ce115-4-timber-1.ppt

1
Timber as Construction Material
Timber and Wood Products
Basic material properties and
characteristics.
By
Engr. Abbas Qureshi

2
Timber in history
• Timber is one of the
oldest building
materials known.
• It has been used in
buildings in most
parts of the world for
thousands of years.
• In low energy ages,
timber was the
natural material of
choice.
Melbourne’s Exhibition Building 1880

3
Timber’s potential today
• Timber is a light
construction material
with a high strength
to weight ratio.
• Economic and easy
to use, it is available
as a structural and
appearance product,
and suitable for a
wide range of uses.
Hardwood in a drying yard

4
The timber – American Wood Council
“Only one primary building material comes from
a renewable resource, cleans the air and water;
utilizes nearly 100% of its resource for products;
is the lowest of all in its energy requirements for
its manufacturing; creates fewer air and water
emissions than any of its alternatives; and is
totally reusable, recyclable and 100% .
And it has been increasing in US net reserves
since 1952, with growth exceeding harvest in the
US by more than 30%.”

5
The nature of timber
• Timber is a natural
product, drawn from
the wood in the
trunks of trees.
• Its character is
consistent with the
species of tree and
the form and growth
over time of the
wood in it.
Regrowth in a native forest

6
Definition
• Wood suitable for building or other engineering
purposes is called timber
– Standing timber – part of a living tree
– Rough timber – part of a felled tree
– Converted timber – sawn to various forms like
beams, battens and planks, etc.
– Dressed timber- sawn timber which has been
placed and worked to the required condition.
– Clear timber- timber free from defects.
– Structural timber- timber used in framing and load
bearing structures.

7
Classification of Trees
• Endogenous. Grow inwards.
– Grow in longitudinal fibrous mass like palm,
cane, bamboo, banana, etc. Except bamboo
unsuitable for engineering purpose.
• Exogenous. Grow outwards.
– Conifers. Evergreen trees yielding soft
wood. Deodar, chir, fir, kail, pine, etc.
Distinct annual rings, straight fibers, light
color, resinous and light weight
– Deciduous. Flat broad leaf trees yielding
hard wood. Oak, teak, shishum, poplar and
maple. Indistinct annual rings, non-resinous,
dark in color and heavy weight

8
Structure of Timber
• Tree – Crown, Trunk, Roots
• Cross section of a tree
– Bark (outer and inner) – outermost protective layer
of cells and woody fibers that splits and peels off
– Cambium – outermost one ring between the bark
and sap wood not yet converted into wood
– Sap wood – outer annual rings that transmit sap
from root to branches
– Heart wood – innermost rings surrounding pith
– Pith or Medula – first formed portion of stem of tree,
initially to convey sap
– Annual rings
– Medullary rays – thin horizontal veins from bark to
pith to carry sap inwards

9
Tree growth
• Spring season
– Salt and water solution sucked by the roots
– Under sunlight absorbs CO2 from air
– Viscous solution is called sap
• Autumn season
– Sap descends under the bark and leaves a
thick layer
– Sap layer transformed into wood as
cambium layer
– New layer added each year as annual ring
– Sap carried through medullary rays from
bark to interior

10
Tree growth
• New wood on outside of tree
– oldest wood on the inside
– youngest wood on the outside
– diameter largest at the base
– one ring (layer) per growing season
• Tree in forest grows toward light
– trunk is straight
– lower branches die
– leaving small knots in wood
• Bark protects wood from damage
– the tree sheds bark each year

11
Major parts of tree

12
Major parts of tree

13

14
Production of wood
Cambium - growth cells
• wood cells created on the inside
• bark cells created on the outside
Sapwood - newest wood
• on the outside of tree (~ 1-3 cm)
• takes nutrients from root to leaves
Heartwood - older wood
• cells closed - can’t pass nutrients
• storage for waste - extractives
Pith - the start of growth in the tree
• the original sapling

15
Heartwood and Softwood

16
Hard and Soft Wood Characteristics
Property Soft Wood Hard Wood
• Color Lighter Darker
• Growth Faster Slower
• Weight Lighter Heavier
• Density Low High
• Annual Rings Distinct Indistinct
• Heart wood & Indistinguishable distinguishable
Sap wood
• Strength Strong along grains Strong along
& across grains
• Conversion Easy Difficult
• Resinous Exists in pores Does not exist
• Examples Chir, Fir, Conifer Teak, sal, shisham

