LIST OF FIGURESFigure 1: Mpingo (African Blackwood)2Figure 2:
Mkongo (Pod Mahogany)2Figure 3: Mvule(African Teak)3Figure 4:
Mninga(East African Padauk)3Figure 5: Natural seasoning8Figure 6:
Kiln seasoning8Figure 7: Electric kiln9Figure 8: Radial
shake10Figure 9: Heart shake10Figure 10: Cup shake11Figure 11:Star
shake11Figure 12: Rind gall11Figure 13 : Wall formwork15Figure 14
:stair form work15
Table of ContentsLIST OF FIGURESiCHAPTER
ONE11.0INTRODUCTION11.1DIFFERENT TYPES OF TIMBER FOUND IN
TANZANIA11.2HARDWOODS/CONIFEROUS11.3SOFTWOODS11.4DIFFERENT TYPES OF
TIMBER FOUND IN TANZANIA21.4.1MPINGO (AFRICAN
BLACKWOOD)21.4.2MKONGO(Pod Mahogany)21.4.3MVULE(African
Teak)31.4.4MNINGA(East African Padauk)3CHAPTER
TWO52.0CHARACTERISTICS OF TIMBER52.1INTRODUCTION52.2MECHANICAL
PROPERTIES52.2.1Stress strain relationship52.2.2Compressive
strength52.2.3Tensile strength62.2.4Bending strength62.2.5Shear
strength62.3PHYSICAL PROPERTIES62.3.1Density and specific
weight62.3.2Moisture movement62.3.3Swelling72.3.4Heat
conductivity72.3.5Sound conductivity72.3.6Resistance to action of
acid and alkali72.4SEASONING OF TIMBER72.4.1Advantages of
timber72.4.2Types of Timber Seasoning82.4.3Natural
seasoning82.4.4Artificial seasoning82.4.5Water Seasoning:9CHAPTER
THREE103.0DEFECTS OF TIMBER103.1INTRODUCTION103.2Defects due to
abnormal growth103.3Defects due to conversion123.4Defect due to
seasoning12CHAPTER FOUR134.0TIMBER
PRESERVATION134.1INTRODUCTION134.2OIL TYPE PRESERVATIVE134.3ORGANIC
SOLVENT PRESERVATIVES134.4ACETIC ANHYDRIDE TREATMENT134.5WATER
SOLUBE PRESERVATIVES134.6Methods of applying
preservatives144.7DIFFERENT USES OF TIMBER IN
CONSTRUCTION154.8Challenges of using timber in
construction165.0CONCLUSION176.0REFFERENCE18
CHAPTER ONE1.0 INTRODUCTIONTimber refer to the wood used for the
construction work in fact the word timber is derived from an old
English word TIMBRIAN which mean to build, on the other hand timber
is material delivered from trees high plant gymnosperm and
angiosperm division. Hundred year timber has been used as building
material in the country, timber as the building material it used
for varieties structural work such as trusses for roof
construction, beam, column, railway bulk and bridges structural
element. Timber can be highly being durable when properly treated
been the case proper understanding of nature, limitations,
properties of timber is required in order have a good safe timber
structures1.1 DIFFERENT TYPES OF TIMBER FOUND IN TANZANIA There
several types of timber found in the country, they names given as
results of the trees names such as mninga , mpodo, mvule and
Mpingo. Timber used for engineering works is divided into two
classes1.2 HARDWOODS/CONIFEROUSThese are dicotyledonous plants
characteristically with broad leaves, these tree give non resinous
wood, the timber obtained from these trees are strong along and
across fibers it is also flexible, Strong and tough capable of
bearing tension, compression or shear.Examples of hardwood found in
Tanzania are Mninga(East African padauk), Pangapanga, and Msenjele
(African lignum vitae) which are normally found in Eastern South of
Tanzania.1.3 SOFTWOODS Softwood is wood from gymnosperm trees such
as conifers. Softwood is the source of about 80% of the world's
production of timber, with traditional centers of production being
the Baltic region. Softwoods are not necessarily softer than
hardwoods. In both groups there is an enormous variation in actual
wood hardness, with the range in density in hardwoods completely
including that of softwoods.
