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Outline Presentation

WOOD AS A STRUCTURAL MATERIAL

1.1. Introduction 1.2. Wood structure

1.2.1. Anatomy of wood 1.2.2 Methods of conversion 1.3. Natural defects 1.4. Wood preservation 1.5. Fire retardants 1.6. Wood axes 1.7. Lumber grading

1.8. Timber constructions development 1.8.1. Timber frames for houses

1.8.2. Timber frames for bridges 1.8.3. Great timber structures

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WHY TIMB ER AS A BUILDING MATERIAL ?Since ancient times, wood and stone have been important

construction materials.

ADVANTAGES: The simplicity with which it can be worked by hand or

by machine.

The tooling costs are relatively low compared with

competitive materials. Wood is ideal if it is necessary to erect an individual

structure for a particular purpose but it is equallysuitable for small batch or mass production.

Wood remains the cheapest of all structural materials Its excellent thermal insulation.

The unique aesthetic properties of finished wood.

Its high strength to weight ratio:

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Wood is strong with outstanding rigidity in bendingand strength in compression.

Wood has exceptional stability in the longitudinal

direction, even when subjected to fluctuatingmoisture content .

Wood is free from corrosion .

The variability between woods of different species

may appear to be a disadvantage to theunintelligent user but it is, in fact, a distinctadvantage, as different species have differentproperties and there is almost always a suitable

wood for a particular purpose.

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However, there is one feature of wood that isunique amongst all structural materials:

it is a CROPthat can be obtained whereas its competitors

such as stone, brick, metal and plastics areall derived from exhaustible resources.This feature is alone sufficient to ensure thatwood will continue to be used as a structural

material virtually forever.

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WOOD STRUCTURE

As a plant a tree consists of acrown of branches with leaves,generally supported on a singlemain stem known as the trunk (or

bole) which connects the crown tothe roots in the ground

Anatomy of wood

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Longitudinal axis

Radial axis

Tangential axis

WOOD AXES

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Heartwood is the olderwood in the central portion ofa tree, which has ceasedparticipating actively in thephysiology of tree life.

Sapwood is the newerwood, which usually appearsas a lighter coloured band

immediately within the bark,extending inward from a fewtoo many annual rings,depending upon species.

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Woods are commonly divided into softwoods (thecone-bearing plants that are conifers) andhardwoods (the broad-leaved plants meaning

dicotyledonae and monocotyledonae).

Typical softwood speciesare the pines (trad.- pin),firs (trad.- brad), spruces (trad.- molid), and

redwoods (trad.- soiuri de conifere), while typicalhardwood speciesinclude the oaks (trad.- stejar),maples (trad.- artar), beeches (trad.- fag) andbirches (trad.- mesteacan).

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Methods of conversion

The first level of wood for construction is the log (trad.- bustean,barna). The logs are converted into sawn wood (trad.-cherestea, lemn ecarisat) by means of conversion saws.

TRUNK is barked (peeled) LOG is converting SAWN WOOD

There are different ways to cut the log to produce timber.The

manner in which the log is sawn is usually considered to berelatively unimportant. It is not so, because the manner caninfluence the behaviour of the sawn wood.

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Methods of conversion The simplest technique is to make a large number of parallel

cuts, a method known as through-and-through, flat sawn or backsawn. The outer boards are largely cut in the tangential plane whilethe middle board is in the radial plane, the angle between the annualrings and the surface of the board progressively varying through theintermediate boards.

Another manner is quarter-sawn boards. It is particularlysuitable for use as flooring as they do not suffer the cupping that is acharacteristic of tangential or outer flat sawn boards.

An alternative method of conversion known as billet sawn is tomake three through-and-through cuts to provide two flat sawnboards from the centre of the log. These will naturally include any

heart defects, which can then be removed when the boards are re-sawn. The remaining wood consists essentially of two half logsoften known as wainscot billets. These billets are then turned on totheir flat face and re-sawn to give a number of boards.

