Wood, timber and timber products

Wood, Timber

and Timber Products

Applications of Timber

USDA


USDA

Rafter-type roof


Traditional Buildings

Norway

Padmanabhapuram Palace, Kerala/Tamilnadu.

17th century.


Traditional Buildings: Composite Construction with Masonry

Bourges, France.

15th century.

Liuheta pagoda, Hangzhou, China

Present form dates to 1152.


USDA

Commercial buildings (USA)


Bridges

www.quns.cam.ac.uk/Queens/Images/WinBridg.html

Wooden bridge, Queen’s College, Cambridge, UK.

Built originally in 1749 (oak), repaired in 1866 &

rebuilt in 1905 (teak).

Covered wooden bridge, Lucerne, Switzerland

Built originally in the 1300s, burned down in 1993,

was rebuilt.


NAFI

Railway Bridge


NAFI

Marine/Waterfront Structures


NAFI


Formwork and scaffolding

Wood

• Wood is a naturally occurring, biological material. It is

probably the world’s oldest structural material.

• Since it is easy to produce and handle, it is a widely

used construction material.

• The annual production of wood is about 1 billion

metric tons.

• Wood has good structural properties, is aesthetically

appealing and relatively cheap. Though it is

vulnerable to fire and decay through biological attack,

it can last for a long time if properly maintained.

Young et al.

Wood

Wood is more complex than many other materials because:

• There are at least 30,000 species of trees, and this alone leads

to a tremendous variation in the properties of wood.

• Wood is a composite material, with a variety of properties at

different scales.

• It has a lot of flaws and imperfections, which can control its

structural behaviour.

• It is anisotropic because of the way in which a tree grows.

Young et al.

Comparison of the properties of wood with those of some other materials

(E: Young’s modulus; ρ: density; σtensile: tensile strength; σcompressive: compressive strength, KIC: fracture toughness)

Wood Species

Trees are divided into two broad classes:

• Hardwoods: Tropical, broad-leaved, deciduous (shed leaves

annually), porous (contain vessel elements). Examples: Teak,

Sal, Oak.

• Softwoods: Conifers, have needle- or scale-like evergreen

leaves, non-porous. Examples: Fir, Pine, Cedar.

No reference to actual hardness of wood !

Some Common Indian species:

• Teak: Good dimensional stability and natural durability.

Heartwood varies from yellow-brown to dark golden-brown,

eventually becomes darker upon exposure to air.

• Sal: Strong and hard wood. Dark brown in colour.

• Deodar: Light and durable. Light brown in colour.

• Rosewood: Heavy wood with high strength. Heartwood varies

in colour from golden brown to dark purplish brown with

blackish streaks. USDA, Varghese

Structure of Wood

Young et al.

Macroscopic level

Outer bark: dense

rough layer of

protection.

Inner bark: transports

sap from leaves to

growing parts of the

tree.

Cambium: layer of

tissue, one to ten cells

thick, between bark and

wood.

Sapwood: wood on the

outside, conducts

moisture from roots,

stores food.

Heartwood: inner core,

nonliving, more

resistant to decay, drier

and harder.

Rays: small amount of

cells that grow in the

horizontal direction

Structure of Wood

Macroscopic level

USDA, Young et al.

The annual rings are the most distinct

feature of a tree trunk.

As the cells of the cambium grow and

divide during the growing season, they

form a ring of cells around the trunk.

In spring, during the period of rapid

growth, these cells are larger with thin

walls, and are referred to as springwood

or earlywood.

Later in the growing season, the cells are

smaller and with thicker walls, and

therefore harder and stronger. This is

called summerwood or latewood.

Structure of Wood

Macroscopic level

Illston and Domone

Radial growth of the truck must accommodate

existing branches of the tree.

This is achieved by the structure known as the knot.

If the cambium of the branch is still alive when it

fuses with that of the trunk, there is continuity in

growth, and a green or live knot is formed.

If the cambium of the branch is dead, there is

absence of continuity, and the trunk grows around

the dead branch and even the bark. Here, a black or

dead knot is formed. Such knots may drop out of

the plank on sawing.

Green or live knot

Black or dead knot

Structure of Wood

Young et al.

• Wood may be modelled crudely as a bundle of aligned tubular cellulose

cells or fibres, glued together.

• The middle lamella bonds the neighbouring cells.

• The primary wall is thin with randomly oriented microfibrils.