1.4 DIFFERENT TYPES OF TIMBER FOUND IN TANZANIA1.4.1 MPINGO
(AFRICAN BLACKWOOD)Mpingo is one of the most expensive timbers in
the world and is preferred wood of the musical instrument trade
because of its high density, fine texture and exceptional
durability. Due to unsustainable extraction the tree is threatened
with commercial extinction, but a sustainable trade is possible,
providing a secure long term future for both woodwind musicians and
communities who live around the forests where it grows and they are
found in the southern part of Tanzania .
Figure 1: Mpingo (African Blackwood)1.4.2 MKONGO(Pod Mahogany)
Although prized locally, pod mahogany is rare in international
trade. Small quotas for the species restrict its application to
specialist markets. The most suitable sectors for development are:
(1) musical instruments, particularly in the manufacture of guitars
and xylophones, and (2) furniture, where it could be used to
substitute marabou, bubinga and/or rosewood. FSC 100% supplies of
pod mahogany could also be used to replace doussie, merbau and
bubinga in the construction, household and consumer goods, and
flooring sectors (depending on the volumes required).
Figure 2: Mkongo (Pod Mahogany)1.4.3 MVULE(African Teak) African
teak has strong dark brown hardwood resistant to termites and is
used for construction, furniture, joinery, paneling, floors and
boats. The tree can be used in the control of erosion. It makes a
good shade tree and is useful as a roadside tree in urban areas. It
grows rapidly, can be coppiced and is ready for cutting after about
fifty years. The tree is nitrogen fixing and the leaves are used
for mulching. Figure 3: Mvule(African Teak)1.4.4 MNINGA(East
African Padauk) East African padauk is already used to manufacture
flooring, furniture, construction materials, household and consumer
goods, and musical instruments. It is easily worked and versatile
which, together with the large quota available, permits its
potential entry into numerous sectors, including large scale and
structural applications (subject to the dimensions required). FSC
100% stocks could be pitched as a supplement or alternative to
current supplies and similar species in all of the existing
industries, as well as in the manufacture of windows and doors,
musical instruments and boats and yachts. Figure 4: Mninga(East
African Padauk)i. The system is not expensive to be implemented
into offices.ii. It offers good records keeping in form of images
because it has the memory card for storage purpose.iii. It provides
enough evidence in lawsuits this is due to fact that the system is
capable of saving the image of the victim.iv. It is easy to do the
maintenance and repair
CHAPTER TWO2.0 CHARACTERISTICS OF TIMBER2.1 INTRODUCTIONThe
principal characteristics of timber with which specifiers may be
concerned on are strength, durability and finished appearance
however timber can be characterized based on the following 2.2
MECHANICAL PROPERTIES Engineers, architects and carpenters must be
well versed with the mechanical properties oftimber. In order that
the engineer may properly design columns and beams for various
partsof wooden structures, he must be thoroughly conversant with
the strength and stiffness of theavailable classes of timber. The
architect must not only appreciates the beauty of various species,
the relative ease with which each may be worked, the tendency to
shrink, warp, and check; but he must likewise be prepared to
proportion joints and rafters to carry the imposed loads
withoutexcessive deflection. The wheelwright must understand how
the toughness and strength of hisaxles, spokes, and shafts are
influenced by species, rate of growth, density, and defects.
Thecarpenter and the craftsman must also have knowledge of the
mechanical properties of woodin order that they may work it to best
advantage. The mechanical properties of timber thatneed elaboration
are as follows.
