Wood & Timber Wood & Timber
CIVE 202 – Construction Materials
Wood & Timber
CIVE 202 – Construction Materials
Wood & Timber Introduction
Important points concerning wood: 1. Many kinds (>30,000 species of trees) 2. Wood is a composite material 3. Natural material (many flaws, imperfections) 4. Anisotropic (mechanical properties vary with respect to load orientation)
CIVE 202 – Construction Materials
Wood & Timber Introduction
Terminology: Wood - (Materials) refers to small clear specimens free of any macroscopic defects. Timber - (Structural), refers to sawn structural members and may contain a wide variety of macroscopic defects
‘Specific’ Values – per unit weight basis
CIVE 202 – Construction Materials
Wood & Timber Introduction
Three distinct parts: 1. Root System 2. Trunk 3. Crown
CIVE 202 – Construction Materials
Wood & Timber Introduction
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
1. Outer Bark - protection 2. Inner Bark - sap transport 3. Cambium - growth 4. Wood: Sapwood - food storage, moisture transport Heartwood - nonliving cells
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
5. Annual Rings: Springwood, earlywood - rapid growth, large cells, thin walls Summerwood, latewood - slow growth small cells growth, cells, thick walls 6. Pith - primary tissue, original growth
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure – Growth Rings Growth rings vary in width depending on species and site conditions. diti Rings formed during short or dry seasons are thinner than those formed when growing conditions are more favourable. Rings formed in shady conditions are usually thinner than those formed by the same species in sunny conditions. It is commonly believed that the age of a tree may be determined by counting these rings but this method can lead to errors because abnormal environmental conditions can cause a tree to produce multiple-growth increments or even prevent growth entirely for a period.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure – Growth Rings
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure – Growth Rings
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber Microstructure
Softwoods: S ft d Have H needle-like dl lik or scale-like l lik lleaves and d a relatively l ti l simple cell structure. Typically coniferous. Ex. Spruce, Pine, Fir, Cedar, Hemlock Hardwoods: Have broad leaves and a more complicated cell structure. Typically deciduous. Ex. Oak, Maple, Walnut, Ash, Birch, Elm Though hardwood tends to be denser, stronger and more resistant to decay, the vast majority of lumber is made from softwood for economic reasons.
CIVE 202 – Construction Materials
Wood & Timber Microstructure
Wood is primarily composed of hollow hollow, elongated elongated, spindle spindle-shaped shaped cells that are arranged parallel to each other along the trunk of a tree (similar to a bundle of drinking straws). When lumber and other products are cut from the tree, the characteristics of these fibrous cells and their arrangement affect such properties as strength and shrinkage as well as the grain pattern of the wood. The directionality of these long narrow cells result in a anisotropic materials, whose mechanical properties vary depending upon the direction in which they are tested.
CIVE 202 – Construction Materials
Wood & Timber
Microstructure – Hardwood
CIVE 202 – Construction Materials
Wood & Timber
Microstructure – Softwood
Wood properties are highly directional !!
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
Bundles of aligned thin walled tubes, glued together • Middle lamella (lignin): between cells • Primary wall: Cellulose microfibrils, random irregular network • Secondary wall: S1, S2 (thickest layer), S3
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
Previous slide: (a) Simplified model of the microstructure of wood showing bundles of aligned thin-walled tubes (known also as cellulose cells) and the structure of the cell wall. ((b)) Transmission electron micrograph g p of a cell wall cross-section,, showing layered structure, including middle lamella (ML). primary wall (P), and the three layers of the secondary cell walls, S1, S2, S3.
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber
Microstructure - Microfibrils • The secondary layers (walls) consist of helically arranged cellulose microfibrils oriented toward the long axis of the tracheid. • Microfibrils are threadlike bundles of cellulose relatively parallel to each other. • Microfibril angle is measured as the angular deviation from the vertical cell wall of the microfibrils in the S2 layer. • Orientation of the S2 microfibril angle has a significant influence on tensile strength, stiffness, and shrinkage. • Longitudinal shrinkage increases sharply while tangential shrinkage decreases at microfibril angles greater than 25 degrees.
