Sab 2112 - L11 Timber

TIMBER / WOOD

Timber Structure

TIMBER Wood is one of the oldest known materials used in construction It is the only naturally renewable building material Species of wood in Malaysia – more than 2500

Names of wood Vernacular – given by the local community e.g. Cengal – Cengai or Penak Botanical – given by botanical member e.g. Balanorcarpus Heimeii (Cengal) Commerce – given for trading e.g. Cengal

Design Concept

Wood popular for Lightweight Construction Simplicity in fabrication Lightness Reusability Material availability Simpler connections Environmental compatibility Adaptability to modifications and remodeling

Wood in Construction In the form of lumber – pieces of wood cut from tree trunks Wood products – glue laminated timber, pressed wood, chip board etc. Tree trunks → Log → Lumber

USE OF TIMBER

Classification of Wood Hardwood – tree has broad leaves, seeds with hard shell, mostly grow in tropical climate e.g. jati

Softwood – species that has needlelike leaves, seed without hard shell, mostly grow in seasonal climate e.g. Pine trees

This classification is based on the Botanical aspects, not the hardness of the wood

Cont. Generally, most hardwoods are harder to work with than softwood The term hardwood and softwood do not necessarily indicate the relative hardness or density between the two categories

Growth Ring/Annual Ring

Growth ring = earlywood + latewood

Earlywood – cell with larger opening, during winter Latewood – cell with smaller opening, during summer

Growth Ring

Growth Ring

Wood Cell

Cell Cavity

Cell Wall

Wood Cell

Forest Research Institute of Malaysia (FRIM) Hardwood in Malaysia – classified based on the density and durability Three categories Heavyweight hardwood – density greater than 880 kg/m3, very durable e.g. balau, cengal Normal/Intermediate weight hardwood – density in the range of 720 to 880 kg/m3, less durable compared to heavyweight hardwood, e.g. kempas, keruing Lightweight hardwood – density less than 720 kg/m3, not durable except being preserved, e.g. meranti

Properties of Wood The most important properties that affect strength and durability of wood are: 1) 2)

Moisture content Specific gravity

Effect of Moisture Content

Moisture content The weight of water in wood expressed as a percentage of its oven-dry weight Determining moisture content – oven or moisture meter Wood is a hygroscopic substance – it can absorb water/moisture

The ability to absorb or lose moisture depends on environmental conditions Relative Humidity Temperature

Variations of moisture content in living trees Moisture content 400%, specific gravity 0.2 Moisture content 100%, specific gravity 1.15 Typical trees contain water about two times the weight of its solid material

Water exist in wood FREE WATER – in the cell cavity BOUND or ADSORBED WATER – in the cell wall The amount of water in cell cavities varies with the amount of drying Lumber has moisture generally in excess of 50% at the time of its manufacture

FIBRE SATURETION POINT (FSP) FSP is when the cell cavity contains only air and the cell wall is saturated with water Moisture content at FSP can vary from 20 to 30% depending on the species of wood

Effect of Moisture Content Above FSP – volume and mechanical properties unchanged or constant, although the density of wood decrease Below FSP – wood shrink and strength increase

Amount of water in cell will affect: Physical properties Mechanical properties Durability Dimensional stability

Equilibrium Moisture Content The moisture content at which wood neither gains nor loses moisture to the surrounding Its values range between 5 and 17% at 70 °C and relative humidity between 2 to 80%

Specific Gravity Is the ratio of the weight of wood (oven-dry) to the weight of an equal volume of water Specific Gravity = (Ws)/(wwV) Ws – oven-dry weight ww – density of water V – volume of (green) wood

Wood is composed of: Solid matter Water Air Volume = Vsolid + Vwater + Vair

Specific gravity Determined in three conditions: 1) Green (moisture content in excess of 19%) 2) Air-dry (moisture content – 12%) 3) Oven-dry (moisture content 0%)

Cont. Strength and stiffness of wood increase with the increase in specific gravity Physical and mechanical properties of wood are related to specific gravity Density of wood is defined as the mass or weight per unit volume It is directly related to porosity or proportion of voids and calculated using the total weight of wood

PROPERTIES OF WOOD Strength is one of the main properties that determines the suitability of wood in construction

Strength – the ability of material to resist external load without failure

Strength of Timber Tensile strength Compressive strength Flexural strength Shear strength hardness Impact strength Splitting strength

Loading Condition

Strength of wood Varies according to the direction of the applied load 1) Parallel to grains 2) Perpendicular to grains Wood grains – direction of wood fibre with respect to the main axis of wood

Wood Grains Straight grain Edge grain Flat grain Cross grain Spiral grain Diagonal grain The slope of grains will affect the strength of wood

Effect of Wood Grain

Cont. Tensile strength and compressive strengths parallel to wood grains are higher than perpendicular to wood grains. Compressive strength perpendicular to wood grains is between 12 and 18% of compressive strength to wood grains

Tensile strength parallel to wood grains is approximately 2 to 4 times than compressive strength parallel to wood grains Shear strength perpendicular to wood grains is higher than parallel to wood grains

Cont. Hardness of wood is generally defined as how easy the wood to be work with i.e. cut, nailed It depends on the density, stiffness, and bonding between wood fibres

Impact strength – the ability of wood to absorb sudden external load It depends on the hardness, elasticity, plasticity of wood

FACTORS AFFECTING STRENGTH AND DURABILITY OF TIMBER Moisture content Density Wood grains Defects Preservative Treatment

Moisture Content Higher moisture content will reduce the strength and durability of wood Moisture will cause wood to decay

Density Wood with higher density will have higher compressive strength and better durability due to dense microstructure

