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Characteristics of timber - Relationship to properties

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1 Characteristics of timber - Relationship to properties
Maximise performance of timber Intuitive understanding of timber behaviour This lecture introduces some wood science concepts that assist in the explanation of timber properties and performance. In order to be able to use and detail timber effectively, both architects and engineers need to understand the physical characteristics of timber and how they affect its performance and behaviour. These can be learned, but if a practitioner has even an elementary understanding of wood structure, often these limitations can be understood rather than just known. An elementary understanding of wood science can develop an intuitive understanding of wood properties that will allow a designer to maximise the performance of the timber and minimise the impact of limitations. It will underpin the responsible use of this building material. Necessarily, some broad generalisations will be made in this lecture. (Whole courses at university level are aimed at the material we will cover in one session.) Some references to good wood science texts are included at the front of these notes if you want to pursue this fascinating area in more depth than these notes allow. Some references are made in the notes to sections of HB Timber Design Handbook. In these sections you will be able to find some more explanatory information on the topics covered. This slide points out that the properties and behaviour of timber can all be understood in terms of the wood microstructure and the way it interacts with its environment. Perhaps the best place to start explaining this slide is at the bottom: If we want to maximise the performance of timber, we MUST understand the way it behaves. We also need to know about its physical properties. To effectively understand behaviour and know its properties, we need to understand a little about wood science. Knowledge of Properties and Performance Physiology of timber fibres, cells, grain, growth rings Moisture content emc and shrinkage Creep and duration of load effects Natural growth characteristics Structural properties of timber

2 Performance of Timber Appearance/Structural/Durability
Grain and colour Feature Dimensional stability & emc% Structural Essential e.g. strength and stiffness Utility e.g. dimensional stability - shrinkage/emc Straightness - bow, spring, cup and twist Principally, designers are interested in an appropriate level of performance. The type and level of performance varies for the applications of timber in a building. It is important that a designer understands what performance is required for each and every material to be used in the structure. Performance requirements (explored in session 1) will lead to properties that are important for inclusion in the specification. Generally for timber, this can be accomplished by selecting an appropriate grade, species and treatment for the timber. (Grading and specification is covered in session 3). Durability Biological hazards Natural resistance / treatment

3 Microstructure of Timber
Grain direction This is a picture of the cell structure of both softwood and hardwood timber. The essential difference between hardwoods and softwoods is the presence of vessels in hardwoods. These are continuous pipes running the length of the tree and serve as conduits for water and nutrients in the outer layers of wood in a growing tree. The actual cells in the softwood species have the same function as the vessels in hardwoods. The diagrams show a number of important features of wood. The cells constitute fibres and have the main direction of orientation in the longitudinal direction (parallel to the trunk of the tree). The individual cells are stuck together rather weakly but parcels of longitudinal cells are bound together with a few cells that run transverse to the longitudinal axis of the wood. These are called rays. Cells - fibres - mainly longitudinal orientation Bound together with rays Higher strength and stiffness parallel to grain

4 Cells Chemical components of wood - products of photosynthesis
Cellulose - network of molecules cell walls - microfibrils - fibrous Lignin - ‘gel’ - acts as bonding agent which ‘glues’ cells together Hemicellulose - cross linking - binds cellulose into the cell Cells are composed of three main chemicals; cellulose - a long chain polymer with the chains principally aligned with the long axis of the cell. In some parts of the cell wall, the cellulose is spirally wound. lignin - glass-like substance that serves as the filler - like the resin in fibreglass. It is a brown colour. hemicellulose is a cellulose-type molecule, but not as long as the main cellulose. It is a more mat-like molecule that is used to wrap up the cell. The main structural element is the cellulose which is very effective in transmitting tension or compression. The spiral winding of the cell gives buckling resistance to the cellulose that is parallel to the cell axis. Darker colourings are caused by lignin in cells with thin walls, or by extractives deposited in the empty cells. Cells are hollow tubes, so liquids can be taken into the wood along the cells much more easily than across the cells. In keeping water out of timber, we must focus on the ends of the cells or the end-grain of the wood. Likewise, treatment penetrates better along the cells allowing more effective penetration of the end zone. Straight fibres Spirally wound fibres

5 Direction of Strength and Stiffness
Direction of grain Strong parallel to grain & Stiff parallel to grain Weak perpendicular to grain

6 Moisture in Wood Cells 100% Growing tree Unseasoned timber free water
25% fibre saturation bound water Partially seasoned timber removed bound water Seasoned timber 15%