CIVE 202 – Construction Materials
Wood & Timber
Microstructure - Microfibrils • Relative thickness of the P, S1, and S3 layers contributes significantly i ifi tl to t th the variability i bilit off llongitudinal it di l shrinkage. hi k • Microfibrils have lignins and hemicellulose between the cellulose molecules which increases the shear strength between them. • Thus, they contribute to the tensile strength and toughness of wood. wood • The covalent bonding forces along the length of the microfibrils are much stronger than the secondary forces binding them together laterally. • This fact leads to the highly orthotropic behavior of wood.
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
• Wood is composed of cellulose, lignin, hemicelluloses, and minor amounts (5 (5-10%) 10%) of extraneous materials contained in a cellular structure. • Variations in the characteristics and volume of these components and differences in cellular structure make woods heavy or light, stiff or flexible. • 1) Cellulose: 45-50% of wood composition polymer y made up p of thousands of g glucose units – Linear p covalently bonded Æ leads to high tensile strength and stiffness of cellulose. – The cellulose molecules are laterally bonded in linear bundles by a combination of hydrogen bonding and Van der Waals bonding.
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
• 1) Cellulose (continued) – Hydroxyl groups (OH-) in glucose generate Van der Waals and H-bonding to adjacent molecules. • They attract OH- groups of adjacent molecules of cellulose, creating microfibrils. • They attract water molecules and hence are largely responsible for the swelling and shrinking that wood undergoes when it is wetted and dried. • 2) Hemicellulose: 20-25% of wood composition – Important in the paper-making process. – Provide much of the fiber-to-fiber bonding. – Built up of variety of different sugar molecules.
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
3) Lignins: 20-30% 20 30% of wood composition – Have a complex 3-dimensional structure. – Built up from phenyl propane units. – Permeate the matrix of cellulose microfibrils in the cell walls. – Fill the spaces between wood cells. – Impart rigidity and compressive strength to the cell walls. 4) Extractives: 0-10% of wood composition – Do not form a part of the basic wood structure. – Responsible for color, odor, taste etc
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
CIVE 202 – Construction Materials
Wood & Timber Variability
• The properties of a single species are relatively constant within limits; thus the selection of wood by species alone may sometimes be adequate. • To use wood to its best advantage and most effectively in engineering applications, specific characteristics or physical properties must be considered.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain • G Grain i is i a very iimportant t t ffeature t off wood d and d titimber b depending upon its characteristics, some properties of timber can vary significantly. • Wood is an orthotropic material, that is, it has unique and independent mechanical properties in the directions of three mutually perpendicular axes: longitudinal, radial, and tangential. • The longitudinal axis is parallel to the fiber (grain); the radial axis is normal to the growth rings (perpendicular to the grain in the radial direction); and the tangential axis is perpendicular to the grain but tangent to the growth rings.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain
The anisotropic axes of wood structure.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain
The anisotropic axes of wood structure.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain • All mechanical properties are affected by direction of loading loading. • The mechanical properties along the longitudinal axis are normally higher than those in the radial and tangential direction. • When wood is loaded in compression along the grain, most of the cell walls are compressed axially axially, thus failure involves localized buckling of the cell wall. • However, when it is loaded across the grain, the cell walls bend, hollow wood cells simply collapse or flatten: compressive strength is therefore lower.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain
CIVE 202 – Construction Materials
Wood & Timber Defects
Wood refers to small, clear specimens, free of any macroscopic defects defects. Wood specimens are used to study relationships between microstructure and properties. Timber: normal sawn structural members found in a lumberyard. Timber contains a large number of defects which govern the structural properties, like the underlying structure of wood does. Grains in timber can be compared to fibers; they affect tensile, compressive, shear strength and the variation of modulus of elasticity.