• The secondary wall has a thin outer layer, a thick middle layer and a thin

inner layer. These layers have microfibrils oriented in different directions.

Microstructure: Cells

Cellulose cell model Transmission electron micrograph

of a cell wall cross-section

Structure of Wood

Young et al.

Microstructure: Cells

Tracheids Internal cell walls

of earlywood

tracheids

• In softwoods, 90% of the volume

consists of longitudinally

oriented cells called tracheids;

the remaining are transversely

oriented cells called

parenchyma.

• In hardwoods, the

microstructure is more complex

as they contain, in addition to

the tracheids and parenchyma,

fibres and pores.

Structure of Wood

Young et al.

Microstructure: Cellular Arrangement - Anisotropic

Softwood: Scots pine Hardwood: European oak

3-D image of

0.5×0.5×0.8 mm

blocks

Structure of Wood

USDA, Young et al.

• All wood is composed of cellulose, lignin, hemicelluloses and minor

amounts of extraneous materials contained in a cellular structure.

Variations in the characteristics and volume of these components and

differences in cellular structure make the wood heavy or light, stiff or

flexible, hard or soft.

• Cellulose, the major component, constitutes approximately 50% of the

wood, by weight. It is a high-molecular-weight linear-polymer built from the

glucose monomer. During growth the cellulose molecules are arranged

into strands called fibrils (bonded by a combination of hydrogen and van

der Waals bonding), which make up the cell walls of the word fibres. Most

of the cell wall cellulose is crystalline.

• Lignin constitutes 23-33% of the softwood and 16-25% of the hardwood. It

is the cementing agent that binds the cells together. Lignin is a three-

dimensional phenylpropanol polymer.

• Hemicelluloses are branched, low-molecular-weight polymers.

• Extraneous materials in wood include oils, resins, fats, calcium, potassium

and magnesium.

Microstructure: Chemical composition

Orthotropic Nature

• Due to the way trees grow, wood

is highly orthotropic in nature.

• The properties are different along

the longitudinal, radial and

tangential directions.

• Nine independent constants are

needed to describe the elastic

behaviour of wood.

• The way of sawing will affect the

properties of the timber, as well

as the decorative features.

Properties of Wood

USDA, Young et al.

USDA, Illston & Domone, Young et al.

Specific gravity or Relative density

• For all species of wood, the specific gravity of the cell wall

material itself is about 1.5.

• However, the specific gravity of wood varies from 0.04 (for

balsa wood) to about 1.4 (for lignum vitae).

• The differences in the relative densities between species is

related to the variations in the void space or porosity

associated with the geometry of the wood cells and their

grouping.

• Specific gravity is a good indication of the mechanical

properties.

• Within the same species, the mechanical properties vary

linearly with the specific gravity.

• Lower the specific gravity, easier it is to cut the wood with a

sharp tool.

Properties of Wood

Illston & Domone, Varghese

Effect of Moisture Content

• The moisture content of green wood is high, varying

from 60-200%.

• Green timber will yield moisture to the environment with

consequent changes in its dimensions.

• For every combination of relative humidity and

temperature of the environment there is an equilibrium

moisture content of the wood.

• Moist wood is more susceptible to attack by fungi.

• For all these reasons, it is desirable to dry timber before

its use.

Seasoning is the process of controlled drying to

remove sap and reduce moisture without causing

cracks and distortion.

Properties of Wood

Young et al., Illston & Domone

• Moisture in wood exists in two forms:

• Free water within cell cavities

• Bound water adsorbed in the cell

walls

• As green wood dries, the free water

evaporates first. Fiber saturation point

is reached when all the free water has

been removed but the cell walls are

still saturated. This generally occurs

at moisture contents of 25-30%.

• Further removal of water compacts

the molecular structure, leading to

additional hydrogen bonding.

Therefore, the wood shrinks and

becomes stronger. This process is

reversible.



Young et al., USDA

Shrinkage

• Changes in moisture content above the fibre saturation point do

not affect the dimensional stability of wood.

• Below the fibre saturation point, the

volumetric shrinkage of wood is

approximately proportional to the

volume of water lost.

• Shrinkage is not the same in all

directions.


Young et al., USDA

Shrinkage

• The longitudinal shrinkage is normally negligible.

• However, the values of

tangential and radial shrinkage

can be in the range of 3-12%.

Tangential shrinkage is higher

than radial shrinkage.