7 Moisture in Timber Moisture content (mc) = weight water weight wood
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 grain Moisture in timber: When timber is growing in trees, the cells are all full of sap, water and extractives. (In a live tree, there is actually a much bigger mass of water than wood!) Once the tree is felled, the moisture will start to move out of the wood. The first moisture to go is the free water - this is the water that is held in the voids of the cells (equivalent to the holes in the drinking straws). Once the moisture content is at fibre saturation point, (around 25%) all of the free water has gone and the remainder is bound water that is held in the structure of the cell walls. (On our model, this would be water held within the plastic of the drinking straws). Seasoning is the process of removing moisture from timber. Drying of timber to remove firstly the free moisture, then the bound moisture can be accomplished in several ways. The aim is to remove the moisture at a uniform rate through the piece, so that there is a minimal moisture content differential within the piece. Where a large moisture content differential exists, then degradation of the timber can occur, causing twisting, cupping, checking, spliting, collapse, etc. The most appropriate method of seasoning to use varies with species. However, the method used is generally not of importance to the user. However, the moisture content of the timber delivered is of importance in many applications. See Section HB108 - Timber Design Handbook for further information on methods of seasoning. Seasoning - process of removing moisture from timber Kiln drying (steam, LPG gas, solar) Air drying Other - chemical, microwave.

8 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 to / from wood Moisture in wood at equilibrium with moisture in atmosphere Typical emc Indoor air conditioned emc 8% - 10% Indoor heated emc 8% - 12% External - coastal emc 14% - 18% External - inland emc 10% - 15%

9 Specification of Moisture Content
Usually specified as Seasoned or Unseasoned 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 Moisture has a number of effects on timber : Dimensions - Wood that is taking up moisture swells, and wood that is drying shrinks. Most of the shrinkage will occur when the moisture content is below the fibre saturation point. This is the range of moisture contents that the wood will experience in normal service. Most of the shrinkage occurs normal to the grain. On the drinking straw model, removing water from the cell wall (drinking straw) will cause them to become thinner. The length of the straws will not change significantly, but the width of the bundle will. In fact, there are slightly different rates tangentially to radially. The rays, fibres that run through the wood from the centre of the log to the outside, and shown in the figure on slide 4M-2.4, stabilise the radial dimension a little. Strength - Water in the cell walls tends to make them a little slippery. It acts as a lubricant and allows the fibres to slide past each other a little easier. When we are measuring the fibre strength of wood, we would see a small reduction in strength of timber as moisture content increases. Stiffness - Water lubrication within the cells causes a small increase in elastic deflection under load (this is a decrease in stiffness), however it has a marked effect on creep. With only loose bonds between the cells, (in our model between the drinking straws) as load is applied, the straws rely on friction to stop them sliding over each other. Under long term loading, some sliding will occur. This is creep. Water in the cell walls increases the creep markedly by lubricating the slip interface. Creep is accelerated while water is moving into or out of the wood. Durability - fungi and termites need to have moist wood to thrive. Moist wood is more vulnerable to biological degradation. As well, the timber is protected by coatings and most paints and glues need dry wood to stick to. Moisture therefore can compromise the durability of timber by making conditions more favourable for biological attack and by reducing the effectiveness of protective coatings. Coatings - Finally, unless the protective coatings are flexible, the shrinkage and swelling of timber as moisture moves in and out causes deterioration of the coatings. Once a coating has been broken in places, the water can move into the timber. The undamaged coating can trap moisture in the timber. The swelling and shrinkage of the timber in response to changes in moisture content will cause rapid deterioration of the rest of the coating. Keep paint work in good condition or rapid deterioration of both the paint work and the timber will result. Effect of mc on properties: reducing mc causes an increase in strength stiffness (reduced creep) durability (reduced risk of attack) effectiveness of coatings dimensions A decrease in dimensions b & d (shrinkage mainly perp. to grain)

10 Shrinkage Radial shrinkage Tangential shrinkage Longitudinal shrinkage
Shrinkage from 25% to 12% Radiata Pine Rad 3.5% Tang 4% Hoop Pine Rad 2.5% Tang 3.5% Cypress Spotted Gum Rad 4.5% Tang 6% Karri 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% Softwood Hardwood Reduction in cell wall thickness Loss of moisture in range mc <25% Longitudinal shrinkage Reduction in cross-sectional dimensions

11 Detail to avoid problems
Shrinkage Large timber - large splits Restraint of seasoned timber - splits Specify correctly Detail to avoid problems