CIVE 202 – Construction Materials
Wood & Timber Defects
As the tree grows in height, branching is initiated by lateral bud development development. The lateral branches are intergrown with the wood of the trunk as long as they are alive. After a branch dies, the trunk continues to increase in diameter and surrounds that portion of the branch projecting from the trunk when the branch died. If the dead branches drop from the tree, the dead stubs become overgrown and clear wood is formed.
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
The mechanical p properties p measured on wood are modified by y natural defects and abnormalities, which occur as a function of the way in which a tree grows. In general, the defects all have tendency to decrease the strength of the wood and change its mode of failure. Flaws are not uniformly distributed in trees, thus a large variability in strength can be found within a tree tree.
CIVE 202 – Construction Materials
Wood & Timber Defects
Diagrammatic representation of the density distribution in a spruce stem
CIVE 202 – Construction Materials
Wood & Timber Defects
Fiber and ring orientation: in structural timber, depending on how uniform the grain was in the original tree tree, the grain direction may not coincide with the axes of the board.
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
Knots: the portion of a limb that has been surrounded by subsequent growth of wood wood.
CIVE 202 – Construction Materials
Wood & Timber Defects
Knots: Knots distort the fibers around the knots which leads to
tensile stresses perpendicular to the grain even if it is loaded in tension.
CIVE 202 – Construction Materials
Wood & Timber Defects
Knots: The effects of knots depend on size, type, frequency, and location of the knots knots.
CIVE 202 – Construction Materials
Wood & Timber Defects
Checks: a lengthwise separation of the wood which usually
extends across the growth ring ring, it commonly results from the drying process.
CIVE 202 – Construction Materials
Wood & Timber Defects
Wane: the lack of wood on the face of a piece, for any reason at all all.
CIVE 202 – Construction Materials
Wood & Timber Defects
Shake: separation along the grain, between the annual growth ring ring.
CIVE 202 – Construction Materials
Wood & Timber Defects
Pitch pocket: opening between growth rings containing resins or bark bark.
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
Effects of knots on the failure pattern in timber beams: (a) knot at bottom of beam
CIVE 202 – Construction Materials
Wood & Timber Defects
Effects of knots on the failure pattern in timber beams: (b) knot at top of beam
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Grading
Visual Grading: Density (rate of growth, % of latewood) Decay Heartwood or Sapwood (affects durability) Knots (type, size, location) Slope of Grain Shakes, Checks, Splits Wane Pitch Pockets Mechanical Grading: Stiffness (Related to Modulus of Elasticity) Nondestructive Grading: Modulus of Elasticity
CIVE 202 – Construction Materials
Wood & Timber Grading
CIVE 202 – Construction Materials
Wood & Timber Grading
CIVE 202 – Construction Materials
Wood & Timber Grading
CIVE 202 – Construction Materials
Wood & Timber Grading
CIVE 202 – Construction Materials
Wood & Timber Specific Gravity
Relative density of cell wall material ~ 1.5 for all species. Overall relative density ranges dramatically by species: Balsa wood = 0.04 Lignum Vitae = 1.40 Relative density is a function of void space (or porosity).
S = S 0 (1 − p )
n
S = mechanical property S0 = value of S at zero porosity p = porosity n = empirical constant
CIVE 202 – Construction Materials
Wood & Timber Specific Gravity
CIVE 202 – Construction Materials
Wood & Timber Specific Gravity
CIVE 202 – Construction Materials
Wood & Timber Moisture
Moisture can exist in wood in two states: 1. Free water within the cell cavities (evaporates first). 2. Bound water, absorbed into the cell walls.
Fiber Saturation Point: The condition in which all of the free water in the cell cavities has evaporated, but the cell walls are still fully saturated. Typically 25 - 32% moisture content.