• For example, teak undergoes

radial, tangential and volumetric

shrinkage of about 3%, 6%

and 7%, respectively.

Engineering Properties

Young et al.

Elastic Modulus

• Wood is linear elastic only over a small strain range.

• In general, the elastic modulus is highest in the longitudinal

direction (parallel-to-grain) and lowest in the tangential direction.

• Values of longitudinal elastic modulus ranges from 6 to 17 GPa.

For example, teak has a Young's modulus of about 9.4 GPa in

the green state and about 10.6 GPa for 12% moisture content.


Young et al.

Tensile Strength

• The tensile strength parallel-to-grain is high, ranging from 70 to 150 MPa.

The corresponding failure strain is small, in the order of 1%. Failure

occurs within the secondary wall of the cells that form the fibrils, with the

breaking of primary bonds.

• The strength perpendicular-to-grain is smaller, in the order of 2 to 9 MPa.

Failure occurs through the separation of the microfibrils and breaking of

secondary bonds. The strains can be high due to the distortion of the

cells.

• In bending, the modulus of rupture along the grain is in the range of 40 to

100 MPa. Failure generally begins with crushing in the compressive zone

and ends with tensile rupture of the bottom fibres. Teak has a modulus of

rupture of about 80 MPa in the green state and 100 MPa with a moisture

content of 12%.


Young et al.

Compressive Strength

• The compressive strength parallel-to-grain is only about

half of the tensile strength, in the range of 25 to 60 MPa.

Teak has a parallel-to-grain compressive strength of

about 40 MPa in the green state and about 60 MPa with

a 12% moisture content.

• In the longitudinal direction, failure occurs by the kinking

of the microfibrils and buckling of the cell walls.

• When compressed perpendicular to grain, the cells

begin collapse at a

stress of 3-10 MPa.

After that the

deformation continues

until complete collapse

and a consequent

increase in load.


Young et al.

Shear Strength

• The shear strength of wood depends significantly on whether primary or

secondary bonds are broken during failure. Therefore, the direction of the

failure plane with respect to the grains determines the strength.

• Shear parallel-to-grain is common, and involves the breaking of

secondary bonds. The corresponding strength is in the range of 5 to

15 MPa. The value for teak is about 9 MPa in the green state and about

13 MPa with a moisture content of 12%.


Variability

• The properties of wood vary considerably due its nature.

• The coefficient of variation of the tensile strength is in the order of 25%.

That of other properties can range from 10 to 35%.

• Due to the high variability, the safe (or characteristic) strength used in

structural design is much lower than the mean strength.

Effect of Temperature

USDA

• The mechanical properties of wood

generally decrease when heated and

increase when cooled. This effect is

reversible.

• At high temperatures, there is a

permanent deterioration of wood.

permanent effect on

modulus of rupturereversible effect

of temperature

on modulus of

elasticity,

modulus of

rupture and

compressive

strength at

different

moisture

contents

Creep

USDA

• Creep deformations are

significant in wood.

• Creep increases with

temperature and

moisture content.

Thermal Properties

USDA

Thermal conductivity

• The conductivity of structural softwood timber at 12% moisture

content is in the range of 0.1 to 1.4 W/(m-K), compared with 216

for aluminum, 45 for steel, 0.9 for concrete, 1 for glass, 0.7 for

plaster, and 0.036 for mineral wool.

• Conductivity increases with moisture content, temperature or

specific gravity.

• Since the thermal conductivity and heat capacity of wood are

low, it does not absorb or release heat quickly. Due to this wood

does not feel hot or cold to the touch as some other materials.

Thermal Properties

USDA

Thermal expansion

• The parallel-to-grain values of the expansion coefficient vary in

the range of 30~45 × 10-6 /K.

• Thermal expansion coefficients across the grain are proportional

to specific gravity. They range from 5 to 10 times the parallel-to-

grain coefficient.

Decay due to Fungi

USDA

• Wood that is always dry does not decay.

• When wood is constantly submerged in water, the deterioration

is slow since only some bacteria and fungi can attack under

water.

• Deterioration is more rapid in hot and wet climates than in cool

or dry climates.

• Early stages of decay are difficult to detect before significant

weight loss occurs.

• When weight loss reaches 5-10%, the mechanical properties are

reduced by 20-80%.

Insect Attack

USDA, Illston & Domone

• Wood is consumed by termites,

some beetles and wood wasps.

