2. • Timber is one of the most popular material
in light construction because of :
a) simplicity in fabrication
b) lightness
c) reusability
d) insulation from heat. sound & electricity
e) aesthetically pleasing appearance
f) resistance to oxidation, acid attack & salt
attack and salt water
g) environmental compatibility
4. STRUCTURE OF WOOD
“Bark” is a thin, rough and dense covering
that surrounds the trunk.
“Cambium” is a thin (microscopic) layer of
wood cells exists inside the bark.
The growth of wood takes place continuously
under the bark in the cambium layer resulting
ring knowns as “Annual ring”.
Width of ring depends on the rate of growth of
the tree.
5. “Pith” is the center of the log surrounded by
the annual rings.
The number of rings approximately represents
the age of the tree.
“Heartwood” is the inner part of the trunk is
made of dead tissue which primary function is
to provide mechanical support to the tree.
Heartwood that is the older wood is darker,
drier and harder than the outer part.
“Sapwood” is the outer part and it contains
living cells.
6. The trunk place the role to convey a solution
called sap to the leaves & also to support the
crow at such a height as to ensure a
sufficiency of air & light.
As one layer of woods succeeds another, the
cell in the layers die, cease to function for
food storage and only useful to give the tree
stiffness.
8. Physical & mechanical properties of wood
differ from species to species & also within
species.
Among the factor influencing it’s properties
are climate, density of the surrounding
forest, character of the soil, moisture
content, defects and the area in the log from
which the lumber is derived.
9. FACTORS AFFECTING
STRENGTH OF TIMBER
• Density
• Moisture Content
• Temperature
• Grain structure
• Position in tree
• Condition of growth
• Defects
• Creep
10. • DENSITY
Density of wood is defined as the mass or
weight per unit volume.
Moisture in wood has a very large effect on
the specific gravity as well as the density.
Timbers of young tree has a very low density,
therefore reduced stresses used for such
material.
Weight of timber reduced by drying while
most strength properties are increased.
The higher the density, so the higher it’s
mechanical properties.
11. • MOISTURE CONTENT
Moisture content in a living tree varies with
the species.
Even in the same species, variation in moisture
content depends on the age & size of the tree
and its location.
Mechanical properties of wood influenced by
moisture content but modulus elasticity is less
affected by changes in moisture
Strength of wood increase as the moisture
content decreases
12. Moisture content determined by oven-dry
method or by electric moisture-meter
method
Drying of timber from the green condition
as cut to constructional usable content of
say, 18% moisture content will cause
shrinkage.
13. • TEMPERATURE
Strength of timber decreases, together with
increasing temperature
Permanent loss of strength may happen if
wood is held at high temperature for a long
period.
14. • POSITION IN TREE
In the early tree’s life, wood often tends to
become stronger with increasing distance
from the pith
15. • CONDITION OF GROWTH
Environmental factor such as height above
the sea level, temperature, type of soil,
rainfall, spacing between the trees have
effect on the strength of the properties of
the timber
16. • DEFECTS
Common defect are cracks, knots and slope
of grain and occur principally during the
growing period and the drying process.
It can be classified:
a) knots
b) shake
c) split
d) check
17. Knots
It is a cross-section or longitudinal section
of a branch that was cut with the lumber
It could affect the mechanical properties of
wood
Also allow stress concentration to occur
Effect of knots depends on their position in
the section.
Knots are harder, denser and possess
different shrinkage characteristics than those
of wood tissue.
19. • Shake
Shakes are lengthwise separations in the
wood occurring between and parallel to
annual rings
20. Split
It’s a complete separation of wood fibers
usually at the ends, throughout the thickness
of lumber and parallel to the fiber direction.
could affect the durability of timber.
21. Check
It’s a lengthwise separation of wood
occurring across or through the annual ring
usually as a result of seasoning.
It can occur anywhere on a piece of
lumber.
23. • DECAY or DESTRUCTION
Decay is a decomposition of the wood
substances caused by fungi
Wood suffered fungal attack can becomes
brittle or weak.
Destruction is done by insects or marine borers
Termites normally found in warmer climates &
wood beetles in conditions of high humidity
25. Timber as a structural material
• The oldest construction material and still
one of the most versatile
• A natural material with inherent flaws and
variability
• We need to recognize its strengths and
weaknesses
• Timber design therefore as much an art as a
science
26. One of nature’s most efficient structures:
an Arbutus tree facing the onslaught of West Coast storms
27. Decay of wood
Requirements:
• nutrition (wood)
• modest temperature (~ 20 C)
• moisture (the only one that can be readily controlled)
32. Material
properties of
wood
lignin
cellulose fibres
… imagine a bundle of
straws held together with
elastic bands
• tension parallel to grain
• compression parallel to grain
• tension perpendicular to grain
• compression perpendicular to grain
• shear
33. Consequences of different design
values
• Avoid tension perpendicular and shear stresses at
all cost
• Make use of compression strength of wood as
much as possible
• Simplify connections and use compression load
transfer when possible
• Avoid stress concentrations and complex stress
patterns
35. Visual Grading of Lumber
• Lumber is sorted for a specific application, e.g.
– For tension members all knots and defects have a
significant effect
– For beams and stringers, the grader focuses on
edge knots
– For posts and timbers sloped grain is more
important
• The larger the members, the higher the
probability of missing some important defects
36. The sorting process
• Sorting by species
– Species of similar strength characteristics are
lumped together
• Visual grading
– A certified grader sorts wood by hand according to
visual appearance
– Lumber gets sorted according to end use
– Grading criteria:
• Knots (type, location, size, frequency), wane, checks,
slope of grain, pitch pockets
• Mechanical grading
37. Testing of lumber
Tension test
Bending test
Full size members are tested
(a) To failure (full
distribution is obtained)
(b) Up to a proof load (only
lower tail end of
distribution is obtained)
Strength
distribution
Probabilityof
occurrence
Strength5th percentile value
Proof load
42. Engineered wood products -
pick the best member for each application
plywood
finger-jointed studs
oriented
strandboard
I-joists
laminated veneer lumber
laminated strand lumber
43. Efficient use of timber for a long span roof
(minimal connections)