preferred because good-quality timbers of thetypes traditionally used for marine work are difficult to find and are costly.

- Heavy construction worko e.g. piling, bridgeso Timber is still used where availability and cost of materials are favourable.

However, the use of timber in construction industry is being reduced by precastand/or in situ concrete piles and steel sheet piling.


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- Medium/light constructional worko Timber in this type of construction work has much more range of uses due to

availability and east of working and handling of the cheaper types of timber.o

Roof trusses, partitions, screens, floors and wall panels are often produced infactory in large numbers

PROCESSED TIMBER PRODUCTS

- The use of solid timber in construction has been gradually reduced by a range ofprocessed products (sheet materials and glued-laminated members) which can overcomesome of the disadvantages of using solid timber.

- Sheet Materials(sheet, board and panel products)

o Veneer plywood

Commonly referred to simply as plywood. Plywood is defined in BS6566: Part 2 as a material in which all the plies

are made of veneers (thin sheets of wood) oriented with their planeparallel to the surface of the panel.

Ply is defined as a measure of thickness of wood.

Veneer is defined as a thin layer or sheet of wood.

o Core plywood

Manufactured from strips of solid timber, normally edge-glued togetherto form a solid slab of material which is then surfaced with one or two

cross-banded veneers on each face. Most commonly referred to as blockboard or laminboard. The distinction

between blockand laminboard is based on the width of the core strips.

Blockboard

o

The core strips can be up to 25 mm wide. Laminboard

o Strips less than 7 mm wide.

o Particleboards

The basis of these materials is that by breaking the wood down into relativelysmall pieces, and resembling these pieces with a binder of some kind, the effectsof the original grain structure and natural defects are removed and a more

uniform product can be obtained.

Chipboard

The raw material may be solid wood such as sawmill offcuts,

logging waste, or wood residues. Fed into a chipper which cuts the required sizes and shape of

particle.

The cips are then dried and screened to separate out oversize

chips and fine dust, and sometimes to classify the chips as faceor core, face chips being finer.


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After that the chips are mixed with a carefully controlled small

quantity of resin and then formed into a mat on a flat plate, thechips being oriented totally at random.

o In some cases the mat may be layered with finer chips atthe top and bottom to give a smoother surface to thefinished board.

The mat is then pressed to the required thickness in a high-temperature press which brings the particles into intimate contactand cures the glue.

Manufacture is completed by trimming to size and sanding.

Oriented strand board (OSB)

A type of particle panel product composed of strand-type flakes

that are purposefully aligned in directions that make a panel

stronger, stiffer, and with improved dimensional properties in thealignment directions than a panel with random flake orientation.

Cement bonded particleboard

Quite different from other boards

o Bonded with cement rather than with sythetic resins.o Wood content of the finished board is quite low.o

Density is much higher.

o High resistance to fire.

o Fibreboard

A broad generic term inclusive of sheet materials of widely varying

densities manufactured of refined or partially refined wood (or othervegetable) fibers. Bonding agents and other materials may be added to

increase strength, resistance to moisture, fire, or decay, or to improve

some other property. There are two basic processes

Wet process

o

Fibres are added with water to form a slurry which is fedunder thicknessing rollers on to a wire mesh conveyer

where the water drains out. For hardboards, this sheet is then pressed on

mesh plates to allow the remaining water toescape, and heated to promote rebonding of thefibres by their residual lignin content.

For softboard or insulating board, the wet sheetis simply fed through drying ovens to remove

the moisture and promote the fibre bondingprocess.

Dry process

o The fibres are put into a dryer, where they are mixedwith urea formaldehyde or other synthetic resin, before

being formed into a mat on solid plates.


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o After an initial pressing the mat is cut to size and thensubjected to heat and pressure to compress the fibres and

cure the resin.o

The material produced by this process is called mediumdensity fibreboard (MDF).

-

Glue-laminated sections (Glulam)

o refers to two or more layers of wood glued together with the grain of all layers orlaminates approximately parallel. The laminates may vary as to species, number,size, shape and thickness.

o Accepted as being generally stronger and stiffer than solid timber.o Advantages of glue-laminated timber include:

Flexibility of shape and size.

High standard of material quality.

It is a controlled factory production.

Superior performance in fire.

There is no need for expansion joints.o Low coefficient of thermal expansion.

Defects such as knots are restricted to the thickness of one laminationand their effects overall structural behaviour is significantly reduced.

Chapter 4 - Timber

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