17
Characteristics of Good Timber
• Obtained from heartwood area of tree and free from
sap
• Should have straight and close fibers
• Dark uniform color with uniform texture
• When struck sonorous ringing sound is produced
• Narrow annular rings, closer the rings greater is the
strength
• Compact medullary rays
• Sweet smell and a shining fresh cut surface
• No clogging of saw teeth during sawing
• Free from the defects like dead knots, shakes etc
• Heavy weight
• No woolliness at fresh cut surface
• Bright and smooth surface when planed

18
Defects in Timber – during growth
• Shakes – separations between annual rings
– Star shake – radial splits or cracks wider on surface and
narrow inwards, due to severe heat and frost action
– Heart shake – radial splits in heartwood due to shrinkage
– Cup shake – curved split among annual rings. Also ring shake
• Twisted fibers – wind turning the trunk and twisting
fibers
• Rind gall – growth of sapwood layer over wounds
• Upsets – crushing of transverse fibers due to winds
• Foxiness – yellow or red stains as sign of decay
• Doatiness – speckled stain indicating start of decay
due to over maturity or bad ventilation
• Druxiness – Whitish spot or streaks indicating early
decay
• Coarse grains – wide annual rings due to rapid growth

19
Defects in Timber(Figs.)
Shortcut to 2.lnk
Shortcut to 2.lnk
Shortcut to 2.lnk
Knots

20
Defects in Timber(figs).
Cup shakes Rind galls

21
Defects in Timber – knots
• Knots – root of small branches buried in mother
branch annual rings. This modifies the tissues in
elliptical or concentric circles . Knot disturbs the
homogeneity , strength and workability of wood.
– Dead or loose knot - separable
– Live or sound knot – inseparable
• Size based classification
– Pin knot – under 12 mm
– Small knot – 12 to 20 mm
– Medium knot – 20 to 40 mm
– Large knot – over 40 mm
• Form based classification
– Round knot
– Spike knot

22
Live Knot and Dead Knot
Live Knot Dead Knot

23
Defects in Timber – knots
• Quality based classification
– Sound knot – hard and solid as surrounding wood
– Decayed knot – contains advance decay, softer
than surrounding wood
– Encased knot – annual rings fail to grow into fibers
of surrounding wood
– Tight knot – a knot so securely fastened that it
holds its position in finished product
• Occurrence based classification
– Single knot – wood fibers deflect around one knot
– Cluster knot – wood fibers deflect around two or
more knots as one unit
– Branch knot – two or more knots radiating from one
common center

24
Defects in Timber

25
Defects in Timber

26
Defects in Timber - Diseases & Decay
• Diseases
– Dry rot – fungus turns timber to dry powder
– Wet rot – decay due to alternate wetting and drying
• Decay
– Decay due to fungal and bacterial attack
– Damage due to insects
• Termites, white ants
• Beetles – powder post beetles, long horn beetles,
ambrosia beetle, furniture beetles, death watch beetles
• Carpenter ants
– Damage due to rodents

27
Seasoning of Timber
• Seasoning – extraction of moist sap under
controlled conditions (as nearly as possible at
a uniform rate) from all parts of timber, and to
leave inextricable moisture uniformly
distributed throughout
• Irregular drying causes irregular shrinkage
leading to warping and shakes formation
• Seasoning methods
– Air or natural seasoning – timber stacked in
ventilated shady place. Slow process takes six
months
– Kiln or artificial seasoning – steam chamber with
controllable temperature and humidity. Rapid 4-5
days.

28
Seasoning of Timber(Artificial)
• Water Seasoning :- The logs of wood are
kept in running water. The sap, sugur, and
gum are leached out. The logs are then kept
out in air to dry.
• Chemical or Salt seasoning:- A water +
Chemical( salt, urea) is applied to the timber
for seasoning. It reduces the moisture due to
the difference in vapour pressure of chemical
and pure water. It reduces the internal stress
which are the causes of defects as checks.

29
Seasoning of Timber(Artificial)
• Electric Seasoning:- The logs are placed in
such a way that their two ends touch the
electrodes. Current is passed , wood resists
the flow of current. During this heat is
generated which results in drying of timber.