2.2.1 Stress strain relationship Wood has three principal axes,
longitudinal, radial and tangential along which properties are
fairly constant. Since wood is a noni tropic material, it has three
values of modulus of elasticity varying by as much as 150 to 1,
three shear moduli varying by 20 to 1, and six Poissons ratios
varying by 40 to 1. There is no sharply defined elastic limit in
wood but there is a proportional limit. However, the stress-strain
diagram in any direction is fairly straight over a considerable
range before it gradually curves off. It is a ductile material.The
relative stress-strain curves for direct tension, direct
compression and bending stress intensities parallel to the grain in
Fig. 4.22 show that in both, direct compression and bending, the
proportional limit is in the vicinity of 65 to 75 per cent of the
ultimate strength2.2.2 Compressive strengthWhen subjected to
compressive force acting parallel to the axis of growth, wood is
found to be one of the strongest structural material. Columns and
posts are, therefore, often fashioned of it. However, compressive
strength perpendicular to fibres of wood is much lower than that
parallel to fibres of wood. When wood is subjected to compression
parallel to the grain, it may fail through collapsing of the cell
walls or through lateral bending of the cells and fibres. In wet
wood and in the hardwoods, which are composed of thick-walled
fibres and vessels, incipient failure is due to bending of the
individual fibres. In cross-grained pieces, the failure is likely
to take place through shear parallel to the grain.2.2.3 Tensile
strength When a properly shaped wooden stick is subjected to
tensile forces acting parallel to the grain it is found to have
greater strength that can be developed under any other kind of
stresses. Indeed, the tensile strength of wood parallel to the
grain is so great that much difficulty is encountered in designing
end connections so that the tensile strength of a piece can be
developed. Therefore, wood tension members are rarely used. Tensile
strength parallel to the fibres is of the order 80.0 to 190.0 N/
mm2 . However, wooden parts restrained at their ends suffer from
shearing stresses and crushing which wood resists poorly, and
cannot be extensively used in structure working under tension2.2.4
Bending strength Wood well withstands static bending, owing to
which it is widely employed for elements of buildings, e.g. beams,
slabs, rafters, trusses, etc. The initial failure of long beams of
uniform width is indicated by a wrinkling of the overstressed
compression fibres, much like the failures which occur in
compression prisms. Final failure of such beams is generally in
tension. It is accompanied more or less by snapping as the
individual fibres begin to break when the maximum load is reached.
Very dry specimens sometimes fail very suddenly intension before
any wrinkling of the compression fibres is noticeable. Short deep
beams fail by horizontal shear suddenly, and this is more common in
well-seasoned timber of structural sizes than in green timbers or
in small beams2.2.5 Shear strengthWood has low shearing strength of
6.514.5 N/mm2 along the fibres. Resistance of wood to cutting
across the fibres is 3 to 4 times greater than that along the
fibres, but pure shear generally does not take place since the
fibres are also subjected to crushing and bending.
2.3 PHYSICAL PROPERTIES2.3.1 Density and specific weight All the
physical properties of clear wood are related to its density, which
varies directly with the apparent specific gravity. The true
specific gravity of wood is approximately equal for all species and
averages 1.54, whereas the specific weight and apparent specific
gravity vary with density of wood. The percentage of moisture in
the wood has a very large effect upon the specific weight and hence
true comparisons of this property can only be made on dry
specimens2.3.2 Moisture movement Water is found in three portions
of wood: (1) it constitutes over 90 percent of the protoplasm in
the living cells; (2) it saturates the cell walls; (3) it fills,
more or less completely, the pores of the life less cells. Timber
is liable to shrink or swell with the movement of moisture. This
movement is not the same in all the directions.
2.3.3 Swelling Is the capacity of wood to increase both its
linear and volumetric dimensions when it absorbs water. Swelling of
wood along the length of fibres ranges from 0.1 to 0.8 per cent, 3
to 5 per cent in the radial direction and 6 to 12 per cent in the
tangential direction2.3.4 Heat conductivity Is quite low and the
coefficient of heat conductivity along the fibres is 1.8 times
greater than that across the fibres and averages 0.15 to 0.27 K cal
/mhC. As the bulk density of wood increases and its moisture
content decreases, the amount of air entrapped inside cavities
decreases, the effect being greater heat conductivity of wood.