Wood Defects Any irregularities in the structure of wood or lumber that will reduce the strength and durability Types of defect: 1) Natural defect 2) Defect during processing 3) Due to insects or fungus

Natural Defects

Natural Defects

Design Consideration

Effect of Defect

Processing Defects Generally is due to improper drying methods

Insects and Fungus Wood destroying insects – termites, pin-hole borers, beetles Fungi – brown rot, white rot Conditions for fungal growth 1) Proper temperature 2) Moisture greater than 19% 3) Oxygen 4) Food (wood fibre)

SEASONING OF WOOD The process of controlled drying of lumber (water in cell cavity and wall) to increase its structural properties Reduction in moisture content: 1) Increase strength 2) Reduce shrinkage 3) Reduction in weight 4) More durable

Methods of drying Air drying – drying naturally Kiln/Oven drying – controlled drying (temperatures and humidities)

Stacking lumber

Factors Affecting Drying Process

Temperature Relative humidity Air circulation

Duration of drying Types of wood – softwood, hardwood Size of lumber Surrounding environment Stacking method Air-drying vs Oven drying

Preservative Treatment Is the process of introducing or injecting special chemicals into wood cell by special techniques The purpose is to prevent the destruction from fungi and insects and to inhibit combustion

Classes of Wood Preservatives Creosote oil Water-borne wood preservatives Light organic solvent preservatives It is important that the preservatives must penetrate into the wood and to such extent it is effectively protect the wood

Attributes of Ideal Wood Preservatives Easy penetration into wood cell Permanent in the wood cell Toxic to insect and fungi Safe to handle Colourless Compatible with coatings and finishes Cheap

Ease of Injecting Preservatives Wood density Microstructure of wood Chemical composition of cell wall Size of lumber Moisture content

Methods of Treatment Process Non-Pressure Method Brushing Immersion Hot and cold open tank treatment Pressure Treatments 1) Full cell 2) Empty cell

Cont. The only method to achieve any worthwhile degree of penetration is by way of pressure treatment Brushing – not very effective, chemical will not be able to penetrate into wood cell, need to repeat the process

Immersion Cold immersion – lumber is immersed in tank for few weeks Hot immersion – immersed in tank at higher temperature 90 °C Hot-Cold immersion – immersed in tank at 90 °C, during heating air in cell cavity will expand and pushed out, during cooling the cell will shrink and absorb the chemical

Pressure (full cell) Lumber is put in container Half vacuum (to pull out air) Spray hot chemical, increase pressure (chemical enters wood cell) Half vacuum to push out extra chemical from the cell Cell wall and cavity filled with chemical Suitable for construction in damp soil In buildings, chemical may leach out

Pressure (empty cell) Lumber placed in container Apply pressure (compressed the air) Spray chemical Increase pressure (forcing chemical to penetrate into cell wall but air in cell cavity is compressed) Reduce pressure, half vacuum, air in cell cavity will expand pushing out extra chemical Cell wall filled with chemical but cell cavity is empty More economical, cleaner lumber

TIMBER STRESS GRADE Divided into two categories: Wet condition – moisture greater than 19% Air-dry condition – moisture equal to or less than 19%

Design Purposes FRIM divides stress timber into several grades Basic stress – stress that can be sustained by timber without failure Grade

Design stress

Selected

80% of basic stress

Particular

63% of basic stress

General

50% of basic stress

Strength of Timber Divided into four groups i.e. A, B, C, and D Based on strength Selection of timber based on strength from different groups

MANNER OF CUTTING A LOG The method of cutting a log will affect shrinkage Shrinkage occurs in three mutually perpendicular directions: 1) Along its axis 2) Along a radial direction 3) Along tangent to the radius Tangential shrinkage is the highest, about two times the radial shrinkage The longitudinal shrinkage is negligible

WOOD PRODUCTS Wood in construction can be divided into two forms 1) In the form of lumber 2) In the form of woodbased panel Two types of panels: veneered panels and non-veneered panels

Panels Veneered panels – also called plywood, made from thin sheet of wood Non-veneered panels – manufactured from wood particles or fibres Used for structural applications such as floor and wall panels, non-structural applications such as in furniture and cabinets

Wood-Based Products

Manufactured by bonding together using resin under heat and pressure

Non-Veneered Panels Divided into two major categories: Particleboards Fibreboards

Particleboard Manufactured from discrete pieces of wood particles (chipboard, flakeboard, splinterboard), combined with a synthetic resin or glue, bonded together under heat and pressure in a hot press in which an entire interparticle bond is created Particles – piece of wood smaller than veneer sheets but larger than wood fibre

Classifications Low-density particleboard – density 590 kg/m3 and specific gravity of 0.59 Medium-density particleboard – density between 590 and 880 kg/m3 and specific gravity between 0.59 and 0.80 High-density particle board – density greater than 800 kg/m3 and specific gravity of 0.80

Fibreboard Is a generic term for a homogeneous panel made from wood fibres Fibreboard panels have density between 160 and 500 kg/m3, and specific gravity between 0.16 and 0.50

Cont. Veneer, plywood, and blockboard are natural wood panels Particleboard and fibre building board – are reconstituted wood panels which are manufactured mainly from various wood wastes

Wood Ply

Plywood Is a panel comprising an assembly of plies bonded together

Glulam or Glue-Laminated Timber Consists of sawn lumber laminations bonded with an adhesive so that the grain of all laminations runs parallel with the long direction Can be manufactured in a variety of shapes and sizes

Glulam

Structural Section

Application of Timber Structural – column, beam, truss

Non-structural – frame, partition

Application in Construction

Floor Tile

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