Download - Wood, timber and timber products
Wood, Timber
and Timber Products
Applications of Timber
USDA
Applications of Timber
USDA
Applications of Timber
USDA
Rafter-type roof
Applications of Timber
Traditional Buildings
Norway
Padmanabhapuram Palace, Kerala/Tamilnadu.
17th century.
Applications of Timber
Traditional Buildings: Composite Construction with Masonry
Bourges, France.
15th century.
Liuheta pagoda, Hangzhou, China
Present form dates to 1152.
Applications of Timber
USDA
Commercial buildings (USA)
Applications of Timber
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.
Applications of Timber
NAFI
Railway Bridge
Applications of Timber
NAFI
Marine/Waterfront Structures
Applications of Timber
NAFI
Applications of Timber
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.
Effect of Moisture Content
Effect of Moisture Content
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.
Effect of Moisture Content
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.
Engineering Properties
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%.
Engineering Properties
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.
Engineering Properties
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%.
Engineering Properties
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
Applications of Timber
USDA
Applications of Timber
USDA
Applications of Timber
USDA
Rafter-type roof
Applications of Timber
Traditional Buildings
Norway
Padmanabhapuram Palace, Kerala/Tamilnadu.
17th century.
Applications of Timber
Traditional Buildings: Composite Construction with Masonry
Bourges, France.
15th century.
Liuheta pagoda, Hangzhou, China
Present form dates to 1152.
Applications of Timber
USDA
Commercial buildings (USA)
Applications of Timber
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.
Applications of Timber
NAFI
Railway Bridge
Applications of Timber
NAFI
Marine/Waterfront Structures
Applications of Timber
NAFI
Applications of Timber
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
Applications of Glulam
USDA
Wood-Based Composites
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
Wood-Based Composites
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
Wood-Based Composites
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