30
Seasoning of Timber
Air /Natural seasoning Kiln Seasoning

31
Seasoning of Timber
Air Seasoning
• Slow process
• Simple and economical
• Difficult to reduce
moisture content below
15 to 18 %
• Timber more prone to
fungi and insect attack
• More stacking space
needed
• Stronger timber is
obtained
Kiln Seasoning
• Quick process
• Technical and
expensive
• Moisture reduced to any
desired level
• Timber less prone to
fungi and insect attack
• Less stacking space
needed
• Timber obtained is
comparatively weaker

32
Seasoning of Timber (Objectives)
• Reduce the shrinkage and warping after
placement in structure.
• Increase strength, durability, and workability.
• Reduce its tendency to split and decay.
• Make the timber suitable painting.
• Reduce the weight of timber.

33
Seasoning of Timber
• Preventing drying of logs
– Felled tree roughly squared and sawn
– Ends sealed to prevent rapid evaporation of
moisture
– Logs kept completely submerged in running water
• Stacking of square logs
– Stacked under shade, free from weeds and debris
– One and nine stacking method
– Close crib stacking method - logs, poles, planks
– Open crib stacking method - logs, poles, planks

34
Moisture and drying in wood cells
removed
bound
water
Seasoned
timber
15% MC
100% MC
Unseasoned
timber
Growing
tree
free water
25% MC
fibre saturation
bound water
Partially
seasoned
timber

35
Benefits of drying
• Drying timber
increases its:
– strength;
– stiffness;
– stability
– durability; &
– ease of fastening.
• It is ideally dried to a
moisture content in
equilibrium with its
service environment.
Checking the moisture content

36
Timber Preservation
• Preservation is protection of timber from
attack of insects and internal decay
• Perfect seasoning is the best form of
timber preservation
• Charring – ground embedded ends of
posts. Quenching post ends in water
after charring in wood fire. Prevents dry
rot and attack of worms
• Tarring – coating with tar or pitch

37
Timber Preservation
• Painting – application of paint coat on dry timber to
provide good appearance and protection from attack
by white ants
• Creosoting – application of creosote oil on railway
sleepers, piles and electricity poles. Bad color, bad
smell, stains plaster
• Wolman salt – creosote and salt in water. NaF, ZnCl,
MgSiF, CuSO4
• Ascu treatment – application of powder chemicals
arsenic pentaoxide, blue vitriol, potassium dichromate
• Fire proofing – timber made fire resistance by soaking
in NH4SO4, NH3Cl, NH4PO3, NaAs, ZnCl

38
Tests of Timber
• Specific gravity
• Volumetric shrinkage
• Radial and tangential shrinkage
• Static bending strength
– One point loading test
– Two point loading test
• Impact bending strength

39
Tests of Timber
• Compressive strength
– Parallel to grain
– Perpendicular to grain
• Hardness under static indentation test
• Shear strength test
• Tensile strength test
– Parallel to grain
– Perpendicular to grain

40
Tests of Timber
• Cleavage strength test
• Brittleness test
– Izod impact test
– Charpy impact test
• Torsional strength test
• Moisture content test
– Oven drying method
– Electrical moisture meter method
– Distillation method

41
Tests of Timber
• Moisture content test
– Specimen 5 cm x 5 cm x 2.5 cm
– Specimen weighed fresh as W0
– Oven dried at 103 ± 2° C till no weight
change noted. Dry weight W1
– Percentage of moisture content
= (W1 – W0) / W0 x 100

42
Tests of Timber
• Specific gravity test
– Specimen 5 cm x 5 cm x 15 cm
– Specimen weighed fresh as W1 correct to
0.001 gm
– Specimen volume measured to 0.01 cm3
– Specimen end coated with paraffin wax and
air seasoned to 12% moisture content
– Weight and volume again noted as Wr & Vr
– Oven dried at 103 ± 2° C till weight constant
– Weight and volume noted as W0 and V0

43
Tests of Timber
• Specific gravity test
– Specific gravity at test = W1/V1
– Standard specific gravity = W0/V1
– Oven dry specific gravity = W0/V0
– Moisture content r % = (Wr-W0) / W0 x 100

44
Wood Products
• Veneers
• Plywood
• Lamina Boards
• Fiber Boards
• Block Boards
• Batten Boards

45
Wood Products
• Veneers
– Thin sheets of wood peeled off, sliced or
sawn from a log of wood having attractive or
artistic grain arrangement
– Sheet thickness from 0.4 mm to 6 mm
– Walnut, teak and rosewood veneers used in
plywood, lamina boards and batten boards