CIVE 202 – Construction Materials
Wood & Timber Moisture
CIVE 202 – Construction Materials
Wood & Timber Moisture
• As drying drops below fiber saturation point point, there is removal of water from cell walls which causes compaction of molecular structure and the formation of more H-bonding. Thus, wood will shrink and become stronger. • Above the fiber saturation point, there is little or no effect on mechanical properties. • However, below the fiber saturation point, the mechanical properties of wood are related to the amount of moisture available. • Moisture content of wood is defined as the weight of water in wood expressed as a fraction, usually a percentage, of the weight of wood.
CIVE 202 – Construction Materials
Wood & Timber Moisture
CIVE 202 – Construction Materials
Wood & Timber Moisture
CIVE 202 – Construction Materials
Wood & Timber Moisture
• W Weight, i ht shrinkage, hi k strength, t th and d other th properties ti d depend d upon the moisture content. • In trees, moisture content can range from about 30% to more than 200% of the weight of wood substance. • In softwoods, the moisture content of sapwood is usually greater than that of heartwood. • In hardwoods, the difference in moisture content between heartwood and sapwood depends on the species.
CIVE 202 – Construction Materials
Wood & Timber Moisture
• Green wood is often defined as freshly sawn wood in which the cell walls are completely saturated with water; however however, green wood usually contains additional water within the cell cavities. • During drying, the outer parts of a board can be drier than the fiber saturation point while the inner parts are greater than fiber saturation. • A As moisture i t content t t iincreases, th thermall and d electrical l ti l conductivity increase, as well as the rate of creep. • Wood in service is exposed to both long-term (seasonal) and short-term (daily) changes in relative humidity and temperature of the surrounding air.
CIVE 202 – Construction Materials
Wood & Timber Moisture
• Thus Thus, wood is always undergoing at least slight changes in moisture content. • These changes are usually gradual, and short-term fluctuations tend to influence only the wood surface. • Moisture content changes can be retarded, but not prevented, by protective coatings, such as varnish, lacquer or paint. • Another way to keep the moisture content at a certain percentage is through drying.
CIVE 202 – Construction Materials
Wood & Timber Moisture
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
Volumetric shrinkage – Proportional to volume of water lost • Shrinkage is different in different directions • The loss of water from cell walls induces attractive forces between microfibrils, causing them to bunch together. • Since microfibrils are parallel to grain, there is a large reduction in volume perpendicular to the axis of the cells. • There is a small amount of shrinkage longitudinally. • With respect to shrinkage characteristics, wood is thus an anisotropic material.
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
• Wood is dimensionally stable when the moisture content is greater than the fiber saturation point. • Wood changes dimension as it gains or loses moisture below that point. • It shrinks when losing moisture from the cell walls and swells when gaining moisture in the cell walls. • This shrinking and swelling can result in warping, checking, splitting, and loosening of tool handles, gaps in strip flooring, or performance problems that detract from the usefulness of the wood product.
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
• Wood shrinks most in the direction of annual growth rings (tangentially), about half as much across the rings (radially), and only slightly along the grain (longitudinally). • The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because of the difference in shrinkage and the curvature of annual rings. • Greater shrinkage is associated with greater density density. • The size and shape of a piece of wood can affect shrinkage, and the rate of drying for some species can also affect shrinkage.
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
Fiber Saturation Point
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
CIVE 202 – Construction Materials
Wood & Timber Curing
• Curing timber is important because it improves its properties and protects it from interaction with external elements.| • It also prevents shrinkage because part of the moisture is removed which causes it to shrink (in axial direction). • The wood is dried before usage - by kiln or air. • It is important to dry the wood so as to avoid shrinkage after construction.
