properties of timber - picnic point high school...- moisture in cell cavities lost -> little...
TRANSCRIPT
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Properties of timber
Mahes Rajakaruna
FWPRDC/NTDCTIMBER EDUCATION PROGRAM
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Timber grows on trees
Radiata Pine plantation SA
Softwood
Single species
Native Karri forest WA
Hardwood
Diverse species
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Environmental issues
� In an untouched native forest� growing trees absorb CO2 from atmosphere� decaying trees release CO2 to the atmosphere
� Carbon in ≅ Carbon out� In managed forests, plantations
� growing trees absorb CO2 from atmosphere� mature trees are removed and new trees planted
� Carbon in > Carbon out
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Timber - environment� Growing and harvesting timber - beneficial to
the environment� Low energy of production� Carbon stored in timber away from atmospheric
cycle
Managed ForestsPlantations - single species farmed for timberNative forests managed for many uses
species conservation / biodiversitywater and timber harvestingeducation and recreation
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Hardwoods and Softwoods
� Hardwoods� broad leaf� generally higher
densities� often dark in colour
� Softwoods� needle-like leaves� generally lower
densities� often light in colour
Native Australian hardwood includemountain ash
brown boxspotted gum
jarrah
Native Australian hardwood includemountain ash
brown boxspotted gum
jarrah
common Australian s’wds includeradiata pinecypress pine
hoop pinedouglas fir
common Australian s’wds includeradiata pinecypress pine
hoop pinedouglas fir
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Cross section of a tree
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Sapwood and Tree Growth
� Sapwood -� transfers water and nutrients
from roots to leaves� less dense, lighter colour, cell
wall thickness increasing� susceptible to attack - treatment
required(Allowed in some species of
commercial timber)
■ Cambium -growth layer - new wood laid
down on outside of tree
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Heartwood■ Heartwood -
" cells no longer growing" extractives (growth by-products) can
provide protection from attack
■ Core - (juvenile wood)" oldest wood" at centre - contains pith" laid down when tree young" can be damaged by tree pre-stress during growth
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Performance of Timber� Appearance
� Grain and colour� Feature� Dimensional stability & emc%
StructuralEssential e.g. strength and stiffnessUtility e.g. dimensional stability
- shrinkage/emcStraightness - bow, spring, cup and twist
DurabilityBiological hazardsNatural resistance / treatment
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Microstructure of Timber
� Cells - fibres - mainly longitudinal orientation� Bound together with rays� Higher strength and stiffness parallel to grain� Vessels are only seen in hardwood
rays
rays
cells
fibres
vessels
hardwood
earlywood
rays
latewood
softwood
Graindirection
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Composition of cells
� cellulose� long chain polymer with cells principally
aligned with the axis of cell� very effective in transmitting tension or
compression
� lignin� serves as a bonding agent which glues
the cells together� hemicellulose
� binds cellulose to the cells
Straight fibres
Spirally wound fibres
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Direction of Strength &Stiffness
Direction of grain
Strong parallel to grain & Stiff parallel to grainStrong parallel to grain & Stiff parallel to grain
Weak perpendicular to grainWeak perpendicular to grain
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Moisture in Wood Cells
Unseasoned timber
removed bound water
Growingtree
free water
Seasonedtimber 15%
25%
fibre saturationbound water
Partially seasoned
timber
100%
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Moisture in Timber� Moisture content (mc)
� in growing tree - mc = 50% to > 100%� felled tree - mc begins to decrease
� Fibre saturation point (fsp) (~25%)� above fsp - moisture in cell cavities lost
� -> little change in dimension� below fsp - moisture in cell wall lost
� -> shrinkage perp to grainSeasoning - process of removing moisture from timber
Kiln drying (steam, LPG gas, solar)Air dryingOther - chemical, microwave.