2.3.5 Sound conductivity The velocity of sound in wood is 2 to
17 times greater than that in air and as such wood may be
considered to have high sound conductivity2.3.6 Resistance to
action of acid and alkali Wood is not affected by weak alkali
solution but decays in an acid medium (pH< 4).
2.4 SEASONING OF TIMBER As fresh timber which is obtained from
trees contains about 30 to 40 % sap or moisture. This sap is very
harmful for the life of a timber. Therefore, it is necessary to
remove that sap by applying some special methods. All those methods
which are used for removing the sap from timber are collectively
termed as seasoning of timber2.4.1 Advantages of timber It has
reduced weight, It is strong and durable It has resistance to decay
or rot It takes high polish It is easier to work Its life is
more.
2.4.2 Types of Timber Seasoning Natural Seasoning, Artificial
Seasoning,a. Kiln Seasoning,b. Chemical Seasoning,c. Electric
Seasoning, Water Seasoning2.4.3 Natural seasoning In the air
seasoning or natural seasoning or natural drying, seasoning of
timber, timber is dried by direct action of air, wind and sun. In
this method, the timber logs are arranged one over the other,
keeping some space or distance between them for air circulation of
fresh air.Generally this type of seasoning requires few months to
over a year, this is very slow process.
Figure 5: Natural seasoning2.4.4 Artificial seasoning Kiln
Seasoning:In kiln seasoning timber is placed in a chamber with some
special heating arrangement. In this process one thing should be
kept in mind that heating system should be under control, otherwise
timber will be crack or wrap. The time required for this seasoning
is 3 to 12 days. This is quick process
Figure 6: Kiln seasoning
Chemical Seasoning:In chemical seasoning carbon dioxide,
ammonium carbonate or urea are used as agents for seasoning, those
are applied in dry state, the inter surface of timber dries first
than outer side. This ensures uniform seasoning. The time required
for this seasoning is 30 to 40 days Electric Seasoning:In this
method electric current is passed through the timber logs. The time
required for this seasoning is 05 to 08 hours Figure 7: Electric
kiln2.4.5 Water Seasoning:In water seasoning, timber logs are kept
immersed whole in the flowing water. The sap present in timber is
washed away. After that logs are taken out from water and are kept
in open air, so water present in timber would be dried by air. The
time required for this type of seasoning is 2 to 4 weeks.
CHAPTER THREE3.0 DEFECTS OF TIMBER3.1 INTRODUCTION Defects can
occur in timber at various stages, principally during the growing
period and during the conversion and seasoning process. The defects
in the wood as shown in Fig. 4.4 are due to irregularities in the
character of grains. Defects affect the quality, reduce the
quantity of useful wood, reduce the strength, spoil the appearance
and favor its decay.3.2 Defects due to abnormal growth Following
are some of the important defects commonly found in wood due to
abnormal growth or rupture of tissues due to natural forces.
I. RADIAL SHAKEThese are similar to the star shakes and occur in
felled timber when exposed to the sun during seasoning. Radial
shakes are generally irregular, fine and numerous .In this many
splits are appeared
Figure 8: Radial shake
II. HEART SHAKE Occurs due to shrinkage of heart wood, when tree
is over matured. Cracks start from pith and run towards sap wood.
These are wider at center and diminish outwards.
Figure 9: Heart shake
III. CUP SHAKEAppears as curved split which partly or wholly
separates annual rings from one another. It is caused due to
excessive frost action on the sap present in the tree, especially
when the tree is young
Figure 10: Cup shakeIV. STAR SHAKEAre radial splits or cracks
wide at circumference and diminishing towards the center of the
tree. This defect may arise from severe frost and fierce heat of
sun. Star shakes appear as the wood dries below the fiber
saturation point. It is a serious fault leading to separated log
when sawn
Figure 11:Star shake
V. RIND GALLCharacterized by swelling caused by the growth of
layers of sapwood over wounds after the branch has been cut off in
an irregular manner. The newly developed layers do not unite
properly with the old rot, thereby leaving cavities, from where
decay starts.