CIVE 202 – Construction Materials
Wood & Timber Curing
Wood & Timber
CIVE 202 – Construction Materials
Wood & Timber Introduction
Important points concerning wood: 1. Many kinds (>30,000 species of trees) 2. Wood is a composite material 3. Natural material (many flaws, imperfections) 4. Anisotropic (mechanical properties vary with respect to load orientation)
CIVE 202 – Construction Materials
Wood & Timber Introduction
Terminology: Wood - (Materials) refers to small clear specimens free of any macroscopic defects. Timber - (Structural), refers to sawn structural members and may contain a wide variety of macroscopic defects
‘Specific’ Values – per unit weight basis
CIVE 202 – Construction Materials
Wood & Timber Introduction
Three distinct parts: 1. Root System 2. Trunk 3. Crown
CIVE 202 – Construction Materials
Wood & Timber Introduction
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
1. Outer Bark - protection 2. Inner Bark - sap transport 3. Cambium - growth 4. Wood: Sapwood - food storage, moisture transport Heartwood - nonliving cells
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
5. Annual Rings: Springwood, earlywood - rapid growth, large cells, thin walls Summerwood, latewood - slow growth small cells growth, cells, thick walls 6. Pith - primary tissue, original growth
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure – Growth Rings Growth rings vary in width depending on species and site conditions. diti Rings formed during short or dry seasons are thinner than those formed when growing conditions are more favourable. Rings formed in shady conditions are usually thinner than those formed by the same species in sunny conditions. It is commonly believed that the age of a tree may be determined by counting these rings but this method can lead to errors because abnormal environmental conditions can cause a tree to produce multiple-growth increments or even prevent growth entirely for a period.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure – Growth Rings
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure – Growth Rings
CIVE 202 – Construction Materials
Wood & Timber
Sections of the Trunk
CIVE 202 – Construction Materials
Wood & Timber Microstructure
Softwoods: S ft d Have H needle-like dl lik or scale-like l lik lleaves and d a relatively l ti l simple cell structure. Typically coniferous. Ex. Spruce, Pine, Fir, Cedar, Hemlock Hardwoods: Have broad leaves and a more complicated cell structure. Typically deciduous. Ex. Oak, Maple, Walnut, Ash, Birch, Elm Though hardwood tends to be denser, stronger and more resistant to decay, the vast majority of lumber is made from softwood for economic reasons.
CIVE 202 – Construction Materials
Wood & Timber Microstructure
Wood is primarily composed of hollow hollow, elongated elongated, spindle spindle-shaped shaped cells that are arranged parallel to each other along the trunk of a tree (similar to a bundle of drinking straws). When lumber and other products are cut from the tree, the characteristics of these fibrous cells and their arrangement affect such properties as strength and shrinkage as well as the grain pattern of the wood. The directionality of these long narrow cells result in a anisotropic materials, whose mechanical properties vary depending upon the direction in which they are tested.
CIVE 202 – Construction Materials
Wood & Timber
Microstructure – Hardwood
CIVE 202 – Construction Materials
Wood & Timber
Microstructure – Softwood
Wood properties are highly directional !!
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
Bundles of aligned thin walled tubes, glued together • Middle lamella (lignin): between cells • Primary wall: Cellulose microfibrils, random irregular network • Secondary wall: S1, S2 (thickest layer), S3
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
Previous slide: (a) Simplified model of the microstructure of wood showing bundles of aligned thin-walled tubes (known also as cellulose cells) and the structure of the cell wall. ((b)) Transmission electron micrograph g p of a cell wall cross-section,, showing layered structure, including middle lamella (ML). primary wall (P), and the three layers of the secondary cell walls, S1, S2, S3.
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber Microstructure
CIVE 202 – Construction Materials
Wood & Timber
Microstructure - Microfibrils • The secondary layers (walls) consist of helically arranged cellulose microfibrils oriented toward the long axis of the tracheid. • Microfibrils are threadlike bundles of cellulose relatively parallel to each other. • Microfibril angle is measured as the angular deviation from the vertical cell wall of the microfibrils in the S2 layer. • Orientation of the S2 microfibril angle has a significant influence on tensile strength, stiffness, and shrinkage. • Longitudinal shrinkage increases sharply while tangential shrinkage decreases at microfibril angles greater than 25 degrees.