46
Timber products: veneer
• Thin layers of solid
wood sliced or
peeled from a log
and dried.
• High quality material
is used as a
decorative finish.
• Lower grade material
is used to make
plywood and similar
products.
Veneer leaf

47
Wood Products
• Plywood
– Under pressure gluing together of veneer
plies in odd number to make 3 mm to 6 mm
thick sheet
– Grains of alternate layers at right angle to
each other
– Outer plies called face plies or faces
– Used for covering or paneling of walls,
doors, furniture and shuttering

48
Timber products: plywood
• Layers of veneer
glued together so
that the grain
direction alternates
between layers.
• Usually produced in
sheets.
• Very good in a
structure, as a lining
and as a flooring
surface.

49
Wood Products
• Plywood Advantages
– Better appearance
– Stronger than same thickness wood
– Elastic and may be bent to give any shape
– Not much affected by climatic changes
– Uniform strength in all directions
– Available in large sizes not possible in wood
– Almost negligible shrinkage and expansion
– Does not split under nails

50
Properties are variable
• Drying (seasoning) & shrinkage
• Strength & hardness
• Durability
• Appearance

51
Wood products
Lamin board

52
Wood products
Block boards

53
Wood products
Fiber board

54
Fiber board anchoring

55
Wood products
Hard board Melamine hard board

56
Timber shrinks as it dries
• It shrinks at different rates in
different directions.
• Once dry, it will expand &
contract to be in equilibrium
with changing local conditions.
Shrinkage from 12% to 25% MC
Radiata pine Rad 3.5% Tang 4%
Hoop pine Rad 2.5% Tang 3.5%
Cypress Rad 3.5% Tang 4%
Spotted gum Rad 4.5% Tang 6%
Karri Rad 4.5% Tang 10%
Sydney blue gumRad 5% Tang 9%
Grey Ironbark Rad 5.5% Tang 7.5%
Mountain ash Rad 6.5% Tang 13%
Tangential
shrinkage
Radial
shrinkage
Longitudinal
shrinkage

57
Shrinkage in sawn sections
• Tangential shrinkage
is the highest;
• So, the growth rings
tend to shorten as the
timber dries.

58
Timber dries at different rates
• Pines have a porous cell structure:
– They can be dried very quickly.
– Turnaround time from saw to store can
be as low as a week.
• Hardwoods have a non-porous cell
structure:
– The moisture must diffuse through the
cell wall.
– Drying can be very slow.
– It must be done carefully.

59
Grain, strength & stiffness
Direction of grain
Strong parallel to grain & stiff parallel to grain
Weak perpendicular to grain
Properties of the grain

60
Summary of characteristics
• Timber is a natural material
• Each piece is unique in fibre, texture and
colour
• Its strength is dependant on the
properties of the wood and the slope of
the grain
• It takes up and gives up moisture
• It expands and shrinks as it does so
• It deteriorates in line with exposure

61
Timber products
• Timber is available in a wide range of
products and species including:
– Solid sawn, moulded and laminated timber;
– Veneer;
– Plywood; and
– Wood panels.

62
Timber products: sawn
• Hardwood or
softwood.
• Solid timber cut from
a log and dried.
• It is a versatile
material used for
moulding, frames and
exposed structures.
• Size is restricted.

63
Timber products: mouldings
• Hardwood or
softwood.
• Solid seasoned
timber milled to a
wide variety of
shapes.

64
Timber products: glue laminated
• Small pieces of
timber glued
together to form a
larger element.
• Used as both a
structural and
finishing element.
• Size is limited only
by transport
capacity.
bench top
beam

65
Timber products: curved glulam
• Glue laminated
material can be
curved.
Curved bar

66
Timber products: wood panels
• Panels made from
wood or wood fibres
bound together with
glue, or other binder:
– Particleboard
– Medium Density
Fibreboard (MDF)
– Hardboard
– Core board
MDF sheet

67
Jointing
• Timber is a highly
versatile material
that can be shaped
and joined easily.
Mortice and tenon Housed
Half housed

68
Jointing methods
Overlap Fixing blocks
Dowel
Biscuit

69
Conclusions
• Timber is a natural product.
• It has been used for centuries.
• Skilled use and understanding of the
material can yield excellent results.

ce115-4-timber-1.ppt

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