KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Characteristics we can see Wood s structure and variability Part A The macroscopic structure of wood Characteristics we can readily see 0.0 title ! www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Content Constraints • Structure from the cycle of growth (Part A) • Variation in wood properties (Part B) • There are numerous species of trees, growing in a vast array of locations and climate • Given this genetic and material diversity, only broad issues can be covered here • There will always be exceptions to the items discussed – Species, climate and age effects – Knots and other impacts of natural action on structure and character Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Major botanical grouping of species Major botanical grouping of species • Gymnosperms, softwoods, species of trees bearing seeds in cones, • Angiosperms, hardwoods, flowering trees bearing covered seeds – needle-like leaves – generally lower densities – often light in colour – broad leaf – generally higher densities – often dark in colour • Common Australian softwoods include • Common Australian hardwoods include: – radiata pine – cypress pine – hoop pine – All eucalypts • Jarrah, messmate, mountain ash, yellow gum, blue gum – brown box – spotted gum Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au 1! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Wood s structure and properties • Wood is the natural material drawn from the trunks of trees • Its structure and properties relate directly to the tree s: Structure from the cycle of growth – cycle of growth – species – surrounding environment, particularly climate – age Tree growth and the character of wood www.csaw.utas.edu.au www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Tree growth Tree Growth • Seeds germinate • Seedling establishes • New wood forms on the growing tip and the outside of stem • The young tree grows tall quickly to compete for light • Further wood layers form on the outside of the stem • As the tree ages, parts of the wood mature • New wood on outside of tree – oldest wood on inside of tree – youngest wood on outside – diameter largest at base – one ring (layer) per year • Trees grows towards light – trunk straight – lower branches die – leaving small knots in wood • Bark protects wood from damage – tree sheds bark each year Drawings by Bruce Greaves Tree growth and the character of wood www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Zones of wood Zones of wood Bark • A protection layer New bark is made continually with portions falling off each year Bark Pith / heart Pith / heart • cells from the original sapling or growing tip Cambium • growth cells that created wood cells on the inside face and bark cells on the outside Cambium Sapwood Sapwood • newest wood on the outside areas of the stem that transports nutrients between the root and the leaves Heartwood Heartwood • older wood cells filled with extractives and other material to increase durability and support for the tree Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au 2! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Zones of wood: pith Pith / heart / juvenile wood - material Pith / heart / juvenile wood • The pith or heart is found near the centre of the stem • Made up of soft spongy tissue from the original sapling or growing tip, its function is nutrient storage and transport • Generally, its quality is much lower than mature wood, and this complicates both production and design Wood s structure and variability Compared to mature wood, pith or heart material often: • has shorter length cells and fewer latewood cells • has a tendency for spiral grain • in hardwoods, shrinks and moves excessively during drying • is only 85 to 90% as dense • is only 50 to 70% as strong or stiff • is relatively low durability www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Pith / heart / juvenile wood - material Juvenile wood www.csaw.utas.edu.au Shrinkage of heart wood in floor joists Effect of juvenile wood on physical and mechanical properties: a. Juvenile core located in the interior of the tree bole b. properties that increase from juvenile to mature wood c. properties that decrease from juvenile to mature wood Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Zones of wood: cambium Zones of wood: bark Cambium • A layer of cells capable of repeated division that remains active throughout the tree life • Located at the periphery of the wood, it produces phloem (bark) on one side of tree and sapwood (new wood) on the other • This increases the diameter of the tree and pushes the cambium progressively outward Bark • an outer protective layer on the wood consisting of an outer bark and inner layer called phloem • The phloem conveys nutrients from the leaves to the various parts of the tree • As the tree’s diameter increases, the bark has to stretch, fracture, peel or fall off Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au www.csaw.utas.edu.au 3! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Bark patterns Zones of wood: sapwood Sapwood • The more recently formed wood on the outside areas of the stem • Living and open cells and vessels that transports water and nutrients between the root and the leaves • Containing starches but lacking protective extractives, sapwood is attractive to insects & fungi • It is always classed as low durability • Cell walls increasing in thickness as tree grows • They are often pale in colour and distinct, especially in hardwoods Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Zones of wood: heartwood Heartwood growth Heartwood • The heartwood is dead, extractive-filled cells whose main function is mechanical support to the tree • The tree needs to maintain both adequate conducting tissue and adequate support as it grows • As new wood is formed each year, some part of the inner sapwood becomes redundant for water transport and is converted into non-conducting heartwood • So, the outer heartwood boundary continually moving outwards Wood s structure and variability www.csaw.utas.edu.au • Young vigorously growing trees may contain little or no heartwood and have a broad sapwood band • In some species, heartwood may not develop for many seasons • Once heartwood starts to form, it develops at a greater rate than diameter growth Sapwood band in 19yo E globulus www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Extractives and their effects Heartwood colour • Extractives are the products of decomposition of starches, sugars, fats, waxes, oils, gums, tannins and aromatics, stored in the heartwood • Toxic extractives impart decay and insect resistance This is the case in redwood, Huon pine and most cedars • Aromatic extractives impart a fragrance • Other extractives strengthen heartwood while making it difficult to dry and penetrate with chemical preservatives • Significant amount of extractives can make the heartwood heavier than sapwood Blackwood, Acacia melanoxylon www.csaw.utas.edu.au Darwin Stringybark, E tetradonta • Hardwoods have a wider range of heartwood colouration than softwood • Generally sapwood is distinctly lighter in colour – In some woods, heartwood and sapwood show no colour differences Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au 4! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Comparing sapwood and heartwood • Compared to sapwood, heartwood is generally: – heavier, – stronger, – more highly figured, and – more resistant to decay • Being wood formed in a more mature tree, sapwood in young trees can be denser and stronger than adjacent heartwood – more on this later Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Growth generates directionality The macroscopic structure of wood Characteristics we can readily see softwood rad longitudinal tan ge nti al Wood s structure and variability Part B ial • Wood is anisotropic Its properties vary: – radially: across the wood’s growth sequence – tangentially: around the wood’s growth sequence – longitudinally - along the direction of the wood 0.0 title ! www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Impact of directionality Directionality and strength • Wood directionality is arguably the largest single consideration in timber design and production, influencing: – Strength, – Appearance, – Other primary design considerations – Shrinkage Along the grain: 10 units • from green to EMC and movement in service – Primary production constraints Across the grain: > unit Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au 5! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Directionality and appearance Directionality and shrinkage Timber shrinks and moves at different rates in its principal directions Quarter sawn or quarter cut Species relatively straight, even and parallel grain running along the board Radial% Tang.% Softwood Back sawn or crown cut Radiata pine 3.5 Hoop pine 2.5 3.5 Cypress 3.5 Hardwood mixed grain direction that can include cathedrals, crowns or swirls along the face of the board Spotted gum 4.5 Karri 4.5 10 Sydney blue gum Grey Ironbark 5.5 7.5 Mountain ash 6.5 13 The results of poor installation practice and predictable expansion in a domestic timber floor Shrinkage from 25% to 12% MC Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Growth generates stress Growth and stress Growth for height is a key evolutionary trait www.csaw.utas.edu.au As thin cantilevered columns, trees generate peripheral tensile stresses to resist horizontal forces This is balanced in the standing tree by an inner compressive reaction Longitudinal strain profile in log originating from growth stress, showing pattern for different sized logs (Waugh 2000 from Kubler 1959) Tree growth and the character of wood www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Consequences of growth stress Reaction wood www.csaw.utas.edu.au From Bamber, 2001 • Trees produce reaction wood in response to eccentricity in loads on the stem: wind, branches or slope • Reaction wood in: Sawmill waste: 12-year-old E nitens thinned and pruned Tree growth and the character of wood www.csaw.utas.edu.au – hardwoods is tension wood: Pulling against the load – softwoods is compression wood: Pushing against the load Wood s structure and variability www.csaw.utas.edu.au 6! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Uneven growth and reactions Uneven growth and reactions • Softwoods produce compression wood on the underside of eccentric branches or leans to “push” them back in line • Compression wood is expressed as – An eccentric pith on the upper side of the stem – exceptionally wide growth rings on the lower compression side of the stem • These rings contain a higher proportion of latewood • The cells contains more lignin, and under a microscope, the cells appear more rounded than rectangular • Compression wood has greater longitudinal shrinkage compared to normal wood (1-2% vs 0.1-0.2%) • Usually denser (up to 40% higher at times), its strength is about the same as normal wood • Opposite wood refers to wood formed on the opposite side of the stem from compression wood Drawings by Rob Whiltshire Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Eccentric pith location at the base of a harvested pine log Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Uneven growth and reactions • Hardwoods