CIVE 202 – Construction Materials
Wood & Timber
Microstructure - Microfibrils • Relative thickness of the P, S1, and S3 layers contributes significantly i ifi tl to t th the variability i bilit off llongitudinal it di l shrinkage. hi k • Microfibrils have lignins and hemicellulose between the cellulose molecules which increases the shear strength between them. • Thus, they contribute to the tensile strength and toughness of wood. wood • The covalent bonding forces along the length of the microfibrils are much stronger than the secondary forces binding them together laterally. • This fact leads to the highly orthotropic behavior of wood.
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
• Wood is composed of cellulose, lignin, hemicelluloses, and minor amounts (5 (5-10%) 10%) of extraneous materials contained in a cellular structure. • Variations in the characteristics and volume of these components and differences in cellular structure make woods heavy or light, stiff or flexible. • 1) Cellulose: 45-50% of wood composition polymer y made up p of thousands of g glucose units – Linear p covalently bonded Æ leads to high tensile strength and stiffness of cellulose. – The cellulose molecules are laterally bonded in linear bundles by a combination of hydrogen bonding and Van der Waals bonding.
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
• 1) Cellulose (continued) – Hydroxyl groups (OH-) in glucose generate Van der Waals and H-bonding to adjacent molecules. • They attract OH- groups of adjacent molecules of cellulose, creating microfibrils. • They attract water molecules and hence are largely responsible for the swelling and shrinking that wood undergoes when it is wetted and dried. • 2) Hemicellulose: 20-25% of wood composition – Important in the paper-making process. – Provide much of the fiber-to-fiber bonding. – Built up of variety of different sugar molecules.
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
3) Lignins: 20-30% 20 30% of wood composition – Have a complex 3-dimensional structure. – Built up from phenyl propane units. – Permeate the matrix of cellulose microfibrils in the cell walls. – Fill the spaces between wood cells. – Impart rigidity and compressive strength to the cell walls. 4) Extractives: 0-10% of wood composition – Do not form a part of the basic wood structure. – Responsible for color, odor, taste etc
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
CIVE 202 – Construction Materials
Wood & Timber Molecular Structure
CIVE 202 – Construction Materials
Wood & Timber Variability
• The properties of a single species are relatively constant within limits; thus the selection of wood by species alone may sometimes be adequate. • To use wood to its best advantage and most effectively in engineering applications, specific characteristics or physical properties must be considered.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain • G Grain i is i a very iimportant t t ffeature t off wood d and d titimber b depending upon its characteristics, some properties of timber can vary significantly. • Wood is an orthotropic material, that is, it has unique and independent mechanical properties in the directions of three mutually perpendicular axes: longitudinal, radial, and tangential. • The longitudinal axis is parallel to the fiber (grain); the radial axis is normal to the growth rings (perpendicular to the grain in the radial direction); and the tangential axis is perpendicular to the grain but tangent to the growth rings.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain
The anisotropic axes of wood structure.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain
The anisotropic axes of wood structure.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain • All mechanical properties are affected by direction of loading loading. • The mechanical properties along the longitudinal axis are normally higher than those in the radial and tangential direction. • When wood is loaded in compression along the grain, most of the cell walls are compressed axially axially, thus failure involves localized buckling of the cell wall. • However, when it is loaded across the grain, the cell walls bend, hollow wood cells simply collapse or flatten: compressive strength is therefore lower.
CIVE 202 – Construction Materials
Wood & Timber
Macrostructure - Grain
CIVE 202 – Construction Materials
Wood & Timber Defects
Wood refers to small, clear specimens, free of any macroscopic defects defects. Wood specimens are used to study relationships between microstructure and properties. Timber: normal sawn structural members found in a lumberyard. Timber contains a large number of defects which govern the structural properties, like the underlying structure of wood does. Grains in timber can be compared to fibers; they affect tensile, compressive, shear strength and the variation of modulus of elasticity.