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(a) throughand throughsawn

(b) quarter cut (c) billetsawn

Methods of log conversion(sawn wood)

end grain(transversesection)

flat sawnplain sawnslash sawn

(tangentialsurface)

rift sawnquarter sawn

(radialsurface)

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Wood products and their sizes1. timber slat (strip)

2. boards (lumber) (trad.-

cherestea, scandura).

3. sawn timber collar beams(trad.- grinzi).

4. thick plank (trad.- scanduragroasa, dulapi).

5. sawn timber columns and

beams

mm40h

mm60b

mm40h

mm80b

mm100hmm40

mm80b

mm100hmm40

mm100b

mm100h

mm100b

b = width

h = thickness(height, depth)

b = width

h = thickness

(height, depth)

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THE TIMBER PRODUCTS

- planed square edged board

- planed tongued and grooved board

- planed tongued and grooved with V joint board

(match boarding)- plain weather-board

- rebated weather-board

- boards

- ship-lap weather-board

- sawn timber columns

log

- sawn timber beams - floorboards- doors

- door frames- door stops- architrave- skirting

- panelling

- windows- window frames

- surrounds and faces

- joinery (millwork)

- large-boards and cladding

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W OOD DEFECTSMainly degradation of wood can be grouped into two broad

categories: biological deterioration from fungal decay orinsect attack, and mechanical deterioration.

Wood has various natural defects, which can influence thestrength and thereby arrive at a value, which isacceptable for these defects.

Defects may be classified as natural defects, chemical

defects, conversion defects and seasoning defects.

All the defects may degrade wood, with the degree ofdegradation being reflected in varying degrees of loss inmechanical properties.

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W OOD DEFECTS

Seasoning defects. These defects are bowing, springing,twisting and cupping. Seasoning defects are directly relatedto the movement that occurs in timber due to changes inmoisture content. All such defects have an effect on

structural strength as well as on fixing, stability, durabilityand finished appearance.

Chemical defects may occur in particular instances whentimber is used in unsuitable positions or in association withother materials. Most woods are slightly acidic and produceacetic acid if stored in damp conditions. Timber such as oakcontains tannin, which corrode metals. Gums and resinsadversely affect working properties and ability to take glueand surface finishes, while silica in some hardwoods bluntstools.

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A knot is the part of a branch, which became enclosed in a growing tree.

knot (local disturbance of grain)

NATURAL DEFECTS:Knots, Grain defects (trad. defecte de fibra), Annual ring width,Fissures and cracks, Fungal decay

Grain defects are the measure of the deviation of thefibres from the longitudinal axis of the piece.

Annual ring width can be critical in respect of strength in that excess

width of such rings can reduce the density of the timber.

growth rings

Knots

Grain defects

Annual ring width

Mechanical deterioration

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Fissures and cracks

A fissure is any separation of fibres in a longitudinal plane and includes checks,shakes and splits. Their existence reduces the cross-sectional area,resisting shear and bending stress.

Fungal decay

Wormholes are permitted to

a slight extent provided thatthere is no active infestation.Wood wasp holes are notpermitted. Decayed woodshould not be accepted.

Biological deterioration

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WOOD PRESERVATION The following preservatives are recognised in the standards: Preservative oils: Creosote Creosote-coal-tar solutions Creosote-petroleum solutions Oil-borne preservatives: Pentachlorophenol Copper naphthenate Water-borne preservatives: Chromated zinc chloride (CZC) Fluor chrome arsenate phenol (FCAP)

Tanalith (Wolman salts) Celcure Chemonite Greensalt (Eradlith) Boliden salts

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Fire retardants Recently fire-retardant resin treatments have

been developed.

The alternative is the impregnation of the

wood with fire retardant salts. The fire endurance rating R, in minutes, is:

ZbGR 54.2 in [min.]where: -Z = factor dependent on load applied and member type. It has values

between 1 and 1.5;

-b = width dimension of cross section of beam or of larger dimension

of a column before exposure to fire;

- G = beam or column cross-sectional factor.

