Timber, its use and conections

• 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

STRUCTURE OF WOOD
Cambium Layer
Bark
Pith
Annual Rings

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.

“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.

 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.

 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.

FACTORS AFFECTING
STRENGTH OF TIMBER
• Density
• Moisture Content
• Temperature
• Grain structure
• Position in tree
• Condition of growth
• Defects
• Creep

• 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.

• 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

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.

• 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.

• POSITION IN TREE
 In the early tree’s life, wood often tends to
become stronger with increasing distance
from the pith

• 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

• 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

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.

• Shake
 Shakes are lengthwise separations in the
wood occurring between and parallel to
annual rings

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.

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.

• 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

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

One of nature’s most efficient structures:
an Arbutus tree facing the onslaught of West Coast storms

Decay of wood
Requirements:
• nutrition (wood)
• modest temperature (~ 20 C)
• moisture (the only one that can be readily controlled)

Comparative
material
properties
Strain, %
Stress (MPa)
-20
-100
-200
-400
300
302010-10
400
-300
200
100
mild steel
wood (parallel to grain)
concrete

• One of the biggest challenges in light timber construction
• Also an important benefit of heavy timber construction
Fire resistance

Reliability and Safety
Load
distributions
Strength distributions
Load, Resistance
Probabilityofoccurrence
Probability of failure
(overlap area)

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

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

Grading of timber
Defects that affect the
strength of timber

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

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

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

Use of dimension lumber in
residential construction

Large glulam
beams in
buildings
and bridges
Defects are
distributed
among many
laminations

Engineered wood products -
pick the best member for each application
plywood
finger-jointed studs
oriented
strandboard
I-joists
laminated veneer lumber
laminated strand lumber

Efficient use of timber for a long span roof
(minimal connections)

Timber, its use and conections

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