1. classification and structure of wood 2. properties of...
TRANSCRIPT
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Department of Structures and Materials1
Lecturer:
Prof . Dr. Mohammad IsmailFaculty of Civil Engineering, UTM-Skudai,
Johor Darul Ta’zim,
MALAYSIA
December 6, 2013
TIMBER
1. Classification and Structure of Wood
2. Properties of Timber
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1. Classification and Structure of Wood
2. Properties of Timber
Prof Madya Dr. Mohammad IsmailFaculty of Civil Engineering, UTM-Skudai,
Johor Darul Ta’zim, MALAYSIA
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LEARNING OUTCOME
• At the end of the lecture, student should understand:
� General knowledge on timber
� Structure of wood
� Classification
� Properties
� Application
� Factors affecting strength
� Seasoning and preservative
� Timber product
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INTRODUCTION
• One of the oldest known construction materials
• Employed in many application
� Shipbuilding, bridges, railroad ties, flooring, cabinets,
nails
� Simple in fabrication, light, reuseable, insulation from
heat, electricity and sound, pleasing appearance,
� Resistance to oxidation, acid attack, salt water and
environmental compatibility
• Many wood product have been introduced
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STRUCTURE OF
WOOD
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Growth Structures
Trees composed of three
discrete parts:
the roots,
the trunk and
the crown
which has a function to
perform
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Spread the root acts as
anchorage
Root:absorbs moisture and minerals from soil,
transferred via trunk to the crown
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• Trunk:� Provides rigidity and mechanical strength to maintain
the crown of leaves
� Provides transport system for moisture
� Store the sap and convert it to the form needed for
growth
• Crown� Capture as much energy from sunlight, to enable
photosynthesis process
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Bark
Relatively thin, rough, and
dense covering surround the
trunk
Cambium
A thin layer inside the bark, in
which the growth of wood takes
place continuously – eventually
results in growth ring (annual
ring)
Pith
Centre of the log, surrounded
by the annual rings, the number
of which approximately
represents the age of the tree
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• Heartwood
� Dead tissue/cell, provide mechanical support to the tree
� Darker, drier, and harder than the outer part
• Sapwood
� Contain living cell, density different with heartwood rather small
� Less durable and more permeable
� Impregnation with preservatives is easier
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Cell Structure of Wood
•Cells capable of carrying on the life process, make up
the structural element of wood
•Dry wood cells may be empty or partly filled with
deposits such as gums and resins
•Wood grain refers to the
arrangement of wood fibers
•Each cell has a hollow centre called
lumen or cell cavity
•Majority of wood cells are considerably elongated, commonly
called fibers
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CLASSIFICATION OF TIMBER
All commercial timbers classified into two groups, softwood and hardwood. The term hard and soft refer to botanical origin and not necessarily indicate the relative hardness or density of species
• Softwood
� Needle like leaves generally evergreen
• Hardwood
� Trees with broad leaves
� Most tropical hardwoods retain their leaves all year round
� In Malaysia hardwoods further classified into high density, medium and low
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High density Medium Low density
Balau 1000 kg/m3
Balau Merah 910
Bitis 1100
Chengal 980
Merbau
Resak
Giam
Keranji
Kapur 780kg/m3
Kasai
Kelat
Kempas 910
Keruing 830
Rengas
Simpoh
Tualang
Bintagor 710 kg/m3
Durian
Jelutong
Kedondong
Meranti 700 kg/m3
Pulai
Nyatoh 750
Ramin
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No Commercial
Name
Vernicular
Name
Botanical Name
1.
2.
3.
4.
Kempas
Merawan
Keruing
Nyatoh
Kempas
Merawan Jangkang
Keruing Bulu
Nyatoh Ketiau
Koompassia Malaccensis
Hopea nervosa
Dipterocarpus baudil
Ganua motleyana
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TAKE 5
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PHYSICAL PROPERTIES
Two most important physical properties are moisture
content and specific gravity
• Moisture Content
� Timber is hygroscopic, meaning it has affinity for water
in both liquid and vapour forms
� The ability to absorb or lose moisture depends on the
environmental such as temperature and humidity
� Moisture content = (weight of water/oven dry weight) x 100
� = mass of water present in a sample x 100
mass of sample when oven dry
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Department of Structures and Materials, Faculty of Civil
Engineering
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Moisture content of newly cut timber is between 30-200% depend on the timber types and season
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Fibre Saturation PointAs wood dries, water is driven off the cell cavities. A point is reached when the cavities contain only air and the cell walls are saturated with water which is called Fibre Saturation Point
Saturation point
vary 24-30%,
depend on
species
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Density and Specific gravity
� Green wood is heavier than dry wood. Green sapwood
is heavier than green heartwood, whereas dry
heartwood is heavier than dry sapwood
� Density is defined – mass or weight per unit volume.