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Equilibrium moisture content (emc)� Wet atmosphere / Dry wood → moisture moves to wood
� Dry Atmosphere / Wet wood → moisture moves from wood
� Wood at emc → no moisture movement
Indoor air conditioned 8% - 10%Indoor heated 8% - 12%External - coastal 14% - 18%External - inland 10% - 15%
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Seasoning
� Seasoned timber:� mc < 15% - close to emc indoors� will shrink & swell slightly as humidity changes
� Everything else:� sold as unseasoned timber� shrinks on further drying
Effect of mc on properties: reducing mc causes an increase in
� strength� stiffness (reduced creep)� durability (reduced risk of attack)� effectiveness of coatings
A decrease in dimensions b & d(shrinkage mainly perp. to grain)
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Longitudinalshrinkage
Tangential shrinkageRad
ialsh
rinka
ge
ShrinkageLoss of moisture in range mc <25%Loss of moisture in range mc <25%
Reduction in cell wall thicknessReduction in cell wall thickness
Shrinkage from 25% to 12%
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 Gum Rad 5% Tang 9%Grey Iron Bark Rad 5.5% Tang 7.5%Mountain Ash Rad 6.5% Tang 13%
Softw
ood
Har
dwoo
d
Reduction in cross-sectional dimensions
Reduction in cross-sectional dimensions
Specify correctlyDetail to avoid problems
Specify correctlyDetail to avoid problems
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Duration of Load - Creep
Creep (extra deformation under load)� recoverable - deformation slowly comes out after load
removed� irrecoverable - deformation remains after load
removed� function of moisture movement, magnitude & duration
of load� occurs at all loads� important when calculating deflections of beams
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Application dictates selection of‘clear’ vs ‘feature’
Natural growth characteristics
� Natural Growth Characteristics� Appearance enhanced - timber
shows its character� Strength decreased: dependent on
size and location of characteristic� Knots - part of a branch extending from
pith� Checks - small surface cracks, often
causedin drying
� Included bark - pockets with no wood fibres
Clear
Feature
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Natural features in Sawn Timber
Slope of grain
esp. at edges - low strength perp. to grain decreases strength at angle to grain
Knotscontain weak juvenile wood, cause slope of grain @ edge
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● Knots•discontinuity of grain at edge•cause slope of grain at an edge•often reduce strength and stiffness
● Gum and resin veins•less connection across grain•lower shear strength and stiffness
● Checks•less connection across grain•reduced shear strength and stiffness
● Pith and core wood•contain weak juvenile wood
Natural features and properties
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Processing Sawn Timber
� Trees are prestressed � Cutting boards from
trunks causes stress relief & slow change in shape of boards
� Bent trees can cause slope of grain in products
� Spring is a problem for all timber
cup
bow
twist
spring
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Evaluation of Structural Properties� Small clear specimens -
data only reflects wood fibre strength
� In-grade testing - commercial sized timber under realistic loading conditions
� Commercial timber� tensile strength < compression
strength� tensile failures - splintery,
brittle, sudden, loud� compression failures - wrinkles,
ductile, slow, quiet
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‘Viking Ship’ Lillehammer Norway
" Large span of trusses" Large glulam material used" Innovative connections" Design and fabrication team
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Timber Behaviour
Unique behaviour (strength parallel to grain)
Unique appearance (life, character, warmth)
Unique structure (parallel cells, growth)
Manufactured wood products• made from wood to maximise
the effect of high strengthparallel to grain
• can still be used for appearance applications
• quality control in manufacturecan give very reliable properties
Larger memberscan be manufacturedfrom many smaller
cross-section pieces
Limits on size of trees felledLimits on size of sawn timber
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Manufactured productsManufactured productsUsed for :
� large spans -deep beams� large cross-sections - large span truss
elements� panel members - bracing, architectural � large panel elements - floor, roof, ceiling
cladding (minor axis bending)Includes:
� LVL� Glulam� Plywood� Others
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LVL Laminated Veneer LumberLVL Laminated Veneer Lumber
� made from laminating thin sheets of wood � most laminates in longitudinal direction� very deep and long sections possible� high strength
Laminates prior togluing and pressing
Final LVL sheet
grain in all laminates
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Glulam Glued laminated timberGlulam Glued laminated timber
� made from gluing many small pieces together to form deep member
� Strength > individual pieces� potential weakness - finger joints
� opportunities for creative architectural use� curved, tapered beams horizontally
laminated beam
vertically laminated beam
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Glulam Portals Jellie Park Pool NZGlulam Portals Jellie Park Pool NZ
•Timber suited to swimming pool environments•Attention to connections
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Plywood� made by gluing and pressing thin laminates together to
form a sheet� grain in laminates in alternate directions - strength in two
directions
� select face material for appearance products & glues for environment, durability
� panels (decorative or bracing)� sheets - plate bending (flooring, formwork)� webs (I-beams, box beams)
Final plywood sheet
Seven laminates prior to gluing and pressing
face grain direction
grain in face laminate
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Other manufactured wood products
� Timber flanged steel web joists -lightweight, open webs give access for servicewebs by light tubes, solid rounds, corrugated sheets
I-beam - timber/LVL flanges, plywood webslightweight, suitable for udls on medium spans
Box beams - timber/LVL flanges, two plywood webssuitable for larger spans, torsionally stiff, can use decorative
plywood
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GradingGrading
Sorting of products into groups with similar characteristics and properties
Structural gradeStructural gradeAppearance gradeAppearance grade
Specify product by calling up a specific grade
SortingSorting CriteriaCriteria
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Appearance GradingAppearance Grading
� Classification of timber used for� furniture � joinery and architectural trim � decorative building products
� Grading rules� knot size & frequency (location unimportant)� splits, cracks, checks (size and frequency)� colour, grain uniformity� utility - want, wane, cup, bow,spring, twist
� Feature grade� makes a feature of natural characteristics eg knots
Criterion for sort is appearance of timber surface
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Lower strength
Lower stiffness
Higher strength
Higher stiffness
Structural GradingStructural Grading
� Used for classification of timber with defined structural properties - includes framing for housing
� May include appearance� Each grade associated with a suite of structural
properties - limit states� strength - characteristic value based on 5th %ile
(conservative - involves safety)� stiffness - characteristic value close to average -
realistic for most applications
Criterion for sort is estimated structural properties of timber
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Structural Grading MethodsStructural Grading MethodsStructural grading is based on correlation
between strength and a grading parameter� Visual stress grading - presence or absence of
natural characteristics� Machine stress grading - stiffness on flat (minor
axis MoE)� Proof grading - ability to take a proof load.