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WOOD AXES

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L = longitudinal axis

R = radial axis

T = tangential axis

L = longitudinal axisT = transverse axis

Wood is considered to be orthotropic, havingunique and independent properties in thedirection of three perpendicular axes

WOOD AXES

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SAWN WOOD (TIMBER) GRADINGGrading is the process of classifying timber according to quality for a particular use.

Quality

class

Load type and destination

ITimber elements are subjected to tension and bending(Truss girders, beams and wood dowels)

II a) Timber elements are subjected to compression and

bending

b) Timber elements are subjected to tension and tension +bending where effective stresses are 70% of allowable

wood strengths

III Secondary timber elements (Roof covering)

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SAWN WOOD (TIMBER) GRADING

Structural designers are interested in strength and stiffness, somodern grading rules provide for what is sometimes called stress grading.

The two methods used for stress grading are:visual grading;machine grading.

The minimum requirements for visual grading standards have beenlaid down in the European Code EN-518 Structural timber Grading

Requirements for visual grading standards. Requirements for machinegrading can be found in EN-519 Structural timber GradingRequirements for machine strength graded timber and gradingmachines.

Guidance on the use of timber in building and civil engineeringstructures is given also by the Romanian code SR-EN 1995-1/2004.

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To know where we shallgo, we need to know

where the craft hasstarted.

HISTORY OF TIMBER STRUCTURES

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1.8.1. Timberframes for houses

Primitive structures

Long tent with ridge purlinRound tent

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Early wood structure

Wood structure

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Timber house frame

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Log home

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Timber frame home

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Timber frame structure of the Middle Ages

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Old English style timber frame

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Typical American timberframe house

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1.8.2. Timberframes for bridges

Natural pedestrian bridges

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Primitive timber bridges

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The Drobeta-Turnu Severin bridge designed

by Apolodor and built by Romans

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Different timber bridge structures

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TRADITIONAL ROM ANIAN TIMBERHOUSES

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LOG HOUSE


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Silverthorne, Colorado

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Picnic Pavilion

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SUMM ER HOUSES

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1.8.3. Great timber structures

Bulk storage building built by Bunnings Limited

for Texada Mines Pty. Ltd, - 41 m span

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39. 6 m span truss roof aircraft hangars

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31.7 m span nail jointed arched store

and workshop buildings

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Wood Research and Development, LLC1760 SW 3rd Street

Corvallis, OR 97333, U.S.A.

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Outline Presentation

1. INTRODUCTION2. PHYSICAL PROPERTIES

Hardness and toughness Thermal properties

Electrical properties Acoustical properties Density and specific gravity Moisture content

3. MECHANICAL PROPERTIES Stiffness properties Strength properties

4. STRENGTH CLASSES

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5. INFLUENCE OF VARIOUS FACTORS ONWOOD PROPERTIES

Density

Moisture content

Knots

Fibre and ring orientation

Temperature

Duration of load Chemicals and decay

1. INTRODUCTION

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The arrangement of fibres in wood suggests that wood may have differentcharacteristics in the various directions within itself. Specifically wood isconsidered to be orthotropic, having unique and independent properties in thedirection of three mutually perpendicular axes.

The mechanical properties of wood used in design process of a buildingelement are usually referred to the following axes: longitudinal axis andtransverse axis. The transverse axis is used instead of tangential or radial axesbecause the variability of the same property about them is less and of minorimportance in timber element design.

The strength of wood is highly dependent upon direction tensile strengthvalues in longitudinal:radial:tangential directions on average are in the ratio of20:1.5:1.


R = radial axis

T = tangential axis


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Properties of wood a key for civil engineers to

use wood as a building materials

Physical properties Density. Moisture content. Hardness andToughness. Electrical properties. Acoustical

properties. Thermal properties. Behaviour in

fire. Resistance to corrosion and environmentalfactors.