Figure 12: Rind gall
3.3 Defects due to conversion Conversion is the term used to
describe the process whereby the felled tree is converted into
marketable sizes of timber. Conversion defects are basically due to
unsound practice in milling or attempts to economize during
conversion of timber. A wane occurs in timber which contains, on
one or more faces, part of the bark or the rounded periphery of the
trunk. This reduces the cross sectional area, with consequent
reduction in strength in the parts affected. Excessive slope of
grains may also be classed as a conversion defect when conversion
has not been done parallel to the axis of the trunk.
3.4 Defect due to seasoning These defects are directly caused by
the movement which occurs in timber due to changes in moisture
content. Excessive or uneven drying, exposure to wind and rain, and
poor stacking during seasoning can all produce distortions in
timber. These defects result in loosening of fixings or disruption
of decoration, or both. The common types of seasoning defects are
checkslongitudinal separation of fibres not extending throughout
the cross-section of wood, splittingseparation of fibres extending
through a piece of timber from one face to another , warp
ageconsists of cupping, twisting and bowing.
CHAPTER FOUR4.0 TIMBER PRESERVATION4.1 INTRODUCTION Timber
preservation generally refers to the application of treatments
(chemicals) to timber to stop the attack of woodworm, fungal decay
(wet rot/dry rot) and to protect it from the effects of dampness.
Alternative methods of timber preservation can be employed without
the use of chemicals, these could be to introduce a physical
barrier (such as a membrane) between the timber and a source of
moisture or even something as simple as increasing the airflow
around timbers can prevent or arrest the causes of timber
degradation. The following are the preservative methods4.2 OIL TYPE
PRESERVATIVEApplied over outside of exposed timber, give unpleasant
smell and are not suitable when timber is to be painted. The types
in use are creosote, carbolinium, solignum etc. with or without
admixture with petroleum or suitable oils having a high boiling
range.4.3 ORGANIC SOLVENT PRESERVATIVESPreservatives Insoluble in
Water) consists of toxic chemical compounds, e.g.
pentachlorophenol, benzene-hexa-chloride, dichlorodiphenyl
trichloro-ethane (D.D.T) and copper naphthenate. These are
dissolved in suitable organic solvents like naphtha, or in
petroleum products such as kerosene, spirit, etc. The treated
timber can be painted, waxed or polished.4.4 ACETIC ANHYDRIDE
TREATMENTIs used for protection of veneers, plywood and light
lumbers against decay by acetylation. They are treated with acetic
anhydride vapor, which minimizes swelling and improves resistance
to decay and attack by insects4.5 WATER SOLUBE PRESERVATIVESAre
odourless organic or inorganic salts and are adopted for inside
locations only. If applied over outside surfaces, the salts can be
leached by rainwater. Examples of leachable (3A-water soluble) type
of preservatives are zinc chloride, boric acid (borax), etc. Zinc
chloride, sodium fluoride and sodium-penta-chloro-phenate are toxic
to fungi. These are expensive and odourless (except for
sodium-penta-chloro-phenate). Benzenehexa-chloride is used as spray
against borers. Boric acid is used against Lyctus borers and to
protect plywood in tea chests.
to compile data. However, such standardized answers may
frustrate users. Questionnaires are also sharply limited by the
fact that respondents must be able to read the questions and
respond to them.4.6 Methods of applying preservatives Before
applying preservatives, the timber should be completely seasoned.
There are some importantmethods of applying timber preservatives
which are given below. Painting and dipping method Pressure process
or full cell process Empty cell process Painting and dipping
method: This is the most common method in which the preservative
material is applied by means of a brush several times. The timber
is also immersed in a tank full of liquid (preservative material).