• Timber used in salt water can be

attacked by marine borers, such as

the shipworm and the gribble.

Fire

Illston & Domone

• Timber is a combustible material. However, the maintenance of

strength with temperature and time is better than steel.

• As the surface temperature increases beyond 100 ºC, volatile

gases are emitted. In excess of 250 ºC, there is sufficient build

up of these gases for timber to ignite in the presence of a flame.

In the absence of a flame, the temperature has to rise beyond

500 ºC for self-ignition to occur.

• Chemical bonds break and the microstructure degrades in the

temperature range of 175-350 ºC. The degradation of cellulose

results in the production of volatile gases and a reduction in the

degree of polymerisation.

Fire

Illston & Domone

Formation of the char protects

the unburnt timber.

Failure occurs only when the

unburnt section cannot sustain

the applied load.

• Due to pyrolisis there is

darkening of the timber and

emission of volatile gases.

Then, the reaction becomes

exothermic and charring

occurs.

• The volatiles cool the char and

block incoming convective

heat.

• The surface is cracked and

material is lost gradually.

Processing of Timber

• Sawing of logs into suitable pieces of timber is

called conversion.

• Conversion losses vary from 30-50%.

• After sawing, the timber is graded depending on

type, grain direction, knots, sapwood, worm holes,

etc.

• In the USA and other countries, timber for

construction is stress-graded based on strength,

stiffness and uniformity of size.

• Non-destructive tests may be used to verify the

mechanical integrity.

Varghese, USDA


USDA


USDA

Rafter-type roof


Traditional Buildings

Norway

Padmanabhapuram Palace, Kerala/Tamilnadu.

17th century.


Traditional Buildings: Composite Construction with Masonry

Bourges, France.

15th century.

Liuheta pagoda, Hangzhou, China

Present form dates to 1152.


USDA

Commercial buildings (USA)


Bridges

www.quns.cam.ac.uk/Queens/Images/WinBridg.html

Wooden bridge, Queen’s College, Cambridge, UK.

Built originally in 1749 (oak), repaired in 1866 &

rebuilt in 1905 (teak).

Covered wooden bridge, Lucerne, Switzerland

Built originally in the 1300s, burned down in 1993,

was rebuilt.


NAFI

Railway Bridge


NAFI

Marine/Waterfront Structures


NAFI


Formwork and scaffolding

Wood-Based Composites

Glued-Laminated Timber (Glulam)

• Timber manufactured by gluing together a large

number of relatively short pieces of timber.

• Glulam timber can be upto 40 m in length and over 2 m

deep. They can be straight or curved.

• The pieces are glued together such that the grain

directions are generally parallel.

• More expensive than sawn timber.

• Advantages:

• Size capabilities

• Architectural effects

• Seasoning advantages (pieces seasoned individually)

• Varying cross-sections

• Varying gradesYoung et al., USDA

Applications of Glulam

USDA


Plywood

Panels or sheets made from

wood by gluing together thin

veneers in layers. The layers

are placed such that the grains

of the successive plies are at

right angles to each other.

Manu Santhanam


Plywood

Advantages:

• Can be produced in large sheets.

• Split-resistant

• Have same properties in both directions of sheet.

• Effect of knots are limited to one ply.

• Shrinkage and swelling are minimised.

Varghese, USDA


Particle Board

• Chips are soaked in water, dried, mixed with resin and

pressed together to form boards.

• Typical particle boards have three layers: the faces

consist of fine particles and the inner layer consists of

coarser material.

Other Composites

• Fibreboard

• Strandboard

• Cement bonded particle board

• Wood fiber – Thermoplastic composites

Illston & Domone, Varghese, USDA

References

• Construction Materials: Their nature and behaviour, J.M. Illston and P.L.J. Domone, Spon Press, 2001.

• The Science and Technology of Civil Engineering Materials, J.F. Young, S. Mindess, R.J. Gray and A. Bentur, Prentice Hall, 1998.

• Building Materials, P.C. Varghese, Prentice-Hall India, 2005.

• Wood Handbook: Wood as an engineering material, US Dept. of Agriculture, Report FPL-GTR-113, www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm, 1999.

• Timber – Design for durability, National Association of Forest Industries, Australian Government, 2003, http://www.timber.org.au/resources/timber - design for durability.pdf

• Timber Decks, National Association of Forest Industries, Australian Government, 2004, www.timber.org.au/resources/datafileSS4.pdf

Wood, timber and timber products

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