Mechanical properties Strength properties. Elastic properties. Fatiguestrength. Fracture toughness.

Manufacturing

properties

Ability to be shaped by machines. Ability to be

joined by adhesives.

Economic properties Processing cost. Availability.

Aesthetic properties Appearance. Texture and ability to accept specialfinishes.

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2. PHYSICAL PROPERTIES Hardness and toughness

Sawn-wood is frequently described in terms of itshardness and toughness, but these are terms thatare difficult to define. Sometimes wood is said to bevery tough because it is difficult to saw or plane, or it

has good resistance to abrasion, splitting or shockloads.

The ability to resist excessive shock is probably the property

that can best be described as toughness.

Hardness is the ability of wood to resist penetration

Thermal properties

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Specific heat is the term used to describe the amount of heat energy that is required toraise the unit mass of the material through one degree of temperature. The specific heatof wood is comparatively high, four times as high as that of copper, but this relates tothe mass of material.

The thermal conductivity of wood is approximately 0.4% of that of steel and 0.05% of thatof copper. The thermal conductivity varies approximately in proportion to density .

Thermal conductivity in the longitudinal direction (L) is 2.25 to 2.75 times the value givenfor the (T) or (R) directions.

The average Longitudinal coefficient of thermal expansionL =3.6 x 10-6 /oC

Radial and tangential coefficients of thermal expansion- for softwoods:

T= 1.8 x 45 x specific gravity x 10-6 [/oC]R= 1.8 x 31 x specific gravity x 10-6 [/oC]- for hardwoods:

T= 1.8 x 32 x specific gravity x 10-6 [/oC]R= 1.8 x 32 x specific gravity x 10-6 [/oC]

p pTemperature affects both dimensional stability and strength of wood. Woodexpands as its temperature increases, as do other construction materials. Itscoefficients of expansion vary with direction, being largest radially andtangentially, and least longitudinally. Wood is a good insulator, that is, it has a

high resistance to heat flow.

El t i l ti

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Electrical properties

Wood at a low moisture content is normally classifiedas an electric insulator, or dielectric, rather than as aconductor

Tangential and radial resistance exceed longitudinalresistance for wood.

Wood density, moisture effects and temperature haveeffects on resistance of wood to electrical current.

The direct current properties of materials aremeasured by resistivity or by its reciprocal,conductivity.

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Acoustical properties

Sound insulating values are related to the soundtransmission. The reduction of sound in itstransmission through a material is dependentupon the weight of the material. Since wood has alower density than many structural materials, itseffectiveness in blocking transmitted sound is nothigh.

Sound absorption coefficient for a material isused to determine the total magnitude of theabsorption property of the material.

i d ifi i

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Density and specific gravity

Specific gravity (G) or relative density is theweight of a substance to that of an equal volumeof water.

Density is the mass per unit volume normally

expressed as kg/m3. Basic specific gravity isdefined as:

waterdisplacedofmass

massdry

V

m

Ggww

g

00

where w is the density of water and

Vg is the green wood volume.

Th d it i ht it l f i f d i ti l l

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The density or weight per unit volume of a piece of wood is a particularly

important property. Density [kg/m3], wherem is the mass of timber [kg] and

Vis its volume [m3] is defined as:

Wood substance has a density of about 1500 kg/m3.

Wood itself consists of a mixture of wood substance and spaces,

therefore the amount of wood substance per unit volume decidesthe dry density, which can vary in common species from about 300kg/m3 to 800 kg/m3.

Wood is considered to have moderate density if its dry density liesbetween about 360 and 500 kg/m3, so that woods below this rangeare light woods and those above are heavy woods.

V

m

Density at a moisture content [%] is expressed, related to volumetricshrinkage V, as:

VV .

.

.V

.m

V

m

0101

0101

0101

01010

0

0

Moisture content

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100

weightdry

weightdryweightoriginal

Moisture content, MC or, is the weight of water in thewood expressed as a percentage of the weight

of the oven-dry wood

During seasoning, most of the water in the cell cavities is lost,leaving a condition known as the fibre-saturation point (FSP).