In both types the penetration hardly exceeds 1/16. The duration of
immersion and temperature of solution is increased the penetration
rate.Pressure process or full cell process: In this process, the
timber is placed in an air tight chamber, from which air is
withdrawn by creating a vacuum. The cells are full emptied to
receive preservative material. After that preservative material is
pumped under pressure of 100 to 200 psi and at a temperature of
120degreeF. As the timber contains required quantity of
preservative a low vacuum is maintained to remove excess
preservative. Such a timber is generally used in case of piles in
salt water and railway sleepers.Empty cell process:This method is
similar to the full cell process but initial vacuum is not to be
maintained and no attempt is to be made to remove the air from
cells. The preservative material is applied under pressure of 200
psi
4.7 DIFFERENT USES OF TIMBER IN CONSTRUCTIONi. wood frame work
constructionIn certain part of the world such as Scandinavian
countries, houses will be entirely built of timber because it is
suitable for climatic conditions.Elsewhere, house builders can
choose to support the house by wooden frames or stud walling. Roof
Truss rafters are made entirely of wood and timber shuttering can
be chosen for concrete work; in addition some construction plans
require a massive bearing beam that will balance structure
Figure 13 : Wall formworkii. outdoor features
constructionConstruction of commercial and some private projects
will also include exterior work. Outside features such as patios
and decking will be made of wood. Additionally, garden architects
will require timber for raised plant containers and fencing, while
garden shed and garages are often constructed of timber.iii. used
in decorative worksThe most visible use of timber is displayed in
the finishing process of a construction project. Stair case, door
frames, skirting and floor boards as well as boiler, mater and pipe
boxes are wooden. Custom-built cupboards are also mostly wooden as
are fitted kitchen appliances
Figure 14 :stair form work
iv. Glued wood components e.g., beams, trusses, arches, frames
and roofs of buildings andInstallations are very effective in
chemically aggressive media because their service life is 1.5 times
greater than that of steel or reinforced concrete. However, the use
for wood should be economically justified and the possibility of
replacing it with prefabricated concrete, asbestos cement, gypsum,
plastics and other items should be carefully considered.
v. The use of fibreboard, ply-boards in building practice
provides a substantial saving both in capital investments and
running costs. The economy is provided, in the first place, by a
more complete utilization of raw materials for the manufacture of
building materials and items. The use of boards made of pressed
wood shavings in dwelling house construction has a great economical
effect. Currently, wood waste is utilized to manufactures polymer
and cement based fibreboard and wood shavings board. This also
allows manufacturing materials of better physical, mechanical and
decorative properties than wood.
4.8 Challenges of using timber in construction Safety from Fire
To provide an environment for the occupants inside or near a
building that is reasonably safe from fire and similar emergencies.
To provide reasonable safety for fire fighters and emergency
responders during search and rescue operations.
Safety from Structural Failure Provide a high confidence of a
low probability of structural failure resulting in local or global
collapse, or the creation of falling debris hazards that could
threaten life. Provide a high confidence that the structure will be
capable of resisting regularly occurring loads and combinations of
loads without significant damage or degradation.Shrinkage and
Swelling of Timber Timber is a hygroscopic material. This means
that it will adsorb surrounding condensable vapors and loses
moisture to air below the fiber saturation point.
Safety during Building Use Provide an environment for the
occupants of the building that is reasonably safe during the normal
use of the building.
5.0 CONCLUSION
In modern building practice, timber and other wood product are
extensively useful and normal they are used for walls and floors of
buildings, carpentry and graded plank items, as well as
prefabricated standard wooden cottages. This high usage of timber
has brought great growth of economy of a country also growth of
cities and town . A great quantity of wood is consumed in building
and installation work for making piles, poles, various load-bearing
components formworks, scaffolds. Currently, wood waste is utilized
to manufactures polymer and cement based fibreboard and wood
shavings board. This also allows manufacturing materials of better
physical, mechanical and decorative properties than wood.