Related to porosity, or proportion of voids
� Both density and volume of wood vary with moisture
content
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• Wood density = (ovendry weight)/(vol. of green wood)
• Specific gravity = Ws (ovendry weight)/wwV (density of water and vol. of green wood)
• The specific gravity determined in three conditions
� Green
� Air-dry
� Oven-dry
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Movements in Service
• Timber in service experience movement or strain due to applied loads, changes in moisture content or changes in temperature
• Coefficient of thermal expansion
� Across grain 30-70 x 10-6 K-1
� Along the grain 3-6 x 10-6 K-1
• The deformation response of timber to load is quite complex. Under low level of load applied for short duration, timber deforms elastically
• Time-dependent deformation is referred to as creep
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TAKE 5
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MECHANICAL PROPERTIES
Wood has unique, independent properties in the
three mutually perpendicular axis: Longitudinal,
tangential, and radial
• Modulus of Elasticity
� MOE in the longitudinal direction is the highest,
depending on species
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Compressive Strength
• Strength parallel to the grain much higher than that
perpendicular to the grain
• Column, post, and members of a truss are
subjected to axial loads parallel to the grain of the
wood
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Bending/Flexural Strength
• Timber has very good bending strength, because
of this and its lightness, it remains one of the
important flexural materials in light construction
• It is used for joist, rafters, headers and other
members that are subjected to bending moments
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Tensile Strength
• Tensile strength parallel to grain is about 2 or 4
times the compressive strength parallel to grain
� Knot greatly reduced the tensile strength
• Tensile strength perpendicular to the grain is very
small
� Knot, shakes, and checks further reduce tensile
strength perpendicular to the grain
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Shear Strength
• A flexural member is always subjected to shear
forces.
• The resulting horizontal shear stress at the neutral
axis of a wood beam may cause shear failure
• Defects such as knot and shakes decrease the area
under shear, and the shear strength of lumber with
these defects is lower than that of clear wood
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• MECHANICAL PROPERTIES increase linearly with specific gravity and decrease with increase in moisture content
• Beyond the fiber saturation point the mechanical properties remain independent of changes in moisture content
• Knot weaken the lumber, and at section containing knots, most mechanical properties are lower than those in clear wood
� The effect of knots are more on axial tensile strength, less on bending strength, and very little on axial compressive strength
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GRADING
� Stress-grading rules incorporate a sorting criterion, a set of allowable values (mechanical properties for engineering design), and a grade name. Up to six allowable properties are associated with a stress grade
�Modulus of elasticity, tensile stress parallel to grain, compressive stress parallel to grain, compressive stress perpendicular to grain, shear stress parallel to grain and bending stress
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• Physical and mechanical properties of wood differ
from species to species and also within each
species
� It is influenced by condition of growth (climate,
density of surrounding forest, character of soil,
moisture content and defect)
• To simplify the structural design the species are
divided into several group
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Group
A
Group
B
Group
C
Group
DLasak
semulajadi
Need
curing
Lasak
semulajadi
Need
curing
Need
curing
Need
curing
Balau
Bitis
Chengal
Keranji
Kandis
Kempas
Kulim
Tualang
Balau
Merah
Merbau
Resak
Bekak
Berangan
Dedali
Kapur
Bayur
Bitangor
Durian
Kayu getah
Ara
Damar minyak
Geronggang
Jelutong
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FACTORS AFFECTING
STRENGTH
Moisture Content
• Main factor that affect strength
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Density and Specific Density
• This properties can be taken as an index to the
strength of timber.
• High strength timber means that the cell wall is
thick and the strength of timber is depend on the
thickness of the cell wall
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Defect
• The existence of defect such as knot, sloping
grain, cracks, fissures, wane, distortions, insect
damage, fungal decay, reaction wood and sapwood
will contribute to the weakness of timber
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ADVANTAGE AND
DISADVANTAGE OF TIMBER
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Thank You