Changes in dimensions tend to be linear with moisture in therange of 5 to 20% moisture content. In this range movementscan be calculated from:

where: - h1 and h2are the dimensions at moisture 1 and 2;- is the coefficient of swelling (positive) or shrinkage(negative).

1212100

1 hh

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[N/mm2]

MC

20 30 4010

Fibre-saturation point

[%]

Variation of strength versus moisture content

The graph shows that the fibre-saturation point occurs at around 25-30% and 25% is generally accepted as being a norm in sawn lumberand timber strength assessment. Between the fibre-saturation point

and zero moisture content, wood shrinks as it loses moisture andswells as it absorbs moisture. Above the fibre-saturation point, thereis no dimensional change with variation in moisture content.

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3. MECHANICAL PROPERTIES

(behavior of wood under applied forces)

MECHANICAL PROPERTIES

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MECHANICAL PROPERTIES

The strength and stiffness properties of most interest instructural design are:

compressive strength parallel to the grain; compressive strength perpendicular to the grain; tensile strength parallel to the grain;

bending strength; shear strength;

modulus of elasticity parallel to the grain; shear modulus.


R = radial axis

T = tangential axis


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Stiffness properties

The modulus of elasticity, also called Youngs modulus, usually used in

the design process is taken as a longitudinal modulus, EL. Data for ERand ET are not extensive and usually they are not presented asallowable properties for species. However, where a transversemodulus, ET(or E), is essential in design, an approximation often usedis 0.06 times the longitudinal value.

GLR, GLTand GRTdenote the three moduli of rigidity, or shear moduli, in

the (LR), (LT) and (RT) planes respectively.

The six Poisson's ratios are denoted by LR, RL, LT, TL, RT andTR.

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Prop.

Wood

Modulus of elasticity [N/mm2] Shear modulus

[N/mm2]GRTEL (E//) ET (E)

Softwood 10,00011,300 300 500

Hardwood 11,50014,300 600 1000

Romanian codes present the design values of:- elasticity modulus in longitudinal (parallel) direction, EL- elasticity modulus in transverse (perpend.) direction, ET- shear modulus GRT for softwood and hardwood.

Strength class (charactersic values) system established in

SR EN 338 St t l ti b St th l (EC5)

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SR-EN 338 Structural timber Strength classes (EC5)

C14 C16 C18 C22 C24 C27 C30 C35 C40

[N/mm2

]fm,k 14 16 18 22 24 27 30 35 40

ft,0,k 8 10 11 13 14 16 18 21 24

ft,90,k 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.4

fc,0,k 16 17 18 20 21 22 23 25 26

fc,90,k 4.3 4.6 4.8 5.1 5.3 5.6 5.7 6.0 6.3

fv,k 1.7 1.8 2.0 2.4 2.5 2.8 3.0 3.4 3.8

[kN/mm2]

E0,mean 7 8 9 10 11 12 12 13 14

E0,05 4.7 5.4 6.0 6.7 7.4 8.0 8.0 8.7 9.4

E90,mean 0.23 0.27 0.30 0.33 0.37 0.40 0.40 0.43 0.47

Gmean 0.44 0.50 0.56 0.63 0.69 0.75 0.75 0.81 0.88

[kg/m3]

k 290 310 320 340 350 370 380 400 420

D30 D35 D40 D50 D60 D70

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m = bending;

t = tension;

c = compression;

v = shear;f = strength

k = characteristic;

0 = parallel to the grain;

90 = perpendicular to the grain.

