4 TIMBER & WOOD

LEARNING OUTCOMEAt the end of this lecture, students should be

able to understand and explain;

i. Structure of wood

ii. Types of wood

iii. Physical properties of wood

iv. Wood product

v. Preservation treatment of timber

vi. Testing of timber and wood

COURSE OUTCOME

CO2: Describe the basic science and engineering fundamentals pertaining to other construction material namely steel, timber, bricks and bitumen and its influence to their properties.

programme OUTCOME

PO1: Ability to acquire and apply basic knowledge of science, mathematics and engineering

INTRODUCTION

One of the oldest structural materials still in use.

Timber is unique in its structure and mode of growth.

Timber characteristics and properties are distinct and more complex than those of other common structural materials such as concrete, steel and brickwork.

Timber can be used for many purposes in building construction. Some of the applications are:

i. Foundation piles ii. Structural components such as beams, columns,

etc. iii. Roofing – rafters, ties, struts, purlins and

bracing. iv. Flooring v. Walling vi. Ceiling vii. Door and windows

It has:I. Light weightII. Excellent insulating propertiesIII. Good fire resistanceIV. Aesthetic appeal

However, timber is very sensitive to environmental conditions

E.g. timber is very sensitive to the moisture content which it gives a direct effect on the strength and stiffness, swelling or shrinkage of the timber.

STRUCTURE OF WOODThe source of structural timber are coming

from the mature trees (wood) of any species.Essential for the engineer to have a knowledge

of the nature and growth patterns of trees.Basically, there are 3 subsystems:1. Roots2. Trunk3. Crown

The trunk is the most important for engineers.Wood is composed of long thin tabular cells.The cross section of a trunk:

In temperate countries, a tree produces a new layer of wood just under the barkin the early part of every growing season. This process will produce a visible concentric rings known as annular rings, annual rings or growth rings.

In tropical countries where trees grow throughout the year, a tree produces wood cells that are essentially uniform. The age of a tree may be determined by counting its growth rings.

The annular band of cross-section nearest to the bark is called sapwood. It acts as a medium of transportation for sap from the roots to the leaves.

The central core of the wood which is inside the sapwood is heartwood. It functions mainly to give mechanical support or stiffness to the trunk.

TYPES OF WOOD1)Softwood • Low density timber with relatively low strength.• Poor durability qualities unless treated.

2) Hardwood • High density timber and high strength.• Less dependency in preservatives for improving

durability in properties.• Tropical wood• Expensive

CLASSIFICATION OF MALAYSIAN TIMBERS

Malaysian timbers are classified into four categories:

1. Heavy Hardwoods (HHW) 2. Medium Hardwoods (MHW) 3. Light Hardwoods (LHW) 4. Softwoods (SW)

The classification of the three categories of Hardwoods is based largely on the average density of the timbers at 15% moisture content.

LAMIN BOARD

GLUE LAMINATED WOOD

PHYSICAL PROPERTIES OF WOOD

Timber’s strength is dependent on many factors which act in dependently or in conjunction with others which is:

1. Moisture content2. Density 3. Slope of grain4. Timber defect5. Timber grading6. Position in tree7. Condition of growth

1) Moisture contentThe behavior of timber is significantly

influenced by the existence and variation of its moisture content.

Moisture contained in ‘green’ timber is held both within the cells (free water) and within the cell walls (bound water).

The condition in which all free water has been removed but the cell wall are still saturated is known as the fibre saturation point(FSP).

At levels of moisture above FSP, most physical and mechanical properties remain constant.

Variation in moisture content below the FSP cause considerable changes to properties such as weight, strength, elasticity, shrinkage and durability.

The controlled drying of timber is known as seasoning.

Air seasoning• The timber is stacked and layered with air-

spaced in open sided sheds to promote natural drying.

• This method is relatively inexpensive with very little loss in the quality of timber if carried out correctly.

• It has disadvantage that the timber and the space which it occupies are unavailable for long periods.

2) DensityMajor factor determining its strength.Specific gravity or relative density is a measure of

timber’s solid substance.Expressed as the ratio of the oven-dry weight to

the weight of an equal volume of water.Since water volume varies with the moisture

content of the timber, the specific gravity of timber is expressed at a certain moisture content.

Basic SG of commercial timber ranges from 0.29-0.81.

3) Slope of GrainGrain is the longitudinal direction of the main

elements of timbers.If the angle of deviation increases, the

strength will decreases.For timber having sloping grain of as little as

1/18, the bending strength is reduced by 15%.

4) Timber DefectsDefects in timber whether natural or

caused during conversion or seasoning will have an effect on structural strength as well as on fixing, stability, durability and finished appearance of timber.

5) Timber GradingIn order to have a way of selling or buying the

timber product to various parts of the world, the timber have to undergone the grading system for structural applications for determining the permissible stresses to a series of grades.

2 types of grading:• Visual grading• Machine grading

6) Position in TreeIn soft wood, the wood from near

the butt of the tree is usually somewhat denser, stronger and harder than that from near the top of the tree.

In the early years of a tree’s life, the wood often tends to become stronger with increasing distance from the pitch.

7) Condition of GrowthEnvironmental factors affecting tree

growth, such as height above sea-level, temperature, rainfall, type of soil, spacing between trees, etc.

Have an effect on the strength of the properties of the timber, probably mainly through their effect on density.

Defects in TimberWood have a lot of defect due to the growing

period and during the conversion and seasoning process.

This defect may cause reduction to its strength or impairing its appearance.

Defects may be classified as:i. Natural defectsii. Chemical defectsiii. Conversion defectsiv. Seasoning defects

1) Natural DefectsOccurs during the growing period.Cracks and fissuresOccurs in various parts of the tree and the

indication of decaying process.

Knots Common features of a wood where this is the

portion of a branch embedded by the natural growth of the tree.

Grain defectsRefers to the direction of the fibre

arrangement in wood. May occurs in the form of twisted-grain, cross-

grain, flat-grain and spiral-grain.

Fungal decayOccur in growing mature timber or

even in recently converted timber. It is good practice to reject such timber.

Annual ring widthThis can be critical in respect of

strength in that excess width of such rings can reduce the density of the timber.

2) Chemical Defects• Occurs in particular instances when timber is

used in unsuitable positions or in association with other materials.

• Timbers such as Oak and Western Red Cedar contain tannic acid and other chemicals which corrode metals.

• Gums and resins can inhabit the working properties of timber and interfere with the ability to take adhesives.

3) Conversion Defects• Due to misleading in the milling techniques in

attempting to use every possible piece of timber converted from the trunk. A wane is the example of the conversion defects.

4) Seasoning Defects• Related to the movements that occurs in a timber

due to changes in moisture content.• Excessive or uneven drying, expose to wind and rain,

may produce defects or distortions in timber.• All such defects have an effect on structural strength

as well as on fixiting, stability, durability and finished appearance

FACTOR AFFECTING STRENGTH OF TIMBER1) Density• The higher the density, the higher the mechanical properties.• Young tree –very low density and reduced stresses are used for such

material• The weight of timber is reduced by drying and most strength

properties are substantially increased

2)Moisture Content• Moisture content of wood is defined as the weight of water in wood

expressed as a fraction, usually a percentage, of the weight of oven dry wood.

• Can be determine by the oven-dry method or by the electric-moisture-meter method

• Weight, shrinkage, strength, and other properties depend upon the moisture content of wood.

3) Temperature• With increasing temperature, the

strength of timber decreases. • A permanent loss of strength may result

if wood is held at a high temperature for a long period.

4) Grain Structure• Grain structure determines the

permissible and modulus of elasticity.

5) Position in Tree• In softwoods, the wood from near the butt of

the tree is usually some what denser, stronger and harder than that from near the top of the tree and in the early years of a tree’s life the wood often tends to become stronger with increasing distance from the pitch.

6) Conditions of Growth• Environmental factors affecting tree growth,

such as height above sea-level, temperature, rainfall, type of soil and spacing between trees.

WOOD PRODUCTWood based panel products are sheet

materials which contain a significant amount of wood in the form of strip, veneers, chips, flakes or fibres.

The categories usually recognized within this group of board materials are plywood, glue laminated timber, particleboard and fibre building boards.

1) PlywoodPlywood is defined as a wood based panel

product consisting of an assembly of plies bonded together.

Plies maybe some or all of which are wood.Includes core plywood such as block board and

laminboard.Plywood is a versatile product that can combine

attractive surface appearance with superior performance under hazardous conditions.

2) Glue Laminated TimberRefers to structural and non-structural members

formed by gluing together boards in such manner that grain directions in all boards are substantially parallel by the use of arrange of adhesive to:

i. Increase strengthii. Improve functional propertiesiii. Enhance permissible stresses This depend on:i. Laminated form of construction ii. Availability of suitable adhesivesiii. The geometry of the built-up forms

Method of treating timber:1. Pressure Processes.- Apply pressure greater than atmospheric.

2. Non-Pressure Processes.a) The hot and cold bath treatment.

- Placed in a tank of hot preservative and heated there for some hours.

- At the end of the heating period, the timber removed to a separate tank containing cold preservative or left down in the same tank.

PRESERVATION OF TIMBER

b) steeping/soaking in water soluble preservative.

- This method consist of immersing the dry timber in a tank of preservative, removing, draining and allowing to dry.

- Duration normally 2 weeks and usually done with preservative of the aqueous solution type.

Method of Treating Timber• Brush and spray.• Immersion.• Hot and cold open tank.• Pressure vacuum treatments.• Diffusion process.

Testing of timber & woodMechanical Testing Wood is tested to predict performance two main techniques

testing of timbers of structural sizes (ASTM D 198)testing of representative, small, clear specimens (ASTM

D 143) Testing of structural-size members is more important –more

applicable to design values Tests include flexure, compression, tension, etc. Flexure test is more commonly used than the other tests Two-point, third-point, or center-point loading

Third-point bending test on a 4 x 6 wood lumber

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Compression parallel to grain

Tension perpendicula

r to grain

Compressionperpendicular

to grain

Tension parallelto grain

Hardness perpendicular

to grain

Hardness parallel to grain

Bending

Testing Representative, Small, Clear Specimens

(Q&A)