CIVE 202 – Construction Materials
Wood & Timber Defects
As the tree grows in height, branching is initiated by lateral bud development development. The lateral branches are intergrown with the wood of the trunk as long as they are alive. After a branch dies, the trunk continues to increase in diameter and surrounds that portion of the branch projecting from the trunk when the branch died. If the dead branches drop from the tree, the dead stubs become overgrown and clear wood is formed.
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
The mechanical p properties p measured on wood are modified by y natural defects and abnormalities, which occur as a function of the way in which a tree grows. In general, the defects all have tendency to decrease the strength of the wood and change its mode of failure. Flaws are not uniformly distributed in trees, thus a large variability in strength can be found within a tree tree.
CIVE 202 – Construction Materials
Wood & Timber Defects
Diagrammatic representation of the density distribution in a spruce stem
CIVE 202 – Construction Materials
Wood & Timber Defects
Fiber and ring orientation: in structural timber, depending on how uniform the grain was in the original tree tree, the grain direction may not coincide with the axes of the board.
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
Knots: the portion of a limb that has been surrounded by subsequent growth of wood wood.
CIVE 202 – Construction Materials
Wood & Timber Defects
Knots: Knots distort the fibers around the knots which leads to
tensile stresses perpendicular to the grain even if it is loaded in tension.
CIVE 202 – Construction Materials
Wood & Timber Defects
Knots: The effects of knots depend on size, type, frequency, and location of the knots knots.
CIVE 202 – Construction Materials
Wood & Timber Defects
Checks: a lengthwise separation of the wood which usually
extends across the growth ring ring, it commonly results from the drying process.
CIVE 202 – Construction Materials
Wood & Timber Defects
Wane: the lack of wood on the face of a piece, for any reason at all all.
CIVE 202 – Construction Materials
Wood & Timber Defects
Shake: separation along the grain, between the annual growth ring ring.
CIVE 202 – Construction Materials
Wood & Timber Defects
Pitch pocket: opening between growth rings containing resins or bark bark.
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Defects
Effects of knots on the failure pattern in timber beams: (a) knot at bottom of beam
CIVE 202 – Construction Materials
Wood & Timber Defects
Effects of knots on the failure pattern in timber beams: (b) knot at top of beam
CIVE 202 – Construction Materials
Wood & Timber Defects
CIVE 202 – Construction Materials
Wood & Timber Grading
Visual Grading: Density (rate of growth, % of latewood) Decay Heartwood or Sapwood (affects durability) Knots (type, size, location) Slope of Grain Shakes, Checks, Splits Wane Pitch Pockets Mechanical Grading: Stiffness (Related to Modulus of Elasticity) Nondestructive Grading: Modulus of Elasticity
CIVE 202 – Construction Materials
Wood & Timber Grading
CIVE 202 – Construction Materials
Wood & Timber Grading
CIVE 202 – Construction Materials
Wood & Timber Grading
CIVE 202 – Construction Materials
Wood & Timber Grading
CIVE 202 – Construction Materials
Wood & Timber Specific Gravity
Relative density of cell wall material ~ 1.5 for all species. Overall relative density ranges dramatically by species: Balsa wood = 0.04 Lignum Vitae = 1.40 Relative density is a function of void space (or porosity).
S = S 0 (1 − p )
n
S = mechanical property S0 = value of S at zero porosity p = porosity n = empirical constant
CIVE 202 – Construction Materials
Wood & Timber Specific Gravity
CIVE 202 – Construction Materials
Wood & Timber Specific Gravity
CIVE 202 – Construction Materials
Wood & Timber Moisture
Moisture can exist in wood in two states: 1. Free water within the cell cavities (evaporates first). 2. Bound water, absorbed into the cell walls.