Each piece passed through machine, bending applied at about characteristic strength level. Broken pieces fail - unbroken ones pass
� Quality control - verification of grade properties by testing
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High Stress grade = High strength and stiffness
Timber Stress GradesTimber Stress Grades� Structurally graded products need to be
assigned properties for designers to use� limited number of grade descriptions � most versatile for sawn timber is F-grade
system
Strength Stiffness
Stress Grade Structural propertiesStructural properties
DeflectionBendingTension
CompressionShear
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Stress GradesStress Grades� Stress grade is assigned to a package of
timber� Stress grade gives structural properties
� Each piece in a package can be taken to have those properties
Timber stamped with Stress grade at gradingF-grades - commonly used with all grading methods and with plywoodMGP grades - used only with seasoned pine (machine graded)GL grades - used only for glulam membersLVL -each manufacturer assigns own grade & design properties
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Structural grading
Visual Stress Grading RulesStructural grading
Visual Stress Grading Rules� Very different to appearance grading-
� different characteristics are important� different sizes & location of each feature are
allowed� Each piece of timber examined by a
trained grader for characteristics known to decrease strength, stiffness or utility � knots - size, location, angle and position in
relation to others � slope of grain - on each face or edge� splits and checks
(Checks that may be important to appearance grading may not be important here)
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Visual Stress GradingVisual Stress GradingVisual grading
sorts intoStructural Grades
Visual Visual GradingGradingRulesRules
#1#1#2#2 #3#3 #4#4
#5#5
For each species, Tables assign an F-grade
to each of the structural grades.An F-grade may be
stamped on each piece.eg. For Structural # 3 seasoned jarrah,eg. For Structural # 3 seasoned jarrah,
F14 is stamped onto each piece.F14 is stamped onto each piece.For Structural # 3 seasoned For Structural # 3 seasoned (imported)(imported) SPF, SPF,
F7 is stamped on each piece.F7 is stamped on each piece.
F5F5
F11F11
F14F14
F17F17
F22F22
F8F8
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Relies on correlation between a measuredstructural property and all others
Minor axis E most commonly used � each piece tested in non-destructive bending
about minor axis over most of the length� minimum E value determines grade (MGP
grade) of whole piece� grade stamp often automatically
applied by the machine(visual check after gradingcan over-ride machine grade stamp to downgrade piece)
Machine stress gradingMachine stress grading
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Proof GradingProof GradingGrade verification technique
Timber initially sorted using a documented process
If too many pieces fail, If too many pieces fail, producer must producer must adjust initial adjust initial sorting processsorting process
broken pieces broken pieces rejectedrejected
FailFail
Pass
Pass
Grade verifiedGrade verifiedPieces sold as Pieces sold as Proof graded timberProof graded timber
Significant major axis bending load applied
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DurabilityBiological/ Physical
HazardsWeatheringFireChemical
TIMBERSpeciesNatural durability of heartwood
Maintenance ensures protection remains functional
Treatmentenhances durability
of sapwood
FungiTermites / borersMarine
Design Detailingminimisesexposureto hazards
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Biological/Physical Hazards�Weathering - protection from moisture, sun
•sealants (including paint)•shading (positioning in structure)
�Fire - protection•sprinkler systems (active)•insulation eg. fire-rated plasterboard (passive)•oversized members allow loss through charring (passive)
�Chemicals - timber performs well relative to steel/concrete•resistant to degradation for pH>2 and pH<10•softwoods marginally better than hardwoods
�Fungi�Termites/borers�Marine organisms
�Fungi�Termites/borers�Marine organisms
BiologicalHazard Classification
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Hazard LevelsHazardClass
Exposure ServiceConditions
BiologicalHazard
H1 Inside aboveground
Fully ProtectedWell ventilated
Borers Only
H2 Inside aboveground
Protected fromWetting nil leaching
Borers andTermites