[N/mm2]

fm,k 30 35 40 50 60 70

ft,0,k 18 21 24 30 36 42

ft,90,k 0.6 0.6 0.6 0.6 0.7 0.9

fc,0,k 23 25 26 29 32 34

fc,90,k 8.0 8.4 8.8 9.7 10.5 13.5

fv,k 3.0 3.4 3.8 4.6 5.3 6.0

[kN/mm2

]E0,mean 10 10 11 14 17 20

E0,05 8.0 8.7 9.4 11.8 14.3 16.8

E90,mean 0.64 0.69 0.75 0.93 1.13 1.33

Gmean 0.60 0.65 0.70 0.88 1.06 1.25

[kg/m3]

k 530 560 590 650 700 900

It ranges from the weakest grade ofsoftwood, C14, to the highest grade

of hardwood, D70, currently usedin Europe.

Experimental data show that all-important characteristic strength

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and stiffness properties can be approximated from either bending

strength, modulus of elasticity or density. These relationships,

according to EC5, are:

45.0

,,0,5 kmkc ff

8.0

,, 2.0 kmkv ff

kktf 001.0

,90,

16

,0 mean

meanEG

kmkt ff ,,0, 6.0

kkcf 015.0,90,

meanEE ,005.0 67.0

30

,0

,90

meanmean

EE

5 INFLUENCE OF VARIOUS FACTORS ON WOOD

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5. INFLUENCE OF VARIOUS FACTORS ON WOOD

PROPERTIES

Density ()The relation between mechanical properties and

specific gravity has the form:

where: - S= the value of any particular mechanical property

- G= specific gravity- K, n= constants depending on the particular property beingconsidered.

nKGS

Moisture content

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Moisture content

Mechanical properties increase with decrease in moisture content. Most

clear wood mechanical properties obey the following relation in the vicinityof 20oC:

2

21

2

21

MCMG

MCMC

MG

MC

MCMC P

PPP

- PMG= value of property for all moisture contents greaterthan moisture content MG(slightly below fibre saturation point),at which property changes due to drying are first observed.

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In timber design the influence of moisture is taken intoconsideration by assigning timber structures to

service classes. The European code EC5 and the Romanian anexes

define this modification factor, mui.

Code gives the following values (subscript idefines theload type):

- 1.00 for all types of loads and the first service classof the timber construction;

- 0.90 for all types of loads and the second service

class of the timber construction;- 0.70 0.90 for the third service class of the timber

construction and different loads.

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Knots

Influence of a knot on the mechanical propertiesof a product varies depending upon the size,location, and type of stress that is applied to the

member

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Fibre and ring orientation

The influence of fibre direction on mechanical properties

can be approximated by Hankinson's formula:

nn QP

PQN

cossin

where:

- N= the property at an angle ;- = the angle between property direction and direction

parallel to the grain;-Q= the property across the grain;- P= the property parallel to the grain;- n= empirically determined constant.

Immediate effect of temperature on

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Immediate effect of temperature on

strength properties

Temperature

0 +20 +100-100[oC]

+200-200

100

200

Property[percent of value at 20oC]

Duration of load m generally called working condition coefficient or modification factor

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mdi, generally called working condition coefficientor modificationfactor

Type of

load

Load duration

classSymbol m

di

softwood hardwood

Static bendingShear

Permanent load mdimdf

0.55 0.60

Long term

variable load

0.65 0.70

Short term

variable load

1.00 1.00

Compression Permanent load mdc0.80 0.85

Long termvariable load

0.85 0.90

Short term

variable load

1.00 1.00

Tension Permanent load mdt0.90 0.95

Long term

variable load

0.95 1.00

Short term

variable load

1.00 1.00

Elasticity

modulus

Permanent load mdE1.00 1.00

Long term

variable load

1.00 1.00

Short termvariable load

1.00 1.00

Chemicals and decay

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Chemicals and decay

Chemicals may degrade wood, the degree ofdegradation being reflected in varying degrees of loss inmechanical properties. The effect of chemicals onmechanical properties is highly dependent upon the

specific type of chemicals. Wood-destroying fungi seriously reduce strength.

One measure of the progress of decay is the amount ofweight loss as a result of fungal attack.

Insects may destroy most of a piece of wood, frequentlywithout external evidence of the damage.

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