Fiber Saturation Point: The condition in which all of the free water in the cell cavities has evaporated, but the cell walls are still fully saturated. Typically 25 - 32% moisture content.
CIVE 202 – Construction Materials
Wood & Timber Moisture
CIVE 202 – Construction Materials
Wood & Timber Moisture
• As drying drops below fiber saturation point point, there is removal of water from cell walls which causes compaction of molecular structure and the formation of more H-bonding. Thus, wood will shrink and become stronger. • Above the fiber saturation point, there is little or no effect on mechanical properties. • However, below the fiber saturation point, the mechanical properties of wood are related to the amount of moisture available. • Moisture content of wood is defined as the weight of water in wood expressed as a fraction, usually a percentage, of the weight of wood.
CIVE 202 – Construction Materials
Wood & Timber Moisture
CIVE 202 – Construction Materials
Wood & Timber Moisture
CIVE 202 – Construction Materials
Wood & Timber Moisture
• W Weight, i ht shrinkage, hi k strength, t th and d other th properties ti d depend d upon the moisture content. • In trees, moisture content can range from about 30% to more than 200% of the weight of wood substance. • In softwoods, the moisture content of sapwood is usually greater than that of heartwood. • In hardwoods, the difference in moisture content between heartwood and sapwood depends on the species.
CIVE 202 – Construction Materials
Wood & Timber Moisture
• Green wood is often defined as freshly sawn wood in which the cell walls are completely saturated with water; however however, green wood usually contains additional water within the cell cavities. • During drying, the outer parts of a board can be drier than the fiber saturation point while the inner parts are greater than fiber saturation. • A As moisture i t content t t iincreases, th thermall and d electrical l ti l conductivity increase, as well as the rate of creep. • Wood in service is exposed to both long-term (seasonal) and short-term (daily) changes in relative humidity and temperature of the surrounding air.
CIVE 202 – Construction Materials
Wood & Timber Moisture
• Thus Thus, wood is always undergoing at least slight changes in moisture content. • These changes are usually gradual, and short-term fluctuations tend to influence only the wood surface. • Moisture content changes can be retarded, but not prevented, by protective coatings, such as varnish, lacquer or paint. • Another way to keep the moisture content at a certain percentage is through drying.
CIVE 202 – Construction Materials
Wood & Timber Moisture
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
Volumetric shrinkage – Proportional to volume of water lost • Shrinkage is different in different directions • The loss of water from cell walls induces attractive forces between microfibrils, causing them to bunch together. • Since microfibrils are parallel to grain, there is a large reduction in volume perpendicular to the axis of the cells. • There is a small amount of shrinkage longitudinally. • With respect to shrinkage characteristics, wood is thus an anisotropic material.
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
• Wood is dimensionally stable when the moisture content is greater than the fiber saturation point. • Wood changes dimension as it gains or loses moisture below that point. • It shrinks when losing moisture from the cell walls and swells when gaining moisture in the cell walls. • This shrinking and swelling can result in warping, checking, splitting, and loosening of tool handles, gaps in strip flooring, or performance problems that detract from the usefulness of the wood product.
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
• Wood shrinks most in the direction of annual growth rings (tangentially), about half as much across the rings (radially), and only slightly along the grain (longitudinally). • The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because of the difference in shrinkage and the curvature of annual rings. • Greater shrinkage is associated with greater density density. • The size and shape of a piece of wood can affect shrinkage, and the rate of drying for some species can also affect shrinkage.
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
Fiber Saturation Point
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
CIVE 202 – Construction Materials
Wood & Timber Shrinkage
CIVE 202 – Construction Materials
Wood & Timber Curing
• Curing timber is important because it improves its properties and protects it from interaction with external elements.| • It also prevents shrinkage because part of the moisture is removed which causes it to shrink (in axial direction). • The wood is dried before usage - by kiln or air. • It is important to dry the wood so as to avoid shrinkage after construction.
CIVE 202 – Construction Materials
Wood & Timber Curing