H3 Outside aboveground
Moderate wettingand leaching
Decay borers& termites
H4 Outside inground
Severe wetting& leaching
Severe decay,borers & termites
H5 Ground contact Extreme wetting,leaching &/or criticaluse
Very severe decay,borers and termites
H6 Marine watersNth & Sth
Prolonged immersionin sea water
Marine woodborers and decay
H6SW Marine watersSth only
Prolonged immersionin sea water
Marine woodborers and decay
H ClassesH Classes
H1 least H1 least hazardoushazardous
H6 mostH6 mosthazardoushazardous
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Natural Durability (Heartwood)
Extractives andExtractives andgrowth growth
characteristicscharacteristicsaffect affect
natural durability natural durability of timber speciesof timber species
Class Durability Species1 Highly
durableIronbarkTallowwoodCypressTurpentineForest red gum
2 Durable Spotted gumBlackbuttWestern cedarRiver red gumStringy bark (yellow & white)
3 Moderatelydurable
Brush boxRose/flooded gumSydney blue gumSilver topped stringybark
4 Non-durable Douglas firHoop pineRadiata pineMountain ash/ Tasmanian oakunidentified timbers
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Long-term Performance of Timber
Can match natural durability with Can match natural durability with environmental hazard to estimate longenvironmental hazard to estimate long--term term performance of untreated timber heartwoodperformance of untreated timber heartwood
NaturalDurability
Class
Heartwood Service Life (years)
H1 FullyProtected
H3 Aboveexposed
H5 InGround
Class 1 50+ 50+ 25+
Class 2 50+ 30 15 - 25
Class 3 50+ 15 8 - 15
Class 4 50+ 5 - 8 < 5
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TREATMENTSAll treatments
use and dispose of timber correctlypenetration of heartwood is limitedbetter penetration through ends
LOSPtimber mc unchangeddimensional stability
•clear•limited water repellency•difficult to glue
CCA‘wet’ process
timber needs to be redried
•greenish appearance
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TREATMENTS - penetration
thin treated heartwood
case
Treated sawn section
treated sapwood
Treated round log
treated sapwood
Good penetration of sapwoodPoor penetration of most heartwood
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DESIGN PRINCIPLES FOR DURABILITY
� Isolate timber from insects� for termites, chemical barriers /
physical barriers between ground and timber� Protect timber from water
� fungi require moisture content of wood >20%� swelling, shrinkage can cause cracks
� Protect timber from heat� direct sunlight can cause
excessive drying, shrinkage� direct sunlight breaks down wood / cellulose
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ISOLATE TIMBER FROM PESTS
� Barriers� no direct path from
ground to timber � chemical / physical ant caps
Chemicalblanket
Termimesh, granoguard ant caps, etc
● Inspect■ barriers can be penetrated or bridged■ inspect barrier for signs of penetration (galleries)■ inspect wood for signs of infestation (galleries)
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SLAB BARRIERSmonolithic slabs and mesh
Monolithic slab
Mesh
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SLAB BARRIERSgraded stone and reticulated chemicals
Graded stone
Reticulatedchemicalbarrier
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PROTECT TIMBER FROM WATER
� Protect timber from rain� good paint/Copper
Napthanate seal� caps� roofs/verandahs
� Protect important timber from ponded water
� Elevate timber posts above water
� Detail connections to avoid trapped water
Replaceable capping rail/
sloping
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PROTECT TIMBER FROM WATER
✔✔ ✘✘✔✔✘✘
simple detailing is better!
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PROTECT TIMBER FROM WEATHERINGHot sun can dry timber,
cause shrinkage, cracking.� use top quality
- paints, stains, water repellents� opaque systems work best
light colours cooler, less weathering� repair & maintain paints, seals regularly
� clear finish stains : 3 - 5 year maintenance� light colour paints : 7 - 10 year maintenance
� see detailing for protection from water
PROTECTED
30ºEXPOSED
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Glues - must use/specify exterior glues
Grade - fewest inclusions,splits, nicks in upper surface
EXPOSED MEMBERS
Orientation - heart towards bottomBottomBottomTopTop
✘✘
✘✘
Moisture content-•protect from moisture if possible•seasoned products close to equilibrium for most of Aust.•allow for shrinkage when using unseasoned timber
Size - minimise surface area