produce tension wood on the upper side to “pull” them back • Stems with tension wood are often elliptical • Like compression wood, tension wood: – has wider rings compared to rings on the opposite side of the stem – shrinks excessively along the grain – has strength properties inferior to normal wood of similar density – is not easy to detect and difficult to discard during manufacture • Under a microscope, tension wood cells have very thick walls and very small lumens • Thick and loosely attached secondary walls manifests as fussy and woolly surfaces when the timber is cut, especially when processed green Wood s structure and variability Species, climate, and age effects Drawings by Rob Whiltshire www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Species effects Species effects hardwood vessels fibres earlywoo d rays softwood cells! latewood rays Red gum, 860 kg/m3 Radiata pine, 430 kg/m3 • Each species has a unique genetic make-up • Aspects of material properties vary: – between species groups • softwood/hardwood, collapse prone/non-collapse prone – within species groups Wood s structure and variability www.csaw.utas.edu.au rays Hardwood • Mainly broad leafed • Vessels transport nutrients • The cell structure in the heartwood is generally closed • Most hardwoods have relatively thick cell walls and are darker in colour Tree growth and the character of wood Softwood • Mainly conifers with needle-like leaves • The cell structure is relatively open & all sapwood cells transport nutrients • Most softwoods have relatively thin cell walls and are lighter in colour www.csaw.utas.edu.au 7! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Climate effects: growth rings Climate effects: growth rings • The availability of light, nutrients and water influences cell formation in the cambium • As days become shorter and colder or moisture levels declines, cell division in the cambium slows and can ceases During this slowdown, latewood forms – higher density wood of smaller cells with thicker walls and smaller lumens • Climatic fluctuations such as summer/winter or wet/dry season affect the tree s growth rate • As seasons change and days lengthen and become warmer or moisture become available, cell formation in the cambium increases and earlywood forms – lower density wood of often bigger cells with thinner walls and bigger lumens Photos: Geoff Boughton Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Climate effects: growth rings Age effects: property development zone of unchanging wood properties zone of wood property change www.swst.org/teach/set2/redwoo~1.jpg • In a true growth ring, the latewood to earlywood transition is abrupt • When normal growth is interrupted by drought or late frost, false rings may occur with two or more rings being formed in a single year • False rings tend to have a gradual transition of the cell size on both sides (age of deposited wood – years) (age of log – years) Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood s structure and variability www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Knots Knots and other features The marks of growth, insects and living on the wood • Knots are parts of a branch enclosed within the wood of the stem • If the branch is living, the cambium of the stem and branch are continuous and an intergrown or tight knot resulting • If the branch has died, the cambium is not continuous and an encased or loose knot result • The appearance of the knot depends on the direction of cut through the included branch • If a cut is made along the axis of the original branch, the knot appears as a spiked knot Encased knots 0.0 title ! www.csaw.utas.edu.au Wood s structure and variability www.csaw.utas.edu.au 8! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Occluded knots Insects • If living branches are pruned, the branch stub will occlude (grow over) • If a dead branch is pruned, the branch stub may be drawn out by the growing tree leaving a resin-tract Occluded branch • Various insects live in the bark and stem, leaving trails in the wood The appearance of the tracks varies with insect and the direction of cut Limb trace Wood s structure and variability Kino or resin tract www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Fire, shakes, impacts and other damage Summary www.csaw.utas.edu.au • There are numerous species of trees, growing in a vast array of locations and climate • Wood structure and properties relate directly to the tree s cycle of growth; species; surrounding environment, particularly the climate; and age • Wood is anisotropic Its properties vary radially, tangentially and longitudinally • Each species has a unique genetic make-up and aspects of material properties vary both within and between species groups • These variations can be critical to how we: – make wood products – use them in design Fire, impacts and wind damage can wound the stem The tree responds with sap or kino, especially in eucalypts Wood s structure and variability www.csaw.utas.edu.au Tree growth and the character of wood www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood s structure and variability www.csaw.utas.edu.au 9! ... Hardwoods have a wider range of heartwood colouration than softwood • Generally sapwood is distinctly lighter in colour – In some woods, heartwood and sapwood show no colour differences Wood. .. juvenile to mature wood c. properties that decrease from juvenile to mature wood Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its... distinct, especially in hardwoods Wood s structure and variability www.csaw.utas.edu.au Wood s structure and variability KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin