ETHIOPIAN TECHNICAL UNIVERSITY

CIVIL TECHNOLOGY FACULTY

WOOD TECHNOLOGY DEPARTMENT

Teaching Material on Introduction to WoodScience and Technology

CHAPTER ONE
1. INTRODUCTION TO WOOD AND WOOD ANATOMY

1.1 Unique Characteristics of Wood

 What is wood?
Derived from the word timberian, that means to build. Denotes wood which is suitable for
building or carpentry.
Three terms to be known in connection to the timber Converted timber: sawn and cut into
suitable commercial sizes.
Rough Timber: obtained after felling a tree
Standing Timber: Timber contained in living tree
 Valuable properties:
» Low heat conductivity
» Ability to mechanical working
» Small bulk density
» High Strength
 Drawbacks
» Decay
» Inflammability
• » Fluctuation in properties due to changes in moisture
6/10/2021 Compiled by: Misganu E.
 A: The outer bark is the tree's protection from the outside world. Continually renewed from
within, it helps keep out moisture in the rain, and prevents the tree from losing moisture
when the air is dry. It insulates against cold and heat and wards off insect enemies.

B: The inner bark, or “phloem”, is pipeline through which food is passed to the rest of the tree
and it lives for only a short time, then dies and turns to cork to become part of the protective
outer bark.
C: The cambium cell layer is the growing part of the trunk and it annually produces new bark
and new wood in response to hormones that pass down through the phloem with food from the
leaves.
These hormones, called “auxins”, stimulate growth in cells.

Auxins are produced by leaf buds at the ends of branches as soon as they start growing in spring.

There are several easily visible layers in the tree. The first layer, the outer bark, provides
protection for the tree. For some species, such as Poplar, it is very thin. For others, such as
Douglas Fir, it can be very thick, sometime more than 10 centimeters.

…and sapwood on the inside. The sapwood transports the sap from the roots to the leaves where
it is turned into nutrients. Width of the sapwood may vary from a few centimeters in Douglas Fir
and Spruces to more than 30 centimeters in Ponderosa Pine.
There are many thin layers inside the inner bark. Each indicates a year of growth, commonly
called an annual ring. The total number of layers at ground line tells the tree's age. Thickness of
the layers varies with species, tree age and growth conditions. A thin layer, for example, could
indicate a year of low rainfall.

1.3 Wood as an Industrial Raw Material

/Wood Products/

Primary e.g.
 Round wood
 Sawnwood
 Wood-based panels
 Plywood
 Laminated Veneer Lumber (LVL)
 Particleboard (chipboard)
 Medium density fibreboard
 (MDF)
 Oriented Strand Board (OSB)
 Softboard/hardboard
Secondary processed e.g.
 Laminated panel board
 Glue Laminated Timber (glulam)
 Composite “I” beams
 Coated wood and panels
 Pressure (preservative) treated
 Modified wood (chemical, physical modification)
 Chapter Two
2. Structures of Wood
2.1 Macroscopic structure of wood

The cross-section of a tree is divided into three broad categories consisting of the bark, wood, and
cambium Micro-fibril orientation for each cell wall layer of Scotch pine with chemical
composition as percentage of total weight.
Cell wall layers are primary (P), S1, S2, and S3.
Wood, or xylem, is composed ofthe inner sections of the trunk.
The primary functions of wood are support and nutrient conduction and storage.

 Wood can be divided into two general classes: sapwood and it functions primarily in food
storage and the mechanical transport ofsap.

 The radial thickness of sapwood is commonly 35 to 50 mm but maybe 75 to 150 mm for some
species.

 Heartwood consists of an inner core of wood cells that have changed, both chemically and
physically, from the cells of the outersapwood.

 The cell cavities of heartwood may also contain deposits of various materials that frequently
give heartwood a much darker colour.
 Extractive deposits formed during the conversion of living sapwood to dead heartwood often
make the heartwood of some species more durable in conditions that may induce decay.

 The cambium is a continuous ring of reproductive tissue located between the sapwood and the
inner depending on the season.

 All wood and bark cells are aligned or stacked radially because each cell in a radial line
originated from the same cambial cell.

2.2 Microscopic structure of wood

 The primary structural building block of wood is the tracheid or fibre cells.
 Cells vary from 16 t0 42um in diameter and from 870 to 4000um long.
 Thus a cubic centimeter of wood could contain more than 1.5 million wood cells.
 When packed together they form a strong composite.
 Each individual wood cell is even more structurally advanced because it is actually a
multilayered, closed-end tube rather than just a homogeneous-walled, non-reinforced straw.
 Each layer is composed of a combination of three chemical polymers: cellulose,
hemicelluloses, and lignin.
 The cellulose and hemicellulose are linear polysaccharides (i.e., hydrophilic multiple-
sugars), and the lignin is an amorphous phenolic (i. e., a three dimensional hydrophobic
adhesive).
 Cellulose forms long unbranched chains and hemicellulose forms short branched chains.
 Lignin encrusts and stiffens these polymers.
 Because carbohydrate and phenolic components of wood are assembled in a layered tubular
or cellular manner with a large cell cavity, specific gravity of wood can vary immensely.
 Wood excels as a viable building material because the layered tubular structure provides a
large volume of voids (void volume),
 It has an advantageous strength-to-weight ratio, and it has other inherent advantages, such
as corrosion resistance, fatigue resistance, low cost, and ease-of modification at the job site.
 Pit structure is but oneof the ultrastructure features of the woody cell wall.
 The cell wall is composed of a great number of microfibrils, as indicated by the fine lines in
this diagram.
 A microfibril is a bundle of cellulose polymer chains. Orientation of microfibrils is very
specific for each layer.
 As shown here, microfibrils of the S2 layer run more or less parallel to the long axis of the
cell, whereas microfibrils of the S1 and S3 run more or less horizontally.
 Orientation of microfibrils in the primary wall is random. Minute structure of the cell wall
largely determines properties of individual fibers as well as wood as a whole.

2.3 Different structure of Softwood and Hardwood

 Chapter Three
3. Properties of wood

3.1.1 Physical Properties of Wood

Workability- the relative case in which wood is shaped cut and fastened together than the others

Warping- is the general term used to describe anyvariation from a true surface.

3.1.2 Density and Specific Gravity

Density

 The density of a material is the mass per unit volume at somespecified condition.

 For a hydroscopic material such as wood, density depends on two factors: the weight of
the wood structure and moisture retained in the wood.
 Wood density at various moisture contents can vary significantly and must be given relative to
a specific condition to have practical meaning.

 Density (ρ) of a material is defined to be the mass per unit volume and is calculated using
the following equation:

 Density of the substance that makes up a wood cell wall has been found to be about 1.5
g/cm3. However, an actual sampleof wood also contains air in the cell lumens, so most
woods have a density less than 1 g/cm3

Density of a sample of wood is usually calculated as the weight density instead of mass:
The oven-dry weight of white C. lusitania wood is 600g. What is its density if the final
volume is 600cm3 ?
Solution: Density (ρ) = M/V
ρ = 600g/600cm3
ρ = 1g/cm3

Specific Gravity
 Specific gravity provides a relative measure of the amount of wood substance contained in a

sample of wood.
 It is a dimensionless ratio of the weight of an oven dry volume of wood to the weight of an

identical volume of water.

 In research activities, specific gravity may be reported on the basis of both weight and volume

oven-dry.
 For many engineering/technology applications, the basis for specific gravity is generally the

oven-dry weight and volume at a moisture content of 12%.

 For example, a volume of wood at some specified moisture content with a specific gravity

of 0.50 would have a density of 500 kg/m3.

3.1.3 Wood Hygroscopicity and Moisture Content in Wood

 Hygroscopicity of wood
 Hygroscopicity is the property of wood to attract moisture from surrounding atmosphere and
to hold it in the form of liquid water or vapor.
 The moisture of living trees varies from about 30 to 200%.
 In softwoods, heartwood has a lower moisture content than sapwood (heartwood about 55 %
and sapwood about 149 %).
 The basic reason for moisture entering into the mass of wood is the attraction of water molecules
by the hydroxyls of its chemical constituents, mainly cellulose.
 As a result, a monomolecular layer of water is formed and held by these hydroxyls with strong
hydrogen bonds.
 Living tree may have 30% to 200% M.C.
 Free water
 water contained in cell cavities

 removed without volumetric changes

 Bound water
 water contained within the cell walls

 removal of this water caused volumetric changes

 “Shrinkage” is from the removal of bound water from cellwall

 “Swelling” will occur if water is reintroduced into cellwall
 There is an optimum M.C. for each species(typically between 7-14% m.c.)
 Specification allow 19 percent maximum (somesay 15%)
 High M.C.--- promotes warping, fungus, insects
 Low M.C. --- brittleness
 MC related to Seasoning

Seasoning/Drying is the process of removing moisture from a harvested wood

Green wood contains 30 to 200% moisture by oven-dried weight, this is lowered to
7% for dry areas or up to 14% in damp areas, leaving a saw mill, wood is at 15%
moisture

Air drying (inexpensive and slow)

 Stack boards with air space between them to allow drying
 After 3 to 4 months, it reaches the local humidity level
 Often requires further dying to reach acceptable levels

Kiln drying (scientific and expensive)

 Boards dried at 70-120 F for 4-10 days
 Rapid drying may result in cracks and deformed lumber, and post-process wood
is thirsty, so it must be covered and cared for properly
3.2 Mechanical Properties
 Tensile Strength – Timber is stronger in tension along the rain but it’s quite difficult to
determine this because of the difficulties in conductingtest.
 Compressive Strength – The strength along the grain is important forcolumns, props, and post.
 Shear Strength – Shear strength is important in the case of the beam and slabs.
 Bending Strength– This refers as the strength of the timber as abeam.
 Cleavability – High resistance for cleavage is important for nailing and screwing while low
splitting strength is important for used as firewood.
 Torsion Strength – is used to determine the torsion strength of thetimber and the specimen
is loaded up to failure.
 Hardness – is important in case of timber for paving blocks flooringbearings and other
similar purposes.
Stiffness - This property is important to determine the deflection of atimber under a load.
Concept of Stress and Strain

 Wood can undergo a certain load, or stress, before breaking.

 Stress is a distributed force per unit area, and occurs when a force or load acts on a solid
member, such as a load on a column.
 Stress is often expressed in units of pounds per square inch (psi) or Pascal's (Pa or N m ).
 When stress is applied to a rigid body like a block of wood, distortion of the wood occurs.
 A large stress will cause the wood to fail and break.
 Low stresses will cause the wood to distort (bend or shrink) without actually breaking.
 The change in length divided by the original length is called strain and a unit- less measure.
 These concepts are illustrated in the figure below. On the left, we have a column without stress,
while on the right, the column has been subjected to an 8000 lb force that is compressing the
wood. This compression results in a slightly shorter beam

Why do we care about stress and strain?

Understanding the concepts of stress, strain, MOE, and MOR are important because they are
used in determining allowable design stresses, or the maximum stress the wood can be
exposed to with reasonable assurance that the wood will not fail. This information, in
combination with strength properties, gives us the ability to use wood with confidence that it
will beable to meet the design stresses.