12 Natural growth characteristics
Application dictates selection of ‘clear’ (few characteristics) Clear Feature ‘feature’ (conspicuous characteristics) Natural Growth Characteristics Appearance enhanced - timber shows its character Strength decreased: dependent on size and location of characteristic Natural growth characteristics: Timber is a natural material, and each piece tells part of the story of the tree which produced it. Knots are caused by branches that are engulfed by the growing tree. Insect attack causes the tree to lay down extra wood to protect the tree giving a lump or ‘burl’, or may produce a resin or sap pocket. Fires cause damage that is often reflected in resin pockets, or may result in the tree trying to heal itself by laying down new wood that traps bark. These, and other experiences contribute to the character of each piece of wood, and makes each length of timber an individual. This individuality gives anything built from the wood a character of its own. Architects can choose to express this individuality by selecting ‘feature’ timber which displays these growth characteristics. On the other hand, many appearance grades limit the extent of natural growth characteristics and result in ‘clear’ timber. ‘Clear’ timber often has much higher strength as it is the deviation of grain around growth characteristics that cause a loss of strength. Knots - part of a branch extending from pith Checks - small surface cracks, often caused in drying Included bark - pockets with no wood fibres Others - pith, resin pockets, shakes...

13 Natural features in Sawn Timber
Knots contain weak juvenile wood, cause slope of edge Centre knots edge knot Arris knot Slope of grain Especially at edges - low strength perpendicular to grain decreases strength at angle to grain

14 Natural features and Properties
Knots discontinuity of grain at edge cause slope of grain at an edge often reduce strength and stiffness Included bark Slope of grain does not necessarily detract from appearance, but slope of grain at the edge of a piece of timber can significantly reduce its tensile or bending strength. Slope of grain can be caused by a slight bend in the tree which means that when a straight board is cut out of it, there is a bend in the grain. It can also be a local deviation in grain around a knot that may be in the timber, or just off the edge. Tensile forces parallel to the edge of the board whether from direct tension or flexure, are not parallel to the grain if there is slope of grain at the edge. The tension parallel to the edge causes a component of tension parallel to the grain and a component perpendicular to the grain. The tensile strength of timber perpendicular to the grain is about 1/40th of that parallel to the grain. (This was demonstrated using the straws previously.) The actual capacity of timber to resist tension at a slope of grain shows a significant reduction for slopes of grain of 10 degrees or more. Diagram Knots: Where a branch starts to grow out of the trunk of a tree, the newly laid-down wood cells in the trunk have to bend around the branch. This leads to a knot in the timber that is cut through the branch. Knots are a natural feature of timber and in many cases, enhance the appearance of the wood. They invariably reduce the strength of the timber due to two effects: The wood in the centre of the knot can have similar properties to juvenile wood with the low strength and durability associated with core wood. Effectively a knot introduces some loss of section. The grain in the wood around the knot is deflected around the knot leading to a localised slope of grain in the timber adjacent to the knot. There are many other growth characteristics of timber that can affect both the structural characteristics of the timber and its appearance. In general, these are all covered in the grading rules for timber. These are covered in session 3. . 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

15 Utility of Sawn Timber Producers minimise problems by
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 Processing of timber can produce some problems for both structural performance and appearance. Modern sawing techniques can minimise the problems by using cutting patterns that maximise the usefulness of the sawn product. Quality control measures in mills also aim to remove product with lower utility before it reaches the market place. Cutting a board from part of a pre-stressed tree can cause residual stresses across the board. On drying these stresses are relieved by movement of the initially straight board which produces cup, bow, spring or twist. In general, bow and cup are not severe problems for structural timber as they can be removed by use of appropriate construction techniques and building layouts. Spring is difficult to remove in most applications. In most appearance uses, significant deviation from straightness is unacceptable. It is important to accurately represent the required straightness in a specification. Most hidden structural timber really has little cause for restrictions on cup, bow or twist. Reasonably generous amounts of spring may be tolerated in some circumstances. Many appearance applications require tight control over all of the utility limitations on timber. This slide has some diagrams of boards with cup, bow, spring and twist. I have found it useful to cut out a rectangular strip of cardboard and bend it out the front of the class to show the effect of these. It is impossible to bend even a piece of cardboard so that it has spring. I generally cut out a different curved piece to model spring. Producers minimise problems by good cutting practice quality control - grading

16 Summary - Properties of Timber
Appearance: Colour, grain, features, smoothness of surface Reflect species, growth patterns, history of tree Specification: species, durability, appearance graded Utility: Dimensional stability (shrinkage, twist, bow, cup, spring), surface hardness Reflect stress changes with moisture loss, creep Specification: moisture content (best close to equilibrium moisture content) Structural: Strength (tension, compression, bending, shear, bearing) - stronger parallel to grain Stiffness (MoE) - stiffer parallel to grain Reflect grain structure, slope of grain, features in timber Specification: structural grade and species

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